The findings, interpretations and conclusions expressed in this study do neither necessarily reflect the views of the International Institute for Infrastructural, Hydraulic and Environmental Engineering, nor ofthe individual members of the MSc-committee nor of their respective empJoyers.
Acknowledgement
Acknowledgements
My profound and sineere gratitude is extended to my sponsors Rijkswaterstaat and my employers Geotech Associates for facilitating my study.
I could never have done it without the assistance and guidance of my mentor/supervisor Associate Professor, Ir. Henk Jan Vehagen and my special advisor Ir. J. Hans De Vroeg of Delft Hydraulics who constantly guided me throughout my studies.
A special thanks also to my family and friends both here and at home for their support. A heart warming thank you to all.
Abstract
A
BSTRACT
Offshore breakwaters have been used in many coastal areas for coastal proteetion with greater or lesser success. However to date these have not been used along the Dutch coast for this purpose. This is probably due to the absence of accurate design tools for their functional design, coupled with the uncertainty of whether thismethod of coastal proteetion
is economical, as compared to currently used methods such as beach nourishment.
Additionally, it is rather difficult to predict the morphological response of the coastline to a designed structure due to the large number ofvariables involved, and their inter-dependenee.
The fact that there is also a considerabie tidal range and a significant tidal current further complicates the matter, and makes the analysis even more difficult.
This Study / Thesis presents firstly an assessment of the applicability of detached infinitely
long offshore breakwaters for proteetion ofsections of theDutch coast from coastal erosion. Secondly it gives insight into the way in whicn the different boundary conditions, such as wave elimate and geometrie parameters of both the coastline and breakwater influence the coastal sediment transport and coastal morphology. Finally, it gives general guidelines for carrying out a functional design and for predicting the coastal response; i.e. whether the structure is likely to cause:
(a) Negligible effect on the coast ie. limited shoreline response (b)Formation of salients;
(c)Formation of Tombolos;
In analyzing the likely response to the wave climate, the (one line model) programme Unibestwas used. The results obtained from this model were compared to those obtained by
using general design rules/tools for emerged breakwaters, developed in previous studies. The results are also used to develop and provide new design guidelines for the use of submerged breakwaters for coastal protection.
A calibration of the model was first carried out using the results of previous studies and measured wave elimate near the areas of interest. This calibrated model was then used to estimate the sediment transport ratewitb andwithout the influence of the breakwater. Having estimated these rates, an attempt was made to predict crudely the likely coastline response to breakwaters of different geometrie parameters, assuming a wave elimate similar to that which existed during the lastfew decades.
From the analyses carried out herein it may be concluded that the use of offshore breakwaters for coastal proteetion appears to be technicaliyfeasible. Moreover, various coastline responses are possible, depending on the design parameters of the structure. The most important parameters affectingthe breakwater's performance are breakwater length and height. Further it appears that the breakwater has to be submerged once the length is of the order 1000 m or more.
Because of the variation in hydraulic boundary conditions (in particular tidal levels) along the Dutch coast, and the great influence of this parameter on the whole analysis, the deductionsmade from this studyare onlyapplicable to sites where these boundary conditions are similar.
Contents
Contents
Page
Acknowledgement
Abstract
1.0 Introduction
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1-3
1.1 General
1.2 Purpose and Nature of The Study
1.3 Methodology
1.4 Scope of Study
2.0 LiteratureReview
4-22
2.1 Introduetion
2
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2 Wave propagation and Sediment Transport in the Coastal Zone
2
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3 Coastal Erosion
/
Accretion
2.4 Types of Coastal P
r
oteetion
2
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5 Offshore Breakwaters
2.6 Unibest Modelling Suite
2.7 Genesis Modelling System
2.8 Comparison of Modelling Systems
3.0 Boundary Conditions
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23-28
3.1 Wave Climate
3.2 Sedimentology
3
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3 Tidal Levels
3.4 Tidal Currents
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5 Cross-shore Profile
4.0 The Dutch Coastal System and Sediment
Transport Regime.. ...
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4.1 General
4.2 Sea Defenses and Coastal Proteetion
4.3 Coastal Erosion
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4 History of Implemented Coastal Proteetion
5.0 Preliminary Functional Design
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. 33-36
5
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1 Introduetion
5.2 Functional Requirements
5.3 Length and Location of Structure for Analysi
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5.4 Transmission Coefficients
Contents
6.0 Modelling Using Unibest and
Sediment Transport CaIculations. . . .
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.. 37-75
6
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1 Introduetion
6.2 Cross-shore Profiles and Calculation Grid
6.3 Wave Climate
6.4 Analysis
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Calculations and Results
7.0 Discussion, Conclusions and Recommendations.
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76-88
7.1 Introduetion
7.2 Objectives of The Study
7.3 Discussion
7.4 Limitations Of Study
7.5 Conclusion and Recommendations
References
Appendices
(Separate document)Introduetion
1.0 INTRODUCTION
1.1 General
Shore proteetion and beach stabilization are major responsibilities in the field of coastal engineering. Beach erosion, accretion, and changesin the offshore bottom topography occur naturally, further engineering in the coastal zone also influences sediment movement along and across the shore, therebyaltering the beach plan shape and depth contours. Beach change iscontrolled by wind, waves, currents, water level, nature of the sediments, and itssupply.
These littoral constituents interact with and adjust to perturbations introduced by coastal structures, beach fills and other engineering activities. Most coastal processes and responses are non-linear and have a high variability in space and time. Although it is difficult and a challenging task to predict the course of beach change, such estimations must be made to design and maintain shore proteetion projects.
In the planning ofproteetion works in the near-shore zone,prediction of beach evolution with numerical models has proven to be a powerful technique to assistin the selection of the most appropriate design. Models provide a framework for developing problem statements and solution formulation, for organizingthe collection and analysisof data, and, importantly, for efficiently evaluating alternative designs and optimizing the selected design.
