Stormwater Drainage Manual PDF
Stormwater Drainage Manual PDF
Stormwater Drainage Manual PDF
STORMWATER
DRAINAGE MANUAL
Planning, Design and Management
Fourth Edition, May 2013
CONTENTS
Page
No.
TITLE PAGE
1
2
CONTENTS
1.
2.
3.
INTRODUCTION
13
1.1
SCOPE
13
1.2
ABBREVIATIONS
13
15
2.1
15
17
3.1
GENERAL
17
3.2
SYSTEM PLANNING
3.2.1
Overview
3.2.2
Progressive and early Improvement
3.2.3
Detailed Considerations
3.2.4
Location of Public Drainage System
17
17
17
17
18
3.3
19
19
19
3.4
ENVIRONMENTAL CONSIDERATIONS
3.4.1
Aesthetics/Landscape
3.4.2
Natural Streams and Rivers
3.4.3
Environmental Assessment
20
20
20
21
3.4.4
4.
5.
Environmental Nuisances
21
3.5
SITE INVESTIGATIONS
21
3.6
SAFETY ISSUES
21
RAINFALL ANALYSIS
23
4.1
GENERAL
23
4.2
HISTORIC RAINSTORMS
4.2.1
Applications
4.2.2
Point Rainfall
4.2.3
Areal Rainfall
23
23
23
23
4.3
SYNTHEIC RAINSTORMS
4.3.1
Applications
4.3.2
Intensity-Duration-Frequency (IDF) Relationship
4.3.3
Storm Duration
4.3.4
Design Rainstorm Profile
4.3.5
Areal Reduction Factor
4.3.6
Frequent Rainstorms
23
23
24
24
24
25
25
26
5.1
GENERAL
26
5.2
26
26
26
26
26
5.3
27
27
27
27
5.4
27
6.
28
6.1
GENERAL
28
6.2
28
6.3
28
6.4
29
6.5
FREEBOARD
29
6.6
29
30
30
6.7
7.
30
RUNOFF ESTIMATION
31
7.1
GENERAL
31
7.2
DATA AVAILABILITY
7.2.1
Rainfall
7.2.2
Evaporation/Evapotranspiration
7.2.3
Streamflow
31
31
31
31
7.3
31
31
31
32
7.4
STATISTICAL METHODS
32
7.5
DETERMINISTIC METHODS
7.5.1
Introduction
7.5.2
Rational Method
7.5.3
Time-Area Method
32
32
33
35
7.5.4
7.5.5
8.
Unit-Hydrograph Method
Reservoir Routing Methods
36
36
HYDRAULIC ANALYSIS
37
8.1
GENERAL
37
8.2
FLOW CLASSIFICATIONS
8.2.1
Laminar vs Turbulent Flow
8.2.2
Surcharge vs Free-surface Flow
8.2.3
Subcritical vs Supercritical Flow
8.2.4
Steady vs Unsteady Flow
8.2.5
Uniform vs Non-uniform Flow
8.2.6
Gradually Varied vs Rapidly Varied Non-uniform
Flow
37
37
37
38
38
38
39
8.3
UNIFORM FLOW
8.3.1
Frictional Resistance Equations
8.3.2
Compound Roughness
8.3.3
Partially Full Circular Sections
39
39
39
40
8.4
40
40
40
41
8.5
41
41
42
43
45
8.6
FLOW ROUTING
8.6.1
Introduction
8.6.2
Hydrologic Routing
8.6.3
Hydraulic Routing
46
46
46
46
8.7
47
8.8
48
9.
49
9.1
GENERAL
49
9.2
49
9.3
50
9.4
51
9.5
QUANTIFICATION OF SEDIMENTATION
51
52
10.1
GENERAL
52
10.2
MATERIALS
52
10.3
LEVELS
52
10.4
DEPTH OF PIPELINE
52
10.5
STRUCTURAL DESIGN
10.5.1
Introduction
10.5.2
Design Procedures for Rigid Pipes
10.5.3
Fill Loads
10.5.4
Superimposed Loads
10.5.5
Water Load
10.5.6
Bedding Factors
10.5.7
Design Strength
10.5.8
Effect of Variation in Pipe Outside Diameters
53
53
54
54
56
57
58
58
60
10.6
60
11. MANHOLES
61
11.1
GENERAL
61
11.2
LOCATION
61
11.3
ACCESS OPENINGS
61
11.4
ACCESS SHAFTS
62
11.5
WORKING CHAMBERS
62
11.6
INTERMEDIATE PLATFORMS
62
11.7
62
11.8
COVERS
63
11.9
63
63
65
12.1
GENERAL
65
12.2
65
12.3
DESIGN LOADS
65
12.4
DURABILITY
65
12.5
MOVEMENT JOINTS
66
12.6
FOUNDATIONS
66
12.7
66
66
66
67
67
67
68
68
69
13.1
GENERAL
69
13.2
CHANNEL LININGS
13.2.1
General
13.2.2
Types of Channel Linings
13.2.3
Design of Amour Layer
69
69
69
70
13.3
CHANNEL SHAPE
70
13.4
70
13.5
70
70
71
71
72
72
72
72
72
72
73
13.6
73
13.7
GEOTECHNICAL CONSIDERATIONS
13.7.1
Embankment Design
13.7.2
Factors of Safety
13.7.3
Loading Cases
13.7.4
Methods of Analysis
13.7.5
Seepage
13.7.6
Sensitivity Analysis
13.7.7
Methods for Stability Improvement
13.7.8
Geotechnical Instrumentation
13.7.9
Sign Boards for Slopes
73
73
74
74
74
75
75
75
75
75
13.8
OTHER CONSIDERATIONS
75
73
13.8.1
13.8.2
13.9
75
75
76
78
14.1
GENERAL
78
14.2
78
78
79
79
79
79
80
14.3
80
14.4
81
14.5
81
81
82
82
14.6
83
83
83
84
14.7
TRASH SCREENS
85
14.8
85
14.9
MISCELLANEOUS ISSUES
14.9.1
System Commissioning
14.9.2
Operation and Maintenance Issues
14.9.3
Division of Maintenance Responsibility
14.9.4
Future Extension
85
85
86
86
86
10
87
15.1
GENERAL
15.1.1 Maintenance Objectives
87
87
15.2
87
87
88
88
15.3
89
89
89
89
89
90
91
15.4
92
92
92
15.5
94
94
95
95
95
15.6
95
95
96
96
96
15.7
DRAINAGE RECORDS
96
15.8
SAFETY PROCEDURES
15.8.1
Safety Requirements for Working in Confined
Space
96
96
11
15.8.2
16.
97
TRENCHLESS CONSTRUCTION
99
16.1
INTRODUCTION
99
16.2
NON-MAN-ENTRY TYPE
16.2.1
Slurry Pressure Balance Method
16.2.2
Earth Pressure Balance (EPB) Method
100
100
100
16.3
MAN-ENTRY TYPE
16.3.1
Heading Method
16.3.2
Hand-dug Tunnel Method
101
101
101
16.4
MAJOR CONSIDERATIONS
16.4.1
Planning & Design Stage
16.4.2
Construction Stage
16.4.3
Environmental Issue
16.4.4
Cost Consideration
102
102
103
105
105
12
Page
No.
REFERENCES
106
TABLES
LIST OF TABLES
110
TABLES
112
FIGURES
LIST OF FIGURES
143
FIGURES
144
13
1. INTRODUCTION
1.1
SCOPE
This Manual offers guidance on the planning, design, operation and maintenance of
stormwater drainage works which are commonly constructed in Hong Kong. Such works
include stormwater pipelines, box culverts, nullahs, river training works, polders and
floodwater pumping facilities.
Some sections of the manual are also relevant for the management of natural
watercourses. DSD has also promulgated Practice Note No. 1/2011 for Design Checklists
on Operation & Maintenance Requirements which can be reached on DSDs internet home
page : www.dsd.gov.hk. Readers are requested to go through the Practice Notes in the
course of designing the drainage system to ensure that the final products satisfy the operation
and maintenance requirements of the maintenance authority.
1.2
ABBREVIATIONS
The following abbreviations are used throughout this Manual:
AFCD
Arch SD
BS
BS EN
BSI
CEDD
DLO/YL
DO/YL
DSD
EPD
ETWB
FEHD
FSD
GCO
GEO
GRP
HDPE
HKO
HyD
HKPF
LCSD
LD
MDPE
NENT
NWNT
14
PWD
SCS
TELADFLOCOSS
TD
uPVC
USBR
WBTC
WSD
15
(b)
16
(c)
(d)
17
GENERAL
3.2
SYSTEM PLANNING
3.2.1
Overview
Detailed Considerations
18
(a)
(b)
(c)
(d)
3.2.4
19
a minimum width of 6 m plus the outside diameter of the pipeline or outside width of culvert
is recommended.
For the implementation of public projects, the acquisition and allocation of land
should follow the prevalent Government procedures. Attention should be drawn to the
general principle that the land intake for each project should be kept to the minimum. If land
is required from LCSD, necessary consultation and arrangement with LCSD should be
initiated at the earliest possible stage.
The Hong Kong Planning Standards and Guidelines stipulate that no discharges
from new stormwater outfalls or nullahs should be allowed to drain into a typhoon shelter,
marina or boat park.
3.3
3.3.1
Government regularly publishes maps and town plans from which information on
land use and topography of catchment areas can be extracted. For large-scale works, aerial
photographs may provide an essential source of reference. Reference should also be made to
Drainage Services Departments drainage records for information on the existing stormwater
drainage systems.
3.3.2
Location of Utilities
(a)
General
The procedure for obtaining approval for the removal and/or diversion of existing
services belonging to utility companies can be lengthy and may require the sanction of the
Chief Executive-in-Council in exceptional circumstances. Engineers should therefore make
the necessary arrangement and obtain agreement with the utility companies in concern at the
20
earliest possible stage. Relevant Ordinances block licences and permits should be referred to
if necessary.
(c)
(ii)
3.4
ENVIRONMENTAL CONSIDERATIONS
3.4.1
Aesthetics/Landscape
All the drainage works should be designed to blend in with the environment. Special
attention should be paid to the aesthetic aspects of the structures and landscaping works.
Landscape architect of the relevant office in CEDD, Arch SD or HyD may be consulted for
advice on landscape treatment.
3.4.2
Natural streams and rivers are good habitats supporting a variety of wildlife and may
have important ecological functions and carry high aesthetic and landscape values.
Construction works should be restrained to minimize possible disturbance to the ecosystem.
For projects that may affect natural streams or rivers, the project proponents should ensure
that comments and advice received from AFCD and appropriate departments are
incorporated into the planning, design and construction of the projects as far as practicable.
If there is vegetation or landscaping features forming part of the mitigation requirements, the
21
project proponent should also identify the maintenance party during the design stage.
Designer should refer to DSD PN 1/2005 for more detailed guidelines on environmental
considerations for river channel design.
3.4.3
Environmental Assessment
The necessity for and the extent of a Project Profile and an Environmental Impact
Assessment (EIA) for stormwater drainage projects should be determined in accordance with
the prevailing Government procedures. The Environmental Impact Assessment Ordinance
(EIAO) was enacted on 4 February 1997 and came into operation on 1 April 1998. All
projects and proposals that are covered under Schedule 2 or 3 of the EIA Ordinance shall
follow the procedures as laid down in the Ordinance. In addition to the air, noise, dust and
water aspects which are usually considered for most civil engineering works, issues such as
dredging and disposal of contaminated mud and the impact of large-scale drainage works on
the ecology of the surrounding areas should also require detailed assessment. Mitigating
measures such as wetland compensation should be devised accordingly.
3.4.4
Environmental Nuisances
3.5
SITE INVESTIGATIONS
Reference should be made to GCO (1987) for guidance on good site investigation
practice and GCO (1988) for guidance on description of rocks and soils in Hong Kong.
3.6
SAFETY ISSUES
Every project has its own particular and distinctive features (e.g. general
arrangement/layout of the works, site location and constraints, accessibility of the works by
the public, etc). It is necessary for the designer to identify all potential risks arisen from the
proposed works and to design the works in such a way as to remove, reduce and/or control
the identified hazards present during the course of construction, operation, maintenance, and
finally decommissioning and demolition. In general, consideration should be given to the
following aspects when carrying out risk assessment at the design stage:
22
(a)
(b)
The operation of works warning signs, fencing, life buoys, grilles, means of
emergency communication.
(c)
(d)
Designer may refer to DSD (2010) or its latest version for information on the hazards
of different types of works and the suitable control measures.
23
4. RAINFALL ANALYSIS
4.1
GENERAL
4.2
HISTORIC RAINSTORMS
4.2.1
Applications
Historic rainstorms are used in actual storm event simulations, which are carried out
in conjunction mostly with the calibration/verification of hydrological/hydraulic models, and
to a lesser extent, with flood-forecast and post-event flood evaluations.
4.2.2
Point Rainfall
There are 180 operational rain gauge stations in Hong Kong, as summarized in
Table 1. The locations of automatic reporting rain gauge (i.e. telemetered) and other
conventional rain gauges which include ordinary and autographic types are indicated in
Figure 2 and Figure 3 respectively. Some of the gauging stations may contain both ordinary
and autographic (monthly) gauges at the same location.
The density of rain gauges in Hong Kong is higher than the World Meteorological
Organizations minimum standards. Nevertheless, the variations of local rainfall are rather
extreme both spatially and temporally, and additional rain gauges may still be needed for
individual projects, either on long-term or short-term basis, for defining the areal rainfall.
4.2.3
Areal Rainfall
4.3
SYNTHETIC RAINSTORMS
4.3.1
Applications
24
4.3.2
Despite some variations in extreme rainfall across the Territory, the rainfall statistics
at HKO Headquarters/Kings Park are recommended for general application because longterm and good quality records at other stations are not readily available for statistical analysis.
The recommended IDF Relationship is based on the Gumbel Solution in the frequency
analysis of the annual maximum rainfall recorded at HKO Headquarters and King's Park (RO,
1991). The relationship is presented in both Table 2 and Figure 4 for durations not exceeding
4 hours. The IDF data can also be expressed by the following algebraic equation for easy
application:
i=
where
a
(t d + b) c
i
= extreme mean intensity in mm/hr,
td
= duration in minutes ( td 240), and
a, b, c = storm constants given in Table 3.
For durations exceeding 4 hours, the rainfall depth instead of the mean intensity is
normally used. The Depth-Duration-Frequency (DDF) Relationship for duration exceeding 4
hours is given in Table 4 again based on RO (1991). The IDF data can be generated by
dividing rainfall depth with duration.
4.3.3
Storm Duration
The design rainstorm duration should make reference to the time of concentration or
time to peak water level of the catchment under consideration as appropriate. The time of
concentration is defined as the time for a drop of water to flow from the remotest point in the
catchment to its outlet. For computational modeling analysis, a longer storm duration may be
required if the recess arm of the hydrograph is required.
4.3.4
(b)
For other methods of runoff estimation and for storm durations equal to or
shorter than 4 hours, a symmetrically distributed rainfall is recommended with
the following formulation based on RO (1991):
25
F(t) =
where F(t)
td
=
a, b, c =
0t
F ( t )
td
2
td
t 0
2
The recommended rainstorm profiles for various return periods are given in Figure 5
and a tabulation of the relationship is shown in Table 5. The connection between the
tabulated data in Table 5 and the curves in Figure 5 is elaborated in Figure 6.
For storm durations longer than 4 hours, the rainstorm profile can be derived from
the IDF or DDF relationship for the portions outside the middle 4 hours.
4.3.5
The design rainstorm profile relates to point rainfall only. The areal rainfall of a
catchment can be obtained by multiplying the point rainfall with an areal reduction factor
(ARF). DSD (1990) gave the following ARF based on a Depth-Area-Duration (DAD)
analysis on local rainstorms:
Catchment Area
A (km2)
ARF
25
1.00
> 25
4.3.6
1.547
( A + 28)0.11
Frequent Rainstorms
Sometimes, for the design of certain drainage components, rainfall with a frequency
of more than once per year is used. The IDF data of such frequent rainstorms are given in
Table 6*, according to Cheng & Kwok (1966).
* no recent research on frequent rainstorms has been carried out for updating.
26
GENERAL
5.2
5.2.1
Applications
Historic sea levels are used in actual event simulations for the calibration and
verification of hydraulic models.
5.2.2
Data Availability
There are 15 operational tide gauges in Hong Kong managed by the Hong Kong
Observatory, the Hydrographic Office of Marine Department, the Airport Authority and the
Drainage Services Department. Brief particulars of the tide gauges are given in Table 7 and
their locations are shown in Figure 7.
Tidal data are normally recorded in Chart Datum (CD) which is 0.146 m below
Principal Datum (PD). The relationship between the two datums can be represented as
mCD = mPD + 0.146 m
where mCD is metre above Chart Datum and mPD is metre above Principal Datum.
5.2.3
Astronomical Tides
Tides arise from the gravitational attractions of the moon and the sun on the sea
water masses. Periodic hourly tidal fluctuations are mainly due to the moon's effect. Tides
in Hong Kong are of the mixed dominantly semi-diurnal type with significant daily
inequality. Daily tidal fluctuations throughout the month are due to the combined effect of
the moon and the sun, with spring tides at new and full moons, and neap tides at the first and
last quarters. Each year HKO publishes a tide table giving the astronomical tide predictions
(based on a Harmonic Analysis) for most of the operational tide gauge stations.
5.2.4
Storm Surges
27
A storm surge is induced by a low pressure weather system. The sea level rises
through barometric suction. The associated wind field also piles up water through surface
friction (wind set-up). Other factors affecting storm surges include the Coriolis Effect,
coastline configuration and sea bed bathymetry.
During tropical cyclones, HKO predicts the height of storm surge at Hong Kong
using the SLOSH (Sea, Lake and Overland Surges from Hurricanes) storm surge model
developed by the National Oceanic and Atmospheric Administration (NOAA) of USA.
5.3
5.3.1
Applications
Table 8 shows the design extreme sea levels at North Point/Quarry Bay, Tai Po Kau,
Tsim Bei Tsui and Chi Ma Wan, based on the Gumbel Distribution, with the parameters
estimated by the Method of Moments. The Mean Higher High Water (MHHW) levels for the
4 tidal stations are shown in Table 9. The data have been converted to mPD for easy
application.
5.3.3
5.4
The rate and magnitude of future sea level rise remain one of the largest areas of
uncertainties in climate change impact assessments around the world. Excluding future rapid
dynamical changes in ice flow, IPCC (2007) projected the global mean sea level to rise by
0.18-0.59 m at 2090-99 relative to 1980-99. The upper bound sea level rise projection could
be considerably higher should ice sheet melt at Greenland/Antarctic occur in a rapid
nonlinear fashion. Some recent projections e.g. Jevrejeva, S. (2006), Rahmstorf, S. (2007)
and Pfeffer et al. (2008)) of global mean sea level rise are in the order of between 0.5 m and
2.0 m by 2100. In addition, it is expected that sea level rise will not be geographically
uniform. Much more research is needed to address the rise of mean and extreme sea level in
Hong Kong. The designer should pay attention to the latest findings of this subject and
follow the guidelines given in the prevailing relevant technical circulars.
28
GENERAL
Flood protection standard is generally defined as the design standard for drainage
system that is adequate to accommodate a T-year flood, whereas T is the design return period
of the flood event. Appropriate flood protection standards should be chosen to suit the type,
category and design life of the drainage systems. Definitions of stormwater drainage systems
are discussed in Section 6.6. The consequential losses ranging from major casualties to
minor inconvenience to daily life due to inadequate flood protection standards should be
carefully considered in major improvement works. Suitable freeboard and reduction in flow
capacity due to sedimentation should be allowed in flood level computations.
6.2
6.3
It should be noted that a drainage system designed for a T-year return period event
does not mean that its capacity will only be exceeded once in every T years. Suppose the
drainage system has a design life of L years, the probability (P) of the system's capacity being
exceeded at least once over its design life is given by:
P = 1 (1
1 L
)
T
For instance, for a drainage system designed for a 50-year design life and a 200-year
return period, there is a 22% chance of flooding at least once during the design life. For the
same design life, the chance is 64% for a system sized for a 50-year return period.
