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
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 [HE MSc Thesis R Everon Zachariah 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 [HE MSc Thesis R Everon Zachariah Abstract ABSTRACT 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 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 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 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 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 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 deductions made from this study are only applicable to sites where these boundary conditions are similar [HE MSc Thesis R Everon Zachariah Literature Review • Revetments, Bulkheads and Seawalls Revetments, bulkheads and seawalls are 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 they are 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 IHE MSc Thesis 10 R Everon Zachariah Contents 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 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) IHE MSc Thesis R Everon Zachariab 76-88 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 changes in the offshore bottom topography occur naturally, further engineering in the coastal zone also influences sediment movement along and across the shore, thereby altering the beach plan shape and depth contours Beach change is controlled by wind, waves, currents, water level, nature of the sediments, and its supply 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 of proteetion works in the near-shore zone, prediction of beach evolution with numerical models has proven to be a powerful technique to assist in the selection of the most appropriate design Models provide a framework for developing problem statements and solution formulation, for organizing the collection and analysis of data, and, importantly, for efficiently evaluating alternative designs and optimizing the selected design In the last decades two methods of 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 lHE MSc Thesis R Everon Zachariah The Dutch Coastal System And Sediment Transport Regime Since the seventeenth century the central coast of the Netherlands, between Den Helder in the North and Hoek van Holland in the south behaves as a closed coastal system in astrong interaction with the barrier islands coast in the north and the interrupted Delta coast in the south For hundred of years the northem ( north of Egmond) and southem (south of Scheveningen) sections of the central coastal system have been suffering from structural erosion 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 the coastline in the eroding sections was of the order 3-5 m/year, caused by the flood and ebb currents near the tidal inlets in the south and in the north, and intensified by the stirring action of shoaling and breaking waves 4.4 History Of Implemented Coastal Proteetion ( Groynes, Sea Walls and Revetments, Beach Nourishment) From 1800 onwards the coastline was more actively defended by building groynes and seawalls The number of groynes was gradually increased and the length of the groynes was extended to about 350 m, almost up to the -4 m N.A.P contour at some locations Long harbour dams normal to the shore were constructed around 1870 near Hoek van Holland and IJmuiden to ensure a safe approach of larger vessels to the harbours of Rotterdam and Amsterdam respectively As a result of these man-made structures, the retreat of the coastline was considerably reduced to about 0.5 to 1.5 m/year Around 1910 some negative effects related to the construction of long groynes and harbour dams were first realized These were the erosion and associated profile steepening in the deeper surf zone and shoreface zone, because of wave and tide induced longshore currents which were forced to flow around the structure at high velocities Since 1960 beach nourishment has become a keystone of coastal defence to further reduce the retreat 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 of massive and continuous beach nourishment has been initiated to compensate for the loss of beach and dune sediments caused by natural erosion processes IHE MSc Thesis 32 R Everon Zachariah 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 methodology is 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 genera I aim may however be summarized as follows: (a) To define a simple basic case of an infinitely long offshore breakwater ( to realistic cases) for the ensuing analysis; (b) To investigate 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 numbers of 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 It is 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 by the construction of an offshore breakwater , and further how these structural parameters and boundary conditions influence these results It is pointed 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 IHE MSc Thesis R Everon Zachariah Discussion, Conclusions and Recommendations 7.0 Discussion, Conclusions and Recommendations 7.1 Introduetion In the previous chapter, some discussion of the analysis and calculations was presented, to enable the reader to follow the sequence of analyses carried out The present chapter is geared at expanding this discussion, and presenting firm conclusions, based on the total results In some cases it may be therefore necessary to reiterate some of the discussion presented previously In other cases, where the discussion is considered completed, no further discussion is presented on the subject The organisation of this chapter is as follows: First a summary is given of the required objectives of the study, with emphasis being placed on what was carried out Secondly the methodology used in carrying out the study is outlined The important parameters required in the study are stated A detailed discussion is presented of the influence of these parameters on the transport