coastal protection

138 Pages·2006·4.89 MB·English

COASTAL PROTECTION
Design of Seawalls and Dikes
incl. Overview of Revetments
by
Krystian W. Pilarczyk
Short Course
Content
Part I : Introduction to coastal protection
Part I1 : Design of seawalls and dikes
incl. overview of revetments
Appendix : Data collection and prediction methods
INTRODUCTION
T0
COASTAL PROTECTION
by
K. W. Pilarczyk
Rijkswaterstaat, Road and Hydraulic Engineering Division
Van der Burghweg 1, P.O. Box 5044, 2600 GA Delft, The Netherlands
INTRODUCTION T0 COASTAL PROTECTION
.
l INTRODUCTION
Coastal communities world-wide are faced with difficult problems of
shoreline erosion control. Because of the high land values inherent in
the shore zones, chronic erosion as wel1 as storm erosion can be viewed
as important management problems resulting in attempts to reduce ero-
sion by coastal defence measures.
As a result of natural shore processes, the boundary between land and
water may undergo a shift in position with time. This shift may be sea-
ward with advancing shorelines (accretion) and landward with receding
shorelines (erosion). These processes occur along every shoreline. The
rate at which shores erode or accrete depends on the composition of the
shore zone and its exposure to erosive forces.
Erosion result from two basic causes: forces of nature acting along the
shoreline and the actions of man. Structures created by man can inter-
£ere with the continuing shore process in the following ways (Figure
l):
interruption of littoral drift patterns (i.e. the movernent of sand by
wave action and currents along the shore);
the deflection of shore current patterns;
the removal of sediments by dredging; and,
modification of wave regimes through reflection from and diffraction
around structures.
N..a.t.u.r.a..1. .c.a.u..s.e.s. .o.f.. .e.r.o.s..i.o.n. .i..n clude:
wind (there is a predominant or resultant wave direction which is
n
usually oblique t; the coastline);
waves and their longshore component of wave energy;
water levels incl. tides, storm-surges and sea-level rise;
fluctuations in river discharge of sediment;
rain striking exposed bluff faces;
ground and surface water movements;
ice action;
ground motion;
anima1 activities; etc.
By £ar, the most significant natural erosive force, along our shore-
lines, is wind-driven wave action in combination with water level chan-
ges (tide, wind set-up, sea-level rising).
INTRODUCTION T0 COASTAL PROTECTION
K. W. Pilarczyk
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years. The type, size and location of coastal protection must be based
on actual needs, benefits expected from the method, its effect on adja-
cent shorelines and on economy. There are many types of coastal protec-
tion (seawalls, groynes, nourishment etc.), each suitable for a defi-
nite purpose (Figure 2). However, an improper or ill-advised design may
adversely affect the coast rather than remedy the situation (Per Bruun,
1963).
Before starting any construction of a coastal protection a decision
must be made as to what kind of coastal protection is needed. The right
answer depends very much on what is expected from the coastal protec-
tion. If a beach is the ultimate aim, then there should be source on
the updrift side. If there is a drain, it should be rendered harmless
in one way or other, or a source of material should be established.
If a coastal protection is to be constructed in an erosion area, the
water depth up to which erosion takes place should be known if possible
before starting construction. If erosion takes place to a limited depth
only, there should be a good chance of building up and maintaining a
beach. If erosion takes place up to deep water, coastal protection may
have to be moved back, after a time interval, depending upon the rate
of erosion and the rate of a possible artificial nourishment to the
beach.
If a beach is not wanted, a source on the updrift side is not impor-
tant, but if there is no source, erosion will continue. In this case it
will be absolutely necessary to know the depth up to which erosion ta-
kes place. If erosion takes place to only a limited depth, it should be
possible to stop the erosion at that depth, but if erosion takes place
up to deep water, it will be irnpossible to maintain the beach only by
groins and or seawalls.
After some time such constructions will have to be withdrawn and cer-
tain land area given up unless al1 the material is artificially repla-
ced. In many cases, this will be impossible for economic reasons.
Unless these important facts are taken into consideration, it will be
very risky to build any coastal protection.
1.2 Review of coastal defence methods
Various measures can be used for coastal protection: direct measures
which prevent or alleviate the immediate effects of the problems and
indirect measures to take away the causes of the problern. For indirect
measures one can think of, for instance, measures which stop the reduc-
tion of river sediment supply to the coastal system. Direct measures
include, for example (Figure 2):
- an artificial sand supply used to replenish an eroding part of a
coast. It may seem expensive and the need for repetition may discou-
rage coastal managers. However, careful considerations of capital
and maintenance costs frequently prove that it is, in fact, the op-
timal solution. An added advantage is that the recreational function
of the beach is preserved.
