Scour
Scour
Scour
Scour Under
Structures
TERM PAPER
OE5800: COASTAL ENGINEERING
Prof.V.Sundar
Harikrishnan V
NA10B009
Srinija A
NA10B049
Teja Sree D
NA09B037
Table of Contents
1. Introduction
2. Local scour and global scour
3. Physical processes of scour
4. Geotechnical considerations
4.1 Soil properties
4.2 Soil types affecting scour
4.3 Soil types affecting scour depth
5. Influence of liquefaction on scour
5.1 Liquefaction
5.2 Theory on Liquefaction
5.3 Residual Liquefaction
6. Common scour problems
6.1 Scour at coastal inlet structures
6.2 Scour at structures in deep water
6.3 Scour at structures in shallow water
6.4 Other occurrences of scour
7. Prediction of scour
7.1 Scour at small diameter vertical piles
7.1.1 Pile scour by currents
7.1.2 Pile scour by waves
7.1.3 Pile scour by waves and currents
7.2 Scour at large diameter vertical piles
8. Design of scour protection
8.1 Vertical walls
8.2 Sloping structures
8.3 Piles
8.4 Submerged pipelines
9. Case studies
9.1 Case study 1
9.2 Case study 2
10. References
Figures
Fig 1: Scour induced armour displacement
Fig 2: Seaward tilting & settlement due to scour
Fig 3: Seaward overturning of gravity wall
Fig 4: Failure due to scour
Fig 5: Representation of Local and Global scour development around a jacket
Fig 6: Types of scour that can occur at a bridge
Fig 7: Horseshoe & wake vortices around a cylindrical element
Fig 8: Schematic explanation of terminology
Fig 9: Graph representing variation of scour depth w.r.t time
Fig 10: Flow pattern around monopile
Fig 11: Pattern of shear stress amplification M from cylindrical pile
Fig 12: Coastal scour Problems
Fig 13: Correction factor K1, for Pile/Pier shape
Fig 14: Wave-induced equilibrium scour depth at a vertical pile
Fig 15: Wave and current scour around large vertical piles (Rance 1980)
Fig 16: Stone blanket scour protection for submerged pipelines
Fig 17: Offshore wind farms
Fig 18: Marine mattresses installed on the banks of the Haulover Canal
Fig 19: Triton Marine Mattresses are installed at one of four bridge locations
1. Introduction:
Scour is the removal by hydrodynamic forces, of granular bed material in the
vicinity of a coastal structure. This definition distinguishes scour from the more
general erosion; and as might be expected, the presence of a coastal structure most
definitely contributes to the cause of scour.
When a structure is placed in a marine environment, the presence of the structure
will change the flow pattern in its immediate neighbourhood, resulting in one or
more of the following phenomena:
the contraction of flow;
the formation of a horseshoe vortex in front of the structure;
the formation of lee-wake vortices (with or without vortex shedding) behind
the structure;
the generation of turbulence;
the occurrence of reflection and diffraction of waves;
the occurrence of wave breaking; and
the pressure differentials in the soil that may produce quick
condition/liquefaction allowing material to be carried off by currents.
These changes usually cause an increase in the local sediment transport capacity
and thus lead to scour. The scour is a threat to the stability of coastal structures
such as piles, breakwaters, seawalls, etc.
Scour-induced damage happens at sloping-front structures when scour undermines
the toe so it can no longer support the armor layer, which then slides downslope
Monolithic gravity seawalls can also settle and tilt as a result of scour
Scour at vertical sheet-pile walls can result in seaward rotation of the sheet-pile toe
due to pressure of the retained soil as shown in figure 4:
Likewise, scour at a bridge occurs as local scour and global scour. Local scour
occurs around the individual piers and at the abutments, while the global scour
occurs as the general lowering of the riverbed, as sketched below.
Fig 6: Types of scour that can occur at a bridge. Adapted from Melville
Coleman (2000)
The global scour in this example may, in addition to the contraction scour, occur
due to hydro meteorological changes (e.g., prolonged high flows),
geomorphological changes (e.g., lowering of channel base level due to catchment
wide adjustment in geomorphology), human activities (e.g., dam construction),
bank erosion (caused by channel widening, meander migration, a change in the
river controls, or a sudden change in the river course ,e.g., with the formation of a
meander-loop cut-off) (Melville and Coleman,2000).
In which =the bed shear stress and =the bed shear stress for the undisturbed
flow.