In the last decadestwomethodsof beach coastal proteetion have become increasingly popular for coastal proteetion of sandy shores: beach nourishment and offshore breakwaters . In the Netherlands the former is well known and applied regularly. The second method, Offshore Breakwaters, has not yet been applied. However, with the increasing cost of beach nourishment and the apparent success of offshore breakwaters systems abroad, this method of coastal proteetion is appearing increasingly attractive. With this in mind, The Ministry of Public Works, though Rijkswaterstaat has commissioned a number of studies to investigate the effeets of offshore breakwaters on coastal sediment transport and erosionl accretion of the shoreline.
This is one study among others, whieh is geared at gaining insight into the influence of offshore breakwaters on the coastal morphology, in particular, the influence of the geometrie parameters of the structure. To assist in this study, the numerical model Unibest, developed by Delft Hydraulics has been used. The suitability of another modelling system, Genesis, was also investigated.
• Offshore Breakwaters
Offshore breakwaters are coastal structures, usually shore-parallel, located at a certain distance from the shoreline. These structures are used to dissipate wave energy (by one or more methods ) and change the wave and flow patterns. The main purposes are: (1) to proteet beaches or even widen certain stretches of the eoast; (2) to provide a tranquil environment on the lee side of the structure for harbour protection.
Introduetion
1.2 Purpose and Nature of the Study
Although offshore breakwaters have been used for coastal proteetion in many parts of the
world (Japan,Israel and the USA), to date it has not been used along the Dutch coast for this
purpose. This is probably due to the lack of accurate design methodology coupled with the uncertainty in predicting the functional response of the structure. Another complicating issue is the fact that there is a considerable tidal range along the Dutch coast, which makes the functional response of the structure more difficult to determine.
To date, most of the research on coastal response to offshore breakwaters assumes an emersed breakwater. Therefore the empirical formulae and design tools available are only
applicable to these types of structures, and not to submerged ones. However because of the relative large tidal difference along the Dutch coast ( up to 4.0 m in some areas ), it seems virtually impossible to design an economical structure which will always function as an emerged breakwater and at the same time give the type of coastal functional response required.
The purposes of this Study are therefore to: (1) investigate the applicability of offshore breakwaters for coastal proteetion along sections of the Dutch coast; (2) gain insight into the influence of the various parameters on the transport rates and the consequent morphological change (3) provide preliminary design criteria for their successful application. In doing so
the particular conditions applicable to the Dutch coast (such as tidal range and currents) are considered.
In carrying out the analyses, general site data such as bathymetry and wave elimate were used. The three main areas along the coast of interest are ( see Figure 3.1) :
(a) Callantsoog (Ca) (b) Scheveningen (Sc) (c) Domburg (Do)
1.3 Methodology
In order to assess the applicabilityof offshore breakwaters for coastal protection, a studywas
first carried out on the behaviour of offshore breakwaters and parameters which affect their performance. Next, a preliminary breakwater design which would meet the stated objectives was made. This design was carried out using the currently available design theory which is
applicable to emerged breakwaters.
The coastal response was then analyzed using the software packages Unibest. The model was
first calibrated using previouslydetermined (calibrated) transport rates. Having obtained this
preliminary design, the main parameters affecting the breakwaters' performance were varied
within the range (as dictated by the three sites of interest) to gain insight into the way in
which these parameters influence the functional response. Where possible, the rationale/
motivation for the particular condition investigated is stated, so that a better understanding
of the investigation may be gleaned.
The results are summarised infigures, graphs and charts which may then be used for more
Introduetion
accurate preliminary functional designs. These results, obtained by varying different parameters are compared and analyzed to rationalize the similarities and differences of the results.
A more detailed discussion of the methodologyis given in the chapter on modelling (Chapter
6.0).
1.4 Scope of Study
The scope of the study is limited both by time and the computational tools available for the investigation. It is also guided somewhat by the requirements of the sponsors. The generaI
aim may however be summarized as follows:
(a)Todefine a simple basic caseof an infinitelylong offshore breakwater ( to realistic cases)
for the ensuing analysis;
(b)Toinvestigate the effect of an infinitely long offshore breakwater on longshore transport and coastline behaviour as a function of several hydraulic and geometrie parameters by using
Unibest, and carrying out a large numbersof runs, with varying wave height, wave direction, wave period, and water levels, with and without tidal currents;
(c) To investigate the effect of the cross-shore profile ( in terms of slope, presence and location of breaker bar), the breakwater position and its geometrie parameters (such as
height) on the performance of the breakwater as discussed above;
(d) To discuss the results of the one-line model and the benefits /complications of Genesis
for such analyses.
Itis emphasized that the objective of the study is not to reproduce the exact transport rates
along the different sections of the coast, as this has already been carried out and achieved in numerous studies, but to determine how these transport rates and corresponding coastal morphology would be influenced bythe construction of an offshore breakwater, and further how these structural parameters and boundary conditions influence these results.
It ispointed out that the results, conclusions and recommendations arrived at in this study
are based on the assumption that Unibest is accurate in determining the sediment transport rates under different boundary conditions. Unibest is also assumed to determine the morphologic changes accurately. Verification of the model itself (with respect to the computations it makes) is beyond the scope of this study. Only logical trends were investigated to ensure that the model does in fact give results which are consistent with presently developed theory, and observations.
Literature Review
2.0 Literature Review
2.1 Introduetion
This chapter is comprised of two main parts. In the first sections (2.1 - 2.5) general properties of waves, (including wave propagation, wave breaking, sediment transport and coastal erosion) together with breakwaters and the theory goveming the functional response are discussed. The state of the art knowledge available for functional response has been developed for emerged breakwaters, and not for submerged ones, therefore the theory has to be used with caution when being applied to submerged structures.
In the second part (sections 2.6-2.8) , the mathematical model intended to be used in the analyses of the coastal morphological changes is discussed. The capabilities and limitations are highlighted, and a comparison is made between the various models.
2.2 Wave Propagation and Sediment Transport in the Coastal Zone
Waves which are often seen approaching a shore, are usually generated by wind in an offshore region. These waves propagate towards the shore often travelling several kilometres. As they approach the near-shore they undergo some transformation resulting from refraction, shoaling and even diffraction, if an obstacle is encountered in their path. The fore-mentioned processes result in changes in wavelength, height, and direction. As the waves approach even shallower water, they become unstable and are forced to break, releasing some of their energies in the process.