29
6.4
The following approximate pragmatic rule for determining the T-year flood level in
the fluvial-tidal zone of a drainage system is recommended. The T-year flood level is taken
as the higher of those flood levels due to the following two cases:
Case I
Case II
The design return periods for combined rain and tide events are tabulated in Table
11 for easy reference.
6.5
FREEBOARD
The freeboard is the vertical distance between the crest of a river embankment, or
manhole cover level in the case of an urban drainage system, and the design flood level.
Freeboard should be provided to cover super-elevations at bends, wave run-ups, etc. For
normal condition, a 200mm allowance is generally considered adequate to cover superelevations at bends and wave run-ups if both apply. For locations where excessive superelevations at bends and wave run-ups are expected, these shall be assessed separately.
Allowance should also be made for ground settlement and bank erosion if considered
necessary. In addition to other allowances made, a margin of safety (300 mm minimum) is
recommended to account for inaccuracies in flood level computations. Sediment thickness at
the bed should be excluded from the freeboard calculation and provision for such thickness
may be achieved through a lower design bed level. For the amount of sedimentation in
stormwater drains, please refer to Section 9.3.
6.6
Table 10 stipulates the flood protection standards for five categories of stormwater
drainage systems according to the nature of catchment served or the hierarchy of the drains
within the overall drainage system. These are:
(a)
(b)
(c)
(d)
(e)
While the meaning of Agricultural Land in Category (a) is self-evident, those of the
others are outlined below.
30
6.6.1
Village Drainage refers to the local stormwater drainage system within a village.
A stormwater drain conveying stormwater runoff from an upstream catchment but happens to
pass through a village may need to be considered as either a Main Rural Catchment
Drainage Channel or Village Drainage, depending on the nature and size of the upstream
catchment. In any case, the impact of a 50-year event should be assessed in the planning and
design of village drainage system to check whether a higher standard than 10 years is
justified.
6.6.2
6.7
When part of a drainage basin is a WSD catchment, the stormwater drainage should
be designed for the greater of the following:
(a)
(b)
runoff from the catchment excluding the part of the WSD catchment but
include the estimated overflows from the catchwaters and reservoir spillways
as provided by WSD.
31
7. RUNOFF ESTIMATION
7.1
GENERAL
Methods to estimate runoff from single storm event can be based on statistical or
deterministic approaches. The common deterministic methods are the rational method, the
time-area method, the unit hydrograph method and the reservoir routing method. The two
fundamental pre-requisites for any reliable runoff estimates are good and extended rainfall/
evapotranspiration data and adequate calibration/verification of the rainfall-runoff model
parameters by sufficient number of gauging stations.
7.2
DATA AVAILABILITY
7.2.1
Rainfall
Design
Evaporation/Evapotranspiration
Daily evaporation data are measured at the HKO King's Park Meteorological Station.
Three lysimeters to measure potential evapotranspiration are also available at the station.
7.2.3
Streamflow
WSD operates a network of stream flow gauges for water resources planning
purposes. The locations of these gauges are given in Figure 8. Flow-Duration curves are
available in the Annual Report on Hong Kong Rainfall and Runoff WSD (annual). Rating
curves for the gauges can be obtained from WSD. Locations of DSDs river stage gauges are
also shown in Figure 8. These DSD gauges are primarily for flood monitoring.
7.3
7.3.1
There is no single preferred method for runoff estimation. A chosen model for any
given application should be calibrated and validated with rainfall-runoff data, whenever
possible.
7.3.2
32
(d)
(e)
(f)
(g)
Dilution Method
Ultrasonic Method
Electromagnetic Method
Float Gauging Method
Details of various methods of flow gauging are described in Herschy, R.W. (1985).
7.3.3
Practical Difficulties
There are practical difficulties in gauging the runoff in drainage systems within
floodplains and in those subject to tidal influence. In the former, the flow cross-section may
be too wide for flow gauging to be practical. In the latter case, the discharge in the drainage
system may be affected by the sea level as well as rainfall. In both cases, the parameters in
the rainfall-runoff model have to be carefully calibrated and validated in the hydraulic
modeling process.
7.4
STATISTICAL METHODS
The statistical approach to runoff estimation can give good results if the streamflow
records are long enough. The limitation is that it only gives the peak of the runoff and not the
whole hydrograph. Also, runoff may be subject to changes by urbanization and drainage
improvements. Such changes can better be estimated by Deterministic Methods.
The statistics on the streamflow records are expressed in the form of a frequency
analysis of the flow data. This relates the magnitude of flows to their frequency of
occurrence through the use of probability distributions. The flow data series can be treated in
the following manner:
(a)
(b)
A Partial Duration Series. This consists of data which are selected so that
each is greater than a predefined threshold value.
(c)
7.5
DETERMINISTIC METHODS
7.5.1
Introduction
33
Deterministic methods are based on a cause-effect consideration of the rainfallrunoff processes. Such methods are used when:
(a)
There are limited streamflow records for frequency analysis. Runoff data have
to be generated from rainfall data which are usually more plentiful.
(b)
There are changes to the rainfall-runoff responses due to land use changes,
drainage improvements, etc. which have upset the homogeneity of the
streamflow data. This has introduced complications to the statistical analysis
of the data.
(c)
Rational Method
The Rational Method dates back to the mid-nineteenth century. Despite valid
criticisms, it is a traditional method for stormwater drainage design because of its simplicity.
Once the layout and preliminary sizing of a system has been determined by the Rational
Method, the design can be refined by dynamic routing of the flow hydrographs through the
system. Details on the application of the Rational Method are described below:
(a) Basic Formulations. The idea behind the Rational Method is that for a
spatially and temporally uniform rainfall intensity i which continues indefinitely, the runoff
at the outlet of a catchment will increase until the time of concentration tc, when the whole
catchment is contributing flows to the outlet. The peak runoff is given by the following
expression:
Q p = 0.278 C i A
where
Qp
C
i
A
=
=
=
=
Q p = 0 .278 i C j A j
j =1
34
(b) Runoff Coefficient. C is the least precisely known variable in the Rational
Method. Proper selection of the runoff coefficient requires judgement and experience on the
part of the designer. The value of C depends on the impermeability, slope and retention
characteristics of the ground surface. It also depends on the characteristics and conditions of
the soil, vegetation cover, the duration and intensity of rainfall, and the antecedent moisture
conditions, etc. In Hong Kong, a value of C = l.0 is commonly used in developed urban areas.
In less developed areas, the following C values may be used but it should be
checked that the pertinent catchment area will not be changed to a developed area in the
foreseeable future. Particular care should be taken when choosing a C value for unpaved
surface as the uncertainties and variability of surface characteristics associated with this type
of ground are known to be large. It is important for designer to investigate and ascertain the
ground conditions before adopting an appropriate runoff coefficient. Designers may consider
it appropriate to adopt a more conservative approach in estimation of C values for smaller
catchments where any consequent increase in cost may not be significant. However, for
larger catchments, the designers should exercise due care in the selection of appropriate C
values in order to ensure that the design would be fully cost-effective.
Surface Characteristics
Runoff coefficient, C*
Asphalt
Concrete
Brick
Grassland (heavy soil**)
Flat
Steep
Grassland (sandy soil)
Flat
Steep
0.70 - 0.95
0.80 - 0.95
0.70 - 0.85
0.13 - 0.25
0.25 - 0.35
0.05 - 0.15
0.15 - 0.20
* For steep natural slopes or areas where a shallow soil surface is underlain by an impervious
rock layer, a higher C value of 0.4 - 0.9 may be applicable.
** Heavy soil refers to fine grain soil composed largely of silt and clay
(c) Rainfall intensity. i is the average rainfall intensity selected on the basis of the
design rainfall duration and return period. The design rainfall duration is taken as the time of
concentration, tc. The Intensity-Duration-Frequency Relationship is given in Section 4.3.2.
(d) Time of concentration. tc is the time for a drop of water to flow from the
remotest point in the catchment to its outlet. For an urban drainage system,
n
tc = to + tf
where
to
tf =
j =1
Lj
Vj
inlet time (time taken for flow from the remotest point to
reach the most upstream point of the urban drainage
system)
35
tf
flow time
Lj
Vj
where
7.5.3
0.14465L
H 0.2 A 0.1
to
Time-Area Method
This method is modified from the Rational Method. It consists of the combination
of a rainstorm profile with an incremental time-area diagram. Given a rainstorm profile in
which the average rainfall intensities within successive time increments are i1, i2, i3, the
successive ordinates of the runoff hydrograph can be written as:
Q1 = 0.278 C i1 A1
Q2 = 0.278 (C i1 A2 + C i2 A1)
Q3 = 0.278 (C i1 A3, + C i2 A2 + C i3 A1)
where
. etc.
C
= runoff coefficient
A1, A2, etc = successive increments of the time-area diagram
The above formulation is the basis of the Hydrograph Method in Watkins (1962)
used in the United Kingdom for urban drainage design since it was published in the first
edition of Road Note No. 35 in 1963. Flow routing in pipes was later incorporated in the
second edition of Road Note No. 35 in 1974.
36
7.5.4
Unit-Hydrograph Method
The classical theory of unit hydrograph refers to the relationship between net rainfall
and direct runoff. The catchment is treated as a black box with the net rainfall as input and
the direct runoff as response. If the input is a uniform net rainfall with a duration tdur and a
unit depth, the response is the tdur - unit hydrograph. Moreover, the system is considered
linear and time-invariant. The direct runoff due to any net rainfall with different depths for
successive increments of tdur is obtained by linear superposition of the responses of the
various net rainfall depths at each increment of tdur. This process is called convolution. The
direct runoff is added to the base-flow to give the total runoff. Application of the Unit
Hydrograph Method requires:
(a) Loss Model. There are 3 classical methods of determining the net rainfall
hyetograph from the rainfall hyetograph:
(i)
(ii)
(iii)
The loss model parameters can be derived from rainfall-runoff data. The above three
methods are explained with worked examples in Chow, Maidment & May (1988).
(b) Unit Hydrograph. The unit hydrograph for a catchment can be derived from
rainfall-runoff monitoring. For an ungauged catchment, the unit hydrograph may be derived
synthetically from known unit hydrographs of gauged catchments of similar characteristics.
In Hong Kong, the WSD mean dimensionless unit hydrograph was developed for upland
catchments. Details are given in PWD (1968). Other examples of synthetic unit hydrographs
are those according to Soil Conservation Service (1972).
7.5.5
The net rainfall-direct runoff routing can be looked at as a reservoir routing process
with the inflow (I) due to the net rainfall falling on the catchment and the outflow (Q) as the
direct runoff from the catchment. The flood storage volume (S) in the catchment is assumed
to be a function of the outflow. Linearity of this function determines whether the reservoir is
linear or non-linear. Moreover, the reservoir can either be single or a series of reservoirs in
cascade. Examples of Reservoir Routing Methods are the Australian RORB model for rural
catchment (Laurenson & Mein (1986)) and the hydrological component in the Wallingford
Procedure (HRL (1983)) for urban catchment. As with unit hydrograph method, reservoir
routing methods need to work in conjunction with appropriate loss models.
37
8. HYDRAULIC ANALYSIS
8.1
GENERAL
Hydraulic analysis for drainage planning or design makes use of the runoff results of
the various subcatchments and the characteristics of the drainage system to determine flood
levels throughout the system. In the tidal reaches of the system, flood levels are also affected
by the downstream boundary condition at the drainage outfall as defined by a sea level
analysis.
8.2
FLOW CLASSIFICATIONS
8.2.1
Laminar flow is characterized by fluid moving in layers, with one layer gliding
smoothly between the adjacent layers. In turbulent flow, there is a very erratic motion of
fluid particles, with mixing of one layer with the adjacent layers. Nearly all practical surface
water problems involve turbulent flow. The Reynolds Number (Re) is used to distinguish
whether a flow is laminar or turbulent.
Re =
where
A
P
V
=
=
=
=
=
=
VR
500 to 2,000
In surcharged flow (or pipe flow), the whole conduit conveys flow and there is no
free surface. The flow cross-section is the cross-section of the conduit and this does not vary
with the flow. In free surface flow (or open channel flow) which predominates in stormwater
drainage systems, there exists a free surface and the hydraulic cross-section varies with the
flow.
38
8.2.3
In open channel flow, it is important to compare the mean flow velocity ( V ) and the
surface wave celerity (c). The Froude Number (Fr) is defined as:
Fr =
V
c
Q
A
=
gA
Fr 2 =
where
B
Q
A
g
BQ 2
gA3
When Fr < 1, the flow is subcritical and a wave disturbance can travel both upstream
and downstream.
When Fr = 1, the flow is critical. Critical flow has a minimum energy for a given
discharge or a maximum discharge for a given energy.
When Fr > l, the flow is supercritical and a wave disturbance can only travel
downstream.
Chow, V. T. (1959) quotes values of .
8.2.4
In steady flows, flow conditions (viz. discharge and water level) vary with the
position only. In unsteady flow, flow conditions vary with position as well as time. Steady
flow can be either uniform or non-uniform.
8.2.5
If the flow is also independent of position, the flow is uniform. Otherwise, the flow
is non-uniform.
39
8.2.6
In non-uniform flow, if the flow conditions vary slowly with location and bed
friction is the main contribution to energy losses, the flow is gradually varied. Otherwise, the
flow is rapidly varied.
8.3
UNIFORM FLOW
8.3.1
Most of these equations apply to turbulent uniform flow in open channels. The
common equations are given in Table 12 using a consistent set of notations. All the
equations are converted to the Chzy form for easy comparison.
The notations are:
V
R
Sf
C
n
f
ks
g
CHW
=
=
=
=
=
=
=
=
=
=
Amongst the equations in Table 12, Manning and Colebrook-White are the most
popular in local applications. Design values of n and ks are given in Tables 13 and 14
respectively 1 . Manning equation is more convenient to work with in open channel flow
calculations. Colebrook-White equation is presented in design charts in HR Wallingford
(2006). In Table 14, the term sewer should include both sanitary sewers and stormwater
drains with possible polluted flow.
8.3.2
Compound Roughness
Suppose the flow area is divided into N sub-sections of which the wetted perimeters
P1, P2, ..., PN and areas A1, A2, ..., AN are known. If the corresponding Manning roughness
coefficients are n1, n2, ..., nN, the equivalent roughness coefficient is
A5 / 3
2/3
n = P 5/3
A
i 2/3
ni Pi
1
H.R. Wallingford Ltd., Barr, D.I.H. and Thomas Telford Ltd. are acknowledged for their consent to the
reproduction of the Table on Recommended Roughness Values in the publication Table for the Hydraulic
Design of Pipes, Sewers and Channels, 8th Edition (2006) in Table 14 of this Manual.
40
If the surface roughnesses are k1, k2, ..., kN, the equivalent surface roughness is
ks =
8.3.3
Pi ki
P
Charts for partially full circular sections are available for both Manning and
Colebrook-White equations. See Chow, V.T. (1959) and HRL (1990).
8.4
Basic Formulations
Using the definition sketch at Figure 9, the basic equation is:
(a) In differential form:
dy so s f
=
dx 1 Fr 2
Q2n2
A2 R 4 / 3
=
Q 2 B
1
gA 3
so
so x + y1 +
1V1 2
2g
= s f x + y 2 +
2V22
2g
or
s o x + y1 +
8.4.2
1Q 2
2 gA12
= s f x + y 2 +
2Q 2
2 gA22
41
Depending on the bed slope and the flow depth, 13 types of flow profiles can be
distinguished. The classification is described in Chow, V.T. (1959).
8.4.3
Solution Techniques
First, it is necessary to establish the normal flow depth (yn) and critical flow depth
(yc) by solving:
so =
Q 2n2
An2 Rn4 / 3
and
1=
Q 2 Bc
gAc3
For supercritical flows, the calculation starts at some known section at the
upstream side and proceeds in a downstream direction.
(b)
For subcritical flows, the calculation starts at some known section at the
downstream side and proceeds in an upstream direction.
(b)
solution of the unknown flow section from the known one using the difference
equation above and the Standard Step Method or the Direct Step Method.
8.5
8.5.1
General
Weirs or spillways
Gates
Sudden channel expansions or contractions
Hydraulic jumps
Bends
Stepped channels
Channel junctions
Constrictions due to bridge piers, culverts, etc.
42
8.5.2
Rapidly varied supercritical flows may involve large changes in momentum with
large changes in flow depth, formation of waves and vortices, aeration, etc. and may result in
splashing, overflow, the flow flying into the air, rapid erosion of the pipeline/channel, etc.
These effects should be duly taken into account in the analysis, and measures should be taken
to reduce such risks. Some of these measures are as follows. Further guidelines are given in
DSD Practice Note No. 3/2003.
(a)
(b)
Horizontal bends should have radius not less than three times the width
of the channel (for velocity of flow up to 2 m/s). For flows of higher
velocity and/or bends of large angle, flow separation may occur at the
inner bend and significant increase in flow depth may occur at the outer
bend. In addition, shockwaves, choking phenomenon and spiralling of
flow (in pipes) may result. For channels, increase in wall height at the
outer bend (Vischer & Hager) or cover-up of channel top may be
required. Chokage and flow spiralling may also need to be addressed;
(d)
43
8.5.3
(e)
Bridge piers or other obstructions (including trash grilles) should not be placed
inside channels with high velocity supercritical flow. If the flow velocity is
not too high, piers/obstructions may be allowable if they have streamlined
sections and are properly designed so as not to create unwanted hydraulic
jump, cause overshooting of flow or trap floating debris/vegetation.
(f)
If the gradient is very steep (inclination to horizontal greater than, say, 65o)
and there is insufficient space to allow for splashing etc., surface
channel/conduit may be replaced by a downpipe or covered up on top. In such
case, the downpipe/covered channel should be designed to prevent blockage
and to facilitate inspection/clearance if necessary.
(g)
If soil, boulders and debris may be washed down due to erosion or landslide
especially during rainstorms, erosion protection measures such as lining the
embankments and slopes with concrete or shotcrete, use of gabions, erection
of retaining walls, tree planting, hydroseeding and provision of check dams
should be adopted where appropriate.
Stepped Channel
Stepped channels are commonly used to convey flow along slopes. They are
effective in dissipating the energy and in reducing the velocity of the flow. Flow in step
channels can be classified into 3 regimes :
(a) Nappe flow regime The water drops freely at each step, sometimes with a
hydraulic jump. The nappe flow regime occurs in low flows or flow at slopes
of flatter gradient.
(b) Skimming flow regime The water flows down in a coherent stream
skimming over the steps cushioned by recirculating vortices at the steps and
significant air entrainment. The skimming flow regime occurs in high flows
or in step channels of steeper gradient.
(c) Transition flow regime Change from the nappe flow to skimming flow
regime due to increase in flow or increase in slope will pass through the
transition flow regime. Significant spray is present and the flow pattern may
vary significantly from step to step in transition flow regime.
GEO Technical Guidance Notes No. 27 (TGN 27) provides a formula for checking the type
of flow regime applicable to the design and the situation concerned.
Stepped channels should be designed according to GEO TGN 27 in general. For
stepped channels under the nappe flow regime or transition flow regime, reference should be
made to Chanson (1994) and Chanson (2002). Stepped channels of width greater than 900
mm and under the skimming flow regime should be designed in accordance with Annex TGN
27 A2 of TGN 27 except that :
44
(a)
The discharge per unit channel width qw should not be greater than 10 m2/s
(Chanson & Toombes);
(b)
The minimum L/h ratio and the minimum channel length L in order to
establish uniform aerated flow should be found from the following formula
based on Chanson (2002)
where
L
h
dc
=
=
=
=
=
qw = discharge per unit channel width
In addition :
(a)
The possibility of jet deflection at the crest of the channel, i.e., the flow
shooting out as a free-falling jet bypassing the steps, should be checked. For
the flow to remain on the steps, the following equation should be satisfied :
where
If the above condition is not satisfied, the step height h of the first few steps
should be reduced.
(b)
If the residual head and the Froude no. of the flow at the bottom of the stepped
channel are still high, additional energy dissipation device should be provided
downstream of the stepped channel.
