Influence of Offshore Breakwater on coastal sediment transport rates

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 this method 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 interdependenee. 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 of sections of the Dutch 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 Unibest was used. The results obtainedfrom this model were compared to those obtained by using general design rulestools for emerged breakwaters, developed inprevious 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 rate witb and without 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 last few decades. From the analyses carried out herein it may be concluded that the use of offshore breakwaters for coastal proteetion appears to be technicaliy feasible. Moreover, various coastline responses are possible, depending on the design parameters of the structure. The most important parameters affecting the breakwaters 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 (inparticular tidal levels) along the Dutch coast, and the great influence of this parameter on the whole analysis, the deductions made from this study are only applicable to sites where these boundary conditions are similar

A special thanks also to my family and friends both here and at home for their support.

A heart warming thank you to all

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

Offshore breakwaters have been used in man y coastal areas for coastal proteetion w ith greater or lesser success Howe er to date these have not been used along the Dutch coast for this purpose This is probabl y due to the absence of accurate design tools for their funct onal design, coupled with the uncertainty of whether this method of coastal proteetion

is economical, as compared to currently used methods such as beach nourishment Additionall y, 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-dependen e 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 infinitel y long offshore breakwaters for proteetion of sections of the D utch 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 likel y to cause :

(a) Negligible effect on the coast ie limited shoreline response

(b) Formation of salients;

(c) Formation of Tombolos;

In anal yz ing the likel y response to the wave climate, the (one line model) programme Unibest was used The results obtained from this model were compared to those obtained b y 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 rate witb and without the influence of the breakwater Ha v ing 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 last fe w decades.

From the analyses carried out he r ein it may be concluded that the use of offshore breakwaters for coastal proteetion appears to be technicaliy feasible Moreover, various coastline responses are possible, depending on the design parameters of the structure The most important parameters affecting the 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) alo g the Dutch coast and the great influence of this parameter on the w hole anal y sis, the deductions made from this study are only applicable to sites where these boundary condi ions are similar.

Literatur e Re v iew

• 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 distinctionbetween 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 fullforce of the waves Bulkheads are next in size; their primary function is to retain fill andwhile 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 aredesigned 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

6.0 Modelling Using Unibest and

Sediment Transport CaIculations 37-75

6 1 Introduetion

6.2 Cross-shore Profiles and Calculation Grid

6.3 Wave Climate

6.4 Analysis , Calculations and Results

7.0 Discussion, Conclusions and Recommendations 76-88

Appendices (Separate document)

isco trolled 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 responsesare non-linear and have a high variability in space and time Although it is difficult and achallenging 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 withnumerical models has proven to be a powerful technique to assistin the selection of the mostappropriate desig Models provide a framework for developing problem statements andsolution formulatio , for organizingthe collection and analysisof data, and, importantly, forefficiently evaluating alternative desig s 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 theNetherlands the former is well known and applied regularly The second method, OffshoreBreakwaters, has not yet been applied However, with the increasing cost of beachnourishment 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 investigatethe effeets of offshore breakwaters on coastal sediment transport and erosionl accretion ofthe shoreline

This is one study among others, whieh is geared at gaining insight into the influence ofoffshore breakwaters on the coastal morphology, in particular, the influence of the geometrieparameters 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, wasalso investigated

• Offshore Breakwaters

Offshore breakwaters are coastal structures, usually shore-parallel, located at a certaindistance from the shoreline These structures are used to dissipate wave energy (b one ormore methods ) and change the wave and flow patterns The main purposes are: (1 ) toproteet beaches or even widen certain stretches of the eoast; (2) to pro ide a tranquil

environment on the lee side of the structure for harbour protection

The Dutch Coastal System And Sediment Transport Regime

Since the seventeenth century the central coast of the Netherlands, between Den Helder inthe North and Hoek van Holland in the south behaves as a closed coastal system in astronginteraction with the barrier islands coast in the north and the interrupted Delta coast in thesouth For hundred of years the northem ( north of Egmond) and southem (south ofScheveningen) sections of the central coastal system have been suffering from structuralerosion This continuous structural erosion is partly due to the sediment-importing capacity

of the neighbouring coastal systems During the Period 1600 to 1800 the retreat of thecoastline in the eroding sections was of the order 3-5 m/year, caused by the flood and ebbcurrents near the tidal inlets in the south and in the north, and intensified by the stirringaction of shoaling and breaking waves