Waves bring an enormous amount of energy to the coast. This energy which is dissipated in the wave breaking process, causes water level changes, turbulence, heat generation, current generation, and sediment movement. The sediment movement or sediment transport is governed by laws which are not well known. What is known however is that it involves complex 3D circulating patterns of various spatial and time scales (SPM 1984).
If the tidal range is large the wave propagation ( refraction and breaking ) will vary significantly according to the water level. If this is the case, refraction simulations with different water levels may be necessary. Water levels also play a major role in wave overtopping and transmission through breakwaters, sediment overtopping and bypassing shoreward and seaward of groynes.
A number of formulae have been developed to estimate the sediment transport rate in coastal areas. These formulae vary widely in complexity and to some extent, the parameters used in their formulation. Amongst the more popular sediment transport formulae are :Cerc,
Bijker, Van Rijn, Engelund-Hansen, and Bailard.
The advantages and limitations of the CERC and Bijker formulae will now be discussed.(See section 6.4.2 on "Selection of Transport Formula"). Only these two formulae are discussed since the CERC formulae is use by Genesis exclusively, and for Unibest, the Bijker formulae gives the best results for the situation being studied.
Literature Review
• The CERC formula
The Cerc formula (SPM 1984)isprobably the most simple of the fore-mentioned formulae used for estimating coastal sediment transport. It simplystates that the sediment transport rate isproportional to the breaking wave height, and angle . The equation is:
Sc = A*H2*2Cgo*sine cos-+-~b ~o
where :
Sc = sediment transport rate (m'/s)
A = a Calibration coefficient (usually between 0.013 and 0.025)
H = significant wave height atdeep water (m)
Cgo= deep water group velocityof the waves (mis)
cf>o = wave angle at deep water (degrees)
cf>b = Wave angle at breaker depth (degrees)
The breaker depth at which cPb is required, isdefined by: h, = H, l-y
-y is usually given a value of 0.4 to 0.7 .
The CERC Transport is a cross-shore integrated longshore transport due to wave action. It does not take into account the effects of tidal currents.
This simple formula is easy to use and is especiallyconvenient when onlyan overall estimate of sediment transport is required, and only hand calculations are available. However due to its simplicity it does have quite a few drawbacks, two of the most significant ones being: (a) The fact that it does not give the cross-shore distribution of the transport, which is quite
often important if not essential for any form of coastal transport analysis/design .
(b) lts non inclusion of the tidal currents in the calculations means that it is not valid for situations where tidal currents are significant, as is sometimes the case.
(c) There is no direct relationship with grain size.
•The Bijker formula
Bijker 1968 proposed a formula for the total sediment transport due to currents and waves. This formula is based on the formula of Kalinske-Frijling for bed load due to currents and
waves, incorporating a formulation and a suspended load component based on the Einstein Rouse concentration verticals. (Delft Hydraulics )
The formula which states that:
S = Sb
+
Ss (mvrn/s, including pores) where:Literature Review
where:
Sb = bottom sediment transport Ss = suspended sediment transport
d50 = median (50 %) grain diameter (m) d90 = 90% grain diameter (m)
Á = relative density
re = bottom roughness (:::::0.5- 1.0 times ripple height) (m) o, = sediments density (kg/m.)
w = fall velocity (mis)
u, = orbital velocity near the bottom (mis)
w = wave frequency (radsis)
C = Chezy coefficient = 18 log(12dlre) C90 = 18 log(12d/~o)
f.L = (CIC90)3/2
fw = exp[-5.977
+
5.213(Ub/wretOI94~ = C(fwl2g)I/2
B = coefficient which varies between 1 and 5 11and 12 = Einstein-Rouse integrals
The Bijker formula which is perhaps one of the more comprehensive formulae gives rather good results. For low transport capacities, the transport rate is generally overestimated due to the absence of a critical mobility parameter within the formula. (A comparison is made with the Van Rijn formula to determine if this lack of a critical mobility parameter adversely affects the determined transport rates.) The formulae also involves difficult integrations, which does not makes it very useful for hand calculations. This problem is ho wever solved by carrying out numerical integration using a computer.
2.3 Coastal Erosion\ Accretion
Whenever there is a gradient in the sediment transport capacity along a coast, eros ion or accretion occurs, provided sufficient sediment is available to supply the capacity. If the sediment transport rate decreases along the coast, accretion occurs, where as erosion results if there is an increase.
Pelnard-Considere (1956) originated a mathematical theory of shoreline response to wave action under the assumption that the beach profile moves parallel to itself, that is, it translates
Literature Review
shoreward or seaward without changing shape in the course of eroding and accreting. He
verified this mathematical model by comparison to beach changes produced by waves
obliquely incident to a beach with a groyne installed in a movable-bed physical model. If the profile shape does not change, any part on it is sufficient to specify the location of the entire profile with respect to a baseline. Thus, one contour line may be used to describe changes in the beach plan shape and volume as the beach erodes and accretes. This contour is usually taken as the readily observed shoreline, the zero depth contour. The models are therefore termed the "Shoreline change" or "Shoreline response" model. Sometimes the terminology
"One-line" model, a shortening of the phrase "One contour line" model is used with reference to the single contour line. A second geometrical type assumption is that sediment is transported alongshore between two weIl defined limiting elevations on the profile. The shoreline limit is located at the top of the active berm, and the seaward limit is located where no significant depth changes occur (the so-called depth of profile closure).
• Governing Equation for Shoreline Change
The equation goveming shoreline change is formulated by conservation of sand volume.
Consider a right-handed Cartesian coordinate system in which the y-axis points offshore and the x-axis is oriented parallel to the trend of the coast (Figure 2.1). The quantity y thus denotes shoreline position, and x denotes distance alongshore. It is assumed that the beach profile translates seaward or shoreward along a section of coast without changing shape when a net amount of sand enters or leaves the section during a time interval ~t. The change in shoreline position is ~y, the length of the shoreline segment is ~x, and the profile moves within avertical extent defined by the berm elevation DB and the closure depth DC, both measured from the vertical datum (for example, MSL or MLLW).
