Sea Defences

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SEA

DEFENCES

by

Krystian W.

Pilarczyk

R

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kswaterstaat

(Dutch

Pub

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c Works Department )

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SEA DEFENCES

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DUTCH GUlDELlNES ON DlKE PROTECTlON

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by Krystian W. Pilarczyk Report WB-NO-87110

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Rijkswaterstaat (Dutch public Works Department)

Road and Hydraulic Engineering Division P.O. Box 5044, 2600 GA Delft, The Netherlands

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CONTENT ABSTRACT 1• INTRODUCTION

2. DESIGN PHILOSOPHY OF COASTAL DEFENCE STRUCTURES

3. SHAPE AND HEIGHT OF A DIKE 3.1 Loading zones

3.2 Dike shape

3.3 Dike height and run-up 4. STRENGTH OF REVETMENTS 4.1 General approach

4.2 Failure modes and determinant wave load 4.3 Wave loading and wave structure-interaction 4.4 Stability of loosely materials

4.5 Uplift forces. Block and impervious revetments 4.6 Impact forces. Asphalt revetments

4.7 Revetments under ship's induced loads 4.8 Stability of grass-slopes

4.9 Example of probabilistic calculations of revetment 5. DESIGN CONSIDERATIONS

5.1 General requirements 5.2 Dimensioning

5.3 Choice of revetment

5.4 Composition of dike and revetment 5.5 Subsoil requirements

5.6 Joints and transitions 6. MANAGEMENT AND MONITORING 7. CONCLUS IONS

REFERENCES APPENDICES:

I A. Bezuijen, M. Klein Breteler and K.J. Bakker, Design criteria for placed block revetments and granular filters

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DUTCH GUIDELINES ON DIKE PROTECTION

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ABSTRACT

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The increased demand on reliable design methods for protective structures has resulted in the Netherlands in preparing a set of design guidelines for revetments of the sea-, and river-dikes, and for bank protection. These guidelines are intended for technicians and organizations directly involved in the design and management of protective structures. In this report a brief review on general de-sign philosophy, different hydraulic and geotechnical aspectsland design criteria for various types of revetments is given. The sta-bility criteria based on small and large scale tests are formulated for the following systems: rip-rap, concrete units, asphalt and grass-mats. Developments for some other systems are also briefly mentioned.

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4

-Outeh cocst, ~osional OI"QQS

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-t""

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km _........ .,.. ~ , .

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./ =NORTH SEA .,

H

__ dikQS

--_.~ sandy bc20ehQS end dUnQS

!!!.!!~ QI"OSlonOI"flOS.1to 5m PQI" y=1"

tore-shore _{beach dune}

landwards (secundary)

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Fig. 1 DUTCH COAST, EROS/ONAL AREAS

swt

protection

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qroms or permeable groins sea-dike

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SWL.

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---~==~---

--

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toe-/bottom-protection

---~---~~---".., sea-walt

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Fig

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EXAMPLES OF SEA-PROTECTION

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1. INTRODUCTION

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A large part of the Netherlands lies below the mean sea Le v eLj it is protected by dikes, dams and dunes (fig. 1 en 2). The country is therefore dependent on good (safe) sea defences. Driven by the nec-cessity to withstand the water, during centuries the Dutch engi-neers built up their knowledge on hydraulic engineering, and parti-cularly on constructing of dikes and protection measures (revet-ments). However the design of dikes and their revetments was mostl.y based more on rather vague experience than on the gener al valid calculation methods. Due to the increasing demand on reliable de-sign methods, i.e. as a result of more "hard" safety requirements, the Dutch Ministry of Transport and Pu b Li c Works (Rijkswaterstaat) and the Technical Advisory Committee on Water Defences have initia-ted a long term research program on preparing the guidelines for the design of sea and river defence structures. Some of these gui-delines have been reported recently (26), (27) , (28) , (29), (30) , (32).

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In the report aset of basic design guidelines for revetments of the sea dikes, based on the published and unpublished sources, will be given. This set of guidelines is intended for e nqin e e r s and technicians directly associated with the design and management of dikes. Is is not intended as a scientific work dealing exhaustively with theoretical fundamentals. It has been endeavoured as far as possible to give the gener al practical design guidelines with some background information but without offering a solution for every conceivable problem. For a treatment of these matters in greater depth the reader is referred to the original reports.

