April 2014

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

Fig 1: Scour induced armour displacement

Scour impacts vertical-front caissons and other gravity-type structures if the

structure is undermined to the point of tilting as illustrated below in figure 2:

Fig 2: Seaward tilting & settlement due to scou

4

Monolithic gravity seawalls can also settle and tilt as a result of scour

Fig 3: Seaward overturning of gravity wall

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:

Fig 4: Failure due to scour

Coastal structure damage or failure brought about by scour impacts coastal
structures in several ways including: structure functionality is decreased; costs will
be incurred to repair or replace the structure, and scour related damage is often
difficult and expensive to repair; upland property being protected by the structure
may be lost or inundated; clients and cost-sharing partners will lose confidence in
the project's capability to perform as required.

2. Local Scour and Global Scour

Consider a piled steel plat form comprising horizontal and vertical members. When
this structure is exposed to flow action, two kinds of scour will take place; the local
scour around the individual structural elements such as that around the supporting
piles, and the global scour beneath and around the structure in the form of a saucershaped depression, as illustrated in the conceptual picture.
The global scour here is due to the combined action of all the flow effects
generated by the individual structural elements, namely the contraction of flow and
the "turbulence" generated by the structural elements.
Fig 5:

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).

3. Physical processes of scour

Consider a structure placed in a marine environment. The presence of the structure
will cause the flow in its neighborhood to change. This local change in the flow
will generally cause an increase in the bed shear stress and in the turbulence level.
The sediment transport close to the structure is increased mainly because:
1. the average bed shear stress is increased close to the structure ,and
2. the degree of turbulence is increased in the vicinity of the structure.
Both features will lead to an increase in the local sediment transport capacity.
Today, however, much more knowledge is available about item (1) than about item
(2).
Usually the increase in the bed shear stress is expressed in terms of the so-called
amplification factor defined by

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

b) Sea wall / Breakwater type of structure

Fig 8: Schematic explanation of terminology

Owing to the local increase in (i.e. >1), the sediment transport capacity will
increase and presumably the bed will be eroded, the scour process. This process
will continue until the scour reaches such levels that the bed shear stress around the
10

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:

Fig 9: Graph representing variation of scour depth w.r.t time

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.

11

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
12

 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.

13

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.

4.1 Soil properties

Properties of sediments are important in phenomenon of scour. Sediment transport
characteristics are determined by namely
Particle size
Particle shape
Sediment concentration
Fall velocity
Specific weight of particles
Grain size distribution
Threshold of movement where hydrodynamic stresses exceed the seabed stresses
depends mainly on the particle size, density, shape, packing and orientation of
seabed material. They control the threshold movement i.e. initiation of sediment
motion.

14

4.2 Soil types affecting scour

The percentage of soil types with scour problems are listed in the table below. It is
seen that sand foundations have 48% of scour problems while slit foundations do
not display any scour problems.
Soil types with scour problems
Sediment Type
Sand
Cohesive
Mixed
Gravel
Bed rock
Uncertain
silt
total

Percentage (%)
48
19
13
10
05
05
00
100

4.3 Soil types affecting scour depth

The intensity and duration of current will affect the rate of scour in the seabed. The
following table represent the duration of maximum scour depths in different soil
conditions.
Durations for maximum scour depths in different soils
Type
Duration
Sand and gravel bed materials
In hours
Cohesive bed materials
In days
Glacial tilts, poorly cemented
In months
sandstones and shale
Hard, dense, well
sandstones and shale
Granites

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.
15

5. Influence of liquefaction on scour

The effect of liquefaction is done from vibrations of offshore monopile
foundations on scour by performing scaled flume experiments where
liquefaction is induced by a monotonic excess pore water pressure. Many
scour problems are investigated by the use of physical modelling. So far only
a few studies have been performed where both liquefaction and scour were
observed in one experiment. Only little is known on the combinations of both
phenomena .This study focuses on the influence of structural vibrations
trough liquefaction on scour. In order to exclude the effects that do not relate
to liquefaction, a pile modelling a monopile foundation is fixed and a
monotonic excess pore water pressure applied. If this pressure is large enough
and lasts for a sufficiently long time, the sediment grains lose their mutual
contact and the soil becomes liquefied.

