Beach Nourishment

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BEACH NOURISHMENT

K

ees

d' Angremond

Professor ofCoastalEngineering Delft University oÎTechnology

Delft, The Netherlands

I.INTRODUCTION .

2. ANAL YSIS OFCAUSE ANDCONSEQUENCE 2

3. DISCUSSION OF REMEDIAL MEASURES ... . ... ... ... . ... . . . ... 3

4. DESIGN OF SOFT SOLUTIONS .. . . ... ... . . .. ... . .• . ... ... .... 6

4.1Reviewof DesignTools .. . ... ... .. .... .. ... ... . . ... 6

4.2Practical Consequences ofMorphologicalTheory .... ... .. .. ... ... 7

4.3 FaiJure Modes .. ... . ... . .... ... . .. ... .. . . .... ... ... . .. 8

.5. CONSTRUCTION METHODSSOFT SOLUTIONS ... .. ... . ... ... ...•... 9

.5.1 Review of avaiJable equipment " 9 .5.2 Consequences ofequipmcntchosen ...•... 10 6. MEASUREMENT, COST AND PA YMENT ... .. ... ... . . . .... ... .. . . .. 12 6.1 Risk Allocation 12 6.2 Measurement ... ... . . .... .. .. ... ... ... ... ... . ... ... 13 6.3 Cost ... ... .. . .... .. . .. ... ... ... . . . ... ... .... ... ... 14

7. COMBINING HARD AND SOFT SOLUTIONS " 14

8. CASE STUDIES 1.5

LIST OF REFERENCES .... ... . .. . .. ... . .. . .. . ... .. ... 17 1, INTRODUCTION

For many centuries, mankind has been attracted by water,whether it be rivers,lakes or the sea. The reasons were and stillare evident. Water was required for drinking,growing of food, raising cattle orcatchingfish. Gradually other functions were added such as transport and trade, mining and lately recreation.

This meant that the boundary between land and water became a popular and crowded area. More or less automatically, people chose to live close to the waterfront, ignorant of or ignoring deliberately the risks involved. Risks that consist of the fact that the boundary is not stabie, not clearly defined.

Depending on unpredictable natura! events the water gives and takes.In the best case, there is a fluctuation around an equilibrium, but alltoo often the size of the fluctuation is under-estimated.This leadsinevitably to disaster, loss oflife andloss of property.

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416 KEES D'ANGREMOND

Fighting these uncertaintiesis the basis of civil engineering, one of the oldest handicrafts in the history of man,

The tasks of the craftsman have become more complicated over the years, however,The growing population and increasing investments have demanded better guarantees for safety, in particular along the sea shores, The fragile equilibrium existingat many locations was ever more disturbed by human interference with the natural system. And finally, the traditional techniques for coastal proteetion were no longer accepted due to other interests such as recreation and environment.

Nowadays, we can only tackle problems of coastal defense in a wider social context. In fact, an integrated managementpolicy for the coastal zone is indispensableto achieve a proper balance between the various conflicting interests.

1. ANALYSIS OF CAVSE

ANP

CONSEOVENCE

To avoid overlap with other leetures the causes and consequencesof a regression of a sandy coast will be briefly touched only. A long discussion is possible about the definition of the

word coastline, One can seleet the LW line, the HW line, or because the safety of the

hinterland is at stake, the location of the dune base.

Whichever parameter is chosen to characterize the position of the coastline, one may discern various types of changes:

* Fluctuations around a (stable) average location under the influence of variabie

(seasonal) wave conditions

* Systematic regression (or accretion) under the influence of waves

either by:

gradientsin the (wave driven) longshore transport

or by:

cross shore transport.

*

Systematic regression due to longshore tidal currents.

* Systematicregression due to reduced sand supply (retentionof sedimentsin reservoirs

of rivers)

* Systematic regression due to sand mining

*

etc.

The remedy against erosion of a sandy shore depends many times on the causes of the

regression. Though it is not always easy, any remedial measures shall be based on

comprehensive observations,careful diagnosis,and a thorough knowledge of the medicine

chest. .