4.4 History Of Implemented Coastal Proteetion

From 1800 onwards the coastline was more actively defended by building groynes andseawalls The number of groynes was gradually increased and the length of the groynes wasextended to about 350 m, almost up to the -4 m N.A.P contour at some locations Longharbour dams normal to the shore were constructed around 1870 near Hoek van Holland andIJmuiden to ensure a safe approach of larger vessels to the harbours of Rotterdam andAmsterdam respectively As a result of these man-made structures, the retreat of the coastlinewas considerably reduced to about 0.5 to 1.5 m/year

Around 1910 some negative effects related to the construction of long groynes and harbourdams were first realized These were the erosion and associated profile steepening in thedeeper surf zone and shoreface zone, because of wave and tide induced longshore currentswhich were forced to flow around the structure at high velocities

Since 1960 beach nourishment has become a keystone of coastal defence to further reduce theretreat of the coastline In 1990 a historical decision was made to maintain the coastline

position as at that date by all means Since implementation of this policy a program ofmassive and continuous beach nourishment has been initiated to compensate for the loss ofbeach and dune sediments caused by natural erosion processes

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 generaIaim may however be summarized as follows:

(a)Todefine a simple basic caseof an infinitelylong offshore breakwater ( to realistic cases)for the ensuing analysis;

(b)Toin estigate the effect of an infinitely long offshore breakwater on longshore transportand coastline behaviour as a function of several hydraulic and geometrie parameters by usingUnibest, and carrying out a large numbersof runs, with varying wave height, wave directio ,

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 andlocation of breaker bar), the breakwater position and its geometrie parameters (such asheight) on the performance of the breakwater as discussed above;

(d) To discuss the results of the one-line model and the benefits /complications of Genesisfor such analyses

Itis emphasized that the objective of the study is not to reproduce the exact transport ratesalong 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 coastalmorphology would be influenced bythe construction of an offshore breakwater, and furtherhow these structural parameters and boundary conditions influence these results

It ispointed out that the results, conclusions and recommendations arrived at in this studyare based on the assumption that Unibest is accurate in determining the sediment transportrates under different boundary conditions Unibest is also assumed to determine themorphologic changes accurately Verification of the model itself (with respect to thecomputations 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 withpresently developed theory, and observations

IHE M S c Th esis 3 R E v eron Za c hariah

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 theanalyses of the coastal morphological changes is discussed The capabilities and limitationsare 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 anoffshore 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-mentionedprocesses result in changes in wavelength, height, and direction As the waves approach evenshallower water, they become unstable and are forced to break, releasing some of theirenergies 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 involvescomplex 3D circulating patterns of various spatial and time scales (SPM 1984)

If the tidal range is large the wave propagation ( refraction and breaking ) will varysignificantly according to the water level If this is the case, refraction simulations withdifferent water levels may be necessary Water levels also play a major role in waveovertopping and transmission through breakwaters, sediment overtopping and bypassingshoreward and seaward of groynes

A number of formulae have been developed to estimate the sediment transport rate in coastalareas 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 bediscussed.(See section 6.4.2 on "Selection of Transport Formula") Only these two formulaeare discussed since the CERC formulae is use by Genesis exclusively, and for Unibest, theBijker formulae gives the best results for the situation being studied

Mod e lling Us ing U n i b es t and S e dim e nt Tran s port Calculation s

Influence of Breakwater Length

Besidesthe limited influence due todiffraction effectsat the ends, the breakwater length and

distance from sh re, together with the incident wave directions, directly detennine which

waves will enter undisturbed intothe surf zone behind the structure This isdemonstrated in

Figure 6.1 where different zones are depicted, showing areas where waves from different

directions reach the shoreline (in fact the centre of the region where most of the transport is

believed to occur) The regions where all the waves are not reduced may be considered atransition zone leading to the central area where they are all reduced ( as would occur with

an infinitely long structure) The breakwaters considered are initially located at theapproximate beginning of the surf zone ( in water 6.8 m), and has a lengths of 500, 1000,