Fig 10: Flow pattern around monopile

16

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.

Fig 11: Pattern of shear stress amplification M from cylindrical pile

Here with flow velocity U = 30 cm/s and water depth H = 10 cm

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

Soil liquefaction is a process which is closely related to scour. A liquefied bed

behaves as if it was a liquid with different properties than water and has barely any
shear strength. Consequently, bed forms strongly depend on the flow pattern and
the capacity of the soil to support vertical or horizontal load is lost (Whitehouse,
1998)
Two types of liquefaction can be distinguished, namely:
1. Momentary liquefaction, which is an upward force from excess pore water
under a single wave trough.
2. Residual liquefaction, where pore pressure is built up by cyclic loading.
Further distinctions can be made on the type of conditions
Normal or shear loading
Easy drainage to no drainage
Soil properties like the relative soil density or gas content.
.

Momentary liquefaction is caused by the passage of a large single wave and

involves cohesion less material with a larger permeability than residual
liquefaction. Under the wave crest the pore pressure in the soil builds up, while
under the wave trough the water flows out of the soil. Under the outflow there is a
negative pore pressure gradient in the soil. When the soil is fully saturated the
pressure gradient in the soil will not be enough to move particles. However, when
the water contains some trapped air bubbles, even when the air content is less than
1%, the pore water combined with the air act as being compressible. Consequently,
the pore water dissipates much faster with the depth and has a much higher
gradient at the top layer of the soil.
18

5.3 Residual 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.

Among others this can be caused by the following phenomena:

Waves above relatively uncompacted soils

Earthquakes
Shocks (for instance from slope failure)
Rocking of a structure.

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.

When a slender monopile is placed in a marine or fluvial environment, both

horseshoe and lee-wake vortices arise originating from waves and currents. These
vortices are in addition to flow contraction which always takes place due to the
separation of flow. As a consequence the shear stress increases locally. When the
critical shear stress is exceeded, additional erosion takes place and a scour hole
forms. The description of scour falls apart in the quantification of the equilibrium
depth, time scale and shape of the scour hole. Generally the quantification of these
is based on scaled laboratory experiments. In the field a number of other processes
play a role and the combination of processes may give different behavior. This
includes the difficulties with scaling from small scale laboratory experiments to
19

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.

Depending on the hydrodynamic and geotechnical circumstances this can be both

residual liquefaction and momentary liquefaction. Scour around a monopile and
liquefaction has been shown in one experiment by Sumer et al. (2007). They
concluded that a scour hole would only develop after the excess pore water
pressure was dissipated such that liquefaction was not present anymore. On the
other hand Whitehouse (1998) stated that since a liquefied soil has lost most of its
shear strength, the soil can be eroded more easily. More research on the effects on
scour should provide more certainty in the design of offshore wind turbine
foundations.

20

6. Common scour problems

Common coastal engineering situations where scour may occur are illustrated in
the figure below and described as follows:

Fig 12: Coastal scour Problems

6.1 Scour at coastal inlet structures
Kidney-shaped scour holes are sometimes present at the tip of one or both inlet
jetty structures. These scour holes are usually permanent features of the inlet
structure system, but there have been instances where seasonal infilling occurs due
to longshore sediment transport. In some cases scour holes have been deep enough
to result in partial collapse of the jetty head, while in other cases the scour holes
21