rates and consequently the coastline change Finally the conclusions and recommendations resulting from this study are presented 7.2 Objectives of Study As stated in the Introduetion (Chapter 1) the objectives of this study are to investigate the influence of the various geometrie parameters on the functional performance of an offshore breakwater , taking into consideration the variability of the different natural boundary conditions; and the applicability of such structures for coastal proteetion along sections of the Dutch coast The breakwater parameters investigated are : (1) Height (2) Length (3) Crest width (4) Armour unit size (5) Slope (6) Location The natural boundary conditions consists of hydraulic boundary conditions and Littoral/ Geometrie boundary conditions The hydraulic boundary conditions is comprised of sequences of wave climate; which in itself consists of wave height, direction, period, and duration; mean water level; tidal variation with time and related tidal currents; and Set-up [HE MSc Thesis 76 R Everon Zachariah Discussion Conclusions and Recommendations 7.3 Discussion 7.3.1 General The rationale behind using off-shore breakwaters for coastal proteetion is utilizing the reductive (sediment transport) capacity of the breakwater to modify the naturalor existing transport rates alongshore such that the gradient is reduced The ideal situation to be achieved is one whereby the transport capacity is decreased in an eroding area ( area of interest) and the consequent lee erosion transferred to adjacent areas which may be experiencing accretion or less severe erosion If the morphological conditions not allow this, it may be necessary to use beach nourishment in association with offshore breakwaters to combat the likely lee erosion which usually occurs It is c1ear that to achieve this, not only is the location of the structure important, but also its length The required length is partly determined by the natural coastal morphology and the relative importance of the different sections of the coastline The length in turn affects the relative location of the structure, and the required breakwater height For this study, no specific site condition in terms of eroding / accreting areas is being considered Just general trends (to gain insight into the interdependence of these various design parameters) were investigated The module Uni_Lt of the Unibest Software Suite was used to make all sediment transport rate calculations The modelling of the coastline response was done by the module Uni Cl [HE MSc Thesis 77 R Everon Zachariah Discussion, Conclusions and Recommendations 7.3.2 Effect of an Infinitely Long Structure If the breakwater is sufficiently long, and or sufficiently close to the shore, waves from all directions will be reduced by the structure The breakwater may in this case be considered to function as an infinitely long breakwater Critical analysis of Graph 6.8 reveals that for a partiele size of 0.2 mm, a breakwater which is higher than 5.5 m produces a maximum transport capacity which is less than the present incoming capacity regardless of what orientation the coastline makes This implies that breakwater of this height and higher are likely to produce tombolo since the incoming transport is greater than the outgoing A similar deduction can be made for a partiele size of 0.3 mmo In this case, the corresponding breakwater height is 6.0 m These values should not be taken as absolute, but rather as guides which indicate the likely behaviour of the structure If the breakwater is constructed in shallower water such as -5 m contour, as expected, the crest height at which tombolo formation starts is reduced For a breakwater located at this depth a crest height higher than 4.0 m is likely to result in tombolo formation for both partiele sizes This is clearly seen from Graphs 6.8 and 6.9 which shows the transport rates for various breakwater heights, located at the -5 m contour Summarizing the information extracted from above it may be stated that for an infinitely long breakwater located at a depth of 6.8 m (with a relative crest height of (6.8 - 6.0) = 2.0 m say), the transport rates behind the structure are reduced to an extent that tombolo formation is likely to start For a structure located at the -5 m contour the corresponding crest height required for a similar development is approximately 4.0 m, which translates to a relative crest height of (5.0 -4.0) = 1.0 m It may be concluded from this that the maximum allowable relative crest height to prevent tombolo formation decreases as the structure is placed nearer to the shore The coastline response appears to be sensitive to the breakwater height consequently relative crest height This sensitivity further increases as the breakwater is shifted shorewards For this reason it is risky to locate the breakwater near to the shore as inaccuracies in construction or design could result in a coastline response substantially different from the predicted or desired one IHE MSc Thesis 78 R Everon Zachariah Discussion Conclusions and Recommendations 7.3.3 Structures of Finite Length For a breakwater of finite length the coastal response depends on the transport rate at the different sections along the coast This as explained earlier, depends on which waves reach the surf zone without being reduced by the breakwater Hence, the length and its location are very important Figs 6.2 to 6.10 show the calculated coastline position after 50 years, for breakwaters of various lengths and heights, located at the seaward limit of the surf zone These figures indicate that various coastline responses are possible, depending of course on the breakwater length and height This fact may be actually put to use in designing a specific coastal proteetion scheme From Figure 6.2 to 6.4 it may be deduced, that if the structure is relatively short (less than 1000 m) and not too high (less than 6.0 m) a simple salient is