2000 and 4000 m

An infinitelylo gbreakwater would reduce the waves from all directions As the breakwaterlength gets sh rter though, for particular locations within the surf zonet shoreline, there are

wave directions which are not reduced bythe breakwater

It is quite clear then that to model the effectsof a breakwater of finite length, it is necessary

to take the aforementioned factors intoconsideration This necessitates carrying out analyses

to reflect the fa t that only the wave of particular directions corresponding with a particular

location are actually reduced Such analyses may be carried out either by carrying outentirelynew Unibest-Lt runs with thevarious wave elimate for each breakwater height being

considered, or b using the utility program SHOWTS and applying a transport reductionfactor tothe waves which are actually reduced The latter method was selected based on thefact that carrying out entirely new Unibest runs for all scenarios would be extremely time

consuming, and is unlikely to give better results than the second method The results as

obtained using the SHOWTS programme are consistent to those obtained using completely

new Unibest Lt runs for the limited scenarios checked

The transp rt coefficients for the various scenarios are presented in Tables 6.4A and B, forbreakwaters oflength 1000 mand 2000 m This maybe used together with Figure 6.1A and

B to determine the transport rates in the different zones in the lee of the structure

• Morphological Response to Various Breakwater Heights

Unibest_Cl module was used to model the coastline change of an assumed initially straightcoast Analyses were carried out for various breakwater heights for a period of 50 years A

50 year period was selected since, in most cases, equilibrium appears to be reached by this

time Additionally, 50 years is a reasonable planning period and lifetime for the structure.The coastline responses are shown in Figures 6.2 to 6.10 A discussion of these results ispresented in Chapter 7.0

Literatur e Review

where :

S b = bottom sediment transport

Ss = suspended sediment transport

d 50 = median (50 %) grain diameter (m)

u, = orbital velocit y near the bottom (m i s)

w = wave frequenc y (rads i s)

C = Chezy coefficient = 18 log(12dlr e )

C 90 = 18 log(12d / ~ o )

f.L = (C I C 90 ) 3 / 2

fw = exp[-5.977 + 5 213(U b/ wr e t OI94

B = coefficient which varies between 1 and 5

1 1 and 1 2 = 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 capacit y If the sediment transport rate decreases along the coast , accretion occurs , where as erosion result s

in the beach plan shape and volume as the beach erodes and accretes This contour is usuallytaken as the readily observed shoreline, the zero depth contour The models are thereforetermed the "Shoreline change" or "Shoreline response" model Sometimes the terminology

"One-line" model, a shortening of the phrase "One contour line" model is used withreference 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 Theshoreline 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 andthe x-axis is oriented parallel to the trend of the coast (Figure 2.1) The quantity y thusdenotes sh reline position, and x denotes distance alongshore It is assumed that the beachprofile 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 inshoreline position is ~y, the length of the shoreline segment is ~x, and the profile moveswithin avertical extent defined by the berm elevation DB and the closure depth DC, bothmeasured 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 Anothercontribution can arise from a linesouree or sink of sand q = qs + qo , which

adds 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 therate of change of shoreline position:

o /ot + lI(DB + DC) [ oQ/o - 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

2.4 Types of Coastal Proteetion

each with its own advantages and disadvantages The following are among the most commonly used.

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 t o bl o ck a designed percentage of the l o g- 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 maj o r 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.

Gro y nes do not appreciabl y 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 h y draulicall y , using a s y stem of pumps and pipelines to transfer the liquified sand from the sou ree

Literatur e Re v iew

• 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 distinctionbetween 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 fullforce of the waves Bulkheads are next in size; their primary function is to retain fill andwhile 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 aredesigned 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 assegmented or non-segmented

Unlike shore perpendicular structures they may be designed to allow variabie amounts ofmateriaI to pass, thereby affecting the amount of sediment that is available for littoralprocesses 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 detachedbreakwater reveals tbat at least 14 parameters are of importance (Hanson and Kraus) Thebeach response is a function of the breakwater properties, the beach properties, and tbe waveclimate Or stated more explicitly in symbols:

Beach response = F(X,Y,Kr,G,) , (D,.1D,S) , (H, T, 0, O s , aH ' a e , ar)

These symbols are defined below and in Figure 2.2

° = incident wave angle (degrees)

aH = standard deviation of H; etc

+-G y

"' s

l nl tl a l S h ore ll ne

For 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 extentdepends on wavelength L, where the energy of the longer waves penetrate further into theshadow 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 tberatio X/L