have resulted in no structure damage. Hughes and Kamphuis (1996) observed in

movable-bed model experiments that the primary hydrodynamic process
responsible for kidney-shaped scour holes appears to be flood currents rounding
the jetty head and entering the channel. Sediment mobilization, rate of scour, and
extent of scour are increased by wave action, particularly waves that are diffracted
around the jetty tip into the navigation channel. Waves breaking across the jetty
head in the absence of currents will also cause scour of a lesser magnitude
Substantial scour trenches are known to form along the channel-side toes of jetty
structures. These trenches are caused either by migration of the navigation channel
(by unknown causes) to a position adjacent to the jetty toe or by ebb-flow currents
that are redirected by the jetty structure. Hughes and Kamphuis (1996) argued that
ebb flows deflected by a jetty are analogous to plane jet flow exiting a nozzle with
similar geometry. As the flow cross section decreases, the flow velocity increases
proportionately to maintain the ebb flow discharge.
Scour trenches can also form along the outside toe of the updrift jetty. These
trenches might be formed by the seaward deflection of longshore currents that
causes a local flow acceleration adjacent to the jetty toe, or the scour may stem
from high peak orbital velocities resulting from the interaction of obliquely
incident and reflected waves. A likely scenario is scour hole formation due to both
hydrodynamic processes with the waves mobilizing sediment and the current
transporting the material seaward. Scour trenches on the outside toe of a jetty may
be seasonal at locations experiencing seasonal reversal of predominant wave
direction.
Scour holes occur regularly around bridge pilings and piers that span coastal
inlets. Generally, this situation is similar to scour that plagues bridge piers on
inland waterways. Additional factors complicating scour at inlet bridge piers are
the unsteady and reversing nature of tidal flows, and the possible exposure to
waves and storm surges.

22

6.2 Scour at structures in deeper water

Scour can occur at the toes of vertical-faced breakwaters and caissons placed in
deeper water. Wave-induced scour results from high peak orbital velocities
developed by the interaction of incident and reflected waves. If a particular
structure orientation results in increased currents along the structure toe, scour
potential will be significantly enhanced. Localized liquefaction due to wave
pressure differentials and excess pore pressure within the sediment may cause
sediment to be removed by reduced levels of bottom fluid shear stress.
Characteristic scour patterns may occur around the vertical supporting legs
(usually cylinders) of offshore platforms. Under slowly-varying boundary layer
flow conditions, the platform leg interrupts the flow causing formation of a
horseshoe vortex wrapped around the structure just above the bed. This secondary
flow intensifies the bottom fluid shear stresses, and erodes sediment. The quasiequilibrium scour hole closely resembles the shape of the horseshoe vortex. In the
absence of currents, waves can cause scour in the shape of an inverted, truncated
cone around the vertical cylinder provided the bottom orbital velocities are
sufficiently high.
Pipelines laid on the sea bottom are susceptible to scour action because the pipe
cross section obstructs the fluid particle motion developed by waves and currents.
6.3 Scour at structures in shallow water
Piers and pile-supported structures in shallow water react to currents and waves
just as in deep water. However, the shallow depth means that orbital velocities
from shorter period waves can cause scour. Therefore, vertical piles are vulnerable
to scour caused by a wider range of wave periods than in deeper water.
Scour can occur along the seaward toe of detached breakwaters due to wave
reflection. The scour process will be enhanced in the presence of transporting
currents moving along the breakwater. Scour holes may be formed at the ends of
the breakwater by diffracted waves. In shallow water, breaking waves can create
high turbulence levels at the structure toe.
23

 Vertical-front and sloping-front seawall and revetments located in the vicinity of

the shoreline can be exposed to energetic breaking waves that produce downwarddirected flows and high levels of turbulence which will scour the bed. Scour could
also be produced by flows associated with wave downwash at less permeable
sloping structures.
Vertical bulkheads are usually not exposed to waves capable of producing scour;
however, it is possible for scour to occur by local current accelerations.
Scour around pipelines will occur by the same mechanisms as in deeper water
with shorter period waves becoming more influential as water depth decreases.
Buried pipelines traversing the surf zone can be at risk if beach profile erosion
exposes the pipeline to pounding wave action and strong longshore currents.
Depending on specific design details, coastal outfalls may develop scour patterns
that jeopardize the structure.
6.4 Other occurrences of scour
Any type of flow constriction caused by coastal projects has the potential to cause
scour. For example, longshore currents passing through the gap between a jetty and
a detached breakwater at Ventura Harbor, CA, accelerated and caused scour along
the leeside toe of the detached breakwater (Hughes and Schwichtenberg 1998).
Storm surge barriers, sills, and other structures founded on the sea floor can
experience scour at the downstream edge of the structure. Small pad foundations
can be undermined by waves and currents.
Structure transition points and termination points may produce local flow
accelerations or may focus wave energy in such a way that scour occurs.
Scour may occur as a transient adjustment to new construction. For example,
Lillycrop and Hughes (1993) documented scour that occurred during construction
of the terminal groin at Oregon Inlet, North Carolina. Despite maintenance of a
scour blanket in advance of construction, the project required 50 percent more
stone because of the scour.
24