45
The residual head Hres at the bottom of a long stepped channel in which
uniform aerated flow is reached can be found from the following formula :
where
dc = critical flow depth for the given discharge per unit width
=
qw = discharge per unit channel width
= channel angle to the horizontal
fe = Darcys friction factor of aerated flow
8.5.4
Stilling Basin
The most common types of stilling basin make use of the hydraulic jump to dissipate
energy and change the flow regime from supercritical to subcritical. Hydraulic jump will
occur when the flow channel flattens out (the slope of the channel becomes hydraulically
gentle) and the tailwater depth (downstream flow depth) is sufficiently large (if the tailwater
depth is too small, the jump will be swept out to the downstream). A hydraulic jump can be
induced near the point where the channel slope changes from steep to gentle through :
a)
b)
c)
Reference should be made to U.S. Bureau of Reclamation (1960) and Vischer & Hager
(1998). Figure 11 show stilling basins of types I, II and III developed by the U.S. Bureau of
Reclamation, together with the design procedures and guidelines.
46
The impact type stilling basin can be used in both open and closed conduits, and is
effective for flow velocity which is not so large. Details are given in Figure 12 (U.S. Bureau
of Reclamation (1960)).
8.6
FLOW ROUTING
8.6.1
Introduction
8.6.2
Hydrologic Routing
Depending on the choice of the storage function, two hydrologic routing methods
can be distinguished:
(a)
Reservoir Routing
S = f (Q)
(b)
Muskingum Method
S = K [XI + (1-X) Q]
where
8.6.3
K = proportional constant
X = weighting factor, 0 X 0.5
Hydraulic Routing
The basis of hydraulic routing is the solution of the basic differential equations of
unsteady flow (the Saint Venant Equations). Using the notations in Figure 13, these
equations can be written as follows:
Continuity Equation:
47
Q A
+
=q
x t
Momentum Equation:
1 Q
1
+
A t
A x
Local
acceleration
term
Convective
acceleration
term
y
+ g
g so s
Pressure Gravity
force
force
term
term
)=
Friction
force
term
Kinematic Wave
Diffusive Wave
Dynamic Wave
8.7
In order to minimize the head losses in pipe flows, the selection of the pipe materials
and the joint details are very important. The resistance in pipes will be influenced by the pipe
material but will be primarily dependent on the slime and sediment that deposit on the pipe
surface. Other factors such as discontinuities at the pipe joints, number of manholes, number
of branch pipes at manholes and their directions of flow in relation to the main stream, etc
will all affect the head losses.
48
Other sources of resistance which occur in pipes include inlets, outlets, bends,
elbows, joints, valves, manholes and other fittings and obstructions can all be referred to as
head loss and formulated as:
hL =
KV 2
2g
in which K may refer to one type of head loss or the sum of several head losses.
The head loss coefficient K can be found elsewhere in the literatures of hydraulics.
Table 15 contains some of the most commonly used head loss coefficients in Hong Kong as
abstracted from the Preliminary Design Manual for the Strategic Sewage Disposal Scheme.
Reference should also be made to Streeter, V.L. and Wylie, E.W. (1985), BSI (1997/2) and
Chow, V.T. (1959).
8.8
49
GENERAL
Erosion of natural and artificial sediments in a drainage basin, their transport along
the drainage systems, and their subsequent deposition at the lower reaches of such systems
are natural processes in the hydrological cycle. This is an evolving subject known as
sediment transport. This section deals with its common applications in the drainage field
including, amongst others, river bed and bank protection, velocity design in channels and
pipes, scour around bridge piers and the quantification of sedimentation at the lower reaches
of drainage systems.
There are different forms of river bank protection available, such as concrete lining,
masonry facing and gabion wall. Chapter 13 of this Manual gives a more comprehensive list
of different forms of channel linings. Designers shall check the allowable maximum velocity
with the supplier or manufacturer when selecting the form of channel lining.
9.2
The sizing of non-cohesive stones for river bed and bank protection against scouring
induced by river flows is given by the following expression adapted from Zanen (198l):
Dm
where
Dm
K
K
g
(a)
=
=
=
=
=
=
=
V2
1 1
2 g K K
Lane/Shield
0.3 to 0.5
Isbash
0.7
y (1/3)
USBR
50
(b)
Material
dense sand, gravel
1.65
concrete
1.2 to 1.4
asphalt concrete
1.3 to 1.4
granite
1.5 to 2.1
(c) K values. K adjusts for reduced shear stress on the bank and reduced
stabilizing forces due to side slope. This factor is not applicable to the bed, for which a factor
of 1 can be assumed.
K = 1
sin 2 1
sin 2 0.8
where
(d)
sinuosity:
K values. Lane suggested the following table for K to account for river
Degree of Sinuosity
straight canal
1.00
0.90
0.75
0.60
The sizing of armouring stones for wave resistance in the estuarine reach of drainage
channels can be carried out in accordance with guidelines in CED (1996).
9.3
51
Recent research on sediment movement in channels and pipes has shown that there
is no unique design self-cleansing velocity since it depends on sediment type, grading,
concentration, and transport rates as well as the size of the channel or pipe. Details can be
found in DSD (1990).
Even if self-cleansing velocities could be derived, it would be difficult to achieve
them in designs except in steep upland catchments. Large stormwater drainage systems,
particularly those within new reclamations, generally have the potential for siltation due to
flat gradients and also due to the phasing of their handing over upon completion.
Sedimentation must therefore be expected in the middle and lower reaches of drainage
systems. While some allowance for this could be made in sizing the channels and pipes,
facilities must also be provided for regular desilting works to safeguard the drainage
capacities.
9.4
Local scour near the bridge pier caused by the disturbance of the flow field
around the pier.
(b)
Long term degradation of the river bed due to increased flow velocity caused
by the contraction of the river cross section at the bridge site.
(c)
Short term degradation of the river bed around the bridge site during floods.
9.5
QUANTIFICATION OF SEDIMENTATION
52
GENERAL
This Chapter provides guidelines on the materials, level and structural design of
buried gravity pipelines laid by cut and cover method.
In recent years, there have been technological improvements on the use of trenchless
methods including pipe jacking, microtunnelling, directional drillings, auger boring and online replacement techniques for laying pipelines in congested urban areas. Reference should
be made to the relevant literature and manufacturers catalogue in designing pipelines laid by
trenchless method.
10.2
MATERIALS
In general, concrete pipes have been used extensively for stormwater pipelines
throughout the Territory and are normally available in sizes up to 2500 mm diameter in the
local market. Where the stormwater flow is severely polluted, consideration may be given to
the use of vitrified clay pipes to provide better protection against corrosion. Other pipeline
materials are available and may be considered in relation to their advantages and
disadvantages for particular situations. If such alternative materials are proposed, full
account should be taken of their acceptability from the operation and maintenance point of
view.
10.3
LEVELS
10.4
DEPTH OF PIPELINE
Designers should avoid deep underground pipeline. In general, the maximum depth
of a pipeline should not be more than 6 m. Below such depth, maintenance and
reconstruction of the pipeline will be very difficult. If the situation warrants such deep
pipeline, one should always consider other alternatives including the use of intermediate
pumping station.
Normally, the minimum cover from the surface of the carriageway to the top of the
53
pipeline shall be 900 mm. For footway, the minimum cover shall be 450 mm.
10.5
STRUCTURAL DESIGN
10.5.1
Introduction
Pipes can be categorised into rigid, flexible and intermediate pipes as follows:
(a)
Rigid pipes support loads in the ground by virtue of resistance of the pipe
wall as a ring in bending.
(b)
Flexible pipes rely on the horizontal thrust from the surrounding soil to
enable them to resist vertical load without excessive deformation.
(c)
Intermediate pipes are those pipes which exhibit behaviour between those in
(a) and (b). They are also called semi-rigid pipes.
Concrete pipes and clay pipes are examples of rigid pipes while steel, ductile iron,
uPVC, MDPE and HDPE pipes may be classified as flexible or intermediate pipes,
depending on their wall thickness and stiffness of pipe material.
The load on rigid pipes concentrates at the top and bottom of the pipe, thus creating
bending moments. Flexible pipes may change shape by deflection and transfer part of the
vertical load into horizontal or radial thrusts which are resisted by passive pressure of the
surrounding soil. The load on flexible pipes is mainly compressive force which is resisted by
arch action rather than ring bending.
The loads on buried gravity pipelines are as follows:
(a)
The first type comprises loading due to the fill in which the pipeline is
buried, static and moving traffic loads superimposed on the surface of the
fill, and water load in the pipeline.
(b)
The second type of load includes those loads due to relative movements of
pipes and soil caused by seasonal ground water variations, ground
subsidence, temperature change and differential settlement along the
pipeline.
Loads of the first type should be considered in the design of both the longitudinal
section and cross section of the pipeline. Provided the longitudinal support is continuous and
of uniform quality, and the pipes are properly laid and jointed, it is sufficient to design for the
cross-section of the pipeline.
In general, loads of the second type are not readily calculable and it affects the
longitudinal integrity of the pipeline. Differential settlement is of primary concern especially
for pipelines to be laid in newly reclaimed areas. The effect of differential settlement can be
catered for by using either flexible joints (which permit angular deflection and telescopic
movement) or piled foundations (which are very expensive). If the pipeline is partly or
wholly submerged, there is also a need to check against the effect of flotation on the empty
pipeline when it is not in operation or prior to commissioning.
54
The design criteria for the structural design of rigid pipes are the maximum load at
which failure occurs while those for flexible pipes are the maximum acceptable deformation
and/or the buckling load. The design approach for rigid pipes is not applicable to flexible
pipes. For the structural design of flexible pipes, it is necessary to refer to relevant literature
such as manufacturers catalogue and/or technical information on material properties and
allowable deformations for different types of coatings, details of joints, etc.
10.5.2
(b)
(c)
Cd w Bd2
Cd
1
2k
2
2
[1 - exp( - 2k
1
1
H
Bd
)]
55
where
Wc
w
Bd
=
=
=
Cd
H
k
,
=
=
=
=
For practical applications, take = , and use Figure 14 to obtain values of Cd.
Embankment condition. When the pipe is laid on a firm surface and then
(b)
covered with fill, the fill directly above the pipe yields less than the fill on the sides.
Shearing friction forces acting downwards are set up, resulting in the vertical load transmitted
to the pipe being in excess of that due to the weight of the fill directly above the fill. The
load on the pipe will then be determined as in the embankment condition. The equation for
the embankment condition as proposed by Marston is as below:
Wc
Cc w Bc2
2kHe
)-1
Bc
2k
exp (
Cc
+(
H He
Bc
) exp (
2kHe
Bc
It is given by:
exp (
rsd p
3
where
2kHe
)-1
Bc
2k
H
Bc
Wc
w
Bc
Cc
He
H
rsd
p
=
=
=
=
=
=
=
=
=
=
He
Bc
][
) exp
1
2k
2kHe
Bc
H - He
+
Bc
He
2kBc
rsd p
3
1
He
(
2
Bc
)2
H He
Bc2
= rsd p
H
Bc
56
rsd
=
=
=
Narrow trench and embankment conditions are the lower and upper limiting
conditions of loading for buried rigid pipes. Other intermediate loading conditions are not
very often used in design.
One method for deciding whether the narrow trench condition or embankment
condition of the Marston equations is to be used to determine the fill load on pipes was
proposed by Schlick. Calculations are carried out for both conditions. The lower of the two
calculation results is suggested to be adopted in design. Method of construction will be
specified in accordance with the design trench conditions if necessary.
Under certain site conditions, when restricting the trench width is not practical
because of the presence of underground utilities, consideration should be given to design the
pipe for fill loads under the worse scenario of narrow trench and embankment conditions.
If the width of the trench, Bd, and external diameter of the pipe, Bc, are fixed, there is
a unique value of cover depth at which the embankment or narrow trench calculations
indicate the same load on the pipe. This value of cover depth is termed the transition depth
Td, for this trench width and external diameter of pipe.
At depths less than the transition depth, the pipe is in the embankment condition
and the fill load will be dependent on the external diameter of the pipe. No restriction to
trench width is required. In other cases, when the depth is greater than the transition depth,
the fill load is dependent on the assumed trench width. The tabulated fill load on the pipe in
Table 16 will be exceeded unless the trench width is restricted to the assumed value in order
that the pipe is in the narrow trench condition.
The fill load on a pipe and value of transition depth, assuming a saturated soil
density of 2000 kg/m3, are shown in Table 16. If the actual soil density differs from 2000
kg/m3, the fill load may be adjusted by a multiplying factor of /2000. The values of k
assumed in deriving this table are 0.13 for narrow trench condition and 0.19 for embankment
condition. rsd p for embankment condition is taken as 0.7 for pipes up to 300 mm nominal
diameter and 0.5 for larger pipes.
10.5.4
Superimposed Loads
The equivalent external load per metre of pipe transmitted from superimposed traffic
loads can be calculated by the Boussinesq Equation, by assuming the distribution of stress
within a semi-infinite homogeneous, elastic mass:
p= (
3L
2
)(
H3
)
Hs5
57
where
L
p
H
Hs
=
=
=
=
Wp = p Bc
where
Values of traffic loads for design are shown in Table 17 with the following
assumptions:
10.5.5
Main road :
Light road :
Water Load
The weight of water in a pipe running full generates an additional load, the
equivalent external load on the pipe can be calculated from the following equation:
2
3 D
Ww = 9.81
4 4
where
Ww
D
58
In general, the water load is not significant for small pipes of less than 600 mm
diameter. The equivalent water load of pipes of 600 mm to 1800 mm diameter are as below:
10.5.6
600
750
900
1050
1200
1350
1500
1650
1800
2.1
3.3
4.7
6.4
8.3
10.6
13.0
15.8
18.8
Bedding Factors
The strength of a precast concrete or vitrified clay pipe is given by the standard
crushing test. When the pipe is installed under fill and supported on a bedding, the
distribution of loads is different from that of the standard crushing test. The load required to
produce failure of a pipe in the ground is higher than the load required to produce failure in
the standard crushing test. The ratio of the maximum effective uniformly distributed load to
the test load is known as the bedding factor, which varies with the types of bedding
materials under the pipe and depends to a considerable extent on the efficiency of their
construction and on the degree of compaction of the side fill.
The various methods of bedding used with precast concrete pipes are shown on the
relevant DSD Standard Drawing. The values of the bedding factors below are average
experimental values and are recommended for general purposes: (a)
(b)
(c)
(d)
granular bedding
120o plain concrete bedding
120o reinforced concrete bedding with
minimum transverse steel area equal to
0.4% of the area of concrete bedding
concrete surround
1.9
2.6
3.4
4.5
On the basis of the experimental and numerical modelling work carried out, bedding
factors used with vitrified clay pipes for class F, B and S bedding are shown in Figure 16.
10.5.7
Design Strength
For design, it is required that the total external load on the pipe will not exceed the
ultimate strength of the pipe multiplied by an appropriate bedding factor and divided by a
factor of safety.
The design formula is as follows:
59
We
where
We
Wt
Fm
Fs
=
=
=
=
Wt Fm
Fs
Based on the assumed design parameters in paragraphs 10.5.3, 10.5.4, 10.5.5 and
10.5.6, values of the total external design loads in main roads and light roads are shown in
Table 18.
Alternatively, Table 19 may be used for direct evaluation of the minimum crushing
strength or grade of precast concrete or vitrified clay pipes using different bedding factors in
main roads.
Worked Example: Given:
60
10.5.8
The outside diameters in Table 18 are the general maxima for the majority of pipes.
However, a few pipes with outside diameters exceeding the tabulated dimensions may be
encountered. Provided the excess is not greater than 5%, the effect can be ignored. If the
pipes employed have an outside diameter less than that being assumed, the load in Table 18
will then err on the safe side. It may be worthwhile making a more accurate computation of
the design load as described in sections 10.5.3 and 10.5.4 with a view to achieving economy
where the difference in outside diameter is considerable.
10.6
Any leakage from pipeline which is close to the crest of a slope may affect the
stability of the slope. Attention shall be paid to avoid routing of pipeline near slope crest. If
pipeline is to be laid within the crest of a slope, appropriate leakage collection system shall
be provided to prevent any adverse effects to the slope in case of pipe leakage. Reference
shall be made to GCO (1984).
61
11. MANHOLES
11.1
GENERAL
This chapter provides guidelines on the design of manholes.
11.2
LOCATION
Manholes should be provided at:
(a)
(b)
(c)
(d)
<675
80*
100
>1050
120
11.3
ACCESS OPENINGS
Access openings are generally of two types, one for man access and the other for
62
desilting purposes. A desilting opening should not be smaller than 750 mm by 900 mm, and
it should be placed along the centre line of the stormwater drain to facilitate desilting. A man
access opening should not be smaller than 675mm by 675mm. If cat ladders are installed in a
manhole, the minimum clear opening should be 750mm by 900mm. A man access opening
should be placed off the centre line of the stormwater drain for deep manholes and along the
centre line of the stormwater drain for shallow manholes with depths less than 1.2 m.
11.4
ACCESS SHAFTS
11.5
WORKING CHAMBERS
For manholes less than or equal to 1.2 m deep, work in them generally can be
performed from ground level, i.e., the workmen standing on the ground can reach the invert
of the stormwater drain without great difficulty. A working chamber is generally not
required for this type of manhole.
For manholes deeper than 1.2 m, work in them generally cannot be easily carried out
from ground level. Manholes of this type should be provided with working chambers and
access shafts leading from ground level. The working chambers should enable a person to
work inside.
11.6
INTERMEDIATE PLATFORMS
When the invert of a manhole is more than 4.25 m from the cover level, intermediate
platforms should be provided at regular intervals. The headroom between platforms should
not be less than 2 m nor greater than 4 m. The size of the platform should not be smaller than
800 mm by 1350 mm. The platform should be fitted with handrailing and safety chains at the
edge to protect persons from falling down.
In order to facilitate rescue operation in case an accident occurred, designers are
advised to provide an additional manhole opening where space permits.
11.7
Inverts and benchings of the manholes should be neatly formed. The socket ends of
pipes should be cut off and not projected into the manholes. The inverts should be curved to
the radius of the inverts of the pipes and carried up in flat vertical faces, and should match the
cross-sections, levels and gradients of the respective stormwater drain. The benching should
be a plane surface sloping gently downward towards the stormwater drain. Suitable gradient
of the benching is 1 in 12.
63
11.8
COVERS
Manhole covers should be sufficiently strong to take the live load of the heaviest
vehicle likely to pass over, and should be durable especially under corrosive environment.
Heavy duty manhole covers should be used when traffic or heavy loading is anticipated,
otherwise medium duty covers can be used.
Manhole covers should not rock when initially placed in position, or develop a rock
with wear. Split triangular manhole covers supported at the three corners are commonly used
to reduce rocking. The two pieces of triangular cover should be bolted together to avoid a
single piece of the cover being accidentally dropped into a manhole.
Foul sewer and stormwater drain manhole covers should be differentiated by the
grid patterns which are shown on the DSD Standard Drawings.
11.9
11.10
BACKDROP MANHOLES
(b)
A cascade is preferred for drains larger than 450 mm diameter. Downpipes are
suitable for drains less than 450 mm diameter. When downpipes are used, the following are
recommended:
(a)
proper anchoring of the backdrop at the bottom in the form of a 90o pipe bend
surrounded by concrete.
64
(b)
a T-branch at the top fitted with a flap valve inside the manhole to avoid
splashing.
65
GENERAL
Box culverts are required where precast pipes cannot be obtained in a sufficiently
large size or where a box culvert configuration would better suit the available space between
or adjacent to other structures or utilities. For hydraulic design of box culvert, reference can
be made to FHWA (1985), THD (1962) and CIRIA (1997). The selection of the size and
number of cells in a culvert depends not only on the hydraulic capacity but also on the
requirement for maintenance and desilting. To facilitate the use of mechanical plant inside
box culverts, the internal dimensions of each cell of a box culvert should not be less than 2.5
m 2.5 m. The minimum width should be further increased if corner splays are used. For
cells smaller than this size, agreement should be sought from maintenance authority. Smaller
culverts may be used in special situations, such as steep gradients, where siltation or
sedimentation will not be a problem.