The change in volume of the section is ~ V
=
~x~y(DB+
DC) and is determined by thenet amount of sand that entered or exited the section from its four sides. One contribution to the volume change results if there is a difference ~Q in the longshore sand transport rate Q at the lateral sides of the cells. This net volume change is ~Q~t = (oQ/ox)~x~t. Another contribution can arise from a linesouree or sink of sand q = qs
+
qo , whichadds or removes a volume of sand per unit width of beach from either the shoreward side at the rate of ~ or the offshore side at the rate of qo.
These produce avolume change of qàxèt . Addition of the contributions and equating them
to the volume change gives ~V
=
~~y(DB+
DC)=
(oQ/oX)~X~t+
q~~t.Rearrangement of terms and taking the limit as ~t ....0 yields the governing equation for the rate of change of shoreline position:
oy/ot
+
lI(DB+
DC) [ oQ/ox- q]=
0 [1]In order to solve Equation 1,the initial shoreline position over the full beach to be modeled,
boundary conditions on each end of the beach, and values for Q , q ,DB ,and DC must be given.
Literature Review WATER LEVEL DATUM a. Cross-section view DISTANCE OFFSHORE Y w a:: o :::c ti) (.!) z
s
-c UJ o ~ ti) CS---I--_.__-t---:-l
~ ~ qsl1xM Qol1xi1tI
___ .&....--~_.__J
x b. Plan viewFigure 2.1 Definition Sketch for Development of Shoreline Change Equation.
Literature Review
2.4 Types of Coastal Proteetion
Many forms of coastal proteetion are currently being used in various parts of the world ,
each with its own advantages and disadvantages. The following are among the most commonly used.
• Groynes
• Beach Nourishment
• Revetments, Bulkheads and Seawalls
• Off-shore Breakwaters • Groynes
Groynes are shore perpendicular structures, usually constructed of rock or lumber, which block a percentage of the longshore transport. The actual amount of sediment that is blocked is a function of the geometrie and structural properties of the groyne.
The ma in variables influencing this blocking capacity are the height, length and porosity of the structure. Impervious structures extending beyond the seaward end of the surf zone, with a height above the free water surface block almost 100 % of the longshore transport. For shorter structures the actual amount of sediment blocked depends on its cross-shore distribution of the sediment transport. Groynes or a series of groynes may therefore be designed to block a designed percentage of the long-shore transport, thereby reducing or preventing erosion in a given area.
Groynes however have three major disadvantages. Firstly, lee side erosion occurs on the down-co ast end of the structure , which may even endanger the stability of the structure . The second major disadvantage is th at the structure is ineffective against cross-shore transport.
Finally dangerous rip currents may be set up near the tips of the structure. This may be hazardous to users of the beach.
Groynes do not appreciably reduce the wave energy striking the shore where as breakwaters do.
• Beach Nourishment
Beach nourishment may be considered to be a soft form of coastal protection, in contrast to the other structural measures of protection. It basically involves the placement of a volume of sand in the affected area to replace that which has been removed.
It is considered to be environmentally friendly, once executed ina controlled manner. Often it is placed hydraulically, using a system of pumps and pipelines to transfer the liquified sand from the sou ree .
Literature Review
• Revetments, Bulkheads and Seawalls
Revetments, bulkheads andseawallsare all coastal proteetion structures usually placed at the
shoreline or upper shore to proteet the coast from currents and waves. The distinction between revetments, bulkheads and seawalls is mainly a matter of purpose. The structure is
named to suit its intended purpose.
In general, seawalls are rather massive structures because theyare required to resist the full force of the waves. Bulkheads are next in size; their primary function is to retain fill and while generally not exposed to wave action, they still need to be designed to resist erosion by the wave elimate at the site. Revetments are generally the lightest because they are designed to proteet the shoreline against erosion by currents or very mild wave action.
• Off-shore Breakwaters
This subject is discussed in section 2.5 following.
Literature Review
2.5 Offshore Breakwaters
Offshore breakwaters dissipate wave energyby reflecting, diffracting and reducing the height of the transmitted wave that enters the lee side of the structure. They also redistribute the wave and current pattems. Offshore breakwaters may be submerged or emerged, longshore or oblique, shore connected or detached. These structures may be further classified as segmented or non-segmented.
Unlike shore perpendicular structures they may be designed to allow variabie amounts of materiaI to pass, thereby affecting the amount of sediment that is available for littoral processes. Additionally, they are preferred if offshore mode of sediment transport prevails. 2.5.1 Main Parameters AffectingFunctional Response of Off-shore Breakwaters An investigation of the parameters which control the response of a sandybeach to a detached breakwater reveals tbat at least 14 parameters are of importance (Hanson and Kraus). The beach response is a function of the breakwater properties, the beach properties, and tbe wave climate. Or stated more explicitly in symbols:
Beach response = F(X,Y,Kr,G,) , (D,.1D,S) , (H, T, 0, Os, aH' ae, ar) These symbols are defined below and in Figure 2.2 .
.1D = variation in depth at structure (m) (from tide)
D
=
depth at structure (m)Y = original shoreline distance (m)
S
=
sediment availability H=
incident wave height (m) T = wave period (s)°
= incident wave angle (degrees) aH = standard deviation of H; etc.x H.e.T
+-G y "'__s lnltlal ShorellneFor the purpose of analysis, it is Figure 2.2 Definition sketch for parameters of convenient and simpier to group a few of Breakwater
the above variables to form dimensionless parameters as follows.