7. Prediction of scour (limiting to Piles)

There have been many theoretical and laboratory studies conducted examining
various aspects of scour related to coastal projects. Some studies focused on
discovering the physical mechanisms responsible for scour, whereas other studies
were directed at developing engineering methods for predicting the location and
maximum depth of scour. In this section usable engineering prediction methods are
presented for estimating scour for specific coastal structure configurations and
hydrodynamic conditions. We limit our study to pile structures, as to study all the
types of structures in depth would be beyond the scope of this terms paper. To a
large extent the predictive equations have been empirically derived from results of
small-scale laboratory tests, and often the guidance is fairly primitive. In some
situations the only predictive capability consists of established rules of thumb
based on experience and field observation. A comprehensive discussion of scour
mechanisms, theoretical developments, and experiment descriptions is well beyond
the scope of this report. However, there are several publications containing detailed
overviews of scour knowledge for many situations of interest to coastal engineers
(e.g., Hoffmans and Verheij 1997; Herbich 1991; and Sumer and Fredse 1998a).
In the following sections, appropriate citations of the technical literature are
provided for more in-depth study.
The majority of methods for estimating scour at vertical piles were developed for
piles with circular cross section, which are widely used in coastal and offshore
engineering applications. However, there are estimation techniques for piles with
noncircular cross sections and for specialized structures such as noncircular bridge
piers and large bottom-resting structures.
Scour at small vertical piles (pile diameter, D, is less than one-tenth of the incident
wavelength) is caused by three simultaneously acting mechanisms:
formation of a horseshoe-shaped vortex wrapped around the front of the
pile;
vortex shedding in the lee of the pile; and
local flow accelerations due to streamline convergence around the pile.
25

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).

7.1 Scour at small diameter vertical piles

Vertical piles with diameter, D, less than one-tenth of the incident wavelength
constitute the vast majority of pile applications in coastal engineering. Even
cylindrical legs of some offshore oil platforms may fall into this category.
26

7.1.1 Pile scour by currents

Many scour estimation formulas have been proposed for scour caused by
unidirectional currents without the added influence of waves. A formulation widely
used in the United States is the Colorado State University (CSU) equation
developed for bridge piers (e.g., Richardson and Davis 1995) given by the
expression

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.

Fig 13: Correction factor K1, for Pile/Pier shape

The shape factor, K1, is selected from the above figure, and the orientation factor,
K2, can be determined from the following equation given by Froehlich (1988).

27

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

28

Fig 14: Wave-induced equilibrium scour depth at a vertical pile

= 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.
29

At values of KC > 100, Sm /D 1.3, representing the case of current-only scour.

Independent confirmation of the above equation was presented by Kobayashi and
Oda (1994) who conducted clear water scour experiments. They stated that
maximum scour depth appeared to be independent of Shields parameter, grain size
diameter, and whether scour is clear-water or live-bed.
In an extension to their 1992 study, Sumer, Christiansen, and Fredsoe (1993)
conducted additional regular wave live-bed scour experiments using square piles
oriented with the flat face 90 deg and 45 deg to the waves. The following empirical
equations for maximum scour were obtained as best-fits to the observed results:
Square pile 90 deg to flow:

= 2.0[1 0.015(11) ] 11

Square pile 45 deg to flow:

= 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
30

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

7.2 Scour at large diameter vertical piles

Rance (1980) conducted laboratory experiments of local scour at different shaped
vertical piles with diameters greater than one-tenth the incident wavelength. The
piles were exposed to coincident waves and currents. Rance provided estimates of
maximum scour depth as functions of pile equivalent diameter, De, for different
orientations to the principal flow direction. (De is the diameter of a cylindrical pile
having the same cross-sectional area as the angular pile.) These formulas are given
in Figure.