Waves breaking seaward of the detached breakwater have a greater tendency to develo

salients and tombolos than waves breaking on the landward side.This is because there is agreater width for longshore transport The location of the breakwater relative to tbe breakerline isco veniently expressed as the ratio Ho /D

IHE M S c Th esis 11 R E v eron Za c hariah

lit e ra tu re Re v ie w

The shoreline resp nse 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 , H o /D , and Kt

• Wave Transmissivity

• Transmission coefficients

Transmission coefficients Kt are used to determine the height of the transmitted wave behindthe breakwater These coefficients K, are influenced partly by the hydraulic condition, andstructural 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 DnS o

* Permeabiiy of the structure

DnSo = armour unit size (m)

a = coefficient depending on the relative wave height H,I D nSo

b = coefficient depending on the wave steepness, Sop;relative wave height, H/DnS o ; andrelative crest width, B I DN50'

The coefficients for a and bare given by

a = 0.031 H/DnS o - 0.24

b = -5.42 Sop+ 0.0323 H/ DnS o - 0.0017 ( B I DnS o ) 1.8 + 0.51

It 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 desig ,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.

Literatur e Review

2.5.2 Variability of Parameters

Incident waves vary in space and time Their properties also change as they propagate overthe sea bed The beach itself is composed of sediment particles of various sizes and shapeswhich move along and across the shore, controlled by laws which are not fully known orunderstood Further, the beach and back beach also exhibit different textural properties thatvary alongshore and across-shore, and with time In light of this profound variability ofcoastal processes, it is clear that a single answer obtained with a deterministic simulationmodel ( such as that used in this study) must be viewed as a representative result that hasbeen 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 modelresulting from intentional changes in the input If large variations in model predictions areproduced by small changes in the input, calculated results will depend greatly on the quality

of the verificatio , and therefore the model cannot be considered to be accurate Theobjective is therefore to have a model which is not too sensitive to what may be consideredreasonable variability in input parameters

Sensitivity analyses also serves to give insight into the way in which the natural variabilityexisting in the near-shore system affects the predicted results, and therefore enables betterengineering judgement to be made

In section 6.0 the influence of the various parameters are investigated, to determine the moreimportant parameters which govern the K,values

lit e ratur e R ev i ew

2.5.3 Advantages and Disadvantages of Using Offshore Breakwaters

Themain advantages ofusing offshore breakwaters forcoastal prote tion may besummarised

(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 ofconfidence :

(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 andare therefore expensive

2.6 The Uni best Modelling Suite

The UNIBEST software suite (an acronym for UNIform BEach Sediment Transport ) is aone dimensional software suite consisting of three modules, Unibest_LT; Unibest_CL;Unibest_TC This modelling programme was developed by Delft Hydraulics for thesimulation and study of longshore and cross-shore sediment transport processes, and relatedmorphodynamics of beach profiles and beach planform shapes Itis an integrated modellingpackage with diagnostic and prognostic capabilities Unibest is suited when tide and waveinduced longshore currents and sediment transport predominates

The surf zone dynamics are derived from a built-in random wave propagation and decaymodel (Endec), which transforms offshore wave data to the coast, taking into account theprincipal processes of linear refraction and nonlinear dissipation by wave breaking andbottom friction The longshore sediment transport and cross-shore distribution are evaluatedaccording 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 suitableformula selected

The computational procedure may take into account any predefined wave elimate and tidalregime in order to assess gross and yearly transports, seasonal variations and even stormevents

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)

[HE M S c Th esis 14 R E v eron Zachariah

Lir e ratur e R eview

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

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 thatthe 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 d 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

Figure 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 aninput 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 ()'),

Alphạo), fw' k-value Gamma and Alpha are wave breaking coefficients fw is a bottomfriction coefficient in the equation for the energy decay The k-value is used for the bottomfriction 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 valuesare as follows ( Unibest Manual, Delft Hydraulics 1994):

2.6.6 Coefficient for Transport Equation

The coefficient for transport equations allows one to input the parameters for the selectedtransport 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

lit e rature Re v ie w

Tabl e Sho w g R equired Coe fficient Io r V aJiou s Tr ans port Fonnula e

R equi r ed F orm u l a E n ge lund H ansen B ijker Van R ij B a ilard Cerc

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 givessediment transport rates versus coastline angle, if requested This is used asan input file for