• Diffraction
The wave height distribution behind a structure produced by diffraction to a large extent depends on wavelength L, where the energy of the longer waves penetrate further into the shadow region behind the structure. The lengtb of the structure also controls the amount of
energy reaching the beach. Itis therefore likely that shoreline response is a function of tbe ratio X/L
Waves breaking seaward of the detached breakwater have a greater tendency to develop
salients and tombolos than waves breaking on the landward side.This is because there is a greater width for longshore transport. The location of the breakwater relative to tbe breaker line isconveniently expressed as the ratio Ho /D.
literature Review
The shoreline response does not appear to be very sensitive to changes in sediment partiele sizes between 0.3 to 1.0 mmo However grain sizes from 0.1 to 0.2 mm tends to favour tombolo formation. Three important parameters are therefore X/L, Ho/D, and Kt
• Wave Transmissivity
• Transmission coefficients
Transmission coefficients Kt are used to determine the height of the transmitted wave behind the breakwater. These coefficients K, are influenced partly by the hydraulic condition, and structural parameters of the breakwater . The following parameters are among the most
important ones (Delft Hydraulics Publication 453 (1991)).
*
Crest height,R:
above or below the still water level*
Armour Units size DnSo*
Permeability of the structure*
Slope*
Crest width B*
Wave height H,*
Wave length or periodA combination of the more important variables to produce dimensionless parameters and derivation of a formula for the transmission coefficients was carried out by Van der Meer
(1991). The formula which is based on laboratory tests which both himself and others conducted is shown below.
where:
Kt = transmission coefficient
Re
= freeboard height (m)DnSo = armour unit size (m)
Figure 2.2B Definition of Governing Variables Related toWave Transmission
a = coefficient depending on the relative wave height H,IDnSo
b = coefficient depending on the wave steepness, Sop;relative wave height, H/DnSo; and relative crest width, B I DN50'
The coefficients for a and bare given by a = 0.031 H/DnSo - 0.24
b = -5.42 Sop
+
0.0323 H/ DnSo - 0.0017 (BI DnSo) 1.8+
0.51It is evident from the equation that the K, value will vary with, amongst other variables, wave height and period. The significanee of this is that for anyparticular breakwater design, the Kt is a variable, and therefore has to be determined for each wave condition. The
influence or relative significanee of the various parameters 10 the overall transmission
coefficient is investigated in Chapter 6.0.
Literature Review
2.5.2 Variability of Parameters
Incident waves vary in space and time. Their properties also change as they propagate over the sea bed. The beach itself is composed of sediment particles of various sizes and shapes which move along and across the shore, controlled by laws which are not fully known or understood. Further, the beach and back beach also exhibit different textural properties that vary alongshore and across-shore, and with time. In light of this profound variability of coastal processes, it is clear that a single answer obtained with a deterministic simulation model ( such as that used in this study) must be viewed as a representative result that has been smoothed over a large number of unknowns and highly variabie conditions .
Sensitivity
Sensitivity testing refers to the process of examining changes in the output of a model resulting from intentional changes in the input. If large variations in model predictions are produced by small changes in the input, calculated results will depend greatly on the quality of the verification, and therefore the model cannot be considered to be accurate. The objective is therefore to have a model which is not too sensitive to what may be considered reasonable variability in input parameters.
Sensitivity analyses also serves to give insight into the way in which the natural variability existing in the near-shore system affects the predicted results, and therefore enables better engineering judgement to be made.
In section 6.0 the influence of the various parameters are investigated, to determine the more important parameters which govern the K,values.
literature Review
2.5.3 Advantages and Disadvantages of Using Offshore Breakwaters
Themain advantages ofusing offshore breakwaters forcoastal proteetion may besummarised
as follows:
(1) Off-shore breakwaters are useful for both cross-shore and longshore sediment transport
modes;
(2) They reduce offshore losses by actively blocking sediment travelling as bed load; (3) They can reduce the potential for longshore movement, while simultaneously allowing
regional transport pattems to continue;
(4) They may even be used to foster accretion at beaches, thereby nourishing them.
The main disadvantages are:
(1) The response of the breakwater is difficult to predict accurately with a high degree of confidence :
(2) Itis expensive to construct because marine based equipment often has to be employed. (3)Further, in Holland, the required stones for its construct ion are not locally available and are therefore expensive.
2.6 The Uni best Modelling Suite
The UNIBEST software suite (an acronym for UNIform BEach Sediment Transport ) is a one dimensional software suite consisting of three modules, Unibest_LT; Unibest_CL;
Unibest_TC. This modelling programme was developed by Delft Hydraulics for the simulation and study of longshore and cross-shore sediment transport processes, and related morphodynamics of beach profiles and beach planform shapes. Itis an integrated modelling package with diagnostic and prognostic capabilities. Unibest is suited when tide and wave induced longshore currents and sediment transport predominates.
The surf zone dynamics are derived from a built-in random wave propagation and decay
model (Endec), which transforms offshore wave data to the coast, taking into account the principal processes of linear refraction and nonlinear dissipation by wave breaking and bottom friction. The longshore sediment transport and cross-shore distribution are evaluated according to the various formulae, Engelund Hansen, Van Rijn, Bijker, Bailard and Cerc.
This enables a sensitivity analysis for local conditions to be made, and the most suitable formula selected.
The computational procedure may take into account any predefined wave elimate and tidal regime in order to assess gross and yearly transports, seasonal variations and even storm events.
There are two phases in the Unibest_LT user interface.
(a) The definition phase
(b) The run Phase.
In thedefinition Phase the required input data are stored in an input file with an extension
".SCE" (SCEnario).
Lirerature Review
The required input are: (a)Transport mode
(b) Cross-shore profile and calculation grid
(c) Offshore wave elimate
(d) Specification for interface file for Unibest_CL. This file has an extension ".TSC".
(e) Coefficients for the wave equations
(f)Coefficients for the transport equation
2.6.1 Transport Mode
The options in the transport mode allows one to select any combination of the following
formulae to calculate the sediment transport through the cross-section. (a) Engelund Hansen
(b) Bijker (c) Van Rijn
(d)Bailard
(e) Cerc
A comparison can be made ofthe results and the most suited formula(e) selected for further analyses, including the calculation of transport rates for coastline response analysis.
2.6.2 Cross-shore Profile and Grid
Thecross-shore profile isdefined from the seaward limit to the shore. The seaward limit is
chosen such that it isbeyond the depth of closure. The landward extent should be such that the length of shoreline likely to be eroded is included. See Figure 2.3 for graphs showing
definition of cross-shore profile and grid.