A scheme fortheprocess of design,execution and evaluation of engineering works against beach erosion was originally presented by CUR(1987).The scheduleis presented in Fig. 1.

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3. DISCUSSION OF REMEDIAL MEASURES

Once coastal erosionis threatening our settlementsand economie interests,we try to respond adequately. In principle, there are quite a few options. Several categories cao be distinguished,such as hard andsoft measures.(Van de Graaff and Koster, 1990).

The"hard" measures compriseof theconstruction ofprotective structuresmade of quarry

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418 KEES O'ANGREMONO

• perpendicular to the coastline (groins, Fig. 2) • paral1el to the coastline onshore (seawalls, Fig. 3) • paral1el to the coastline offshore (breakwaters, Fig. 4).

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Figure 2Groins; hard structures perpendicular to the coast.

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Figure 3:Seawall; hard structure parallel to the shore; onshore.

_/,.,----~',---_.---... -Figure 4 Offshore-breakwaters; hard structures parallel to the shore; offshore.

The hard structures have in common that they generally prevent ongoing erosion by retaining whatever sand is left. If they accumulate sand, it is only at the expense of adjacent stretches of the coast.

The "soft" measures consist of replenishment of the coast with sand. This sand is mostly

dredged at some distance, transported towards the retreating coast and deposited by hydraulic fil1 methods.

The location of the fil1 can either be (Fig. 5): • at the landward side of the dune (A)

• at the seaward side of the dune, landward of the dune base (B) • at theseaward side of the dune, seaward ofthe dune base (C).

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The choice between all main options has to be based on considerations ofeffeetiveness

and efficiency. In making these considerations, due attention has to be paid to security, recreation, environment,water resources, fisheries, and navigation. Reference is made in this respect to the selection diagram of Kobayashi et aL(l985)

(Fig. 6).

Figure 5 Zonasion of coastal profile in viewof location of beach fill

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4. DESIGN OF SOIT SOLUTIONS 4.1 Review of Design Tools

Before a start canbe made with the design of a beach nourishment project, a clear insight shallbe obtained with respect to the most important morphological processes in the area under consideration.This can be done on the basis oflong term observationsin nature, taking into consideration the gradual changes in bottom topography, the composition of the seabed, hydrographicalconditions such as vertical and horizontaltides, waves,wind, etc.

Generally, field measurementsonly do not reveal the required level of understanding,and the observations will be complemented by calculations of the sediment movement. For this purpose, use is mainly made of computational models according to CERC (1984) and Bijker (1971), which result in the relatively simple quantitative indication of longshore transport. More refined methodsare based on the two line concept, where longshore transport is divided over two zones, and where cross shore transport is introduced as a means of exchange between the two zones. Such method has been proposed by Bakker (1968).

Another important aspect is the beach slope.Ithas been shown by Wiegel (1964) that the beach slope is a function of wave height and grain size. Higher waves lead to more gentle slopes, and coarse sand leads to steeper slopes. (See Fig. 7).

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F'lgUre 7 Beach slope versus grain size and wavelood (Wiegel. 1964)

After a beach fill has been applied,longshoreand cross shore transport are responsible for a gradual redistribution of material. This means that to proteeta certain area, more sand is

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to be applied than theoreticallycaJculated.Itmeans also that dependingon the excess quantity

applied, the beach fill is only effective during a limited period, so that the nourishment

scheme will have to be repeated at regular intervals.In the Shore Proteetion Manual (1984), a method is presented to calculate both, the fill factorRA indicating the required surplus volume, and the renourishment factor RJ indicating the frequency of renourishment. The method takes into account differences in the grain size between the original beach and the fill material.

4.2 Practical Consequences of Morphological TheoO'

In Chapter 3 (Fig. 5) it was discussed where fill material can be placed in the cross section of a sandy coast. Strengthening of the dunes (application in zones A and B) is an exception, as this indicates that serious problems with the safety of the coast have not been remedied in time. The most common area of placing the fill material is therefor zone C. Zone C in itself is consisting of three distinctly different subzones, i.e.:

* on the (wet) beach

* in the breaker zone

* seaward of the breaker zone.