A pictorial representation of processes by which increase in shear stress and
turbulence is given below: Fig 7:
a) Pile structure
structure becomes =0 .The stage where the scour process comes to an end is
called the equilibrium stage.
From the preceding considerations, the scour develops towards the equilibrium
stage through a transitional period. An illustration of the same is given below:
The scour depth corresponding to the equilibrium stage, S is called the equilibrium
scour depth. For a substantial amount of scour to develop, a certain amount of time
must elapse. This time is called the time scale of the scour process. The time scale
of the scour process may be defined in several ways. A representation given by B
Mutlu Sumer et al is
= (1 ( ))
In which T = the time scale of the scour process, and corresponds to the time
period T indicated in the above figure where the dashed line is tangent to the scourdepth-versus-time curve at t=0.
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The aforementioned quantities, namely the equilibrium scour depth and the time
scale, are two major parameters in scour studies. The scour depth is important
because, given the structure and the flow climate, it indicates the degree of scour
potential. The assessment of scour depth is essential in the design of both (1) the
foundation of the structure and (2) the scour protection work.
The time scale is also equally important .A scour hole produced after a storm may
be backfilled. Normally, the question asked in practice is whether any substantial
amount of scour would occur over the backfilled area during the next storm.
Obviously, for a substantial amount of scour to occur, the storm should prevail
over a space of time larger than the time scale of the scour process. Clearly, to
answer the aforementioned question, the time scale of scour must be known.
Clear water scour occurs when bottom shear stresses are high only in a localized
portion of the bed; outside the local region sediment is not moving. This occurs
mostly in uniform, steady flow situations. In live bed scour bottom shear stresses
over the entire bed exceed the level for incipient motion with locally higher shear
stresses where greater scour occurs. An equilibrium is reached when the volume of
sediment being removed from the scour hole is exactly equal to sediment being
deposited in its place.
Understanding the physical processes involved in scour is difficult because the
shear stresses responsible for scour are developed by waves, currents, or combined
waves and currents, which usually are heavily influenced by the presence of a
coastal structure. Because of the distinct influence coastal structures exert on the
hydrodynamics, structural aspects such as geometry, location, and physical
characteristics (roughness, permeability, etc.) impact the scour process. Therefore,
modifying some physical characteristic of a structure may reduce scour potential.
Typical structure and hydrodynamic conditions leading to scour include the
following (acting singularly or in combination):
Localized increases in peak orbital wave velocities due to combined
incident and reflected waves
Particular structure orientations or configurations that focus wave energy
and increase wave velocity or initiate wave breaking
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Structure orientations that direct currents along the structure or cause a flow
acceleration near the structure
Flow constrictions that accelerate the fluid
Breaking wave forces that are directed downward toward the bed or that
generate high levels of turbulence capable of mobilizing sediment
Wave pressure differentials and groundwater flow that produce a quick
condition, allowing material to be carried off by currents
Flow separation and creation of secondary flows such as vortices
Transitions from hard bottom to erodible bed
Even if the hydrodynamic aspects of scour were fully understood, there remains
the difficulty of coupling the hydrodynamics with sediment transport.
Consequently, most scour prediction techniques consist of rules of thumb and
fairly simplistic empirical guidance developed from laboratory and field
observations.
Depending on the circumstances, scour can occur rapidly over short time spans
(e.g., energetic storm events), or as a gradual loss of bed material over a lengthy
time span (months to years). In the short-term case sediment is probably
transported primarily as suspended load, whereas bed-load transport is more likely
during episodes of long-term scour. Scour may be cyclic with infilling of the scour
hole occurring on a regular basis as the flow hydrodynamics undergo seasonal
change.
Most scour holes and trenches would eventually reach a stable configuration if the
same hydrodynamic conditions persisted unchanged over a sufficient time span.
Such an equilibrium is more likely to occur for scour induced primarily by current
regimes than by wave action. It is difficult to determine if observed scour
development at a particular coastal project represents an equilibrium condition.
The scour might be the result of energetic flow conditions that subsided before the
full scour potential was realized. Or it is possible the scour was initially greater,
and infilling of the scour hole occurred prior to measurement. Finally, there is the
possibility that the observed scour is simply the partial development of an ongoing
long term scour process.
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4. Geotechnical considerations
Type of soil supporting a coastal engineering foundation governs erosion potential
and the long term behavior of a foundation.
Cohesive or cemented soils are more scour resistant.
While loose granular soils are rapidly eroded under water motion.
Sandy soils will be eroded earlier than bed rock.