12.2
The design invert level of the box culvert at the downstream end should be kept at a
high level as far as possible to allow for future extension of the culvert. The invert level of
the box culvert should be designed to maintain free discharge of the flow at the outlet and to
avoid backwater effect.
12.3
DESIGN LOADS
If the box culvert is subject to permanent vehicular or pedestrian live loads, Section
2 of HyD (2006) should be followed. Superimposed load from the loading/unloading and
stockpiling of material and the manoeuvring of mechanical plant during desilting operations
should also be catered for. If the filling material above the culvert contributes to the major
superimposed load and the culvert is not classified as a highways structure, the load
combinations and the partial factors as specified in BSI (1997/1) should be adopted in the
design.
12.4
DURABILITY
The durability of a reinforced concrete box culvert depends mainly on the concrete
grade, cover and crack width. Reference should be made to Table 20 of HyD (2006). In
general, as culverts at the downstream reaches of a drainage system are constantly in contact
with water, the Very Severe condition of exposure in the Table is recommended. At the
upstream reaches where the culverts are not subject to sea water attack, the Severe
classification may be used.
Given the required water retaining properties of box culverts, concrete Grade 40 is
recommended.
66
Reference should also be made to the latest guideline on Concrete Specification for
Reinforced Concrete Structures in Marine Environment by the Standing Committee on
Concrete Technology.
12.5
MOVEMENT JOINTS
12.6
FOUNDATIONS
A layer of rock fill material is usually placed below the culvert. Where the subsoil
comprises residual soil (completely decomposed volcanic or completely decomposed granite)
or suitable filling materials, a 300 mm to 500 mm thick layer of rock fill material will
generally suffice. For adverse foundation conditions, consideration should be given to
removing the unsuitable sub-soil and replacing it with rock fill. The thickness of rock fill to
be used in such cases depends very much on the subsoil conditions and needs to be assessed
under the particular circumstances.
Generally, culverts do not need to be supported on piles. One of the exceptions is in
newly reclaimed land or in other areas where substantial or unacceptable differential
settlement is expected. For the design of piled foundations, guidance is given in BSI (1986).
However, Section 9 of BSI (1988/1) concerning the applicability of limit state design to
foundations shall also be referred to.
12.7
12.7.1
Access
Desilting Opening
Desilting opening is normally designed for box culvert to facilitate inspection and
maintenance. In consideration of the methods commonly used for desilting, desilting
opening should be provided at each cell at a maximum interval of 160m for large or multicell box culvert, or 120m for single cell box culvert of cross sectional area less than 5m2.
Desilting openings of 900 mm x 750 mm and multi-part cover type of either 2m x 3m or 3m
x 4m should be provided alternatively along the box culvert. For multi-cell box culvert,
consideration can be given to align the desilting openings of 900 mm x 750 mm and multipart cover type at alternate position across the box culvert to facilitate maintenance.
67
Access Shafts
Access shafts of 900 mm 750 mm should be provided for each cell of a box
culvert at no more than 160 m intervals for large or multi-cell box culvert, or 120m for single
cell box culvert of cross sectional area less than 5m2. The shaft can serve as an inspection
manhole and for ventilation purpose during maintenance operation. A cat ladder should be
provided in the culvert on one side of the shaft. Safety hoops/cages should be provided for
ladders except that the installation of the safety hoops/cages inside the box culvert or flow
area section would not be reasonably practicable such as catching the debris carried by the
flood flow. Under such circumstances, other suitable safety precautions shall be considered
when maintenance work is carried out. The location of access shaft should be suitably located
in line with or at close distance to the desilting opening to facilitate desilting operation and
inspection.
12.7.4
Internal Openings
Freeboard
68
should not be lower than the normal high tide level (2.5mPD). In case of site constraints or
topography rendering the minimum freeboard cannot be practically achieved, the relevant
maintenance parties should be consulted. However, the soffit of tidal influence box culvert
should not be lower than the normal high tide level of 2.5mPD as far as possible.
12.7.6
Safety Provisions
Grilles should be provided at the entrance to box culverts from open channel to
prevent people from being washed into the culvert. The grilles should be so designed that
they will not prevent mechanical plant from entering the culvert to carry out maintenance
operations. The height and spacing of the grilles should avoid the excessive collection of
debris/vegetation which may cause stormwater overflowing from the system. The grilles
should be placed at least 2 m upstream from the box culvert entrance to avoid sealing up the
inlet when the grilles are substantially blocked by debris carried along by the flood flow.
12.7.7
To facilitate maintenance operation, provisions for the installation of stop log or gate
and winching equipment both in the structure of the box culvert and at the seawall should be
considered. Maintenance access must be provided at each seawall outfall. Access to the
culvert shall be by full width openings to allow the installation of stop logs or gates and to
allow pumping over the stop logs or gates. For the maintenance of box culvert affected by
tidal flows, an area for temporary storage of silt removed from the box culvert should be
considered.
69
GENERAL
Where land use permits, open channels should be the preferred option when
compared with underground pipelines and culverts since the latter are more expensive to
construct and maintain than open channels, in particular for those box culverts affected by the
tide.
In general, open trapezoidal channels provide the most economical cross-section for
the conveyance of stormwater, both in terms of construction and maintenance costs.
Rectangular open channels are usually most costly to construct and have limited scope for
improving the aesthetics when compared with trapezoidal channels. An analysis of land
availability, acquisition and the channel appearance should be carried out before a
rectangular section is adopted. Whichever section is adopted, the design should make due
allowance for the appearance of the channel, the selection of suitable lining materials and the
provision of landscaping.
In order to ensure effective protection against flooding of low-lying area behind a
drainage channel, which is also adjacent an estuary, the top level of the proposed channel
embankment or wall should tie in with that of the seawall at the estuary.
13.2
CHANNEL LININGS
13.2.1
General
Side slope and bottom lining should normally be provided along the whole channel
if the flow velocity exceeds 1 to 2 m/s (Chow, V.T., 1959). In the downstream reaches where
the flow velocity is likely to be low, bottom lining is usually not required.
13.2.2
(a) Rigid Linings. Rigid linings are usually made of concrete, shotcrete, precast
concrete slabs, stone masonry or grassed cellular concrete paving. They should only be used
in locations where little settlement is anticipated. Stone masonry is preferred for aesthetics
reasons in the past, while grassed cellular concrete paving is becoming more popular for
ecological reasons. If grassed cellular concrete paving is to be used in locations subject to
tidal influence, careful considerations should be given to ensure that there are suitable grass
species to be established under such condition. Weep-holes should be provided in the lining
for the free passage of groundwater.
(b) Flexible Linings. Flexible linings may consist of rip-rap, grass, gabions or
random rubble. They are used in locations where large embankment settlement is anticipated.
70
If the channel is subjected to tidal influence, the flexible linings should be designed to
withstand wave action.
13.2.3
The armour layer of a flexible lining is susceptible to erosion by wave forces and
flow-induced drag forces. Proper design of the armour layer is essential to protect the
stability of the embankment/revetment. For the design of the armour layer for the tidal reach
of drainage channels, reference to CED (1996) is recommended. For protection against scour
due to river flows, the guidance in Section 9.2 of this manual should be followed.
13.3
CHANNEL SHAPE
The lining is generally the most expensive component of a lined channel. For
economical reasons, the perimeter of channel cross-section should be minimised.
Theoretically, a semi-circular shape provides the maximum hydraulic capacity for the
minimum channel perimeter. A trapezoidal section which is a pragmatic approximation to
the semi-circular shape is often adopted because it is easier and cheaper to construct, and can
accommodate a wider range of flows than the simple rectangular channel. The side slope of
the trapezoidal section normally ranges from 1 in 1.5 to 1 in 3.0, depending on the subsurface condition and maintenance method.
13.4
13.5
13.5.1
Access Ramp
Concrete or similar hard paved access ramps should be provided along the drainage
channel at intervals of about 600 m for the access of maintenance vehicles. For locations
where there is a maintenance access at channel bottom and that the channel bottom is neither
subject to tidal effect nor submerged most of the time, access ramps may be provided at more
than 600m apart. The access ramp should have a width of 3.5 m and a slope ranging from 1
in 12 to 1 in 15. It should slope down in the same direction of flow. Along the edge of the
access ramp, concrete upstands instead of railings are preferred for ease of maintenance.
71
For channel which base slab is always submerged due to tidal effect, intermediate
platform should be provided in the ramp at level of 2.5 mPD with minimum size of 5 m x
20 m to facilitate mobilization of dredging plant and loading and unloading of dredged
materials.
13.5.2
Maintenance Road
72
A crossing slab over the dry weather flow channel should be provided at convenient
locations for use by maintenance personnel and vehicles.
13.5.4
In order to safeguard the safety of the maintenance personnel and the public, both
sides of the channel should be provided with handrailings or parapets. Gates with locks
should be provided at the entrances of access ramps to prevent vehicles from inadvertently
entering the channel.
Staircases should be provided at the channel sides at intervals of 400 m. The
staircases should not protrude from the surface of the channel sides to obstruct the flow. No
opening shall be provided in the parapet or handrailing as entrance to these staircases.
Warning signs should be erected at the parapet or handrailings near these staircases and other
prominent locations to remind the public not to enter into the channel.
13.5.5
For drainage channels at the upstream reaches of a drainage basin, grit traps/sand
traps should be provided to intercept and collect the silt and grit conveyed along small
watercourses in times of storms. The grit trap/sand trap is normally in the form of a sump or
a chamber which should be accessible by grab-mounted lorries with for easy desilting. Some
guidance on design details of the grit trap/sand trap can be found in GCO (1984).
13.5.6
Tidal Channels
For tidal channels where maintenance dredging is envisaged, prior consultation with
Port Works Division (CEDD) is required to determine the minimum water depth for the
marine plant. Consideration should also be given to the effect of the design invert level of a
tidal channel on its rate of sedimentation which is the prime factor affecting recurrent cost of
the channel.
13.5.7
Staff Gauge
Staff gauges should be installed at the channel sides for the checking of water level
in the channel. Details and installation locations of staff gauges should be agreed with the
respective operation and maintenance division of DSD.
13.5.8
Chainage markers and survey markers are to be installed at 100 m and 200 m
intervals respectively on the coping of both sides of the channel. Exact details and locations
of these markers should be agreed with the respective operation and maintenance division of
DSD.
13.5.9
For large drainage channel near the sea where the quantity of desilting is anticipated
to be enormous, marine access should be considered to facilitate future desilting operation.
73
Marine Department should be consulted regarding the requirements for marine traffic
management if marine access is necessary.
13.5.10 Maintenance and Management Responsibilities among Departments
The maintenance and management responsibilities of various departments concerned
including DSD should be clearly defined in early planning/design stage especially in
abandoned meanders, fish ponds, wetlands adjacent to the drainage channels, maintenance
roads and landscaping works. An example of the schedule of responsibilities for a completed
main drainage channels project is shown in Table 20.
13.5.11 Operation and Maintenance Manual
For major drainage channel, an operation and maintenance manual should be
provided by the design office upon the handing over of the project. It should include as-built
channel profiles, system hydraulics, spare parts provided, division of maintenance
responsibility among departments, trigger levels for maintenance dredging, suggested
monitoring schedule during operational phase, environmental issues relating to maintenance
dredging, geotechnical monitoring schedule of channel embankment, safety requirements in
relation to the operation and maintenance of the works and other maintenance items.
13.6
The soffit of all bridges crossing open channels should be designed such that no part
of the bridge soffit will be submerged in water under the design rainstorm. Due
consideration should be given to ensure that sufficient headroom is provided for the passage
of maintenance plant and equipment.
As far as possible, bridge supports should not be positioned within channels. But if
this is unavoidable, the supports should be designed and streamlined so that the obstruction to
the water flow is minimal and debris and boulders are not easily trapped.
Utilities crossing a drainage channel which will reduce the design flow capacity and
prevent the passage of desilting plant should be avoided. Such utilities should be
accommodated as part of the highway or utility bridge crossings at suitable locations so that
no part of the services will be below the bridge soffit. Alternatively, the utility crossings
should be placed beneath the open channel with a minimum cover of 1 m. Such undercrossings must be designed and constructed to a standard to minimize the chance of digging
up the channel for repair or replacement in the future.
13.7
GEOTECHNICAL CONSIDERATIONS
13.7.1
Embankment Design
The embankment design should be checked for the stability of each of the following
failure modes for both the short-term and long-term conditions and the finalized design
should be endorsed by GEO:
(a)
74
(b)
(c)
For the construction of a channel embankment over a layer of soft marine deposits,
excess pore water pressure would be built up instantaneously during the filling of the
embankment. Thus, undrained shear strength of the marine deposit should be adopted for the
short-term condition. For the long-term condition, the consolidation process of soil
underneath the embankment would be completed or nearly completed. As such, the longterm stability should be checked for the drained condition. Moreover, the settlement of the
embankment and its rate of consolidation should also be checked and allowed for in the
design.
13.7.2
Factors of Safety
The factor of safety for an embankment design is defined as the ratio of average
available shear strength of the soil along the critical failure surface of the embankment to that
required to maintain equilibrium.
The adopted factor of safety against the failure of an embankment would be related
to risks causing loss of life or property. It is recommended to follow the guidelines as
stipulated in GCO (1984).
However, in case of a temporary condition such as during the construction stage of
the embankment, a balance should be made between the potential economic loss in the event
of a failure and the increased costs of construction required to achieve a higher factor of
safety.
13.7.3
Loading Cases
The design loading cases should taken into considerations the following:
13.7.4
(a)
(b)
(c)
the effects of steady seepage through the embankment when the river is at high
flows (critical for leeward side slope stability).
(d)
the effects of rapid drawdown during flood recession (critical for riverside
slope stability).
Methods of Analysis
Various methods commonly adopted for slope stability analysis can be applied for
analyzing the stability of an embankment. The method that should be applied depends on the
potential failure mode of the embankment. For those potential failures with a circular slip,
the method by Bishop, A.W. (1955) is simple to apply; for those with a non-circular slip, the
methods by Janbu, N. (1972) or by Morgenstern, N.R. & Price, V.E. (1965) are commonly
75
Seepage
Where appropriate, subsoil drains with proper filters should be provided at the toe of
the leeward slope of the embankment to keep the phreatic surface within the embankment.
The water quality of floodwater is not suitable for fish farming. Thus, the problem
of seepage through the channel embankment should be addressed especially at locations
where fish ponds are reinstated behind the embankment.
13.7.6
Sensitivity Analysis
A sensitivity analysis may be warranted to account for the variability of the ground
conditions and the uncertainty associated with the design values of soil strength. Some
guidance on the sensitivity analysis can be found in GCO (1984).
13.7.7
Various methods have been developed for improving the stability of an embankment
on soft foundation soil as shown in Table 21.
13.7.8
Geotechnical Instrumentation
All embankment slopes formed in association with the construction of the channel
should be registered with GEO according to the relevant technical circular. Standard DSD
sign boards for slopes should be erected alongside the slope edges according to the relevant
technical circular before the slopes are handed over to the maintenance department.
13.8
OTHER CONSIDERATIONS
13.8.1
76
For tidal channel where the downstream receiving water body is polluted or for
some other reasons that the channel at upstream has to be kept dry, an inflatable dam together
with a low flow pumping station installed near the downstream end of the channel may be
adopted to prevent tidal water from flowing into the channel. The inflatable dam will be
automatically inflated/deflated and the operation of the low flow pumping station will be
suspended according to the pre-set conditions.
Under normal weather conditions, the dam will be inflated to prevent tidal water
from flowing into the channel and the dry weather flow retained at upstream of the dam will
be pumped into downstream receiving water body.
During severe rainstorm, the inflatable dam will be deflated and the operation of the
pumping station will be suspended.
Inflatable dam shall be equipped with an alarm system to inform the
operator/controller/public in case of any unexpected deflation of the dam or loss of pressure
inside the dam. This is to ensure that necessary measures can be taken immediately to
evacuate any people who may be present either upstream or downstream of the dam and who
may be threatened by the sudden release of flood water.
Based on experience in Hong Kong, inflatable dam is costly in construction,
operation and maintenance. It is subject to ageing and damage of various kinds which may
require total replacement of the whole dam. There is only very limited suppliers in Hong
Kong thus the long term availabilities of reliable suppliers and maintenance services are in
doubt.
If a project requires the installation of large scale tidal control structures, the pros
and cons of different tidal control structures should be assessed in detail before a particular
type is adopted. Life cycle cost of the control measures shall be studied and the potential
impacts of various tidal structures on the local river-estuarine ecology should also be
addressed. Such assessments are also needed in case any of the existing installations become
dilapidated and need replacement.
In searching for potential alternatives, other types of tidal control structures such as
stop-log, flap valve, self-regulating tidegate, penstocks, radial gate, Obermeyer Spillway
Gate could be considered. It is necessary to evaluate the use of different tidal control
structures based on the following factors:
(a) Specific needs of the drainage project;
(b) Hydraulic performance of the tidal control structures;
(c) Hydraulic impact of the structures, especially during heavy rainstorm;
(d) Effectiveness of the structures to stop tidal water flow;
(e) Constructability and maintainability;
(f) Reliability and fail safe provision;
(g) Environmental considerations; and
(h) Life cycle cost.
13.9
Decking of existing nullahs is not preferred in view of the adverse hydraulic impact,
problems associated with tracking water pollution and utility crossings as well as
77
maintenance difficulties of a decked nullah. An open nullah has the advantage of an effective
flood relief path which can capture overland flows from both sides of the nullah and help to
mitigate the heat island effect.
Water pollution problem in a nullah should be tackled at source by a proper
wastewater collection system, instead of decking. If water pollution cannot be stopped, other
options such as dry weather flow interception systems for collecting and diverting the
polluted flow to the sewerage system should be considered.
When decking of a nullah is proposed, the proponent shall submit a Drainage
Impact Assessment (DIA) to DSD for agreement pursuant to EWTB TC (W) No. 2/2006 and
DSD Advice Note No. 1 to identify the beneficial use and ownership of the decking, and to
ensure that the decking proposal must not cause an unacceptable increase in the risk of
flooding in areas upstream of, adjacent to or downstream of the decked section.
Where utility crossings and obstructions are found in the nullah, their effects on the
hydraulic performance shall be checked to ascertain the acceptability of the drainage impacts
due to the proposed decking and these utility crossings. Opportunity shall also be taken to
eliminate the utility intrusions under the nullah decking project.
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GENERAL
Polder and floodwater pumping schemes have been in use in Hong Kong since the
early 1980s. They have been adopted to protect villages in low-lying catchments in NWNT
and NENT. A polder refers to a piece of lowland enclosed within an embankment, in which
the water level is independent of that outside the embankment. It can be formed by the
construction of flood protection embankment or similar structures in association with roads
and other developments. External floodwater are prevented from entering the polder and
surface runoff collected inside the poldered area will be pumped to nearby existing
watercourses outside the poldered area.
Basically, a polder and floodwater pumping scheme would comprise the following
components:
(a)
(b)
(c)
(d)
The flood protection structure polders and separates the low-lying villages from the
surrounding land and prevents external floodwater from entering the poldered area. Under
normal rainfall conditions, surface runoff within the polder would drain by gravity via the
internal village drains to the floodwater storage pond for storage and subsequent disposal.
When the water level has risen to a pre-determined level, the pumps will be operated
automatically to discharge the stored stormwater to nearby watercourses outside of the polder.
However, it is always favourable to provide some flow control devices to facilitate gravity
drainage of stormwater when rainfall is small and the water level in the nearby watercourse
outside the polder is not high. In this case, the flow control devices enable the runoff to bypass the storage and pumping facilities, and to directly discharge into the downstream
watercourse.
Typical layout and arrangements for a polder and floodwater pumping scheme are
shown in Figures 17 & 18. Although floodwater pumping schemes in Hong Kong have
mostly been implemented for villages under rural settings, it has scope for implementation in
the urban areas.
14.2
In the planning and design of a polder and floodwater pumping scheme, the
following should be taken into consideration:
14.2.1
Land Requirement
79
the size of these major components. For example, deep floodwater storage tank could be
adopted to replace the shallow floodwater storage pond, while underground pump chamber
can be built with its cover reserved for other structures and facilities.