- a row of groynes used on an eroding part of the coast to locally
reduce the longshore sediment transport capacity and thus the coas-
tal erosion. In this way erosion areas can be shifted to less harm-
ful locat ions.
-
a detached breakwater (parallel to and at a certain distance from
the shore) used to change the transport capacities both along and
perpendicular to the coast, resulting in accretion in the lee of the
breakwater, referred to as a tombolo. Such structures have an effect
which is comparable to groynes.
INTRODUCTION T0 COASTAL PROTECTION
K.W. Pilarczyk
-
a seawall used to fix the shoreline. In this way the shoreline ero-
sion is replaced by erosion of the sea bed immediately in front of
the wall. Stability problems may occur, unless the foundation of the
seawall is wel1 below the sea bed.
-
- - existing profile
erosion profile
m ssnd ff11
a 8iniauar profile ddth
b ~idthof .?Md fl11
design storm surge level
A
dune/beach
nourishwnt
B
detached breakwater
groynes or
C
permeable groynes
toe-protection of dune
d~ne/dike Iandward
fore-shore beach - sea-wall -- s.-ecu naar-
I *
protection
Figure 2 Examples of sea-protection
1.3 Design considerations and methodology (Weide, van der, 1989)
When designing coastal structures, following aspects have to be consi-
de red :
The function of the structure
Coastal structures are used for many purposes, such as hi-~ h-water
- - -
protection, coastal-defence, wave-attenuation, flow-guidance etc.
INTRODUCTION T0 COASTAL PROTECTION
K. W. Pilarczyk
For each of these functions different functional requirements have to
be met and consequently different design-specifications are formula-
ted.
T--h-e- p hys-i-c-a-l- -e-n-v-i-r-o-n-m-e-n-t-
As stated above, the design-loads on coastal structures are mainly
determined by hydrodynamic forces. Moreover, hydrodynamic processes
may induce morphological changes which may affect the performance and
the integrity of structures. Finally, the geotechnical characteris-
tics of the site are of importance as they determine the bearing-
capacity of the subsoil, an important parameter for the overall-sta-
bility of the structures.
T..h.e. .c.o..n.s.t.r.u..c.t.i.o.n..-.m.e.t.h..o.d. .e.n visagei
The availability of material, equipment, and labour determines to a
large extent the construction procedure. This procedure in its turn
determines the possibilities of the designer. In those areas where
there are severe constraints in construction-procedures the designer
should be aware of this and should revise his design accordingly.
O..p.e.r.a.t..i.o.n. .a.n..d. .m.a.i.n..t.e.n.a nce
Once the structure is finished, the
structure will be used and mainte-.
nance will be required to secure its
per£ ormance.
The designer should be aware, there-
fore of the maintenance- procedures
and should take care that mainte-
nance is possible. In order to make
a wel1 balanced design al1 aspects
have to be integrated in a frame-
work, which describes the various
aspects and the mutual interaction
between these aspects. Ideally, the
above aspects should be considered
during al1 the stages of the design-
process.
During the design-process, following stages are indentified:
C-o-n-c-e--p-t-u-a-l- d-e-s-i-g-n-
In this stage a number of different alternatives are generated, which
al1 mee.t the functional requirements. Designs are general and only
main dimensions are given. At this stage the relevant aspects should
be identified, which determine the technical and economical feasibi-
lity. Their relative importance should be evaluated and rough figures
should be obtained to quantify them.
P-r-e--l-i-m-i-n-a-r-y- d-e-s--i-g-n
In this stage. a limited number of alternatives is selected after a
screening procedure which focusses on the technical feasibility. I£
required the conceptual stage should be repeated when the first set
of alternatives does not meet these criteria. At this stage, structu-
ral dimensions are quantified in some detail and a check is made on
the economic feasibility. Again the design process may have to be
repeated when the project becomes economically unfeasible.
D-e-t-a--i-l-e-d- -e-n g-i-n-e-e-r-i-n g
At this stage the structural details are designed and detailed design
drawings are made. Parallel to this the possibilities for financing
INTRODUCTION T0 COASTAL PROTECTION
K.W. Pilarczyk
are explored, and environmental and socio-politica1 aspects are con-
sidered in greater detail. Technical, economical and socio-politica1
criteria are used in this stage to screen the feasibility of the pro-
posed design.