For various reasons, it is often decided to place the material on the beach. One of the reasons to do this is the difficulty of working in the breaker zone, another the fact that by placing the material on the beach, recreation is directly benefitting.

In practice, it means that in a large number of cases, a cross section is created which is steeper than the equilibrium governed by the Wiegel data as presented in Fig. 7. This leads to a rapid adjustment of the profile under the influence of cross shore transport. Material deposited between the HW and LW line is redistributed over the full active part of the profile. For the superficial observer it seems that erosion is taking place more rapid than ever. Numerous cases are known that public opinion is very negativeabout the result of beach nourishment because the original spectacular gain of beach width is lost in a short period. A complete survey of the foreshore reveals in most cases that the material is not lost, but is redistributed and fulfills its natural role on the under water slope. (Fig. 8).

One may wonder whether it would not be better to place the material directly in the sections of the profile where it will eventually arrive. It has been indicated already that working in the breaker zone is problematic. Placing the material above the LW line is not easy either, as in this case the breaker zone has to be crossed when

bringing the material in from the seaside. A Figure 8 Redistribution of fill material logical (and potentially economical)

alternative would then be to place the

material just seaward of the breaker zone.The question is whether material placed in this region will ever reach the active zone in order to counteract the loss of sand on the beach.

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On a trial basis this has been attempted recently in Queensland (Australia), as described by Jackson and Tomlinson (1990). The results have been so promising that the method will also be tested in the Netherlands. Preliminary investigations have demonstrated that a sand berm which was accidentally formed off the breaker zone of the island of Ameland really moved towards the beach. (Van der Woude, 1992).

The behavior of such berms is hardly predictable with the traditional one and two line morphological modeIs. It is therefor essential that work on the fundamental aspects of sediment movement under waves and currents is continued, leading to more sophisticated descriptions of the morphological processes. New two and three dimensional models are being developed based on these theories, allowing to calculate changes in the topography in greater detail. The UNIBEST package of Delft Hydraulics / Rijkswaterstaat is an example of such development (Roelvink, 1989-a, 1989-b). For the time being, care is recommended as the (in)accuracy of these mathematical models is not known.

4.3 Failure Modes

When discussing the failure modes for beach nourishment schemes, it must be stressed that any soft solution is a temporary solution. In no way, it may be expected that beach nourishment will make an end to continuing erosion, unless the cause of the erosion is eliminated. This is clearly demonstrated by the calculation of the renourishment factor.

Failure of the nourishment scheme is thus related to a life time of the fill which is considerably shorter than anticipated. In practice, this is mostly caused by a faulty result of the morphological analysis, or by a borrow area not producing the expected quality (grain size) of material.

Unfortunately, morphological calculations are often of the black box type or full of assumptions. The nature of the models makes them rather unaccessible for modem analysis

of reliability and probability of failure. Without proper calibration on the basis of historical development, the design ofbeach nourishment schemes must be regarded with due suspicion. A similar situation exists with respect to the quality of fill material. Indications on nautical charts, superficial (grab) samples etc. are inadequate indications for the real composition of material in the borrow area. The only reliable method to limit the risk of failure due to faulty material is the execution of a comprehensive soil survey, consisting of borings, and possibly supported by a geophysical survey.

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St CONSTRUCTION METHODS SOIT SOLUTIONS 5.1 Review of available eguipment

The main types of dredging equipment that are used in beach nourishment projects are:

• trailing suction hopper dredges • Stationary suction dredges

The trailing suction hopper dredge (TSHD) is a seagoing vessel, equipped with (trailing) suction pipes. While sailing at slow speed, a sand-water mixture is pumped into the cargo hold. The sand is settling in the cargo hold and the excess water is flowing overboard. When fully loaded, the suction pipes are lifted and the vessel sails to the destination. There, the load can be discharged either by opening bottom dOOTSand dumping the sand, or by mixing the sand with water again, and pumping the mix out via a pipeline.