This the reason for using natural rocks for scour protection. To develop or promote
some infrastructure on sandy beaches, it will be better to watch out for scour.
Again there is a difference between scour and erosion. Scour is a kind of a local
phenomenon, more intense with respect to local near obsession. But, erosion can
be felt over the long distances.
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Percentage (%)
48
19
13
10
05
05
00
100
cemented
In years
In centuries
Some of the used scour depths are local scour depth and ultimate scour depth
which is almost beyond steady state.
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This result in a shear stress pattern as for example given by Hjort (1975), see figure
Here the shear stress distribution is shown around a pile as being measured during
experiments.
5.1 Liquefaction
Excess pore water pressures can affect the foundation in a number of ways
Generation of net uplift pressures on the foundation
Changes to the skin friction on the foundation wall
Potential for seabed liquefaction.
Horizontal stiffness of foundation to deflections
This study investigates on the influence of seabed liquefaction on scour. Under the
influence of waves, earthquakes or structural vibrations the pore water pressure
may increase such that the seabed becomes liquefied. Since a liquefied bed barely
has any shear strength, the sediment can be eroded more easily than a non17
liquefied bed Liquefaction means that the material behaves as if it were a liquid.
For cohesion less soils this is defined by Marcuson as being the transformation of
a solid state to a liquefied state as a consequence of increased pore pressure and
reduced effective stress. Consequently the capacity of the soil to support vertical
load is lost and the soil is much more susceptible to erosion.
5.2 Theory on liquefaction
For a pressure variation for example caused by waves or earthquakes, the pressure
between grains alternates continuously. Under a wave cycle the soil may not be
able to fully dissipate its excess pressure built-up under the crest. Therefore the
pore water pressure is able to build up gradually. After a number of waves it may
happen that the accumulated pressure is high enough to exceed the value of the
overburden pressure. Then soil grains become unbounded and start acting as a
liquid.
After the liquefaction takes place, the pore water pressure gradually decreases,
because the pore water slowly flows out of the soil. Finally, the accumulated
excess pore water pressure converges to zero again. This is accompanied with
compaction of the soil.
prototype dimensions. Large safety margins are used to cope with this uncertainty.
The combination of scour and liquefaction has so far rarely been investigated. The
same waves that are causing scour can also cause liquefaction. Apart from waves,
scour can be caused by structural motions and earthquakes.
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The pile does not significantly affect the incident wave. Large diameter piles, in
which the diameter is greater than one-tenth of the incident wavelength, do have an
impact on the incident waves which are reflected by the pile and diffracted around
the pile.
The key parameters governing scour formation appear to be current magnitude,
orbital wave velocity, and pile diameter. Less important parameters are sediment
size and pile shape (if the pile has noncircular cross section).
For detailed descriptions of the physical mechanisms responsible for scour at
vertical piles see Niedoroda and Dalton (1986) or some of the following
references.
A general, and somewhat conservative, rule-of-thumb is: Maximum depth of
scour at a vertical pile is equal to twice the pile diameter.
This rule-of-thumb appears to be valid for most cases of combined waves and
currents. Smaller maximum scour depths are predicted by the equations in the
following sections. Estimation formulas for maximum scour depth have been
proposed for the cases of currents only, waves only, and combined waves and
currents. The flow problem and associated sediment transport are beyond a
complete theoretical formulation, and even numerical modeling attempts have not
been able to describe fully the scour process at vertical piles (see Sumer and
Fredse 1998a for a summary of numerical modeling approaches).
0.65 0.43
= 2.0 1 2 ( )
Where
Sm
= maximum scour depth below the average bottom elevation
h
= water depth upstream of the pile
b
= pile width
Fr
= flow Froude number [Fr = U/(g h)0.5]
U
= mean current velocity magnitude
K1
= pile shape factor
K2
= pile orientation factor
This equation is a deterministic formula applicable for both clear water scour and
live bed scour, and it represents a conservative envelope to the data used to
establish the empirical coefficients.
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0.62
L
K 2 = (cos + sin)
b
Where L /b is as defined in the figure and is the angle of pile orientation. K2
equals unity for cylindrical piles. Other modifying factors have been proposed to
account for sediment gradation and bed forms, but these factors have not been well
established. An additional factor is available for use when piles are clustered
closely together.
Johnson (1995) tested seven of the more commonly used scour prediction
equations against field data and found that the CSU equation (Equation VI-5-265)
produced the best results for h/b > 1.5. At lower values of h/b a different empirical
formulation offered by Breusers, Nicollet, and Shen (1977) provided better results.