Unibest CL for calculatio of coastline changes o er 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 initialand boundary conditio s 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 andseawalls, 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 transp rt rate versus coastline orientation, the so-called S- <I> curve Different S - <I>

curves may be calculated with Unibest_LT along a no -uniform coast and the coef icients

defining this S- <I> curve imported in Unibest CL In this way n n-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 a cretion and erosion

areas Afileisalso generated which tabulatesthe erosionla cretion rates alo g the coastline

Iiteratur e 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 anumerical 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 waveinputs, initial beach configurations, coastal structures and beach fi11s.The main utility of themode11ingsystem 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 detachedbreakwater, jetties and groyne Shoreline movement such as that produced by beach fills andriver 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 thesediment transport rate is proportional to the breaking wave height and angle It is alsoinfluenced by the gradient in breaker height

where: Q = sediment transport rate (rrr v s)

H = breaking wave height (m)

Cg = wave group speed (m i s)

Obr= angle of breaking wave (degrees)

al and a2 are empirical (calibration) coefficients

X is the distance along the shoreThe non-dimensional parameters al and a2 are given by :

where k, and k2 are empirical coefficients, treated as calibration parameters

P s = density of sand :::::2650 kg/rn"; P= density of sea water, 1030 kg / m3 ;

tan {3 = average bottom slope; p = porosity of sand, 0.4

The value of k,usually lie in the range 0.58 to 0.77 Thevalue ofk , is typically 0.5 to 1.0times that of k

Literature Review

A close examination of the above formula for the sediment transport reveals that it in no waytakes into account tidal currents or tidal variation Hence this formula and consequently themodel 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 withstorm events and seasonal variations in wave elimate cannot be simulated Such cross-shoreprocesses 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 nortombolo development into account The most restricting limitation is that there is no directprovision 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 modellingeffort 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 beemployed in a reconnaissance study to define the problem better and to identify potentialproject altematives A scoping mode application is a schematic study with suchsimplifications made as initially straight shoreline and idealised wave conditions representingfor example, predominant seasonal trends in wave height, direction and period In thescopingmo e, 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 asubstantial modelling effort is required

The objective is to obtain correct shoreline change as well as magnitude and direction of thelongshore sand transport rate The design mode of operation proceed systematically throughdata collections, model set up,calibration and verification, then to intensive work to evaluatedesign, 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 ofthe local shoreline is determined from a small-scale chart The regional trend is used toidentify the orientation of the offshore contours for wave refraction modelling, whereasshoreline positions, structure configurations, and other project - specific information arereferenced to the small-scale chart

A longshore xaxis is chosen parallel to the trend of the coastline, and a shore-normal yaxispointing 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 engineerin activities

such as beach nourishment, sand mining etc

[HE MS c Th esis 19 R E ve ron Zachariah

~

PROJECT LATERAL BOUNDARY

is small, the simple wave transformation routine (intemal model) in Genesis may be used torefract, shoal and diffract waves, thereby transforming the offshore wave data to produce thepattem of breaking waves alongshore This pattem is used for calculating the longshore sandtransport rate If the offshore contours are irregular, or the project is of wide extent, aspecialized wave transformation programme must be used to propagate the waves fromoffshore 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 becorrectly 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 agroyne field during the simulation interval,or multiple placement of beach fill Therefore indata collection and project planning, the location configuration, and times (and volumes inthe 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 refractiondetermination Bathymetric and Profile data are also used to establish a general sedimentbudget, to identify local areas of deposition and erosion and to qualitatively estimate anddistinguish 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 twogeneral 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 sedimentbudget is balanced at the boundary segments of the beach A pinned beach boundary mayalso be imposed if the beach is constrained (e.g by a rocky cliff or seawalI), but sedimentcan still move alongshore and past the boundary area A gated boundary condition describesthe case of some preferential gain or loss of sand at the boundary, in other words, theboundary 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 andanalysis, whereas Genesis onlyuses one formula

(b) Unibest is capable of directly co sidering 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-sectionalprofile Unlike Genesis, in Unibest the grid sizes does not have to be uniform, and maytherefore be smaller in sensitive areas such as the surf zone This allows optimum utilizationand a more accurate calculation to be made