As a result of these working characteristics, the trailing hopper dredge has certain advantages and disadvantages. As a seagoing vessel, the TSHD has excellent seakeeping properties, which allows continuation of work, even under adverse weather conditions. Dredging canhe

carried outinwave heights up to several meters, transport of sand can be done under all weather conditions, similar to the dumpingof the load. Unloading by pumping out, however, mayhehampered by wave action, due to the coupling procedure with the pipeline. The main disadvantage of the TSHD is its draft.In order to dredge, sail and discharge, the vessel needs a water depth greater than its loaded draft, which is in the order of 10 m for medium sized dredges. When pumping ashore, all available power has to be used to overcome the flow resistance in the pipeline. This means thatit is advisable to create a mooring facility for the pump ashore operation to avoid the need to keep propellers and bow thruster running. Stationary suction dredges are either cutter suction or plain suction dredges. Both types are equipped with a ladder mounted suction pipe, and one or more dredge pumps. A cutter suction dredge is equipped with a cutterhead, rotating in front of the suction mouth to disintegrate the virgin bottom material and facilitate the mixing of soil and water into a pumpable substance. A plain suction dredge has no such mechanical aids to dig, it mayhe

fitted out, however with powerful waterjets to facilitate mixture formation. By itsnature, the cutter suction dredge sheaves off layers from the bottom, whereas the plain suction dredge is forming craters. All stationary suction dredges are vulnerable to wave action.Thelarger types can certainly survive moderate storm conditions though dredging has to be interrupted. For both types of dredges, the material is tohetransported by a separate contraption. This can be either a pipeline or transport barges.

When a pipeline is chosen, one can still distinguisha submerged (steel) pipeline and a floating one. Nowadays, floating pipelines are mostly selffloating with rubber sections for flexibiIity. A floating pipeline, and more specifically its connections with vessel and shore is vulnerable to wave action. The submerged pipeline is less vulnerable in this respect, but requires a large investmentand reasonable working conditionsduring installation. To overcome the resistance of flow, a high pumping power is required, and in case of sharp material, wear is an important oost element.

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When barges are chosen,one may opt for self propelled or 10wbarges. Disadvantage of the

use of barges is their vulnerabilityfor wave action, mainlyat those moments that the barges have 10come alongsideother equipment to be loaded or unIoaded.The unit cost of transport,

however is much lower for barges than for pipelines. Similar to TSHO's barges need water depth 10sail, though the requirements are less than for the medium size TSHO. Unloading the barges can be realized by dumping via bottom doors, or through a separate barge unloading dredge. Examples of working methods have been given by d' Angremond et al.(1987).

5,2Conseguencesof eguipmentchosen

The choice of equipmentis strongly determined by the prevailing conditions at the work site.

To clarify the relation the sequence of the work willbefollowed, starting at the borrowing process and ending at the disposal.

• Borrow area

Decisive conditions at the borrow area are: water depth, wave and current climate, and soil condition. Shallow water prevents the use of a TSHD, while rough hydrographic conditions prevent the use of stationary dredges. As TSHO and barge are filled by a process which involves overflow,due attention is required for the presence of fines in the borrow area. Their presence (in large quantities) may lead to environmentallyundesirable turbidity. The overflow process reduces the percentage of fines in the mixture, so that coarser material is available for the actual nourishment. Gradually, however, the borrow area will be contaminated with the finer material that is rejected.

Cutter suction dredges and trailers remove a layer of limited thickness from the upperpart

of the soil strata. They leave a flat bottom,so that the borrowing process can be combined with dredging for other purposes such as the creation of harbor basins or shipping channels.

Their dredging depthis limited to about 30 m. When the useful material is covered by unsuitables (silt or clay),the choice of cutteror traileris out of the question.In such condi-tions,the plain suctiondredge can dredge the proper material from great depths (up to 70 m),

evenif it is covered by unsuitables. .

• Transport

The mode of transport is determined by the distance between borrow and disposal area,the routes that are available(either for vesselsor for pipelines),wave and current conditions along the route(s),etc. When the distanceis longit becomesincreasingly difficult to use a pipeline because booster stationsare required to providethe necessary pumping power.When current velocity and wave height en route are high,the use of a floating pipeline is rather difficult,and a submergedpipeline becomes more attractive. Such solution is also required when the pipeline is crossing a navigation route.