31

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.
.

32

8. Design of scour protection

Toe protection in the form of an apron is needed to prevent toe scour which may
destabilize or otherwise decrease the functionality of a coastal structure. The apron
must remain intact under wave and current forces, and it should be flexible enough
to conform to an initially uneven sea floor. Scour apron width and required stone
size for stability are related to wave and current intensity, bottom material, and
structure characteristics such as slope, porosity, and roughness. Design guidance
for scour protection is based largely on past successful field experience combined
with results from small-scale laboratory tests. Special attention is needed where
scour potential is enhanced such as at structure heads/ends, at transitions in
structure composition, or at changes in structure alignment. This section provides
general design guidance for scour aprons; however, this guidance should be
considered preliminary. Projects requiring absolutely stable scour blankets should
have proposed designs tested in a physical model. Hales (1980) surveyed scour
protection practices in the United States and found that the minimum scour
protection was typically an extension of the structure bedding layer and any filter
layers.
The following minimum rules-of-thumb resulted from this survey:
Minimum toe apron thickness - 0.6 m to 1.0 m (1.0 m to 1.5 m in northwest
U.S.);
Minimum toe apron width - 1.5 m (3 m to 7.5 m in northwest U.S.);
Material - quarry stone to 0.3 m diameter, gabions, mats, etc.
These rules-of-thumb are inadequate when the water depth at the toe is less than
two times the maximum nonbreaking wave height at the structure or when the
structure reflection coefficient is greater than 0.25 (structures with slopes greater
than about 1:3). Under these more severe conditions use the scour protection
methods summarized coastal engineering manual
Listed below are common methods adopted for scour protection for various costal
structures. Design aspect of these must comply with geotechnical and
hydrodynamic considerations.

33

8.1 Vertical walls

Toe protection design
Scour apron constructed of quarry stone
.
Determining the toe apron quarry stone size depends on the hydrodynamic
conditions, based on whether they are subjected to waves, currents or combination
of
8.2 Sloping structures
Typically provided by the toe protection.
Additional scour protection to prevent scour by laterally-flowing currents
and strong tidally-driven currents
backfill the scour hole and placing scour blanket

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).
34

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.

Fig 16: Stone blanket scour protection for submerged pipelines

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.

35

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.

Fig 17: Offshore wind farms

36

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

9.2 Case study 2

Application
A bridge scour project at the NASA Kennedy Space Center (KSC) in Cape
Canaveral, Florida, required a remedial and protective solution.
The Challenge
Continuous scour protection was needed around each bascule pier for four bridges
throughout the KSCs channel and fender system. Bridge scour protection is
nothing new for the Triton Marine Mattress System, but the product was to the
owner, thus necessitating an extensive permit process through the USACE and
local municipalities. Positioning and anchoring the mats were also significant
challenges. The Haulover Canal Bridge, in particular, has a narrow channel,
which has high velocities and deep scour pockets, said Kim Rivera of Jones
Edmunds & Associates, Inc., the design engineer for the project.
Site conditions
The project involved installing the geosynthetic revetment system at depths of
approximately 20 feet, amid high water velocities and with limited overhead
37

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.

38

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

39

The Triton system advantage

The owner and consulting engineer Jones Edmunds& Associates, Inc., selected
Triton Marine Mattresses because:
They incorporated Tensar Uniaxial (UX) Geogrids, which have the strength and
flexibility to armorthe bridge piers without damaging them.
The armor units could be locally constructed and customized on shore
prior to installation.
They could provide the most efficient bridge scour protection at less
than half the thickness of riprap, an even bigger benefit for materialconscious NASA.
They are much easier to remove than riprap a factor the agency had to
consider given the potential for future bridge replacement projects.
They offered a one-foot profile that minimized transition from
surrounding riprap grades.
They were constructible even in difficult working conditions.

40

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.

41