likely to be formed This salient is likely to extend to a maximum distance of 500 m from the original shoreline The up-coast area of the salient is likely to show accretion where-as the down-coast area should showerosion The extent of the accreting and eroding areas depend certainly on the crest height and more explicitly, the relative crest height The higher the structure the more severe the accretionJerosion tends to be With increasing breakwater length and relatively low crest height the tendency is for a double salient 10 be formed As the height is increased tombolo formation starts With the formation of a tombolo, the entire sediment transport/coastal morphological development is completely modified, and is significantly affected by the sheltering effect of the formed tombolo This is clearly seen by comparing Figure 6.6 and 6.7 In the former, the double salient is clearly seen, however the right-most salient is eroded as a tombolo develops on the left There is one remarkable point which should be highlighted with respect to the coastline development This is the fact that unlike in the previous cases with the shorter breakwater and the single salient formation, the accretion appears to be more evenly distributed over a wider area (for relatively low breakwater heights) and less severe erosion occurs on both sides of the accreting area This fact may be very useful when trying to proteet an eroding area, which naturally experiences slight to moderate accretion on either side This coastline behaviour can be expected for breakwaters longer than 1000 mand lower than 5.0 m As the breakwater height exceeds 5.5 m for breakwater of length in the region of 2000 m strong salients tend to develop, with the consequence of causing massive accretion up-coast of the salient and severe erosion down-coast The severity of this condition increases with increasing crest height The behaviour of a breakwater with a length greater than 2000 m tends to be very similar to that described above [HE MSc Thesis 79 R Everon Zachariah -_.- Discussion, Conclusions and Recommendations Although the model gives explicit results showing the formation of large salients and tombolo, these results may not be very reliable due to limitations in the model in predicting the correct transport rate when there are large reorientations of the coastline This has to be borne in mind when interpreting the results and making deductions based on these results A indication of this weakness in the model is seen by comparing the respective responses for breakwater heights of 5.5 and 6.0 m Generally the predictions made when breakwater heights greater than 5.5 m ( for a structure located at the outer surf zone.) may not be very reliable, and should therefore be used cautiously Another point of concern is the double salient prediction of the model The double salient, salient and tombolo prediction in the case of a long structure (of the order 2000 mand longer appears somewhat doubtful, and it is not sure if this would actually happen in reality, or if it is due to the rough schematization in wave direction It is possible that because of the limited discrete angles used, ( see table 6.1) the model may have predicted such responses The validity of this may be checked by increasing the number of wave directions used in the schematization, and reducing the number of wave classes to compensate for the number of wave scenarios which are used by the model In fact it would be useful to carry out similar analyses using a completely different schematization, to determine how sensitive the results are to the schematization used [HE MSc Thesis 80 R Everon Zachariah Discussion, Conclusions and Recommendations 7.3.4 Effect of Current • Current The importance and influence of the tidal currents was shown in the previous chapter The influence is even more important with the use of breakwaters, since, in this case currents become the dominant factor affecting the transport, as the effect due to waves is diminished It is therefore important in any study of this kind to take the effects of current into account It is for this reason that the Genesis modelling system although considered and included in the literature was not used further in this study In modelling, the current is assumed parallel to the initial coast This is true initially when the coast is straight However as coast reorientates, this assumption is no longer valid The effect will be that the estimated transport due to currents is different, thereby making the model less reliable for large reorientations 7.3.5 Effect of Breakwater Height and Water level on Transmitted Wave Height The effect of different breakwater heights on the transmitted wave is shown in Graphs 6.11 and 6.12 for a high and low water level As expected the transmitted wave height decreases with increasing breakwater height However a plot showing the transmitted wave for various breakwaters heights at different tidal levels reveal that the important factor affecting the transmission is not so much the breakwater height , but rather the relative crest height 1\: This is clearly seen in Graph 6.13, where there is a wide difference in transmitted wave height (for various water levels) for the same breakwater The significanee of this is that for any particular breakwater design, the coastline response dependant more on the tide and water levels in the area This has to be kept in mind in designing structures for different areas • Storm Set-up During a storm there is a considerable change in water level in the nearshore area Usually, this is set-up (if the wind direction is shorewards) consisting of both wind set-up and wave set-up The wind set-up results from an attempt to balance the shear stress of the wind on the water surface by developing a gradient in the water level The wave set-up results from the wave breaking processes, and the radiation stresses thereby developed The significanee of