When determining the distance for transport by barges or other vessels,one should takeinto account the water depth and other nautical restraints.

The final stage of the transport phaseis the actual delivery of material atthe disposal site.

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the material is to heplaced in the dune or close to the dune base, the use of a land pipe line is almost inevitabIe. The same applies for a location on the beach near the HW line. Only when the material is to be placed below the HW line, or even below the LW line, the use of a pipe line in the last stage of transport canheavoided.

Alllocations in the cross section above the LW line require crossing of the breaker zone with the fulI flow of material. This imposes serious technical complications, high cost and high risk.

'" Disposal

Therefore, in a number of cases it is not possible to cover the full distance by the same means of transport. Then, a combination has to be selected, involving rehandling of material. In this respect one should mainly think of transfer of material from a vessel or a barge into a pipeline. The most simple solution is offered by the direct coupling and pump-out from a

TSHD, except for the fact that the coupling procedure is weather sensitive. Moreover, the

pump capacity of most TSHD's is restricted, so that booster stations are required when the natura! depth prevents the vessel from coming near. The location of a booster may yield particular problems in the offshore region. Recently, a jack-up platform was used in the Netherlands to create a safe location for such booster. (Fig. 9).

Figure 9 Jack Up platform as booster station

When barges are used without a pump-out facility, one might use a separate barge unloading

dredge. This.solution is only feasible in very sheltered waters, for instanee when a transfer

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In a number of cases, it is more attractive to let the hoppers or barges dump the material in a location from where it is rehandled by a separate dredge. It is to be seen whether a protected location is available for such rehandling operation.

There is a completely different method to cross the breaker zone, i.e. by applying astrong jet, which is "rainbowing" the sand water mixture through the air into the beach. This method

is much less controlled than any of the others, and it may lead to considerable loss of material, even up to 20 or 30 %.

It cao be concluded that traversing the breaker zone isa risky, and therefor costly affair,

whether it be done by pumping through a submerged pipeline or by the rainbow method. The cost of this phase may be half of the total cost. Which is worse is that deposition of the fill material on or above the LW line leads to beach slopes steeper than the natural equilibrium. In the foregoing, we have seen that wave action is correcting such deviation from nature in a short period. The engineer realizes that the material "lost" is still performing its task at a lower level of the slope, but public opinion is difficult to convince that there is no waste of taxpayers' money.

• Overall workability and risks

So far, risks and workability have been indicated for separate phases of the material handling. In reality, however, we are not talking about different independent processes, but about a logistic system, whereby delays or breakdowns incurred by any piece of equipment influences all others. The only sound way to look at workability is to look at the whole operation, inclusive the mutual dependance. The more complicated the logistic chain, the more chances there are for delays, and the lower the working rate to be expected. The classical theory of multiplying the workability of all pieces of equipment, considering their delays as independent has created lots of disappointments. Those who have worked frequently along the shore know by experience that long periods of workable conditions are lost because first damage incurred in the previous storm has to be repaired.

A large amount of money cao eventually be saved if it were possible to dump the sand relatively close to the shore, but still just outside the breaker zone. Such solution works only if it cao be proved that the material is transported to the shore under the influence of the natural sediment transport processes. In fact, we are back here at the more theoretical discussion about sediment movement and its predictability, which was touched in section 4.

6. MEASUREMENT. COST AND PAYMENT

6.1 Risk Allocation

Design of coastal engineering works is not a scientific problem only. Whatever solutions are designed, they have to be executed, mostly by a contractor, and after a competitive tender procedure. It means that one is forced to determine the rules of the game. Decisions have to

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information about the natural conditions these more administrative data determine the risks involved.

Though many engineers tend to say that it is the nature of a contractor to accept the risks involved in a contract, one must realize that most contractors will attach a price tag to the risk they have to take. Itis therefore extremely useful that the employer and/or his engineer take a deliberate decision as tothe risks they want to bear themselves and the risks they want to convey to the contractor.