14.2.2
14.2.3
The most commonly used pump for floodwater pumping stations is the Archimedian
screw pump. However, centrifugal pumps have also been used in some of the floodwater
pumping stations in Hong Kong. Both types have their own merits and demerits. The
Archimedian screw pump has been proven to be robust and efficient. It is most suitable for
situation in which large pumping rate at low head is required. However, a screw pumping
station is generally massive, noisy and visually intrusive. On the contrary, centrifugal pump
is prone to damage due to clogging. It is most suitable for pumping water at high pumping
head. Due to the high pressure involved, design of a centrifugal floodwater pumping station
and its associated rising mains should be carried out carefully to avoid hydraulic problems
caused by hydraulic surge, cavitation and creation of vortex in pump sump. Model tests may
be required if such major problems are anticipated. To reduce electricity consumption and
start-and-stop frequency of the pump motor, variable frequency electric motor could be
considered in particular for pumping station with large flow variation. Consultation with the
relevant maintenance parties should be sought in case of doubt.
14.2.4
Environmental Considerations
14.2.5
The drainage impact of the proposed floodwater pumping scheme has to be assessed
thoroughly with the necessary remedial actions considered. In the detailed design stage, it is
also necessary to consider the effect of implementation programmes of other nearby projects.
80
The construction of a polder and floodwater pumping scheme will inevitably alter
the hydrology of the drainage basin. Firstly, flood storage for the basin may be reduced and
flood depth in the flood plain outside the polder may be increased. Under such circumstance,
it is necessary to refer to the relevant Drainage Master Plan for the overall flood risk situation
of the catchment. Secondly, concentrated discharge from the floodwater pumping station to
an existing streamcourse with inadequate capacity would aggravate the flooding in the
surrounding area. The designer should initiate necessary steps to upgrade the downstream
streamcourse to accommodate the pumped discharge.
The size of the floodwater storage pond as well as the pump cut-in/cut-out settings
can also affect the flooding situation in the discharging streamcourse. It is worthwhile to
manipulate the pump setting so as to sensibly alter the outflow hydrograph of the polder to
avoid clashing with the peak flow of the streamcourse. With careful design, the peak water
level in the streamcourse could be reduced.
14.2.6
Harbourfront Enhancement
For new polder and floodwater pumping facilities to be provided on the Victoria
harbourfront, the principles and guidelines sets out in General Circular No. 3/2010 issued by
the Government Secretariat of the HKSAR on Harbourfront Enhancement shall be observed.
In general, the occupation of harbourfront land by public facilities that are environmentally
unpleasant or incompatible with the harbourfront are not supported. Where there are no
better alternatives after taking into account cost and other relevant factors, the project
proponent should keep the footprint to a minimum as far as possible, and implement
necessary mitigation measures to reduce the impact on the harbourfront. In addition,
harbourfront access should be reserved where practicable for public use and the project
proponent should landscape the harbourfront access to compensate for its occupation of the
harbourfront land.
14.3
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14.4
The objective of the internal drainage system is to collect and to convey surface
runoff inside the polder to the floodwater storage pond for storage and subsequent disposal.
The design standard for internal drainage system should be equivalent to that of the village
drainage as shown on Table 10.
Flow control devices, such as penstocks and flap valves, have to be installed to
facilitate flow diversion during routine maintenance and emergency repair of the floodwater
pumping station and floodwater storage pond. Such flow control devices should preferably
be connected to the floodwater pumping station by telemetry for central monitoring and
control. Whenever possible, the design of the internal drainage system and the flow control
devices should enable the by-passing of dry weather flow away from the floodwater storage
pond and the pumping station. When necessary, water level sensors can be used to enable
automatic operation of these flow control devices.
14.5
The open-air floodwater storage pond is less expensive to build and easy to maintain.
When there is land constraint, deep floodwater storage tank can be considered. However, the
construction and maintenance cost of a storage tank is generally much higher.
The floodwater storage pond should best be located at the lowest point of the polder.
During the initial planning stage, the villagers view on the type, size, location and any other
proposed facilities should be sought.
14.5.1
A wet floodwater storage pond is a pond purposely kept wet by allowing some
floodwater to remain in the pond, and it is often continuously recharged by dry weather flow
and groundwater seepage. It is usually adopted for ecological reason, or upon the request of
residents on fung shui reason. In the design of wet pond, care must be taken to ensure that
water inside the pond will not stay stagnant and become septic.
A dry floodwater storage pond is normally kept dry. A dry pond is preferable to a
wet pond from the maintenance point of view. However, proper signage should be provided
to warn the public against the possibility of flash flood.
In general, the floodwater storage pond (wet or dry) shall be fenced off against entry
by the public as far as practicable. It can also be utilised for other purposes such as basket
ball field and playground. If for whatever reason it must be opened for public use, special
precautionary measures (other than signage) shall be in place to ensure that all persons
staying within the storage pond will be evacuated at times of heavy rainfall.
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14.5.2
The sizing of storage pond for a given pumping capacity is given by the following
equation:
Qin Qout
where
Qin
Qout
S
=
=
=
dS
dt
Figure 19 illustrates the case for a pumping scheme with two duty pumps. Smax
represents the storage volume required.
The pumping capacity and the storage volume have to be balanced to arrive at a
most cost-effective combination. If a larger storage is chosen, a smaller pumping capacity
can be provided and hence there will be savings in future operation and maintenance costs.
Of course, this will incur a higher land resumption cost. On the other hand, a smaller storage
can be chosen if larger pumps are provided. The minimum cost can be obtained with the
optimisation of the storage facilities and the pumping capacity.
14.5.3
The operation and maintenance requirements of the floodwater storage facilities are
as follows:
(a)
Peripheral surface channels should be provided around the bottom of the floodwater
storage pond to convey runoff to the gravity outlet under normal situations. Vehicular access
to the pond should be provided for maintenance and desilting purposes. In addition, warning
signs are required to warn the public of the fact that the area may be subject to flash flooding
in case of heavy rain and of the slippery condition of the pond area. Pictorial illustrations to
help illiterate people are suggested.
If the dry pond is to be used also for other purposes, e.g. a playground, a satisfactory
arrangement with the relevant authorities should be made concerning the future management
and maintenance of the pond and the facilities before this concept can be adopted.
(b)
The public must be kept away from the pond area by warning signs, safety fences
etc., which also serve to prevent children from venturing in the pond. On the other hand,
vehicular access to the pond shall be provided to enable mechanised maintenance at regular
interval.
(c)
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14.6
14.6.1
General Requirements
Design Capacity
The design capacity for all duty pumps should be adequate to handle rainstorm
runoffs collected inside the polder with a return period of 10 years. Stand-by pumps must be
available and should be able to automatically take over the failed duty pumps. Both duty
pumps and stand-by pumps should be interchangeable. The standby pumps should be so
designed such that they can also be activated in case of exceptionally severe rainstorms.
In the determination of the pumping requirements, the following guidelines should
be followed:
(a)
The desirable design freeboard for floodwater storage facilities should be 300
mm for a storm with a 10-year return period.
(b)
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(c)
The total maximum pumping capacity (both duty pumps plus stand-by pumps)
of a pumping station should be able to accommodate a storm with a 50-year
return period.
The flood depths inside the polder during a 50-year and a 200-year rainstorm under
the proper functioning of duty pumps and the floodwater storage facilities should be checked.
If the flood depth is considered intolerable, the capacity of the storage facilities should be
increased to suit.
14.6.3
In designing a floodwater pumping station, vehicular access and parking area should
be provided for maintenance vehicles. Unhindered direct access from a public road to the
pumping station is required for the acceptance of FSD for fire fighting purpose. Gateway of
adequate size should be provided for vehicles to deliver and remove the bulky equipment.
To facilitate maintenance of the pumps, penstocks should be installed within the
pump sump for isolation of pumps from the floodwater storage pond/tank. Lifting appliances
should also be provided for lifting of pumps and other heavy equipment. Water level sensors
controlling the operation of pumps should be installed at suitable locations which will not be
subjected to local fluctuation of water level and the interference of floating debris. Water
level sensors should also be adjustable such that the pump cut-in/cut-out levels could be
varied to suit different operating conditions. For future maintenance of screw pumps, a
loading platform is normally required.
Power supply should be adequate for running the control system and all the pumps.
In addition, an emergency power generator must be provided within the station compound to
provide back-up electricity automatically during power failure. The generator must be
designed to supply sufficient power to operate the control system and the pumps. A fuel oil
storage tank is required to be installed in compliance with FSDs requirements. The
minimum capacity of the fuel storage should allow for 36 hours of operation of the generator
when running all duty and standby pumps. It would be preferable that a dual-feed power
supply could be obtained from the electricity company.
In addition to the above, the following points should also be considered for the
operation and maintenance of a pumping station: (a)
(b)
(c)
The configuration, structures, pump sump and designed level of lift of the
pumping station should be such that in the event of a dire emergency, a
portable pump can be put in place to pump as much floodwater as possible
from the storage pond to the pumping station outlet.
(d)
Details of the pump house including parapet walls on the roof, louvers, etc.
should be agreed with the relevant maintenance parties.
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14.7
TRASH SCREENS
Trash screens at the inlets to the pump sump and those at the outlets from the
floodwater storage pond are required to prevent large flooding objects from damaging and
clogging the pumps. The screens are normally of manual hand-raked type. The bar spacing
and the bar size of the trash screens should be properly designed to avoid possible blockage
of the pumps. It is essential that proper access to each screen be provided and a working
platform installed over every screen above flood levels to facilitate routine and emergency
raking. If it is found to be cost-effective, mechanical raking devices should be provided to
save manpower in raking.
The trash screen is considered to be one of the crucial components of a floodwater
pumping scheme, and should be monitored by video surveillance.
14.8
a telemetry system for monitoring and control of plant operation status of the
pumping station as well as other hydraulic structures, such as the inlet
chambers. The monitoring and control signals should include water levels in
the floodwater pond, status of power supply, pumps, penstocks, screens,
generator set, fuel oil storage tank, fire alarms, telemetry fault, etc.
(b)
a video surveillance system for the visual monitoring of crucial electrical and
mechanical (E&M) and civil components (such as the pumping station outlets
to existing watercourses) to ensure proper functioning of the whole scheme at
any time.
14.9
MISCELLANEOUS ISSUES
14.9.1
System Commissioning
86
arrangement for the provision and storage of water and the re-circulation facilities have to be
made to enable smooth running of the pumps.
14.9.2
The major operation and maintenance parties for floodwater pumping schemes are
the Sewage Treatment Division, the respective Operation and Maintenance Division and the
Building & Civil Maintenance Team of DSD. Their comments should be sought as early as
the design stage to agree on the construction details.
Upon commissioning of a floodwater pumping scheme, system manual of the
scheme, operation and maintenance manual of the plant equipment, a full set of civil,
electrical and mechanical drawings, and all required test reports and warranty documents
should be provided to the operation and maintenance parties to facilitate the future operation
and maintenance activities.
In general, flood protection embankment slopes and associated retaining walls have
to be registered in GEO according to the relevant technical circular. Prior arrangements have
to be made with GEO and the maintenance agency as to the registration of the slopes, the
maintenance responsibilities and the display of the registration numbers on site. Standard
DSD sign boards for slope should be used for the display of the registration numbers.
14.9.3
14.9.4
Future Extension
Although the area of catchment inside a polder is usually constant, change in land
use, especially from fish pond and agricultural land to paved area, will affect the response of
the catchment and thus the amount of surface runoff to be pumped. In designing the general
layout of the floodwater pumping scheme, the pumping station and the floodwater storage
facilities should be carefully sited to enable future extension, such as the additional of some
more pumps and the deepening of the storage pond.
87
GENERAL
Maintenance Objectives
The objectives for proper maintenance and operation include:
(a)
to offer a quality of service that is acceptable, having regard to costs and the
effects on the environment, and to remedy recognised deficiencies.
(b)
to monitor the capacity of the system and to restore the flow capacity by
removal of excessive accumulation of silt and grease, etc.
(c)
(d)
(e)
(f)
(g)
to achieve the above service objectives making the best possible use of
manpower and resources at the least cost and least disruption to the public.
15.2
15.2.1
To ensure that the works can be readily handed over to the maintenance authorities
on completion, the standard of design and maintenance requirements laid down in this
Manual must be fully complied with. Additionally, close consultation and liaison should be
maintained between the design office and the maintenance authorities at each stage of the
project. During the planning and design stages of a project, a design memorandum should be
prepared so that the design parameters, handing over requirements or partial handing over
arrangement of large project can be agreed by the maintenance authority. For non-standard
drainage items, detailed consultation is required such that the operation and maintenance
requirements can be incorporated into the design. If unforeseen problems are encountered
during construction and changes have to be made, the maintenance authority must be
consulted as soon as possible so that the changes can be accepted. On completion, any
88
changes made should be incorporated in the design memorandum before handing over of the
completed works to the maintenance authorities.
Prior to handing over of the works, joint inspection must be carried out and any
outstanding works agreed. On substantial completion of the works, a handing over
inspection should be carried out to ensure that all outstanding works have been completed
before the issue of the completion certificate. Within 3 months of issuing the completion
certificate, the final operation and maintenance (O&M) manual for electrical and mechanical
(E&M) works, as-built drawings and calculations should be submitted. Prior to the end of the
Maintenance Period, a joint inspection should again be carried out to check if further works
are required and that all outstanding or remedial works have been completed.
Reference should be made to the Project Administration Handbook and the relevant
technical memoranda for details of handing over and taking over procedures.
15.2.2
All pipes, channels and culverts, etc. to be handed over should be inspected in dry
conditions wherever possible. In the case where the pipes, culverts or channels have to be
commissioned prior to handing over (e.g. due to the requirement to maintain the existing flow
or staged completion) and a temporary diversion of flow is not feasible, an additional
inspection should be arranged prior to the commissioning. In certain circumstances, closed
circuit television (CCTV) survey of the pipes and internal faces of the manholes showing
each connection pipe before commissioning can be adopted as an alternative to the joint
inspection but prior agreement with the respective operation and maintenance division of
DSD should be sought.
15.2.3
Documents to be submitted
After the satisfactory handing over inspection, the following documents should be
submitted as soon as possible, but it should not be later than 3 months under any
circumstance:
(a)
(b)
(c)
(d)
(e)
In the event that as-built drawings are not available at the time of the handing over
inspection, marked up prints of the working drawings showing the final amendments and the
extent of works to be handed over should be provided. Records of material quality and
acceptance tests should also be available for scrutiny.
89
15.3
15.3.1
Inspection Programme
15.3.2
Apart from general visual inspections, closed circuit television (CCTV) surveys can
also be used to investigate the condition, in particular the structural integrity of the drains in
close details.
It is essential that CCTV surveys are conducted during low flow conditions. If the
flow quantity is large, the drain upstream should be temporarily blocked and the flow
diverted. An adequate lighting system should also be adopted so as to produce a clear picture
of the drain. Pipes which are silted and the surfaces coated with grease should be cleansed
prior to the survey.
The defect coding, structural assessment and scoring system of culvert or pipeline
on which CCTV survey has been carried out shall be done in accordance with Manual of
Sewer Condition Classification and Sewerage Rehabilitation Manual published by Water
Research Centre" to determine the priority of remedial works and the future inspection
programme.
15.3.3
(a)
Red and Pink Routes are classified by HyD as the major road network in Hong
Kong. The Red Routes and Pink Routes are sections of the major road network where the
capacity and nature of the alternative routes is limited and the potential impact is very high if
these routes are either partially or totally closed. Details of the Red and Pink Routes are
shown in the relevant Highways Department Technical Circular. Due to the importance of
these routes, it would be highly undesirable to carry out unplanned works within these areas.
90
Persistent leakage of water from sewers and stormwater drains (including gravity
pipes, channels, tunnels and rising mains) not only causes nuisance, but can also be a serious
risk to the stability of slopes and retaining walls. Such leakage can deliver a significant
amount of water into the ground and its potential effect on the stability of the slope or
retaining wall should not be disregarded. Preventive measures in the form of regular
inspection and maintenance should be carried out with reference to Code of Practice on
Monitoring and Maintenance of Water-carrying Services Affecting Slopes (ETWB) and the
latest Geoguide requirements. Where defective drains are found, repairs should be carried
out immediately.
As a minimum, sewers and drains located within a distance of H from the crest of a
man-made slope/retaining wall, where H is the maximum vertical height of the
slope/retaining wall, should be inspected at the frequency in accordance with Code of
Practice on Monitoring and Maintenance of Water-carrying Services Affecting Slopes
(ETWB) depending on the type of slopes. The distance from the crest of the slope should be
further extended where the sewers and drains are known to be leakage prone. Particular
attention should be paid to pressurized rising mains as their leakage or bursting may lead to
severe damage. More frequent leakage detection may be desirable for those rising mains
behind slopes and retaining walls in the high Risk-to-Life (i.e. Consequence-to-life) Category
as classified by GCO (1984). The frequency shall be decided based on the prevailing
conditions of the slopes/retaining walls and the rising mains. Reference can be made to WB
(1996). Records of inspections should be sent to the maintenance agent of the slope likely to
be affected by the sewers or drains.
15.3.4
Desilting Programme
91
Where open channels, nullahs and rivers are subject to tidal effect, regular
monitoring or survey should be carried out to ascertain the degree of siltation so as to
determine the frequency of desilting.
In open channels and inlets to box-culverts, desilting operations are sometimes
necessary for aesthetic or environmental reasons such as odour problem. The most common
situation is where squatter areas or agricultural activities are present upstream of the
engineered channel. Under such circumstances, desilting operations are often required at a
much more frequent interval and there are areas where desilting is carried out more than once
every month.
15.3.5
Manual rodding and scooping is the simplest method used in pipe cleansing. A
rattan rod with its head mounted with a hook or spike is driven manually into the pipe to
pierce the blockage. Solids produced will be collected at the downstream manhole and
removed by scoops. This method requires the least equipment and the set-up time is minimal.
It is very effective in clearing local blockages caused by refuse or debris. However, it does
not clean the pipe thoroughly and the blockage may reoccur very shortly. Its application is
restricted when the manhole is deep, length of the pipe is long or the pipe size is large.
Water jetting is a common method for pipe cleansing. A hose is led into the pipe,
usually from the downstream, and water is jetted out under high pressure up to 20 MPa
pushing the hose forward while at the same time washing away the substances accumulated
inside the pipe. This method is particularly effective in clearing blockages caused by oil and
grease. It is also very effective in clearing the grease coated onto the interior surface of the
pipes so as to explore the pipe surface condition. However, the effectiveness of water jetting
decreases with the increase in pipe diameter and is seldom used for pipes greater than 900
mm diameter. For pipes of length exceeding 100 m, the use of water jetting is also not
effective due to the excessive headloss in the hose.
Apart from normal cleansing, there are proprietary products available in the market
for mounting onto the head of the water jetting hose for breaking through hard material.
Some of the products have been used in Hong Kong and are found to be useful for breaking
out cement mortar in a semi-solid state deposited inside the pipe.
Winching is the most frequently used method for the thorough cleansing of pipes. A
ball or bucket is towed along a section of drainage pipe between two manholes by a pair of
winches. This action is repeated several times and the silt and debris inside the pipe can be
scraped out. This method can be used for various sizes of pipes and is very effective in
removing silt and medium sized particles inside the pipe. Some specially made ball can
also be used for breaking out hard material.
For large size box-culvert subject to tidal effect, desilting under submerged
conditions is labour intensive and very difficult. It is desirable to desilt the box-culvert in dry
condition. This can be achieved by using stop-logs or other device together with pumping.
92
For large open channels, nullahs and rivers with invert level below the tidal range,
specially made floating pontoons with excavators or cranes mounting on them have been
used in conjunction with grabs, air lift/suction or dredging for desilting.
Clearance of livestocks waste from dry weather flow interceptor should be carried
out according to ETWB TC(W) No. 14/2004 with close liaison with concerned government
departments.
15.4
15.4.1
When pipes are found to be damaged, repair work should be carried out as soon as
possible. Replacement of damaged pipes by open excavation is a commonly used method.