Genesis in contrast, is better capable of taking into account (directly) the influence of thedifference in cross-shore profile with longshore distance Additionally since the equationsused for the sediment transport calculations is simpIer, the computational time is less forGenesis A third advantage of Genesis is that it can take diffraction effects into account moredirectly than Unibest

As a conclusion, Unibest is the more suitable of the two modelling systems, for this studywhere tidal influences is one of the key factors

Boundarv C ond itions

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

The sediment partiele sizes at

most of the beaches along the

coast do not vary significantly

The nominal partiele size dnS o

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 adrooof both 200 and 300 microns areused These values were selected since they represent more or less the average partiele sizerange in the vicinity of Scheveningen

Figure 3.1 Dutch co a s t s ho wi n off-shore mea s uring s t a ti ons a nd

e ro si on / accr e tion rat es

23 R Eve r o Zac ha ria h

E-Boundarv Condition s

3.3 Tidal Levels

The tidal amplitudes vary along the coast Generally the tidal range increases form south tonorth Reported values range from l.1 to 2.6 m during neap tide and l.5 to 3.9 m duringspring 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 theabove 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 Thevariation with water depth is given by the Chezy equation, U= cV(hi); where U is the tidalvelocity, 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 SeeAppendix A The tidal currents vary of course with the magnitude of the tidal difference Thevalues shown are depth average values at a depth of approximately 9 m A plot showing thevariation with time is shown in Figure 3.2 These represent the average tidal currents duringspring and neap tide

3.5 Cross-shore Profiles

The variations of Cross-shore Profiles both with time and longshore distance were obtainedfrom the Jarkus Profiles A plot of a few of these profiles is shown in Fig 3.3 A closeinspeetion reveals that the profiles are not constant, but vary longitudinally andchronologically Further description of the longshore profiles is given in Chapter 5

, , oLO

LOo

•

C\J

(Y) Q) '-

, - ~ - - ~ C)

0')

,-,' , - ,

C) C)

C) .c::

~ Cl)

•

(I) (I)

e

C) U

C) t")

G

- - e ii

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 islower 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 andboundary 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 isuninterrupted by tidal inlets

(3) The Wadden Islands coast, consisting of a series of coastal barriers islands with tidalinlets 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 variousassumptions The transport rates are usually calibrated using measured coastline changes

The longshore transport rates derived from Van Rijn's hindeast study (1994) are in goodagreement 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 thedams In the central section north of the harbour dams of IJmuiden the hindcasted transportrates are not in such good agreement

According to Van Rijn, the net longshore transport rate increases from zero near Hoek VanHolland 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 damswhich extend beyond the surf zone (-8 contour)

The net southward longshore transport north of IJmuiden is related to the presence of theharbour dam reducing the wave energy coming from southwest directions Sirnilarly, the areasouth 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.

IHE MSc The s is 29 R Everon Zachariah

Th e Dut c h C oa st al Sy s t e m And Se d i ment Transport Regim e

C ro ss -s h re Co a stline S teepness Y e arl y - ave ra g ed l ongs h ore tra n spo rt in tegrated o ve r the surf profile orientation of pr ofil e * zo n e u t o -8 m N AP ( m 3 / yea r , excl u ding pores )

The Dutch Coastal S vs tem And Sediment Transport Regime

4.2 Sea Defences& Coastal Proteetion

Various types of sea defences exist along the entire coastline Approximately 252 kmconsist

of dunes, 34 km of dikes, 27 km of other constructions (boulevards, sluices, etc.) andremaining 38 km of beach plains located at the extreme ends of the Barrier Islands of theWadden Coast The latter plains do not proteet polderland but are mainly nature areas Inaddition to the above types of sea defences , a number of artificial coastal proteetion works

have been designed and constructed to combat coastal erosion The latter includesconstruction of groynes and periodic beach nourishments

Certain sections of the coastline are of greater interest to coastal managers / engineers thanothers This is due either directly to the value of the coastal area being considered, or to thevulnerability of the coastal zone (and hence the hinterland being protected ) to inundation.Three areas of the Central coastal stretch of particular interest in this study are:

4.3 Coastal Erosion

Coastal eros ion may be classified as follows

(a) Storm Erosion

(b) Chronic or Structural Erosion

Storm erosion is a fairly sudden local event It generally involves the cross-shore movement

of sediment from the dune foot and the foreshore into deeper water during storms It may bereversible depending on the bathymetry and morphology of the area