this is three-fold Firstly, the set-up causes an increase in the local water level This has the effect of causing waves which would normally break further at sea to so near the shore Hence the surf zone and consequently the active area is shifted shorewards This is tied into the phenomenon of dune erosion, when sediment/sand from the onshore area is transported into the offshore region Secondly, an increase in the water level causes the relative crest height of a breakwater to be reduced, with the effect that wave transmission across the structure increases The effect of this increase has already been discussed IHE MSc Thesis 81 R Everon Zachariah Discussion Conclusions and Recommendations In the event of set-down the first two points discussed are reversed Less sediment is also likely to cross the breakwater because of the reduced wave height and reduced relative crest height Finally, because the breakwater may function somewhat like a dam, cross-shore transport which normally occurs during storm is reduced or blocked, depending on the breakwater height IHE MSc Thesis 82 R Everon Zachariah Discussion, Conclusions and Recommendations 7.4 Limitations of the Study This study was limited by a number of factors, three of the more important ones being time, the capabilities/limitations of the software package used and the fact that a one dimensional model was used to represent a three dimensional process The first of these is self explanatory Attention will therefore be on the latter The number of different wave conditions which can be modelIed by the package is the first concern The maximum number is 99, which, although may appear large at first, is a considerable limitation when one considers that the elimate has to be first schematized into a number of directions Using four directions can in no way be considered too detailed a schematization For the above wave directions there are various wave heights and periods, which must be further schematized into wave class intervals and periods Further, for each wave class, this condition may exist at various tidal levels and corresponding tidal currents Although there is a relationship between the wave height and period, and between the tidal level and currents, schematizing the above to take into consideration the various combinations is a rather difficult task, and results is a rather rough sehematization which is likely to have some effect on the final outcome The schematization implemented in this study is shown in Table 6.1 Another limitation inherent in the programme is th at is not possible to get results if the conditions are such that no equilibrium angle (an angle at which the transport is zero) is achievable This was the case when attempts were made to try to model breakwater higher than 7.0 m In general , onee the influence of the tidal currents is sufficiently great or dominant, the programme is unable to give results The limitation of using a one dimensional model to represent a three dimensional process is that it necessitates using depth integrated values both in the input data, and in the calculations The longshore variation in parameters is ignored Further, the longshore variation in crossshore profile which is important for transforming the offshore data into nearshore wave elimate is not considered, i.e straight and parallel depth contours are assumed In spite of the aforementioned limitations, the study has provided great insight into the likely influence of the breakwaters of different geometrie properties on the coastal sediment transport rates and the ensuing morphologieal development It has also provided valuable guidance for further studies/investigations, by giving the ranges of parameters for which further investigation may he useful The less significant parameters have also been identified and may therefore be ignored in future studies Finally considering the general nature of the study, some of the above limitations should not affect the deductions and usefulness of the results, once it is kept in mind that the study is of a general and preliminary nature IHE MSc Thesis 83 R Everon Zachariah Discussion Conclusions and Recommendations 7.5 Conclusions/ Recommendations (1) lt is possible to design offshore breakwaters to proteet seetions of the Dutch coast from erosion (2) Having gained some insight into the coastal response of an offshore breakwater , an environmental impact assessment and an economie analysis should be earried out to ensure that the use of breakwaters for coastal protection, is both economieally and environmentally feasible (3) Different responses are possible, depending on the length, location, and relative crest height of the structure (4) Designs may be site specific, and it may not be possible to implement a design from one site to an adjacent site, if the conditions differ (5) One of the most important conclusions that ean be made from this study is that it is extremely diffieult if not impossible to state categorically the influenee of a particular breakwater parameter on the transport rate or the morphological response of the coastline To make reasonable predictions all the major parameters have to be eonsidered simultaneously This fact is clearly demonstrated by a comparison of the results for different breakwater lengths and heights The reason for this difficulty is that the geometrie together with the hydrologie conditions determine the initial response of the eoastline This response in turn affects the subsequent transport rates This iterative effeet/ response relationship whieh eontinues until equilibrium is reached, further complieated by the faet that the processes involved are non-linear and not fully understood, is what makes general prediction of coastline behaviour so difficult (6) Without considering the major design parameters, it is not possible to determine the effect of offshore breakwaters on the sediment transport rates and consequent