The calculation grid is defined by a grid size and the number of grid sections, n. The grid
sizes do not have to be uniform along the entire length of the cross-sectional profile. They
may be designed such that areas with greater energy decay or steeper contour changes are
given smaller grid sizes. This allows more optimum modelling of the system.
2.6.3 Offshore Wave Climate / Wave Scenario
The wave scenario isdefined byasequence of wave conditions.Awave condition is defined by :
(1) A significant wave height
(2) Water level (3)Peak period
(4)Wave angle at the seaward boundary (5) Tidal flow velocity at a reference depth
(6) Duration in days of the above conditions
A definition sketch for the input and convention used is also shown in Figure 2.3 below
Iiterature Review
Hsig WAVE SCENARIO
T angle
I
...0.0 J-L-\,..+-~~&.I....I-'~""'_--- -7--10.0....
I/) es8
I tidal V \ at depth ] 0.0 5000 lIXXJ.O 1:lOO.o """"' rayx' Coast orientation xFigure 2.3 Definition diagram for input data.
2.6.4 Specification for Interface file
Specifying that an interface file" *.TSC" is required, causes Unibest_LT to generate an input file for Unibest_CL used for carrying out calculations on coastline changes. This " *.TSC " file contains sediment transport rates as a function of coastline angle.
2.6.5 Coefficient for Wave Equations
There are four (4) wave equation coefficients which must be specified, namely Gamma ()'),
Alphaïo), fw' k-value. Gamma and Alpha are wave breaking coefficients. fw is a bottom friction coefficient in the equation for the energy decay. The k-value is used for the bottom friction term in the longshore momentum equation. It is also used initially for the translation of the tidal velocity to the surface slope term of that equation. Recommended default values are as follows ( Unibest Manual, Delft Hydraulics 1994):
Gamma = 0.80 Alpha = 1.00 fw = 0.01 k-value
=
0.10The value may be treated as a calibration coefficient and adjusted slightly during the calibration process.
2.6.6 Coefficient for Transport Equation
The coefficient for transport equations allows one to input the parameters for the selected transport formulae. The coefficients required are as tabulated below. These parameters
describe the physical properties of the transported material and transporting medium (
seawater) along with the bed properties of the coast. The symbols used are as defined before.
literature Review
TableShowingRequired CoefficientIorVaJiousTransport Fonnulae
Required Formula Engelund Hansen Bijker Van Rijn Bailard Cerc
d", v v v' v ~ v v fall velocity v v' v r, v v v' r; v Rho v v v' V Boecr V B" V Viscosity v Alpha v aJh v' Porosity v v' v' Epsilon" v Epsilon" v Tan f v' A(.014 -.025) v'
2.6.7 Output of Unibest LT and Interface with Unibest CL
_
_
The Output of Unibest LT shows the cross-shore distribution of sediment transport rate. This
may be viewed and analyzed as required. An output file "*.TSC" is generated which gives sediment transport rates versus coastline angle, if requested. This is used asan input file for
Unibest CL for calculation of coastline changes over a period of time.
Unibest_Cl is designed to calculate coastline changes due to longshore sediment transport gradients along a nearly uniform coast, on the basis of the single line theory. Various initial and boundary conditions can be introduced to represent a variety of coastal situations. The model is capable of simulating the morphological effects of various coastal engineering
measures such as, headlands, permeable and non-permeable groins, coastal revetments and seawalls, breakwaters, harbour moles, river mouths, training walls, artificial bypass systems
and beach nourishments.
The results of the sediment transport calculations are conveniently expressed in graphs of
sediment transport rate versus coastline orientation, the so-called S-<I> curve. Different S-<I>
curves may be calculated with Unibest_LT along a non-uniform coast and the coefficients defining this S-<I> curve imported in Unibest CL. In this way non-uniform or spatial
variation in wave clirnate and or cross-sectional profile may be taken into account.
The output of Unibest CL shows graphs of the shoreline indicating accretion and erosion areas. Afileisalso generated which tabulatesthe erosionlaccretion rates along the coastline.
Iiterature Review
2.7 Genesis Modelling System 2.7.1 Capabilities and Limitation
GENESIS is an acronym for GENEralised model for SImulating Shoreline change. It is a numerical mode11ingsystem designed to simulate long-term shoreline change produced by
spatial and temporal differences in longshore sand transport. The modelling system is
generalised in that it allows simulation of a wide variety of user-specified offshore wave inputs, initial beach configurations, coastal structures and beach fi11s.The main utility of the mode11ingsystem lies in simulating the response of the shoreline to structures sited in the
nearshore. The model is capable of simulating diffraction and wave transmission by detached breakwater, jetties and groyne. Shoreline movement such as that produced by beach fills and river sediment discharges can also be represented.
The longshore extent of a typically modelled reach may be in the range 1 - 100 km. The time frame may be in the range 1-100 months.
Genesis uses a "Cerc-like" formula for the calculation of sediment transport, where the sediment transport rate is proportional to the breaking wave height and angle. It is also influenced by the gradient in breaker height.
where: Q = sediment transport rate (rrrvs) H = breaking wave height (m) Cg = wave group speed (mis)
Obr= angle of breaking wave (degrees) al and a2 are empirical (calibration) coefficients
X is the distance along the shore The non-dimensional parameters al and a2 are given by :
where k, and k2 are empirical coefficients, treated as calibration parameters. Ps
=
density of sand :::::2650 kg/rn"; P=
density of sea water, 1030 kg/m3;tan {3
=
average bottom slope; p=
porosity of sand, 0.4The value of k,usually lie in the range 0.58 to 0.77 . Thevalue ofk, is typically 0.5 to 1.0 times that of k..
Literature Review
A close examination of the above formula for the sediment transport reveals that it in no way takes into account tidal currents or tidal variation. Hence this formula and consequently the model may onlybe used in situationswhere the tidal currents, and tidal range are considered to be small and have negligible effect on the sediment transport rate.