Structural erosion is more a gradual long-term phenomenon, which results from a gradient

in the longshore sediment transport rate Both modes of erosion may interact

Ifan area is an accreting coastal stretch, it may initially show serious damage due to stormerosion, but this would be followed by gradual restoration of the original dune / beach profilewith time However if the coast suffers from structural erosion, storm erosion will result instructural loss of sediment, because the dune / beach sand will (partly) be used to replenishthe sand deficit of the shoreface

The Dutch Coastal System And Sediment Transport Regime

Since the seventeenth century the central coast of the Netherlands, between Den Helder inthe North and Hoek van Holland in the south behaves as a closed coastal system in astronginteraction with the barrier islands coast in the north and the interrupted Delta coast in thesouth For hundred of years the northem ( north of Egmond) and southem (south ofScheveningen) sections of the central coastal system have been suffering from structuralerosion This continuous structural erosion is partly due to the sediment-importing capacity

of the neighbouring coastal systems During the Period 1600 to 1800 the retreat of thecoastline in the eroding sections was of the order 3-5 m/year, caused by the flood and ebbcurrents near the tidal inlets in the south and in the north, and intensified by the stirringaction of shoaling and breaking waves

4.4 History Of Implemented Coastal Proteetion

From 1800 onwards the coastline was more actively defended by building groynes andseawalls The number of groynes was gradually increased and the length of the groynes wasextended to about 350 m, almost up to the -4 m N.A.P contour at some locations Longharbour dams normal to the shore were constructed around 1870 near Hoek van Holland andIJmuiden to ensure a safe approach of larger vessels to the harbours of Rotterdam andAmsterdam respectively As a result of these man-made structures, the retreat of the coastlinewas considerably reduced to about 0.5 to 1.5 m/year

Around 1910 some negative effects related to the construction of long groynes and harbourdams were first realized These were the erosion and associated profile steepening in thedeeper surf zone and shoreface zone, because of wave and tide induced longshore currentswhich were forced to flow around the structure at high velocities

Since 1960 beach nourishment has become a keystone of coastal defence to further reduce theretreat of the coastline In 1990 a historical decision was made to maintain the coastline

position as at that date by all means Since implementation of this policy a program ofmassive and continuous beach nourishment has been initiated to compensate for the loss ofbeach and dune sediments caused by natural erosion processes

Preliminarv Functional De s ign

5.0 Preliminary Functional Design

5.1 Introduetion

The morphological response of a coast to an offshore breakwater is a function of energy

transmitted to the lee side of the breakwater This in turn is determined bythe relative length

of the breakwater, its permeability, its height above or below water, armour unit size DnSo,

distance from the shore, wave elimate and to a lesser extent the slope and width of thestructure The functional requirements stated below in section 5.2 will govern the preliminaryfunctional design which is presented herein lnitially the intention is to investigate aninfinitely long structure, then to reduce the length to realistic cases, to gain insight into theinfluence on the functional performance

An infinitely long structure would necessitate that it would have to be submerged orsufficiently porous to allow transmission of sufficient wave energy to prevent excessiveaccretion

5.2 Functional Requirements

The intention of the proposed works, as stated before, is to maintain the Dutch coastline atits 1990 position, with the use of no or as little as possible other forms of additional coastalproteetion (such as beach nourishment) This implies that the functional response required islimited salient formations with minimum sinuosity Hence the preliminary design for theensuing analysis will be governed by this fact Some general requirements/ design boundaryconditions are listed below

• The length of shoreline to be protected is generally of the order 2000 m

• The design beach width is of the order 50 m

• Very little adjacent beach erosion is tolerable

Case to be considered

Since the requirement is to determine the applicability of an infinitely long breakwater forcoastal protection, diffraction which is an edge effect is not of great concern This is justifiedsince diffraction will only occur at the ends of the structure and therefore will only affect ashort section of the shoreline near the outer limits of the structure

Additionally, diffraction effects of submerged objects is considerably reduced whencompared to emerged ones, consequently the effect of diffraction is even less This is becausediffraction is essentially a redistribution of wave energy around an obstacle, therefore if alarge percentage of the incident wave energy is transmitted, the remaining energy available

to be diffracted is considerably reduced,consequently the diffraction effects will be less Themajor concern is therefore transmissions at the structure