Risks that are evidently for the contractor are risks connected to his equipment (availability, production rate, breakdowns, etc.). Risks that are mostly accepted by the employer are few, but include generally permits to use a particular borrow area, escalation due to inflation, etc. A grey area is the quality of the fill material, weather conditions, loss of material during execution, etc. It is worthwhile to pay proper attention to these items, as it is not always attractive to leave the risk with the contractor.

Starting from the assumption that most risks can be identified, that they are statistically independent, and preferably Gaussian distributed, it is possible to treat the outcome of a cost estimate as a probabilistic exercise.(See Verhoeven et al., 1992). It largely depends on the market situation how the contractor will react to the statistical distribution of the calculated cost. Generally, he will not be prepared to accept a considerable chance of running into a loss. It means that he will tend to remain on the safe side and for instanee that he will caiculate weather conditions more conservative than average. In this way, the employer pays a certain premium for bad weather. When this is a one time job, this premium may be just and acceptabie. If the job iscarried out year after year by different contractors, the employer pays an annual premium for bad weather which taken over several years is never worse than average. In such case, the employer should consider to take over this risk from the contractor, and make sure that he will not be charged for any worse than average weather. This example can of course be extended to virtually all risk bearing elements, though the outcome may be different from case to case.

6.2 Measurement

Place and mode of measurement determine payments to the contractor. They also determine partially the allocation of risk between contractor and owner. Place and mode of measurement are mostly determined on technical grounds. It can be done by comparing in-and out- survey, either in the borrow area or in the reclamation area. This method may lead to considerabie errors, as large areas have to be surveyed twice, the difference being the contract quantity.

The most obvious souree of errors is the inaccuracy of the survey process itself, but even if this were full proof, there would be discussion about natura! changes (erosion or accretion) in the area of measurement. Certainly in cases when barge transport is applied, it is therefor attractive to measure payable quantities in the means of transport. In theory, this is possible for pipeline transport as weil, though this requires sophisticated procedures to measure discharge and sediment content, with adequate facilities for calibration.

Whatever method of measurement will apply, the contractor wilJ assess in how far the measured quantities reflect his efforts. Any efforts under the contract that will not lead to

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payable quantities will be considered as losses.Itwill be attempted to reduce these losses to the absolute minimum.Similarly, the employer will consider in what way the payable quantities serve his purpose. He will see that in reducing the "losses"the contractor will not circumvent the intentions of the contract.

6.3 Cost

Paymentstothe contractor are not the basis for cost comparison with other altematives.For such comparison the net present value of construction cost, maintenancecost and monitoring cost is the only basis. For hard structures methods have been developed to base the design on probabilistic theories, and to take into account the statistically foreseeable cost of maintenance in calculating the optimum design. (van de Kreeke and Paape, 1964, De Haan

1991, Vrijling1992a.o.).

Unfortunately, the design tools that are used in the design of soft structures have not been developed so far yet that it is possible to indicate confidence levels in a probabilistic approach. Mostly, the result is accepted as it is, and one can only introduce a relatively large scatter around the calculated result to account for the uncertainties.(D'Angremond et al.,

1991).

This situation is highly undesirable, anditis of the utmost importance that attention is paid to the calibration and reliability analysis of computer models in coastal morphology.

7. COMBINING HARD AND SOIT SOLUTIONS

So far, hard and soft solutions have been presented asalternatives to each other. One may consider a combined application, however. The specific properties of hard structures,i.e. their capability to retain sand, make them extremely suitable to elongate the life of a beach nourishment project.. A condition for such combination is that the advantages of the soft solution are not nullified by the hard solution it is combined with. The main advantage of the soft solution is the preservation of the recreational value of the coast, and therefor a combination of beach nourishment with the construction of a seawall or a series of groins is not likely.

It is perfectly conceivable, however, that beach nourishment is combined with the construction of offshore (overtopping) breakwaters.The breakwaters need not destroy the recreational qualities of the beach,and by their presence they mat mitigate the wave driven erosion.Further, they may prevent the unwanted spreading of freshly applied sand overthe foreshore, thus creating a sort of hanging beach. (Fig. 10).