To replace defective pipes by open excavation method, attention should be drawn to
the following:
(a)
(b)
(c)
(d)
(e)
excavation dewatering
(f)
Close liaison with the utility undertakings and traffic authorities is required before
the replacement work is carried out so that suitable construction methods can be determined.
The drawback of the open excavation method is that it may occupy substantial road
space for a long period of time. In the urban area with heavy traffic, the economic loss due to
traffic disruption as a result of open excavation is becoming hard to justify. At the same time,
peoples aspiration is rising and they are becoming less tolerant of traffic disruption. As a
result, when drainage repair or improvement is required, trenchless methods for pipe
rehabilitation should be considered as alternatives to open excavation.
15.4.2
For most trenchless methods, the scope to increase the flow capacity is rather limited.
In general, the unit cost of trenchless renovation is higher than conventional open cut
technique where the pipeline to be replaced is shallow and there is no obstruction due to
underground utilities or other physical structures. However, when the need for increased
flow capacity is not a deciding factor, trenchless renovation method can be employed with
benefits of keeping social costs and economic losses to a minimum as well as avoiding
93
physical obstruction problems that would otherwise arise if conventional method is used.
However, if the defective drains in the urbanized area have become under-capacity,
opportunity should be taken during the remedial works to replace them by larger pipes so that
the overall capacity of the network can be increased to cope with any anticipated
developments.
Some typical trenchless methods which can be used for the rehabilitation of
defective drains are described in the following paragraphs. It should be noted that the list is
not exhaustive and other methods may also be applicable.
Localised Repairs and Sealing
(a) Joint Grouting. This method is applicable to drains which are leaking through
the joints but the drains are still structurally sound. Chemical grout is injected into the
leaking joint filling up the void surrounding it to stop further leakage. For small drains, the
chemical grout is internally applied by an inflatable packer guided by a CCTV camera and
the same packer is used to test for air tightness of the grouted joint. For large pipes, it may be
more convenient to send people into the drains to carry out the grouting directly.
(b) Mechanical Sealing. This method involves the installation of a metal band or
clip faced with an elastomeric material at the damaged section of pipe, which forms a seal
with the inner surface of the pipe. It has the advantage of not relying on in-situ chemical
reaction, and can also be installed quickly.
Mechanical sealing systems are available for spot repair of pipe of either man-entry
or non-man-entry. For non-man-entry pipe, the repair modules are installed by means of an
inflatable packer which expands the clip and presses the rubber against the pipe wall. The
packer can then be deflated and withdrawn.
Internal Lining
(c) Internal Lining using Epoxy Impregnated Liner. This method uses a factory
fabricated lining tube conforming with the internal dimension of the drain to be rehabilitated.
The liner consists of one or more layers of polyester felt in contact with impervious
polyurethane membrane, the thickness of which are chosen to suit individual requirements.
The polyester lining is firstly impregnated with specially formulated resin in the factory.
After delivery to site, the liner is inserted into the defective drain and properly expanded so
that its external surface is in contact with the interior of the defective drain. A high
temperature environment is introduced inside the liner to enable the resin impregnated
polyester felt to cure, harden and form a continuous solid pipe inside the original pipe. Any
branch connection to the relined drain can then be reopened with a remotely controlled hole
cutting machine.
The method is generally applicable for small to large size pipes, and even for oval
and egg shaped drains. It can negotiate through smooth bends but wrinkles may develop at
sharp bends. It adds extra structural strength to the original pipe and by proper design of
resin, it offers good chemical and corrosive protection from all sort of environment. It
provides a smooth surface to the pipe, and may even improve the flow capacity.
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15.5
15.5.1
Operation
Polder and floodwater pumping schemes (PFPS) are designed for unmanned
automatic operation under the control of water level electrodes in the floodwater storage
ponds. The monitoring is by way of video surveillance and a telemetry system to transmit
fault alarm signals to the nearest manned DSD installation.
95
As PFPSs are designed for the protection of lives and properties of villagers, it is
considered that a close surveillance is required. The operation and detailed regular inspection
and maintenance activities should be well recorded. Any defect or irregularity identified
should be reported immediately so that prompt action can be taken to maintain the service
conditions and the PFPSs can operate effectively when the need arises.
For proper upkeep and running of PFPSs, it is necessary that regular inspections,
testing and trial operations must be put into effect. The routine operation functions should
include the regular inspection of the floodwater storage pond, drainage channels, operation of
penstocks/flap-gates, test running of machinery, pumps, telemetry devices, etc. and the
schedule of inspections in the following sub-section should be followed.
15.5.2
Schedule of Inspection
15.5.3
Documentation
The staff responsible for the operation and maintenance functions as described in the
above sub-section should record down the inspection results in a brief tabular report.
Dated photo reports with simple descriptions, taken at prescribed locations and
angles, will generally be sufficient for the majority of the uses, especially as regards the
routine inspections. Video recording may be used as a feasible alternative if more extensive
viewing is required. It is suggested that all representative inspection reports should be kept in
file for future review purposes.
15.5.4
15.6
15.6.1
Existing Capacity
When a connection to the existing drainage system is required, the capacity of the
existing system should be checked to see whether it has adequate spare capacity to
96
accommodate the additional flow from the proposed connection and whether enlargement or
duplication work is required.
15.6.2
Terminal Manholes
15.6.3
Provision of Manholes
15.6.4
Exclusive road drains of short length and roadside gullies directly connected to large
diameter drains or box culverts in tidal zone identified to have pollution issue should be
provided with water seal trap in order to alleviate potential odour problem.
15.7
DRAINAGE RECORDS
The existing drainage records should be continually updated to include all the newly
constructed stormwater drains and installations.
For all new works handed over to DSD for maintenance, as-built drawings as
specified in para. 15.2.3, in hard copy and electronic format, containing the geographical and
topographical data should be passed to the drawing office for retention and incorporation into
the existing drainage record drawings. All manhole positions with details on cover levels,
invert levels, diameters and directions of all the connecting pipes should be given in the
drawings. For other special installations and special manholes, detailed drawings are
required. The hydraulic and structural calculations, in hard copy and electronic format
containing the hydraulic models, should be provided to supplement the drawings.
For all repair works, drainage connections and minor improvement works carried
out during maintenance operations, a survey should be conducted on completion of the works
to record all changes in levels, positions and sizes. The results of survey should be passed to
the drawing office of DSD for updating the drainage records.
15.8
SAFETY PROCEDURES
15.8.1
97
Working in a confined space such as an underground drain, box culvert, tanks, etc.,
is potentially dangerous. Great care must be taken at all times, particularly when working
under adverse weather conditions. The legislative requirements of the Factories and
Industrial Undertaking (Confined Spaces) Regulation have to be followed. Reference should
be made to LD (2000), DSD Practice Note No. 1/2007 and DSD Safety Manual (2010) or their
latest versions, for the legislative requirements and good safety practice for working in
confined space. The essential elements of which include: (a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
15.8.2
Officers should always take note of the prevailing warning messages issued by the
Hong Kong Observatory, in particular the following:
(a)
Thunderstorm Warning
98
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
(k)
Some safety guidelines for working under adverse weather conditions are given in
DSD Safety Manual (2010) or its latest version.
99
16.1
INTRODUCTION
Trenchless (no-dig) construction methods have been used in various DSD projects
and some of the successful applications were presented in the technical papers identified in
the reference list. Trenchless construction method in this chapter refers to means for laying
of pipeline (or construction of culvert) of less than 3 metres in diameter without opening up
the ground surface above, which might be a more cost-effective alternative if it is at all
allowed or permissible. The difficulties for opening up the ground surface in Hong Kong
especially in the vicinity to the heavily inhabited area are various such as unbearable
disruption to traffic or business activities, physical obstructions (above or below ground),
prolonged construction period, construction problems and adverse factors on environmental
& other technical grounds.
Trenchless construction methods can be classified based on whether man-entry
would be permitted for the normal excavation operation along the alignment of pipeline (or
culvert). In this regard, they are broadly classified into two main types, namely man-entry
type and non-man-entry type for the context of this Manual. The main reason for such
classification is safety orientated as the safety concern and requirements would be more
stringent for former type.
For the avoidance of doubt, if some smaller diameter inner pipelines are to be laid
within a larger sleeve pipe or lining pre-formed beforehand, only the operation for
constructing the sleeve pipe or lining is to be governed by this classification. The safety
measures or requirements for subsequent laying of the inner pipelines especially when such
operation might render health or safety hazards to workers such as difficulties to swift
evacuation should be dealt with by other provisions under the construction contract.
Similar to other confined space work under DSDs jurisdiction, contractors workers
who need to participate in the trenchless construction would need to comply with the
competence enhancement training requirements stipulated in DSD Technical Circular No.
3/2012, or its latest version, as appropriate.
100
16.2
NON-MAN-ENTRY TYPE
The usual conditions associated with ordering this type of trenchless method
includes pipeline at considerable depth, long distance between adjacent access shafts and
susceptible ground conditions. After stipulating this type of trenchless method in the
construction contract, strictly no man-entry for normal excavation operation of pipeline (or
culvert) should be allowed for unless the Engineer is satisfied that the risks perceived at the
planning and design stages have all been cleared or for performing rescue operation for the
malfunctioning tunnel boring machine or other essential equipment trapped below ground.
Under this type of trenchless method, those types of tunnel boring machine (TBM)
equipped with a remote-control for normal excavation operation can usually be adopted.
However, some models of TBM which require frequent manual removal of foreseeable
obstruction such as hard strata or rock are unlikely to meet the said non-man-entry
requirements in this connection. In addition, other means known as hand-dug tunnels,
headings and hand shield methods that require constant manual input for normal excavation
operation obviously cannot meet such requirements.
Some basic information in respect of these methods that can meet the non-manentry requirements are highlighted below which are by no means exhaustive. Project
engineers and designers are to satisfy themselves that any particular method accepted can
really perform the required functions.
16.2.1
It is a TBM with a bulkhead located behind the face to form a pressure chamber.
Bentonite slurry or other medium is introduced into the chamber under appropriate pressure
to equalise ground pressure and to be mixed with material excavated by rotary cutterwheel.
The bentonite slurry forms a temporary filter cake on the tunnel face which the slurry exerts
pressure to support the ground. The continuously forming filter cake will be cut away by the
rotary cutterwheel as the TBM advances. The excavated spoil will be removed from the
pressure chamber by the slurry circulation system.
16.2.2
The EPB method consists of a cutting chamber located behind the cutterhead. This
chamber is used to mix the soil with water foam/soil conditioner. It is maintained under
pressure by the screw mucking system. The ground at the cutting face is supported by the
resultant pressure balancing the increase in pressure due to advancement of the TBM and the
reduction in pressure due to discharge of the excavated spoil.
The underlying principle of the EPB method is that the excavated soil itself is used
to provide continuous support to the tunnel face by balancing earth pressure against the
forward pressure of the machine. The thrust forces generated from rear section of TBM is
transferred to the earth in the cutterhead chamber so as to prevent uncontrolled intrusion of
excavated materials into the chamber. When the shield advances, the excavated soil is mixed
with the injected special foam/soil conditioner material which changes the viscosity/plasticity
of the spoil and transforms it into a flowing material. With careful control of the advance
thrust force of the TBM and the rate of discharge of the spoil, adequate pressure could be
101
maintained in the pressure chamber for supporting the tunnel face during the excavation
process.
16.3
MAN-ENTRY TYPE
As indicated by the category name, man-entry would be permitted under this type of
trenchless construction. Thus, adequate underground working space should have been
ascertained by the designer at the pre-contract stage. Thorough study on the record drawings
of the existing structures and utilities for the entire alignment along with any necessary
ground investigation (including geophysical survey) should be conducted prior to allowing
this type of trenchless method in the construction contract. According to the current safety
standards, underground access to workers should be of 1.2m diameter minimum. Besides,
sufficient area at the access shafts should also be obtained as back-up area to cater for
emergency.
Some basic information in respect of these methods that can meet the man-entry
requirements are highlighted below which are by no means exhaustive. Project engineers
and designers are to satisfy themselves that any particular method accepted can really
perform the required function.
16.3.1
Heading Method
Hand-dug tunnel (i.e. open mode) is the technique of installing pipes by forming a
tunnel with manual excavation inside the handshield from the entry pits to the end pits. The
excavated materials are transported to the ground level through a trolley system and lifted up
by lifting gantry installed at entry pit. Segments of tunnel frames are constructed one after
the other until reaching the end pits. It is effective when the alignment of a pipeline (or
culvert) has to pass through mechanical obstructions like walls and artificial hard materials
which can be removed by manual means. However, emphasis has to be placed on pretreatment to the ground prior to excavation to avoid instability of tunnelling soil face.
Excavation at tunnel face could be accelerated by using pneumatic tools or by mini-backhoe
if the size of tunnels allows.
102
16.4
MAJOR CONSIDERATIONS
16.4.1
The roads in the urban areas of Hong Kong are generally congested with
underground utilities and services of different types and sizes at different depths. When
planning a pipeline using trenchless construction, the level and alignment shall be designed
to avoid diversion of existing utilities and services as it involves open excavation. Sufficient
information shall be obtained and trial pits or other means shall be carried out to verify the
depth and locations of all underground utilities in the vicinity of the proposed alignment &
level of trenchless pipeline.
(b) Alignment & Level
Construction of a pipeline (or culvert) normally follows the alignment of the road.
Pipe jacking is well applicable for straight driving, though slightly curved driving is possible
with the latest technology; therefore, pipe jacking is not feasible in areas that require sharp
bends unless additional intermediate jacking/receiving shafts are constructed.
Trenchless construction is often applicable for deep pipes in order to avoid
underground utilities, felling of valuable trees and unacceptable ground movement. Hence, it
is not suitable for very shallow pipe installation. Construction of deep pipelines may
encounter bedrock or boulder which presents difficulty in pipe jacking. Hand-dug tunnel
construction would be an alternative in such scenario while microtunnelling could also drill
through rock and artificial hard material. Comprehensive planning and implementation of
site investigation would help to reduce the risk of unexpected ground conditions. It may not
be practical to carry out thorough drilling along the whole alignment of the pipeline. In most
circumstances, directional coring and geophysical methods should be considered along the
whole alignment of the proposed trenchless pipeline.
For pipeline (or culvert) construction in long length, it may be worthwhile to
consider adopting different forms of construction for different sections of the pipeline
between intermediate temporary shafts to suit changing conditions along the pipe alignment.
(c) Locations of Jacking/Launching and Receiving Shafts
Jacking shaft and receiving shaft between a pipeline (or culvert) should be selected
to avoid conflicting with traffic and major utilities or minimize their diversions. The choice
of the locations of the launching shaft and the receiving shaft depends on a number of factors
such as the positions of permanent manholes, the hydraulic design of water flow, the
maximum length of pipelines for the ease of future maintenance and the required working
space. Substantial working space is required for launching shaft, receiving shaft and slurry
treatment plant (for slurry operated TBM method). Thus, the area occupied by the shafts and
slurry plant shall be considered in space limited works fronts during design stage. Due
consideration should be given to temporary traffic arrangement schemes and application of
Excavation Permits in stages.
103
The underground condition generally governs the geometry of the shaft. Rectangular
shaft is usually constructed because it can be modified without much difficulty to
accommodate existing utilities and services. However, at locations where these features are
absent, circular shaft is used due to the smaller member size of temporary works required.
The shape and size of a shaft need to be tailor-made to suit utility constraint,
resulting in the possible use of a combination of sheetpiles and pipe-piles to overcome the
problem. For deep shaft, grouting is always required along the perimeter to ensure
watertightness and hydraulic failure at the base of the shaft, before excavation is to
commence.
(d) Site Investigation and Design
Thorough site investigation shall be carried out at the design stage. The site
investigation result is essential to the design and procurement of machine for trenchless
construction. Designers shall take into account the tunnel face stability, face support
pressure, dewatering effects, ground loss and ground movement estimation, etc in the design.
With regard to ground control slurry TBM method, guidelines for design calculations and
work procedures shall be made reference to GEO Report No. 249.
16.4.2
Construction Stage
(a) Alignment & Level Control
The pipe jacking works in Hong Kong using TBMs generally shall follow the
Specification for Tunnelling (BTS and ICE, 2010) as guideline for controlling tunnel
alignment, in that a tolerance of 50mm is specified for line. However, deviation in more than
the tolerance may occur at locations with unfavourable ground conditions. The tunnel
alignment is corrected by suitable extension or retraction of the steering cylinders installed in
the TBM. The use of TBM with 4 nos. steering cylinders offers better control in alignment
than that with 3 nos. It requires a long period of time to correct any out-of-tolerance in
alignment in order to avoid causing damage to the jacking pipes. However, allowance has to
be made to account for the irregular profile of the tunnel, due to different ground conditions
encountered during excavation, which could affect installation of the permanent pipeline
therein to the required alignment.
The level control shall also follow the Specification for Tunnelling (BTS and ICE,
2010), in that a tolerance of 35mm is specified for level. In case of the pipeline exceeded
the tolerated 0.5 degree angular deflection at pipe joint, there is a need to carry out a detailed
inspection to ensure that there is no dislocation thereat. To better control the hydraulic
performance of a pipeline (or culvert), tighter tolerance on invert level in the order of few
millimeters may be adopted. For excessive opening in pipe joint, remedial measures have to
be carried out. This can be achieved by locally trimming the concrete at the pipe end for
better bonding before applying non-shrinkage epoxy, with a strength equivalent to the pipe,
to fill up the problematic location for prevention of ingress of water.
(b) Safety Concerns
(i)
104
The adoption of compressed air hand-dug tunnelling method would face a problem
associated with air loss in porous ground, giving rise to the necessity of carrying out ground
treatment to safeguard the tunnel and the personnel working inside. The switch-on of
compressors and generators roundthe-clock to maintain the pressure in tunnel also causes
noise problem. Although high cost and relative low production rate make this method only
applicable to short drives, the removal of artificial obstructions can be warranted. However,
following the rapid development of TBM technology which allows pipe jacking drives in
curved alignment and detects obstructions ahead of TBM advancement, personnel working
under compressed air shall be avoided as far as practicable due to the risk involved and the
reasons stated above. In case working under compressed air is found essential, detailed
justifications and risk assessment should be prepared before construction.
Compressed-air tunnel will be adopted for high groundwater table condition.
Depending on depth, an air pressure of 1 to 2 bars is required to balance the water head in the
excavation face and be maintained inside the tunnel round-the-clock to avoid flooding which
may in turn affect tunnel stability. To ensure constant supply of air, a standby compressor is
provided for emergency situations. Pressurization and depressurization process is required in
the air-lock installed on top of an air deck erected in the jacking shaft, for personnel entering
and leaving the tunnel respectively. A medical lock needs to be provided at the shaft location
when the applied compressed-air pressure exceeds 1 bar.
(ii)
Workplaces for trenchless construction including shaft and tunnel are always
enclosed nature and there are reasonably foreseeable risks such as sudden ingress of water
and collapse of tunnel. Procedures for working in confined space shall be strictly followed
including assessment on the tunnel face stability and ingress of groundwater. Reference shall
be made to DSD Practice Note No. 3/2012 Safety Supervision of Work in Confined Space,
or its latest version, DSD Safety Manual on Use of Headings and relevant regulations.
When working in confined space under compressed air condition, there have been
some cases that air was found leaking through the porous ground during tunnel excavation,
resulting in inflow of groundwater. This entails horizontal and vertical grouting from inside
the shield to stabilize the ground before further excavation could be proceeded with.
Flooding of the heading and the access shaft can occur as a consequence of sudden
inrush of water from exposed faces due to bursting of nearby watermains, heavy rainfall, etc.
In the risk assessment, suitable measures to ensure the heading works watertightness to
prevent flooding should be considered.