-
Construction-stage
When the design stage is completed, tender documents are prepared and
the tendering procedure is started, resulting in the construction.
Although this stage is not part of the actual design-process, the
designer should be aware of the constructional constraints imposed by
this stage. Availability of material, accessibility of the area and
the corresponding limitation in the use of equipment, feasibility of
intermediate construction stages are some of the aspects to be consi-
dered. Especially in the detailed engineering stage these aspects
should be evaluated.
Operation and rnaintenance
Again this stage is beyond the actual design-process, but should be
considered by the designer. Especially maintenance can have an impact
on the selection of the design. When possibilities for maintenance
are poor, the initia1 design should be such that the structure can
operate without regular maintenance. ff local labour-cost is cheap
and capital investments are difficult, a cheap structure may be more
appropriate, when regular maintenance can be guaranteed.
The design-methodology is shown schematically on Figure 3. As shown in
this Figure, a simulation-model is required to evaluate the behaviour
of the structure in the various stages of the design.
One of the most difficult aspects of the design is the selection of the
model which should be used in the various stages of the design. In ge-
neral it can be stated that in the course of the design-process more
advanced methods are used. The actual choice, however, is dependent on
the complexity of the problems, the size of the project and the risk-
level which is acceptable.
1.4 Design process of coastal structures
In Figure 4 the process of the design of a coastal project has been
drawn in diagrammatic form (CUR, 1987). The starting point in this dia-
gram is the identification of a beach erosion problem. The second phase
in the design is the selection of the type of protective measure. The
final phase can be the risk analysis of the project.
B..o.u.n.d.a..r.y. .c.o..n.d.i.t.i..o.n.s. .a.n..d. .i.d..e.n.t.i.f.i..c.a.t.i.o..n. .o.f. problem
The actual design of coastal protection is determined by the local con-
ditions, like bathymetry, waves, tides, current~,m orphologic proces-
ses, and the characteristics of sediment and soil. Since these loads
cannot be computed accurately, they are defined in statistica1 terms.
Design-conditions are therefore specified in terms of probabilities,
and a probabilistic design concept is applied. Moreover, other factors
may impose certain constraints on the design like environmental condi-
tions, infra- structure, time of execution, etc.
For the design of a number of tools are available, such as the mathema-
tical models for integrated wave current sediment-transport that can be
used also for the prediction of the coastal development without and
with shore structures, available knowledge about the effect of sand
size on coastal profile development etc.
For the design a clear insight into the hydraulic boundary condition
and the morphologic processes in the area of interest is necessary. The
INTRODUCTION T0 COASTAL PROTECTION
K.W . Pilarczyk
rnorphologic processes not only affect the type of protection to be cho-
sen, but also, in the case of eroding coast, the required volume of
sand to be supplied in order that the project fulfills its purposes
during a specified time. 1'
FUNCTIONAL REOUIREMENTS i
O~JEC~MS shoreiine stabilizotion CRITERIA erformonce
backshore protection fechnicol feasibilit
inlet stabilizotion economic feasibili{y
horbour protectian + ~~cio/poiiticalf eosibility
.t
DE+SIG N SIMULATION EVALUATION
CONCEPTUAL ir MODEL CRITERIA
DESIGN LEV+EL I .
l
PRELIMINARY L MODEL
r
LEVEL i1 REDESIGN
4
I
MODEL GO T0
i
r
ENCiNEE>ING LEVEL I11 NEXT PHASE
P
A
WATER LEVELS. TIDES. WAVES. CURRENTS. SEABED
NATLIRAL ENVIRONMENT
Figure 3 The design rnethodology
The knowledge about the rnorphologic processes can be obtained by a
careful analysis of available data (i.e. soundings and levelling), sup-
plemented by cornputations of sediment transport. Possibly additional
measurements may be deemed necessary. In this connection it is observed
that these activities should be done already before to decide upon an
artificial beach nourishment or another coastal protection measure.
Not only the present evolutions in the coastal zone need to be known,
but also those which are expected to happen in future. Possibly the
observed evolutions may change for instance in the case of the migra-
tion of large scale sand waves along the coast. In fact the present
condition and tiìe future development of the coast should be confronted
with the interests involved, in order to decide whether or not protec-
tive measures should be taken.
The recent publications by Goda (1985), Horikawa (1988), SPM (1984) can
serve as good references for determination of the boundary conditions
for the coastal projects.
INTRODUCTION T0 COASTAL PROTECTION
K.W. Pilarczyk