Another option for combinationofhard and soft solutions is theincorporation of a hard core in a nourishment scheme.Such core is normally covered with sand,but adds to the residual strength of the sea defence under extreme conditions.

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Figure 10 Hanging beach

8. CASE STUDIES

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Some characteristic examples of beach nourishmentwill hediscussed:

- Beach nourishment along the Belgian coast (Fig. 12)

- Beach nourishment at Lido di Ostia,Rome. (Fig. 13),after Toti et al. (1990). - Foreshore nourishment in Queensland, Australia (Fig. 14 and 15)

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LIST OF REFERENCES

Angremond, K.d', Jong, A.J.de, Oorschot, J.H.van (1987), Beach Nourishment, Design Elements and Execution of the Works. Proc. 2nd Int. Conference on Coastal and Port Engineering in Developing Countries,Beijing.A/so published in Terra et Aqua 37,August

1988.

Angremond, K.d', Lavooij, H.A., Ordonez,J. (1991), Coastal Shore Proteetion Works at Corinto, Proc. 3rd Int. Conference on Coastal and Port Engineering in Developing Countries, Mombasa.

Bakker, W.T.(1968), The dynamics of a coast with a groyne system.Proc. 11th Conference on Coastal Engineering,London, pp. 492-517.

Bijker, E.W.(1971), Longshore Transport Computations, Proc. ASCE, Joumal of the Waterways, Harbors and Coastal Engineering Division,WW4,November.

(18)

CURIRIJKSWATERSTAATI DELFf HYDRAULICS (1987), Manual on artificial beach

nourishment, Report 130, ISBN 90 212 6078 6.

Graaff, J. van de, Koster, M.J. (1990), Dune and Beach Erosion and Nourishment, Coastal

Protection, Balkema, Rotterdam, ISBN 90 6191 1273.

Haan, W. de. Deterministic computer aided optimum design of rock rubble mound breakwater cross sections. Coastal Engineering 15 (1991) pp 3-19.

Iackson and Tomlinson (1990), Nearshore Nourishment: implementation, monitoring and model studies of 1.5M m' at Kirra Beach, Coastal Conference, paper no.253.

Kobayashi, H., Tanaka, T., Shoyama, S.(1985), Beach nourishment in Yokohama Marine Park. PIANC, 26th International Navigation Congress, Brussels.

Kreeke, J. van de, Paape,A. (1964), On OptimumBreakwater Design, Proc. 9th Conference on Coastal Engineering.

Roelvink,J.A. (1989-a), Feasibility of offshore nourishmentof the Dutch sandy coast, Proc., Int. Conf. Hydraulic and Environmental Modelling of Coastal, Estuarine and River Waters, Bradford, England, 19-21 Sept. 1989.

Roelvink, I.A. and Stive (1989-b), M.J.F., Bar-generatingcross-shore flow mechanisms on

.a beach, JGR, vol. 94 no.C4, pp 4785-4800, April 15, 1989.

Toti, M., Cuccioletta, P., Ferrante, A.(1990), Beach Nourishment at Lido di Ostia (Rome), PIANC 27th International Navigation Congress, Osaka.

U.S. Army Corps of Engineers (1984), ShoreProteetion Manual, Volumes I and11.U.S. Army Coastal Engineering Research Center.

Van der Woude, G.(1992), OnderwateroeversuppletieNoord Ameland, Master's Thesis, Delft University of Technology, Faculty of Civil Engineering. .

Verhoeven, F.A., Van Deursen, P.P., Meegdes, H.S.L.M.(1992), Decision Support System for Determining Tender Prices of Dredging Projects, Proc. XIIIth World Dredging Congress, Bombay.

Vrijling, J.K., Jorissen, R.E., Klatter, H.E. and Kuijper,H.K.T. (1992),The maintenance of Hydraulic Structures,Proc. 23rd Int. Conference on Coastal Engineering, Venice, Italy.