(c) Ground Movement Monitoring
Tunnelling and pipe jacking would induce settlement in surrounding ground. The
magnitude of settlement is greatly affected by ground conditions, type of tunneling method,
control of inflow of groundwater, depth of tunnel and jacking speed. The presence of
underground utilities and services above the jacked pipeline would lead to undermeasurement of surface ground settlement due to their rigidity. It is necessary to estimate
the settlement influence zone and to assess its effect on nearby roads, structures and utility
105
installations such that they can be safeguarded during the operation and remedial measures
taken, if necessary. Maximum ground settlement occurs at the centre line of the pipeline and
diminishes to zero at a distance from its two sides. Most settlements occur during and
immediately after completion of tunneling and pipe jacking works. Further settlement would
continue, and its stoppage depends on the ground and groundwater conditions above the
jacked pipeline, for a few weeks to a few months.
In many cases, ground settlement is associated with change in groundwater level
due to dewatering or groundwater inflow in the tunnel. The Contractor shall closely monitor
the standpipe and piezometer readings with reference to the baseline record. If there is
significant drawdown of groundwater level, assessment to the effect of settlement and
subsequent remedial measures shall be carried out.
For monitoring settlement, sub-surface settlement markers, in the form of a steel rod,
by coring through rigid pavement, are generally adopted, with their installation at suitable
intervals along the alignment of the pipeline and with sufficient number offset at both sides,
prior to commencement of a pipe jacking drive. In flexible pavement, nail markers are used.
This is supplemented by visual inspection that if settlement occurs, cracks would develop in
pavement. For structures sitting on shallow foundation, their condition has to be assessed
before commencement of tunneling and pipe jacking such that suitable monitoring devices
such as tilt markers and settlement markers can be installed to monitor the ground behavior
during the course of works. If the measured ground settlement exceeds the predicted value,
the tunneling and pipe jacking works have to stop and an investigation on the cause, and the
damage, if any, carried out, with remedial measures such as ground treatment implemented,
as necessary, prior to resumption of works.
16.4.3
Environmental Issue
Slurry pressure balance method using TBM requires a large amount of bentonite
based slurry during the course of driving. Proper consideration should be made to recycle
and dispose of the bentonite slurry after use to minimize the impact to the environment.
The bentonite based slurry of slurry shield TBM is mixed at the slurry tank and
pumped to the work face through the cutterhead of the shield under a recycling system. The
spoil excavated by the slurry shield machine is pumped to the slurry tank for separation and
disposal. Proper monitoring system should be set up to avoid overflow of slurry from the
recycle tank in case the outflow slurry pipe is clogged with spoil. The slurry tank should also
be designed with sufficient free board to avoid the bentonite based slurry spilling out on
public roads and drains.
16.4.4
Cost Consideration
Pipeline (or culvert) laid using trenchless construction method are usually of higher
construction cost than those laid using open trench construction method. Thus, consideration
shall be made on the cost-effectiveness of trenchless construction method and the benefits
that may be brought to the public before adopting trenchless construction method instead of
open trench construction method. However, for case of deep sewer, great difficulty in utility
diversion or temporary traffic arrangement, trenchless construction method may prevail in
terms of cost and constructability.
106
REFERENCES
Adri Verwey (2005). Lecture Notes on Computational Hydraulics, Version 2006-1
Bishop, A.W. (1955).
The Use of the Slip Circle in the Stability Analysis of Slopes.
Geotechnique V, No.1. pp 7 - 17.
Brater, E.F. & King, H.W. (1976).
New York.
British Standards
BSI (1988/1).
BS5400: Steel, Concrete and Composite Bridges. Part 1: General
Statement. British Standard Institution, London.
BSI (1988/2).
BS5911: Precast Concrete Pipes, Fittings and Ancillary Products.
British Standards Institution, London.
BSI (1997/1).
BS8110: Structural Use of Concrete Part 1 to 3. British Standards
Institution, London
BSI (1997/2).
BS EN 752-4: Drain and Sewer Systems Outside Buildings. Part 4:
Hydraulic Design and Environmental Considerations. British Standards Institution,
London
BSI (2008).
BS EN 752: Drain and Sewer Systems Outside Buildings, British
Standards Institution, London
CED (2002).
HKSAR.
107
CIRIA (1997).
Report R168 Culvert Design Manual. Construction Industry Research
and Information Association.
Cunge, J.A. (1969).
On the Subject of Flood Propagation Method (Muskingum
Method). Journal of Hydraulic Research, International Association of Hydraulics
Research, Vol.7, No.2. pp 205-230.
DSD (1990).
Reports of Territorial Land Drainage and Flood Control Strategy Study
- Phase I, by Mott MacDonald as consultants to Drainage Services Department,
Hong Kong Government.
DSD (1992).
Strategic Sewage Disposal Scheme Site Investigation & Engineering
Studies, Stage I Kowloon System, Preliminary Design Manual, by AB2H
Consultants for Drainage Services Department, Hong Kong Government.
DSD (2002).
Research and Development Section, Trenchless Pipe Installation and
Renovation Techniques for Construction of Drainage Pipelines, Research &
Development Report No. RD 1005/2 Drainage Services Department, The
Government of the Hong Kong SAR
DSD (2005).
DSD Practice Note 1/2005 - Guidelines on environmental considerations
for river channel design. Drainage Services Department, Hong Kong SAR
Government.
DSD (2009).
Safety Manual.
Government.
FHWA (1985).
Hydraulic Design of Highway Culverts (FHWA-IP-85-15).
Highway Administration, USA.
GCO (1984).
Geotechnical Manual for Slopes, 2nd Edition.
Office, Hong Kong Government.
Federal
Geotechnical Control
GCO (1987).
Geoguide 2 - Guide to Site Investigation. Geotechnical Control Office,
Hong Kong Government.
GCO (1988).
Geoguide 3 - Guide to Rock and Soil Description.
Control Office, Hong Kong Government.
Geotechnical
GEO (1998).
Geoguide 5 - Guide to Slope Maintenance. Geotechnical Engineering
Office, Hong Kong Government.
GEO (2008).
Ground Control for Slurry TBM Tunnelling, GEO Report No. 249,
Geotechnical Engineering Office, Hong Kong, 37 p.
Herschy, R.W. (1985).
Publishers.
108
The Wallingford
HRL (1990).
Charts for the Hydraulic Design of Pipes and Channels, 6th edition.
Hydraulics Research Limited, Wallingford.
HyD (2010).
Guidance Notes No. 35 (RD/GN/035) Guidance Notes on Road
Pavement Drainage Design. Highways Department, HKSAR.
HyD (2006).
IPCC, 2007: Summary for Policymakers. In: Climate Change 2007: The Physical Science
Basis. Contribution of Working Group I to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z.
Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge
University Press, Cambridge, United Kingdom and New York, NY, USA, p.11.
J.A. Cunge, F.M. Holly, Jr., A. Verwey (1980).
River Hydraulics. Pitman, Boston.
Practical
Aspects
of
Computational
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Pfeffer et al., 2008: Kinematic Constraints on Glacier Contributions to 21st Century SeaLevel Rise, Science, Vol. 321, No.5894, pp.1340-1343
PWD (1968).
Design Flood for Hong Kong. Public Works Department, Hong Kong
Government.
Rahmstorf, S., 2007: A semi-empirical approach to projecting future sea-level rise, Science,
Vol. 315, No. 5810, pp 36870
RO (1991).
A Design Rainstorm Profile for Hong Kong. Royal Observatory, Hong
Kong Government. (Unpublished)
Soil Conservation Service (1972).
Delft
Watkins (1962).
The Design of Urban Sewer Systems (Transport and Road Research
Technical Report No.35). Transport and Road Research Laboratory.
WB (1990).
WBTC No. 6/90, Greenhouse Effect Allowance in Design. Works
Branch, Hong Kong Government.
WB (1996).
Code of Practice on Inspection & Maintenance of Water Carrying
Services affecting Slopes. Works Branch, Hong Kong Government.
WSD (annual).
Hong Kong Rainfall and Runoff.
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Zanen (1981).
Revetments. International Institute for Hydraulic and Environmental
Engineering, Delft, the Netherlands.
110
LIST OF TABLES
Table
No.
1
10
11
12
13
14
15
16
17
Traffic Loads
18
111
Table
No.
Page
No.
19
20
21
22
23
24
25
26
112
20
Figure 3
46
Figure 2
86
Figure 2
28
Figure 2
*26.000
*29.000
*34.000
*36.500
*39.000
41.437
84.8
105.8
*120.0
*145.0
*173.0
*194.0
*220.0
232.6
30++
15++
10
0.50
0.25+++
Notes :
24.500
64.4
60++
356
336
302
274
231
197
179
147
116
83.0
55.5
20
*interpolated data
**based on hourly rainfall records at RO (now HKO) Headquarters (1884 1939 ; 1947 1990)
4.
5.
423
399
362
329
278
240
219
181
144
105
71.8
100
3.
394
372
336
306
258
221
201
167
132
95.5
64.8
50
1
1
ln ln1
T
326
308
276
250
210
179
161
132
103
73.4
48.3
10
2.
295
278
249
224
188
159
143
116
90.4
63.4
40.5
For interpolation/extrapolation, x = +
248
234
207
185
156
130
115
92.5
70.7
48.2
29.3
T(year)
452
427
387
353
299
258
236
196
156
114
78.8
200
1.
21.00
17.312
13.378
43.3
120++
10.085
(mm/h)
(mm/h)
25.45
1/
240**
Duration
(min)
Parameters
490
462
421
384
325
282
258
215
172
126
88.0
500
519
489
446
408
345
300
275
230
184
136
95.0
1000
113
114
10
20
50
100
200
500
1000
548
573
603
639
687
722
766
822
855
5.2
4.6
4.4
4.3
4.2
4.1
4.1
4.1
4.0
0.51
0.47
0.44
0.43
0.42
0.41
0.40
0.39
0.39
82.325
72.137
62.982
56.757
50.910
186.2
168.6
146.7
127.7
116.1
24 hours
18 hours
12 hours
8 hours
6 hours
135
149
170
195
216
253
231
255
288
331
371
435
486
1
1
ln ln1
T
192
213
241
277
310
363
407
523
267
296
334
383
431
505
561
604
634
315
349
392
450
507
595
659
710
744
777
350
389
436
500
565
662
732
789
826
861
1065
1312
100
The Intensity-Duration-Frequency (IDF) data can be generated from the above by dividing depth with duration
96.624
217.7
2 days
288
438
550
666
968
1205
50
3.
104.982
249.3
3 days
310
461
581
839
1063
20
Based on hourly records measured at RO (now HKO) Headquarters (1884 1939 ; 1947 1990)
113.004
268.7
4 days
328
491
739
953
10
2.
117.653
284.9
5 days
356
635
838
119.127
312.6
7 days
478
665
1.
138.702
427.0
15 days
Notes:
152.71
609.4
(mm)
31 days
1/
Parameters
(mm)
Duration
386
428
480
551
622
729
805
867
908
943
1162
1418
200
432
480
538
617
698
818
902
971
1016
1053
1289
1558
500
468
520
582
667
755
885
974
1049
1098
1135
1385
1664
1000
115
116
287
231
188
161
143
130
119
111
105
99
95
90
87
84
81
78
76
74
72
70
69
67
66
64
63
62
61
60
59
58
57
56
55
55
54
53
52
52
51
51
50
49
49
48
48
47
47
46
46
46
45
45
44
44
44
43
43
43
42
42
42
344
278
227
195
174
159
147
138
130
123
118
113
109
105
101
98
96
93
91
89
87
85
83
82
80
79
77
76
75
74
73
72
71
70
69
68
67
66
66
65
64
63
63
62
62
61
60
60
59
59
58
58
57
57
56
56
55
55
55
54
54
399
325
267
232
208
191
177
166
157
150
143
138
133
128
125
121
118
115
112
110
107
105
103
101
100
98
96
95
93
92
91
90
88
87
86
85
84
83
82
81
81
80
79
78
78
77
76
75
75
74
74
73
72
72
71
71
70
70
69
69
68
457
372
307
267
240
221
205
193
183
174
167
161
155
150
146
142
138
135
132
129
126
124
121
119
117
115
113
112
110
109
107
106
104
103
102
101
100
98
97
96
95
95
94
93
92
91
90
90
89
88
87
87
86
85
85
84
83
83
82
82
81
Duration
Interval
(min)
60.5 61.5
61.5 62.5
62.5 63.5
63.5 64.5
64.5 65.5
65.5 66.5
66.5 67.5
67.5 68.5
68.5 69.5
69.5 70.5
70.5 71.5
71.5 72.5
72.5 73.5
73.5 74.5
74.5 75.5
75.5 76.5
76.5 77.5
77.5 78.5
78.5 79.5
79.5 80.5
80.5 81.5
81.5 82.5
82.5 83.5
83.5 84.5
84.5 85.5
85.5 86.5
86.5 87.5
87.5 88.5
88 .5 89.5
89 .5 90.5
90 .5 91.5
91.5 92.5
92.5 93.5
93.5 94.5
94.5 95.5
95.5 96.5
96.5 97.5
97.5 98.5
98.5 99.5
99 .5 100.5
100.5 101.5
101.5 102.5
102.5 103.5
103.5 104.5
104.5 105.5
105.5 106.5
106.5 107.5
107.5 108.5
108.5 109.5
109.5 110.5
110.5 111.5
111.5 112.5
112.5 113.5
113.5 114.5
114.5 115.5
115.5 116.5
116.5 117.5
117.5 118.5
118.5 119.5
119.5 120.5
120.5 121.5
Note: * Rate of Rainfall is the average value over consecutive time intervals.
42
41
41
41
40
40
40
40
39
39
39
39
38
38
38
38
37
37
37
37
37
36
36
36
36
36
35
35
35
35
35
34
34
34
34
34
34
34
33
33
33
33
33
33
32
32
32
32
32
32
32
32
31
31
31
31
31
31
31
31
30
54
53
53
53
52
52
52
51
51
51
50
50
50
50
49
49
49
48
48
48
48
47
47
47
47
46
46
46
46
46
45
45
45
45
44
44
44
44
44
43
43
43
43
43
43
42
42
42
42
42
42
41
41
41
41
41
41
41
40
40
40
68
68
67
67
67
66
66
65
65
65
64
64
63
63
63
62
62
62
61
61
61
60
60
60
60
59
59
59
58
58
58
58
57
57
57
57
56
56
56
56
55
55
55
55
55
54
54
54
54
54
53
53
53
53
53
52
52
52
52
52
52
81
81
80
80
79
79
78
78
77
77
76
76
76
75
75
74
74
74
73
73
72
72
72
71
71
71
70
70
70
69
69
69
69
68
68
68
67
67
67
67
66
66
66
66
65
65
65
65
64
64
64
64
63
63
63
63
63
62
62
62
62
117
Table 6 Intensity-Duration-Frequency (IDF) Relationship
for Frequent Rainstorms
Duration
(min)
10
480++
5.2
8.1
12.1
15.0
420++
6.0
9.1
13.2
16.3
360++
7.1
10.5
15.0
18.4
300++
8.6
12.4
17.4
21.2
240++
10.7
14.9
20.6
24.8
180++
13.1
18.2
25.1
30.2
120++
17.9
24.1
32.3
38.5
60++
27.5
36.1
47.6
56.2
30++
39.2
50.6
65.6
77.0
15++
53.1
67.3
86.1
100.3
10*
61.0
77.0
98.0
114.0
5*
77.5
96.0
120.5
139.0
2*
97.0
119.0
148.0
170.0
1*
114.0
138.1
169.9
194.0
0.25+++
147.9
173.7
207.7
233.5
Notes :
1.
2.
3.
118
Table 7 - Tide Gauges in Hong Kong
Name of Station
Remarks
North Point
1954
Discontinued in 1986
Chi Ma Wan
1961
Discontinued in 1997
Tai Po Kau
1963
1974
1974
Waglan Island
1976
Lok On Pai
1981
Discontinued in 1999
Tamar
1984
Discontinued in 1991
Quarry Bay
1985
Tai O (old)
1985
1994
Shek Pik
1997
1999
Airport Authority
Ko Lau Wan
2000
Hydrographic Office of
Marine Department
Kwai Chung
2001
Hydrographic Office of
Marine Department
Ma Wan
2004
Hydrographic Office of
Marine Department
Cheung Chau
2005
Hydrographic Office of
Marine Department
2004
Drainage Services
Department
Sai Kung*
2004
Drainage Services
Department
Mui Wu*
2004
Drainage Services
Department
Tai O*
2005
Drainage Services
Department
119
Return Period
(Years)
North Point/
Quarry Bay
(1953-1990)
Tai Po Kau
Chi Ma Wan
(1962-1990)
(1974-1990)
(1963-1990)
2.65
2.95
3.05
2.75
2.95
3.35
3.25
3.05
10
3.05
3.65
3.35
3.15
20
3.15
3.95
3.55
3.25
50
3.35
4.25
3.75
3.45
100
3.55
4.55
3.85
3.65
200
3.65
4.75
3.95
3.75
North Point/
Quarry Bay
(1962-1990)
Tai Po Kau
Chi Ma Wan
(1981-1990)
(1983-1990)
(1981-1990)
1.98
1.97
2.28
2.05
120
2-5 years
200 years 4
50 years 4
10 years 1,3
50 years 2,3
Notes:
1. The impact of a 50-year event should be assessed in each village to check whether a higher
standard than 10 years can be justified.
2. Embanked channels must be capable of passing a 200-year flood within banks.
3. For definitions of Village Drainage and Main Rural Catchment Drainage Channels, refer to Section
6.6.1.
4. For definitions of Urban Drainage Branch and Urban Drainage Trunk Systems, refer to Section
6.6.2.
Flood Level
Return Period
Case I
Case II
200 years
100 years
50 years
10 years
5 years
2 years
121
Equations
Chzy
Manning
Darcy-Weisbach
Hagen-Poiseuille
ColebrookWhite
Formulation
Limit of Applications
V = C RS f
rough turbulent
R1 / 6
n
rough turbulent
V =
V =
V =
RS f
8g
RS f
f
laminar/turbulent
gS f R 2
laminar
k
1.255
V = 32 gRS f log s +
14.8 R R 32 gRS f
Hazen-Williams
pipe flow
V < 3m / s,
122
Surface
Best
Good
Fair
Bad
0.012
0.011
0.012
0.013
0.009
0.010
0.013
0.010
0.011
0.011
0.012
0.010
0.011
0.012
0.010
0.010
0.011
0.012
0.012
0.017
0.025
0.013
0.011
0.0225
0.013
0.012*
0.013
0.014
0.010
0.011*
0.015*
0.013*
0.012*
0.012
0.013
0.011
0.012
0.013
0.011
0.012*
0.013*
0.015*
0.014*
0.020
0.030
0.014
0.012
0.025
0.014
0.013*
0.014
0.015
0.011
0.013*
0.017*
0.015
0.014*
0.013*
0.015*
0.012
0.013*
0.015*
0.012
0.013
0.014
0.016
0.016*
0.025
0.033
0.015
0.013
0.0275
0.015
0.017
0.025
0.035
0.0225
0.025
0.025
0.028
0.020
0.030
0.040
0.025*
0.0275*
0.030
0.030*
0.0225*
0.033*
0.045
0.0275
0.030
0.035*
0.033*
0.030
0.033
0.040
0.035
0.025
0.030
0.033
0.040
0.0275
0.033
0.035
0.045
0.030
0.035
0.040
0.050
0.033
0.040
0.045
0.055
0.015
0.017
0.013
0.017
0.017
0.015
0.017
0.013
0.015
0.016
0.013
0.014
0.015
0.018
0.030
0.035
0.017
0.015
0.030
0.025
0.035
123
Table 13 (Contd)
Surface
5. Same as (3) some weeds and stones
6. Same as (4) stony sections
7. Sluggish river reach, rather weedy or with very deep
pools
8. Very weedy reaches
Notes: *Values commonly used for design.