Johnson (1992) developed a modified version of the CSU empirical equation for
use in reliability analysis of failure risk due to scour at cylindrical piles. Her
formula represents a best-fit to the data rather than a conservative envelope. An
application example is included in her 1992 paper.
7.1.2 Pile scour by waves
The physical processes associated with wave-only scour around vertical piles are
reasonably well described qualitatively (See Sumer and Fredse (1998a) for a
comprehensive review and listing of many references.)
In an earlier paper Sumer, Christiansen, and Fredsoe (1992a) established an
empirical equation to estimate scour at a vertical pile under live bed conditions.
They used small- and large-scale wave flume experiments with regular waves, two
different sediment grain sizes, and six different circular pile diameters ranging
from 10 cm to 200 cm. Maximum scour depth (Sm) was found to depend only on
pile diameter and Keulegan-Carpenter number (KC).The experimental data of
Sumer, Christiansen, and Fredsoe (1992a) are shown plotted in figure given below,
and the solid line is the predictive equation given by
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= 1.3[1 0.03(6) ]
Where
D
= cylindrical pile diameter.
KC = Keulegan-Carpenter number, which is defined as
=
Where
Um =maximum wave orbital velocity at the bed (in the absence of a structure)
T
=regular wave period
D
=diameter of the pile
No live-bed scour occurs below values of KC=6, which corresponds to onset of
horseshoe vortex development.
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= 2.0[1 0.015(11) ] 11
= 2.0[1 0.019(11) ] 3
Scour for the square pile oriented at 45 deg begins at lower values of KC, but the
maximum scour at large KC values approaches Sm /D = 2 regardless of orientation.
7.1.3 Pile scour by waves and currents
Kawata and Tsuchiya (1988) noted that local scour depths around a vertical pile
were relatively minor compared to scour that occurs when even a small steady
current is added to the waves. Eadie and Herbich (1986) conducted small-scale
laboratory tests of scour on a cylindrical pile using co-directional currents and
irregular waves. They reported the rate of scour was increased by adding wave
action to the current, and the maximum scour depth was approximately 10 percent
greater than what occurred with only steady currents. This latter conclusion
contradicts Bijker and de Bruyn (1988) who found that nonbreaking waves added
to steady currents slightly decreased ultimate scour depth whereas adding breaking
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waves caused increased scour to occur. Eadie and Herbich also noted that the
inverted cone shape of the scour hole was similar with or without wave action, and
the use of irregular versus regular waves appeared to influence only scour-hole
geometry and not maximum scour depth. They developed a predictive equation
using results from approximately 50 laboratory experiments, but no wave
parameters were included in the formulation. Finally, they pointed out that their
conclusions may hinge on the fact that the steady current magnitude exceeded the
maximum bottom wave orbital velocity, and different results may occur with weak
steady currents and energetic waves.
Earlier work by Wang and Herbich (1983) did provide predictive equations that
included wave parameters along with current, pile diameter, sediment properties,
and water depth. However, there were some unanswered questions about scaling
the results to prototype scale. Consequently, until further research is published,
maximum scour depth due to waves and currents should be estimated using the
formulations for scour due to currents alone
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Fig 15: Wave and current scour around large vertical piles (Rance 1980)
Maximum scour occurs at the corners of the square piles. Estimates of extent of
scour are useful for design of scour blankets. Sumer and Fredse (1998a) provided
additional information about flow around large piles.
.
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8.3 Piles
Scour aprons constructed of stone or riprap, gabions, concrete mattresses, or
grout-filled bags. Options other than riprap or stone should be tested in
physical models.
.
8.4 Submerged pipelines
Burying the pipeline in a trench
Covering the pipeline with a stone blanket
Protective mattress.
Outside the active surf zone, burial depth is a function of local wave and current
climate, sediment properties, and liquefaction potential. Usually the excavated
material can be used as backfill provided it is sufficiently coarse to avoid buildup
of excessive pore pressures which could lead to liquefaction and vertical
displacement of the pipeline (Sumer and Fredse 1998a).
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Pipelines traversing the surf zone should be buried at an elevation lower than the
anticipated erosion that would occur over the projected service life of the pipeline.
Generally, stone blankets or mattresses are not considered effective protection in
the surf zone because the elements must be designed to withstand the intense
action of breaking waves.
Various types of scour mattresses have also been used effectively to protect
pipelines. Mattresses may be economical when stone is not readily available;
however, special mattress placing equipment is usually required.