Shoreline changes such as that produced by cross-shore sediment transport as associated with storm events and seasonal variations in wave elimate cannot be simulated. Such cross-shore processes are assumed to average out over a sufficiently long simulation period.
The programme is however limited in that it cannot take wave reflection at the structure nor tombolo development into account. The most restricting limitation is that there is no direct provision for changing tide levels and taking the influence of tidal currents into account. Genesismay be applied at different time levels, depending on the stage of the project study, amount and quality of data available to operate the modelling system, and level of modelling effort required. These two levels are referred to as the scoping mode and design mode. The scoping mode uses minimal data input needed to characterise the project and might be employed in a reconnaissance study to define the problem better and to identify potential project altematives. A scoping mode application is a schematic study with such simplifications made as initially straight shoreline and idealised wave conditions representing for example, predominant seasonal trends in wave height, direction and period. In the scopingmode, the model is an exploratory tool for obtaining estimates of the relative trend in shoreline change. Tbe design mode is used in feasibility or design studies for which a substantial modelling effort is required.
The objective is to obtain correct shoreline change as well as magnitude and direction of the longshore sand transport rate. The design mode of operation proceed systematically through data collections, model set up,calibration and verification, then to intensive work to evaluate design, finally being used to optimize the final project design.
2.7.2 Input Data
The first technical step in the modelling task is to establish a shoreline coordinate system. The regional trend of the coast is determined from a wide-scale chart, whereas the trend of the local shoreline is determined from a small-scale chart. The regional trend is used to identify the orientation of the offshore contours for wave refraction modelling, whereas shoreline positions, structure configurations, and other project - specific information are referenced to the small-scale chart.
A longshore xaxis is chosen parallel to the trend of the coastline, and a shore-normal yaxis pointing offsbore forms a right-hand coordinate system. See figure 2.4.
The information required may be listed as follows; (a) Shoreline Position
(b)Hydraulic Conditions (waves, water levels etc)
(c) Structure Configurations and other engineering activities such as beach nourishment, sand mining etc.
Literature Review
y
TREND OF OF1'SHORE BOTTOIol CONTOURS
---_. --- --- ---W 0::
s
~ o w o z ~ PROJECT LATERAL ö BOUNDARY --------- ---SHORELINE COORDINATE SHORELINE ~ PROJECT LATERAL BOUNDARY\
~ONGSHORE GRID SPACING I t DISTAN'CE ALÖNGSHORE xFigure 2.4 Definition Sketch For Modelling With Genesis
(d) Beach Profiles
(e) Boundary Conditions .
• Shoreline Position
Shoreline position is referenced to the longshore baseline and refers to the zero depth contour with respect to a certain datum (usuallyMSL). This is similar to the shoreline position used in the Unibest CL module.
• Hydraulic Conditions
• Waves
The wave c1imate input is similar to that required by Unibest. Time series or statistical summaries of offshore wave height, period and direction are required. If the offshore contours are parallel to the trend of the coastline and the extent of the project to be modelled is small, the simple wave transformation routine (intemal model) in Genesis may be used to refract, shoal and diffract waves, thereby transforming the offshore wave data to produce the pattem of breaking waves alongshore. This pattem is used for calculating the longshore sand transport rate. If the offshore contours are irregular, or the project is of wide extent, a specialized wave transformation programme must be used to propagate the waves from offshore to nearshore for use by Genesis.
Literature Review
• Water levels
Genesisdoes not allow direct representation of tidal changes. However, changes in breaking waves as caused by variations in water levels can be represented in the wave input.
• Engineering structures and activities
Structures and other engineering activities, such as placement of beach fills, must be correctly located on the grid both in time and space. Genesis allows representation of changes in structures through time as, for example, extensions of a breakwater, construction of a groyne field during the simulation interval,or multiple placement of beach fill. Therefore in data collection and project planning, the location configuration, and times (and volumes in the case of beach fills, dredging and sand mining) must be assembied.
• Bathymetry and Profiles
Bathymetric measurements of the cross-shore profiles are required for wave refraction determination. Bathymetric and Profile data are also used to establish a general sediment budget, to identify local areas of deposition and erosion and to qualitatively estimate and distinguish cross-shore and longshore effects at structures in some situations.
• Boundary Conditions
Boundary conditions must be specified at the two lateral ends of the numerical grid. Boundary conditions determine the rate at which sand may enter and leave the modelIed area, and may have a profound effect on shoreline change. Genesis allows representation of two general boundary conditions. If the position of the shoreline can be assumed to be stationary, this condition defines a pinned beach. A pinned beach boundary is appropriate if the sediment budget is balanced at the boundary segments of the beach. A pinned beach boundary may also be imposed if the beach is constrained (e.g. by a rocky cliff or seawalI), but sediment can still move alongshore and past the boundary area. A gated boundary condition describes the case of some preferential gain or loss of sand at the boundary, in other words, the boundary influences the transport rate.
2.8 Comparison of Modelling Systems
Both Unibest and Genesis are one dimensional modelling systems, therefore a direct comparison can be made of these two packages.
The most significant advantages of Unibest as compared to Genesis are the fact that : (a) Unibest uses up to five different formulae for the sediment transport calculations and analysis, whereas Genesis onlyuses one formula.
(b) Unibest is capable of directly considering the effect of a tidal range and tidal currents whereas this cannot be done directly with Genesis.
Literature Review
One further advantage of Unibest is due to the method used to define the cross-sectional profile. Unlike Genesis, in Unibest the grid sizes does not have to be uniform, and may therefore be smaller in sensitive areas such as the surf zone. This allows optimum utilization and a more accurate calculation to be made.
Genesis in contrast, is better capable of taking into account (directly) the influence of the difference in cross-shore profile with longshore distance. Additionally since the equations used for the sediment transport calculations is simpIer, the computational time is less for Genesis. A third advantage of Genesis is that it can take diffraction effects into account more directly than Unibest.
As a conclusion, Unibest is the more suitable of the two modelling systems, for this study where tidal influences is one of the key factors.