Best
Good
Fair
Bad
0.035
0.045
0.050
0.040
0.050
0.060
0.045
0.055
0.070
0.050
0.060
0.080
0.075
0.100
0.125
0.150
124
Normal
0.003
0.015
0.03
0.06
0.03
0.015
0.03
0.06
0.15
0.015
0.03
0.06
Wrought iron
0.03
0.06
0.15
0.15
0.6
3.0
Uncoated steel
0.015
0.03
0.06
Rusty steel
0.15
0.3
0.03
0.06
0.15
0.06
0.15
0.3
0.15
0.3
0.6
0.15
0.3
0.6
50
60
25
30
0.6
1.5
3.0
1.5
3.0
6.0
6.0
15
30
15
30
60
0.3
0.6
1.5
Prestressed
0.03
0.06
0.15
0.06
0.15
0.6
0.06
0.15
0.3
0.3
0.6
1.5
0.6
1.5
Poor
125
Table 14 (Cont'd)
Suitable values of ks (mm)
Material
Good
Normal
0.03
0.06
0.15
With spigot and socket joints and O ring seals dia < 150mm
0.03
With spigot and socket joints and O ring seals dia > 150mm
0.06
0.003
0.03
0.03
0.06
0.003
0.006
0.03
0.06
0.03
Glazed
0.6
1.5
3.0
Well pointed
1.5
3.0
6.0
15
30
3.0
6.0
Asbestos cement
3.0
6.0
Clayware
1.5
3.0
uPVC
0.6
1.5
1.5
3.0
Asbestos cement
0.6
1.5
Clayware
0.3
0.6
uPVC
0.15
0.3
Poor
Clayware
Glazed or unglazed pipe:
uPVC
Sewers slimed to about half depth; velocity, when flowing half full,
approximately 1.2 m/s :
126
Table 14 (Cont'd)
Suitable values of ks (mm)
Material
Good
Normal
0.3
3.0
30
0.15
1.5
15
0.06
0.6
6.0
0.03
0.3
1.5
0.015
0.15
1.5
Trowel finish
0.5
1.5
3.3
Float finish
1.5
3.3
5.0
3.3
7.0
18
Unfinished
2.0
7.0
18
5.0
14
43
10
33
70
60
150
300
300
600
15
60
150
150
300
600
Poor
Concrete Channels
Earth Channels
Notes :
1.
The classifications Good, Normal and Poor refer to good, normal and poor examples of their respective
categories unless otherwise stated. Classifications Good and Normal are for new and clean pipelines. The
range of roughness takes account not only of the quality of the jointing but also the variation in surface
roughness to be found in pipes that are normally of the same material.
2.
Figures in shaded bold print are the values particularly recommended for general design purposes.
3.
The hydraulic roughness of slimed sewers vary considerably during any year. The Normal value is that
roughness which is exceeded for approximately half of the time. The Poor value is that which is exceeded,
generally on a continuous basis, for one month of the year. The value of ks should be interpolated for velocities
between 0.75 m/s and 1.2 m/s. In Hong Kong, sewers for permanent use should be classified as slimed sewers.
4.
The hydraulic roughness of sewer rising mains varies principally with the amount of slime that builds up inside
the pipe and is normally not significantly affected by factors such as the jointing or the construction. Primarily,
the increasing roughness values are intended to cover for the loss of flow area. The Normal value represents
the mean value of the measured hydraulic roughness while the Good and Poor values represent the values
which are two standard deviations on each side of the Normal value.
127
Table 15 - Head Losses Coefficient K
Source: DSD (1992)
Entry Losses
Sharp-edged entrance
Re-entrant entrance
Slightly rounded entrance
Bellmouthed entrance
Footvalve and strainer
0.50
0.80
0.25
0.05
2.50
Intermediate Losses
(i)
Elbows
(R/D = 1 approx.)
22.50
450
900
0.20
0.40
1.00
(ii)
22.50
450
900
0.15
0.30
0.50**
22.50
450
900
0.10
0.20
0.40
(iv)
22.50
450
900
0.05
0.10
0.20
(v)
(vi)
Sweeps
(R/D = 8 to 50)
Mitre Elbows
2 piece
2 piece
2 or 3 piece
2 piece
3 piece
2 piece
3 piece
4 piece
Tees
Flow in line
Line to branch or branch to line:
Sharp-edged
Radiused
22.50
300
450
600
600
900
900
900
0.15
0.20
0.30
0.65
0.25
1.25
0.50
0.30
0.35
1.20
0.80
(x)
(xi)
Sudden Contractions*
Inlet dia : Outlet dia.
5:4
4:3
3:2
2:1
3:1
5:1 and over
B.S. Tapers*
Flow to small end
Flow to large end
Inlet dia : Outer dia.
4:5
3:4
1:2
Valves
Gate Valve - fully open
1/4 closed
1/2 closed
3/4 closed
Globe valve
Right angle valve
Reflux valve
Butterfly valve
Exit Losses
Sudden Enlargement
Bellmouthed Outlet
0.35
Figure for enlargements, contractions and B.S. Tapers apply to smaller diameter.
Value modified.
Head Loss
Head Loss
=
V 2 / 2g
Velocity Head
0.40
0.60
0.80
0.15
0.20
0.35
0.60
0.80
1.00
0.15
0.20
0.30
0.35
0.45
0.50
negligible
0.02
0.04
0.12
0.12
1.00
6.00
24.00
10.00
5.00
1.00
0.30
K
1.00
1.00**
128
Assumed
Outside
Dia.
Bc
(mm)
Type of
Load
150
190
225
Assumed
Trench
Width
Bd
(m)
Td
(m)
Narrow
Wide
0.60
-
280
Narrow
Wide
300
380
375
1.2
1.5
1.8
2.4
3.0
4.6
3.7
-
5.4
7.1
13.4
9.0
15.1
10.8
18.1
14.4
20.4
18.1
24.0
27.1
0.70
-
2.4
-
7.9
10.5
15.6
13.1
17.8
15.7
21.4
21.1
24.2
26.4
29.2
39.7
Narrow
Narrow
Wide
0.75
0.85
-
1.5
2.4
-
10.6
14.3
17.8
19.8
17.9
20.3
23.2
21.6
24.6
28.4
28.7
28.3
32.6
36.0
34.6
40.4
54.1
500
Narrow
Narrow
Wide
1.00
1.05
-
2.4
3.0
-
12.8
17.5
24.3
26.5
21.9
27.8
30.8
26.4
35.3
38.4
35.2
40.2
44.9
44.0
52.6
57.6
66.4
450
580
Narrow
Wide
1.15
-
2.4
-
14.3
20.6
28.9
25.8
33.5
30.9
41.9
41.4
49.3
51.9
63.6
78.2
600
790
Narrow
Wide
1.35
-
1.8
-
17.8
25.7
35.6
34.4
41.5
41.5
52.5
55.7
62.3
69.7
82.0
105
750
950
Narrow
Narrow
Wide
1.50
1.60
-
1.5
1.8
-
20.6
29.3
40.1
41.2
39.2
47.0
50.2
50.0
59.5
62.8
67.0
71.0
74.3
84.1
94.6
102
127
900
1120
Narrow
Wide
1.90
-
2.1
-
23.5
33.1
51.3
43.8
60.4
55.5
77.5
78.5
93.0
98.6
127
149
1050
1300
Narrow
Wide
2.05
-
2.1
-
26.7
37.4
55.9
48.9
65.8
61.5
84.6
90.6
102
114
140
172
1200
1490
Narrow
Wide
2.30
-
2.1
-
30.0
41.7
62.7
54.4
74.0
68.0
95.3
98.8
115
130
159
197
1350
1650
Narrow
Wide
2.45
-
2.1
-
33.0
45.6
67.2
59.3
79.2
73.6
102
106
124
143
173
219
1500
1830
Narrow
Wide
2.60
-
2.1
-
36.2
49.7
71.8
64.4
84.8
80.0
110
114
133
153
187
242
1650
2010
Narrow
Wide
2.80
-
2.4
-
39.4
54.0
78.5
69.5
93.0
86.0
121
122
147
162
206
264
1800
2240
Narrow
Wide
3.05
-
2.4
-
43.5
59.4
85.4
76.3
101
94.1
131
132
160
175
226
295
129
Assumed
Outside
Dia.
Bc
(mm)
Type
of
Load
150
190
225
1.2
1.5
1.8
2.4
3.0
4.6
Main road
Light road
16.8
13.6
12.8
9.2
10.4
6.4
8.9
4.8
6.7
2.9
5.2
2.0
3.2
1.0
280
Main road
Light road
24.5
19.7
18.6
13.2
15.3
9.5
13.0
7.0
9.9
4.2
7.7
2.9
4.4
2.3
300
380
Main road
Light road
33.3
26.8
25.4
18.1
20.7
12.8
17.6
9.6
13.6
5.8
10.5
3.9
6.1
1.7
375
500
Main road
Light road
42.9
34.4
32.7
23.2
26.7
16.6
23.0
12.4
17.5
7.6
13.7
5.0
7.9
2.3
450
580
Main road
Light road
50.0
40.1
38.2
27.0
31.5
19.4
27.1
14.6
20.7
8.9
16.2
6.0
9.3
2.8
600
790
Main road
Light road
66.6
53.0
51.5
36.2
42.3
26.0
36.5
19.3
27.7
11.8
21.7
7.9
12.5
3.6
750
950
Main road
Light road
80.2
63.5
62.0
43.2
51.0
31.2
43.9
23.3
33.4
14.1
26.1
9.5
15.0
4.4
900
1120
Main road
Light road
93.2
73.3
72.6
50.2
60.0
36.2
51.3
27.1
39.1
16.8
30.5
11.1
18
5
1050
1300
Main road
Light road
106
82.7
84.3
57.6
69.5
41.5
59.8
31.1
45.2
19.2
35
13
20
6
1200
1490
Main road
Light road
120
92
96.6
65.0
80.4
47.2
68.2
35.6
51.8
22.0
40
15
23
7
1350
1650
Main road
Light road
131
99.5
107
71.0
89.4
51.7
76.4
39.4
58
24
45
16
26
8
1500
1830
Main road
Light road
143
107
118
77.0
99.1
56.6
85.0
43.1
64
27
49
18
28
8
1650
2010
Main road
Light road
154
114
129
82.7
109
61.0
93.8
46.8
70
29
54
20
31
9
1800
2240
Main road
Light road
172
122
144
89.4
122
66.6
104
51.0
79
32
61
22
35
10
130
Table 18 - Design Loads for Rigid Buried Pipelines
Nominal
Pipe
Assumed
Outside
Assumed
Trench
Dia.
DN
(mm)
Dia.
Bc
(mm)
Width
Bd
(m)
0.9
1.2
1.5
1.8
2.4
3.0
4.6
150
190
0.60
22
19.5
19.5
19.5
21
23
27
225
280
0.70
32
29
28
28
31
32
34
300
380
0.75
44
40
39
38
38
39
41
375
500
1.05
55
50
48
50
53
58
66
450
580
1.15
64
58
57
58
63
68
73
600
790
1.35
86
79
79
80
83
86
96
750
950
1.50
105
95
93
95
96
100
115
900
1120
1.90
120
110
110
110
120
130
150
1050
1300
2.05
140
130
125
130
135
145
170
1200
1490
2.30
160
145
145
145
155
165
190
Nominal
Pipe
Assumed
Outside
Assumed
Trench
Dia.
DN
(mm)
Dia.
Bc
(mm)
Width
Bd
(m)
0.9
1.2
1.5
1.8
2.4
3.0
4.6
150
190
0.60
19
16
15.5
15.5
17.5
20
25
225
280
0.70
28
24
23
23
25
27
31
300
380
0.75
38
32
31
30
31
32
36
375
500
1.05
47
41
38
39
42
50
60
450
580
1.15
54
48
45
45
51
55
67
600
790
1.35
73
64
63
63
67
73
87
750
950
1.50
87
76
74
74
77
85
100
900
1120
1.90
100
88
85
87
99
110
135
1050
1300
2.05
115
100
98
99
110
120
155
1200
1490
2.30
130
115
110
110
125
140
175
131
Table 19 - Minimum Strength or Class of Pipes in Main Roads
(a) Class of Precast Concrete Pipes
Nominal
Pipe Dia.
DN
(mm)
Assumed
Outside
Dia.
Bc (mm)
Assumed
Trench
Width
Bd (mm)
Bedding
Factor
Fm
150
190
600
1.9
2.6
3.4
1.9
2.6
3.4
1.9
2.6
3.4
1.9
2.6
3.4
1.9
2.6
3.4
1.9
2.6
3.4
1.9
2.6
3.4
1.9
2.6
3.4
1.9
2.6
3.4
1.9
2.6
3.4
225
300
375
450
600
750
900
1050
1200
280
380
500
580
790
950
1120
1300
1490
670
750
1050
1150
1350
1500
1900
2050
2300
0.9
4.6
132
Table 19 (Contd)
(b) Class of Vitrified Clay Pipes
Nominal
Pipe Dia.
DN
(mm)
Assumed
Outside
Dia.
Bc (mm)
Assumed
Trench
Width
Bd (mm)
Bedding
Factor
Fm
100
130
600
150
190
200 &
245 &
225
280
300
370
375 &
460 &
400
500
450
550
500
600
615
730
600
700
800
1100
1200
1300
1400
1.2
1.5
1.8
2.4
3.0
4.6
1.9
2.5
1.9
2.5
1.9
2.5
1.9
2.5
1.9
2.5
1.9
2.5
1.9
2.5
1.9
2.5
F
F
F
F
F
F
F
Note :
1. L, M, H denote class of concrete pipes in Table 7 of BS 5911:Part100:1988.
2. F and B denote Standard and Extra Strength respectively of vitrified clay pipes in Table 3 of BS65:1988.
133
Below is an example for reference only. Project engineer should liaise with the relevant
authorities to identify the maintenance and management parties for their own works in the
design stage.
Completed Works
Maintenance
Department
Management
Department
DSD(1)
DSD(2)
HyD
TD & HKPF
HyD
TD & HKPF
Arch.SD
HKPF
Run-ins
HAD
HAD
Road cleaning
LCSD
--
DSD
DSD
Borrow area
CEDD
CEDD
LCSD
LCSD
AFCD
AFCD
Mangrove plantation
CEDD
CEDD(3)
Grasscrete softworks
LCSD
LCSD
Grasscrete hardworks
DSD
DSD
134
Table 20(Contd)
+3.4mPD embankment platforms and surplus
strips of land
Lands D
Lands D
Lands D
Lands D
DSD
DSD
Arch.SD
--
Fire hydrants
WSD
WSD
DSD
DSD
Concrete parapets
Water points and water metre boxes
Notes: (1) DSD is responsible for planning and carrying out desilting/ dredging work,
and structural maintenance of the channel banks. FEHD is responsible for the
removal of refuses, animal carcasses, dead fishes and vegetation.
(2) Future management of waters within the main drainage channel will be further
discussed among Marine Department, Marine Police, LCSD and DSD.
DSDs management role is currently limited to flood control aspects.
(3) CEDD will temporarily take care of mangrove plantation. Future operation
and maintenance of this wetland will be addressed as part of Wetland
Compensation Study by AFCD.
135
Application
Limitation
Replacement of
foundation soil
Huge quantity of
unsuitable material
has to be disposed
of and a large
quantity of
imported fill is
required
Modification of
embankment
geometry
Staged
construction
Additional land
resumption/
clearance is
required
Longer
construction period
is required
Provision of
geosynthetic
reinforcement
Not effective in
improving the
embankment
against bearing
capacity failure
Installation of
vertical drain
Installation of
Geocell Mattress
136
Table 22 - Components of a Polder and Floodwater Pumping Scheme
Facility
Protective Bund
Components
Functions
Concrete grass
lining
Hydroseeding
Roads and
footpaths
Inlets to pump
sump
Gravity outlets
from village
floodwater storage
pond
Flexible flap
valves
Penstocks
Concrete pipes
linked with pond
137
Table 22 (Cont'd)
Facility
Floodwater
storage pond
Surface channels
in village
Components
Functions
Pond area
Peripheral
channels
Maintenance
access and
working platform
Concrete grass
lining
Hydroseeding
Peripheral fence
Raised peripheral
amenity planter
Warning sign
boards
Channels,
concrete pipes and
catchpits
138
Table 22 (Cont'd)
Facility
Floodwater
pumping station
Components
Functions
Reinforced
concrete pumping
station structure,
pumps and electric
motors
Penstocks
Ventilation plant
Transformer room
of power company
Emergency power
generator
Underground fuel
oil storage tank
Fire fighting
equipment
Flow
measurement
device
Gravity outlet
Telemetry
control/alarm
system
Security fence
Amenity areas
within station
compound
Landscaping
Video
Surveillance
system
139
Table 23 Some of the Existing Polder and Floodwater Pumping Schemes
in the NWNT and NENT
Village Catchment
Area
Pond Area
55,700 m2
1,500 m2
2 duty + 1 standby
( @ 1,800 l/s )
15,500 m2
4,750 m2
1 duty + 1 standby
( @ 100 l/s )
3. Lo Uk Tsuen
21,500 m2
2,150 m2
1 duty + 1 standby
( @ 280 l/s )
510,000 m2
17,800 m2
3 duty + 2 standby
( @ 2,500 l/s )
5. Sha Po Tsuen
59,000 m2
2,420 m2
1 duty + 1 standby
( @ 1,100 l/s )
260,200 m2
37,000 m2
2 duty + 1 standby
( @ 2,500 l/s )
42,000 m2
2,140 m2
2 duty + 1 standby
( @ 425 l/s )
55,000 m2
2,800 m2
2 duty + 1 standby
( @ 550 l/s )
Location
Pump Capacity
140
Below is an example for reference only. Project engineers should liaise with the relevant
authorities to identify the maintenance and management parties for their own works in the
design stage.
Completed Works
Maintenance
Department
Management
Department
O&M Division
/DSD
O&M Division
/DSD
O&M Division
/DSD
O&M Division
/DSD
HyD
TD & HKPF
Road cleaning
FEHD
--
LCSD
LCSD
AFCD
AFCD
O&M Division
/DSD
O&M Division
/DSD
ST Division
/DSD
ST Division
/DSD
ST Division
/DSD
141
Component
Frequency
Complaint black-spots
1 month to 6 months,
depending on location
Annually
Sewer and drains within a distance of 4H from At least once every 5 years
crest of slope or retaining wall (H = vertical
height)
Other stormwater drains
1-5 years
142
Table 26 - Schedule of Inspection for Polder and Floodwater Pumping Schemes
Facilities/Objectives
a) gravity inlets to floodwater
storage pond
Duties
Check and take necessary actions for siltation, trapped debris at
trash screen and pipe blockage
Frequency
Once every
two weeks
Monthly
Monthly
Check and take necessary actions for siltation at pond area and
peripheral channels
Monthly
Pumps
Once every
two weeks
Once every
two weeks
Once every
two weeks
Once every
two weeks
Once every
two weeks
Once every
two weeks
Once every
two weeks
Monthly
Once every
two weeks
Once every
two weeks
Half-yearly
Yearly
Yearly
Yearly
Once every
two weeks
As required
Telemetry system
143
LIST OF FIGURES
Figure
No.
1
Intensity-Duration-Frequency Curves
10
Stepped Channel
11
12
13
14
15
16
17
18
19
Figure 1.
144
Figure 2.
145
145
Figure 3.
146
147
F(t)
a [ b 2 (1 c ) t ]
( 2 t b ) c 1
548
603
687
766
855
2 yr
10
50
200
1 000
RETURN
PERIOD
4.0
4.1
4.2
4.4
5.2
0.39
0.40
0.42
0.44
0.51
148
Figure 6. A Schematic Diagram showing a Design Storm Profile corresponding to the Step Function of the Same Return Period
149
Figure 7.
150
Figure 8.
151
Locations of WSD Stream Flow Measurement Stations and DSD River Stage Gauges
152
Figure 9.
153
Figure 10.
Stepped Channel
154
Type I Basin (for flow with 2.5 < Fr1 < 4.5)
Table A
Table C
Notes :
1. See general notes, design procedures & guidelines in sheet 4.
Figure 11.
155
Table A
Table B
Table C
LENGTH OF STILLING BASIN
Notes :
1. See general notes, design procedures & guidelines in sheet 4
2. Type II basin is suitable for flow with upstream velocity v1 not exceeding 15 m/s.
Figure 11.
156
Table A
d2
d1
Table C
Notes :
1.
2.
Type III basin is suitable for flow with upstream velocity v1 of all values.
Figure 11.
157
Figure 11.
Figure 12.
158
159
160
161
162
163
164
165