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9. Case studies
9.1 Case study 1
The Use of Geotextile Containers (GTCs) for Scour Mitigation and Protection at
Offshore Monopile Structures
Project Title: Scour Protective Measures for Offshore Wind farms
The Challenge
This project was an R&D project aimed not towards delivery of an active
commercial solution to an existing problem but to explore an offshore option. A
variety of measures and options are available to provide scour protection to
offshore wind farm turbine foundations. These include: rock dumping, frond and
concrete mattressing and anti-scour engineering design. Sand-filled geotextile bags
offer a potentially more cost-effective alternative to these conventional approaches.
Partrac Consulting were commissioned to explore the range of current anti-scour
options (to include a cost-comparison) and to research the issues surrounding the
use of sand-filled geotextile bags within the context of scour protection. In
addition, preliminary conceptual designs for bags were requested.
The Solution
Partrac Consulting summarised the background to marine scour in order for the
client to fully understand the governing physical processes and the current state of
knowledge. The historical use of geotextile bags in common maritime engineering
contexts (e.g. coastline protection, riverbank stabilisation) was also summarised,
and the principal types and characteristics of a specific geotextile bag product was
presented.
Several options for differing configurations of bags together with associated
deployment methods were discussed and compared. Finally, the consenting process
for use of geotextile bags in the sea in UK waters was presented. This data and
information are currently being used by the client to potentially introduce
geotextile bags as an alternative to current scour protection in offshore wind farms.
M
clearance. Narrow channels, strong currents and deep scour pockets demanded
dredge and fill maneuvers or anchoring systems for slopes greater than 2 to 1.
There is extremely low visibility at all bridges and high velocities These
bridges provide the only access into KSC and were required to remain open to
vehicle traffic, said Rivera, who added that there were strict requirements
regarding vessel closure times and channels depths.
Fig 18: Marine mattresses installed on the banks of the Haulover Canal at
Kennedy Space Center
Alternative solution
Bank & shore riprap presented an alternative solution, but its 3.6-foot thickness
would have required excessive channel dredging to ensure proper, USACEapproved depth. Articulated concrete block mats had a much higher cost compared
to Triton Mattresses and could not be custom fit around KSCs fender system.
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The Solution
Triton Marine Mattresses were selected for their constructability, adaptability and
durability in a challenging, submarine environment. Also, the coastal and
waterway revetment system was much more cost-effective than the alternatives.
We found that in high-flow conditions, the marine mattresses tend to be very
stable, said Jeff Fiske, Industrial Manager Coastal and Waterway at Tensar
Corp. Regarding constructability, trying to place material under water and get a
specified thickness of material in adverse conditions is always difficult. Having a
unit like Triton Marine Mattresses that goes in as a discreet size that can be
positioned using GPS, actually gives the contractor and engineer assurances that
what was specified is actually what was installed.
Fig 19: Triton Marine Mattresses are installed at one of four bridge locations
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10. References
Coastal Engineering Manual, CERC, US Army Corps of Engineers,
Vicksberg, USA.
Prof. V. Sundar, (2012), Coastal Engineering, Online Video Lecture,
Webpage: http://nptel.iitm.ac.in , http://www.youtube.com/user/nptelhrd
Hans F. Burcharth and Steven A. Hughes, (2003), chapter 2, 5 & 8, EM
1110-2-1100 (Part VI)
The Mechanics of Scour in the Marine Environment: By B Mutlu
Sumer (Technical University of Denmark) & Jrgen Fredse (Technical
University of Denmark)
SCOUR OF SAND BEACHES IN FRONT OF SEAWALLS by John B.
Herbich, Ph.D., P.E.Professor of Civil Engineering Texas A&M University
College Station, Texas And Stephen C. Ko, M. S. Research Instructor
Lehigh University Bethlehem, Pennsylvania.
LOCAL SCOUR NEAR STRUCTURES by Leo C. van Rijn,
www.leovanrijn-sediment.com, march 2013
Whitehouse, R.S.J.W., 1998. Scour at marine structures. Pub. Thomas
Telford Ltd
Sumer, B.M., Whitehouse, R.J.S. and Trum, A., 2001. Scour around coastal
structures: a summary of recent research, p. 153-190. Coastal Engineering,
Vol 44
Melville, B.W., 1988. Scour at bridge sites, p. 327-362. Technomic
Publishing Company, USA, Civil Engineering Practice, 2
Case studies: TRITON Kennedy Space Center, Cape Canaveral, Florida &
Coastal and Marine Geosciences, Partrac consulting.
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