Boundarv Conditions
3.0 Boundar
y
Conditions
3.1 Wave Climate
Appendix A ( separate document) shows the deep water wave elimate for three observation
(measuring) stations located near the three sites of interest. These stations, Eierland,
IJmuiden and Platform Euro 0are located offshore of Callantsoog (Ca), Scheveningen (Sc)
and Domburg (Do) and are indicated by E, I, and 0 respectively. See Figure 3.1
An inspeetion of the wave
elimate at these three stations reveal that the wave elimate are
similar , and that the
predominant wave directions
are from south-west, west,
north-west, and north. These
directions account for over 85
% of the total waves. A more
in depth analysis reveals that the predominant wave heights
generally varies from 0.5 m to
2.5 m. These wave heights
account for more than 80% of
the waves. A summary of the
most important wave data
applicable for the coastal
orientation under consideration is presented in Tables 3.1A-D.
This data has been used for the analyses presented herein.
3.2 Sedimentology HOLLAND COAST I
•
o
•
~'\The sediment partiele sizes at
most of the beaches along the
coast do not vary significantly.
The nominal partiele size dnSo
generally ranges between 250
and 350 microns. This small
variation has been shown to
have some influence in
sediment transport, when waves
are the main agent for the transport. For the analyses a
dro
o
of both 200 and 300 microns areused. These values were selected since they represent more or less the average partiele size
range in the vicinity of Scheveningen.
[HE MSc. Thesis o
,
legend: lhorellne relreat ~ o·2mlytar ~ 2 • 04mIm year-I
>4mlyear 15 '" Iun !Figure 3.1 Dutchcoastshowingoff-shore measuring stations and
erosion/accretion rates.
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3.3 Tidal Levels
The tidal amplitudes vary along the coast. Generally the tidal range increases form south to north. Reported values range from l.1 to 2.6 m during neap tide and l.5 to 3.9 m during spring tide. The average tidal variation with time for the central section near Scheveningen is shown in Figure 3.2. This variation is considered representative of the areas of concern,
and is therefore used in the schematization of the water levels in the analysis. Comparing the above values show that there is considerable variation of the amplitude not only with position (along the coast), but also with time (neap tide / spring tide).
3.4 Tidal Currents
Tidal currents vary not only with longshore distance but also with depth of the water. The variation with water depth is given by the Chezy equation, U= cV(hi); where U is the tidal velocity, h the water depth, and i is the longshore gradient and C the Chezy coefficient. The tidal currents were obtained from the Stroomatlas for the Scheveningen area. See Appendix A. The tidal currents vary of course with the magnitude of the tidal difference. The values shown are depth average values at a depth of approximately 9 m. A plot showing the variation with time is shown in Figure 3.2. These represent the average tidal currents during spring and neap tide.
3.5 Cross-shore Profiles
The variations of Cross-shore Profiles both with time and longshore distance were obtained from the Jarkus Profiles. A plot of a few of these profiles is shown in Fig 3.3. A close inspeetion reveals that the profiles are not constant, but vary longitudinally and chronologically. Further description of the longshore profiles is given in Chapter 5.
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The Dutch Coastal System And Sediment Transport Regime
4.0 The Dutch Coastal System And Sediment Transport Regime
4.1 General
The Dutch coast consists of approximately 350km of coastline along the North sea. It plays a particularly important role in coastal defence since approximately half of the country is lower than sea level. It is generally a dynamic coast, characterised by alternating coastal
stretches of accreting or eroding areas.
The coastline may be further divided into three segments based on coastal morphology and boundary conditions . These subsegments are:
(1) The Delta coast in the south, consisting (of former delta and islands).
(2) The central coastal stretch between Hoek van Holland and Den Helder, which is uninterrupted by tidal inlets.
(3) The Wadden Islands coast, consisting of a series of coastal barriers islands with tidal inlets in between.
The coastal sediment transport scheme along the Dutch coast has been investigated by many.
Some of the more recent studieshave been done by Roelvink and Stive (1989); Stroo and Van de Graaff (1991); and Van Rijn(1994). These investigators have used a number of methods,
including various mathematical modeIs, sediment budgets models, together with various assumptions. The transport rates are usually calibrated using measured coastline changes.
The longshore transport rates derived from Van Rijn's hindeast study (1994) are in good agreement with the computed transport rates (based on models ) in the central section (chainages 70- 95 km around Scheveningen) sufficiently far away from the influence of the dams. In the central section north of the harbour dams of IJmuiden the hindcasted transport rates are not in such good agreement.
According to Van Rijn, the net longshore transport rate increases from zero near Hoek Van Holland to about 500,000 m'/year near IJmuiden. North of IJmuiden the net transport rate is directed southwards in the section from 35 to 55km and north again in section 0 to 25 km. The sediment transport rate is zero near Hoek van Holland because of the harbour dams which extend beyond the surf zone (-8 contour).
The net southward longshore transport north of IJmuiden is related to the presence of the harbour dam reducing the wave energy coming from southwest directions. Sirnilarly, the area south of the dams is sheltered from waves arriving from north- west directions.
A summary of the transport rates along different sections of the coast is presented in Table
4.1.
The Dutch Coastal System And Sediment Transport Regime
Cross-shore Coastline Steepness Yearly-averaged longshore transport integratedoverthe surf profile orientation ofprofile* zoneupto -8mNAP(m3/year, excluding pores)
(0) (-)
North (positive) South (negative) Net + or - error
14 18" 0.008 680000 -340000 340000(+ ,- 50%) 28 7" 0.008 580000 -290000 290000(+,-100%) 40 8° 0.008 360000 -180000 180000(+ ,-100%) 47 10° 0.006 300000 -150000 150000(+,- 75%) 68
z
i
-
0.005 300000 -150000 150000(+,-50%) 76 26° 0.005 300000 -150000 150000(+,-50%) 92 37· 0.006 380000 -190000 190000(+,- 50%) 103 40° 0.008 720000 -360000 360000(+ ,-50%) 108 41° 0.012 760000 -380000 380000(+,- 50%)·Prollie stee nep ssISslo e op r line throu h -g 1 mand-8 m NAPcontours
Table 4.1Sediment Transport Rates along DutchCoast