morphologie response The major parameters which have to be considered are: (A) breakwater geometrie parameters • Height • Length (B) Hydraulic Conditions • Wave Height • Wave Direction • Wave Period • Frequeney of occurrenee of above conditions IHE MSc Thesis 84 R Everon Zachariah Discussion, Conclusions and Recommendations • Mean waterlevel • Tidal Variation with time (both currents and levels) • Set-up (c) Location of the structure (d) Existing transport regime (7) Other parameters which may be less significant are: (a) Breakwater geometrie parameters; armour unit size slope erest width (b) Physical boundary condition cross-shore profile These parameters may therefore be neglected in a first study (8) For a breakwater of length of the order 1000 m located at the seaward limit of the surf zone breakwater heights of more than 6.0 m tend to result in tombolo formation For a breakwater located at -5 m contour the corresponding crest height is 4.0 m (9) In designing a breakwater, the location of the structure may be used as a design parameter in order to achieve the required coastline response The structure should be placed within the range of the -7 mand -5 m contour Placing the structure nearer to the shore (shallower water) although more economical is not recommended for several reasons Some of the more important ones being; (a) The transport zone affected by the structure is decreased, therefore the breakwater has less influence on the total transport rates, hence also on the coastline response (b) As the structure is placed in shallower water, the structure is more sensitive to changes or inaccuracies in crest height This makes the coastal response prediction more difficult and less reliable (c) If the structure is built in deeper water, it may be possible to the construction in stages so that the final design may be modified to produce the required coastline response (d) The influence of set-up and therefore relative crest height is less for a structure located in deeper water The analysis carried out in this study assumes that the set-up and set-down [HE MSc Thesis 85 R Everon Zachariab Discussion, Conclusions and Recommendations cancel out This in fact is not completely true and should be considered in a more detailed analysis The effect of a set-up is to increase the water level and consequently the relative crest heights and therefore the transmission coefficients This would result in higher transport capacities, and would therefore increase the required breakwater height The above assumption was made because of; (1) the limitations of the programme in terms of the number of wave scenarios that may be inputted (hence the schematization) as discussed in section 7.4 "Limitations of the Study" (2) The difficulty in determining the correlation between set-up and occurrence of particular wave classes (10) To design a breakwater for a particular site, more detailed investigation, possibly with a two dimensional model is necessary (11) Considering the limitation in the number of wave scenarios which may be used with the model, and the rough schematization which resulted, further analyses should be carried out to determine the effect of the schematization on the results In particular, the prediction of a double salient should be examined (12) This study only investigated non-segmented breakwaters, however it is very likely that segmented breakwater mayalso offer a technically feasible solution This option should be investigated IHE MSc Thesis 86 R Everon Zachariah Re eren ces References Battjes, J A 1974 "Computation of set-up, longshore currents, run-up and overtopping due to wind-generated waves", Delft University of Technology, Delft Genesis User Manual, U.S Army Corps of Engineers, U.S.A "Coastal Engineering Note", U.S Army Corps of Engineers, U.S.A Loveless, J H 1991 "Developments in the Design of Offshore Breakwater Schemes" , University of Bristol, London Meer, van der J.W 1991 Stability and transmission at low-crested structures", Delft Hydraulics Publications no 453, Delft Pilarczyk, K.W.(ed.) 1990 Coastal Protection, A.A Balkerna Netherlands Publishers, Rotterdam, Pluijm, M 1993 Onderzoek Naar De Toepasbaarheid van Offshore Golfbrekers Langs De Nederlandse Kust, Frederic R Harris, The Hague, Netherlands Pluijim, M., J.C van der Lem, A.W Kraak and J.H.W de Ruig, "Offshore Breakwaters Versus Beach Nourishments: A Comparison" , 24th International Conference on Coastal Engineering, Kobe, Japan, 1994 Rijn, van L C 1994 "Sand Budget and Coastline Changes of the Central Dutch Coast Between Den Helder and Hoek van Holland", Delft Hydraulics, Delft Rijn, van L C Principles of Sediment Transport in Rivers, Estuaries, Coastal Seas and Oceans, Lecture Notes HH279/93/1, IHE, Delft Rosati, J Dean 1990 Functional Design of Breakwaters for Shore Protection: Empirical Method Coastal Engineering Research Centre, prepared for Department Of The Army, U S Army Corps of Engineers, U.S.A IHE MSc Thesis 87 R Everon Zachariah Re eren ces SPM (1984) Shore Protection Manual, Coastal Engineering research Centre, U.S Army Corp of Engineers Stroomatlas Tilmans, W.M.K 1993 Coastal erosion - The Kerteh case Analysis of causive factors and mitigative measures using dedicated mathematicaI modelling tools, Delft Hydraulics Publications no 477, Delft Unibest Manual: A Software Suite for Simulation of Sediment Transport Processes and related Morphodynamics of Beach Profiles and Coastline Evolution, October 1994, Delft Hydraulics, Delft Zeidler, B R.(ed) Effectiveness of Coastal Measures, Rijkwaterstaat and Delft Hydraulics, Gdansk (1992) [HE MSc Thesis 88 R Everon Zachariah ... 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. .. 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,... basic case of an infinitely long offshore breakwater ( to realistic cases) for the ensuing analysis; (b) To investigate the effect of an infinitely long offshore breakwater on longshore transport