For the revetment, i.e. the protective covering of a waterretaining structure (dike) requirements are formulated with reference to the purpose of the structure and the revetment, the technical features of constructing it, and possible special circumstances involved. The shape of the cross-sectional profile of the dike is of influen-ce on the type of revetment material suitable for revetment con-struction. The design of the shape and the height of a dike are thus also discussed.

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Various types of revetment are distinquished with reference to the properties of the materials and/or the units, and of the base on which they are installed. The following types of revetments are treated: rip-rap and other loosely systems, concrete units, asphalt and grass.

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As an interim result of long-term research being still in progress, some information concerning the stability of the different revet-ments (i.e. new stability criteria) is given. Requirements are ap-plied to the base layers of the revetment because these are impor-tant in maintaining its stability under wave action and in ensuring that the structure will continue to function permanently. In this connection a distinction is drawn between permeable and impermeable bases. It is stated what materials can suitable be used for a per-meable- or impermeable-layer and what requirements they must

satis-fy, more particularly with regard to the material properties and compos it ion, compact ion, pene tra t ion of ma ter i al in to the othe r layers and the manner of use, while the circumstances of the job may impose restrictions on applicability.

In the experience of many dike managers, substantial damage is liable to occur at the transition from one type of revetment to an-other and in zones where the revetment ends. Although it is not practicable to give standard solutions, outright mistakes can be highlighted. The toe construction, the upper boundary of the hard

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-designing

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exeeution

management

problems

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andy coasts (inel.dunes)

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grass/clay dikes

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rigid measures (groins)

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loose materials

rockfill,gravel,

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pitehed stone/eonerete

sand

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asphalt

bloeks

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mattresses I mats

govermental

research

institutions

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in-site measurements

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models

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caleulations

contractors

manufacturers

consultants

COASTAL

PROTECT/ON-INTEGRA TED APPROACH

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considered.revetment and the transition to a different type of revetment are

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Becausesimple-to-useof themathematicalcomplexity modelsof theavailablesubject fortheredealingare withas yetvari-no ous kinds of revetment and subgrade. The actual progress r n this direction is discussed. All the same, with the aid of the data yielded by theoretical/empirical research, and the available expe-r1ence, it is possible to determine approximately the necessary di-menSlons of the given types of revetments.

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Although the Dutch guidelines and other reports subject are based on the research and experiences veloped country, the basic ingredients of this common value for the whole hydraulic engineering the developing countries.

on the discussed of the highly de-knowledge are of world including

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RESEARCH POLICY

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•

WE HAVE TO BUILD A REAL

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2. DESIGN PHILOSOPHY OF COASTAL DE FEN CE STRUCTURES

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Coastal defences are constructed to proteet the population and the economical values against storm surges. However, the absolute safe-ty is nearly impossible to realize. Therefore it is much better to speak about the probability of failure (or safety) of a certain de-fence system. To apply this method, all possible causes of failure have to be analysed and consequences determined. This method is actually under developing in the Netherlands for dike and dune de-sign. The "fauIt three" is a good tooI for this aim (fig. 4). In the fault tree, all possible modes of failure of elements can even-tually lead to the failure of a dike section and to inundation. They can also badly influence the behaviour of the revetment even

if properly designed.

Although all categories of events, that may cause the inundation of a polder, are equally important for the overall safety, the engi -neers responsibility is mainly limited to the technical and struc-tural aspects. In the case of the sea-dike the following main events can be distinguished (see also fig. 3):

- overflow or overtopping of the dike

- erosion of the outer slope or loss of stability of the revetment - instability of the inner slope leading to progressive failure

instability of the foundation and internal erosion (i.e. piping)

- instability of the whole dike

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~ ~ 1 EROSION OF CREST ~ overtopping settlement 2 EROSION OF ~ ~ ~ INNER SLOPE

-wave overtopping slip eirele outer slope 3 MICRO STABILITY

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-~ ~ ....-1 4 SLIDING

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-slip eirele inner slape _{Ique ae Ion}

5 INTERNAL EROSION

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micro instability drifting iee

6 FLOW SLIOE ~ ~ ILJaUEFACTIONI ~ 7 WAVE IMPACT

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"piping" _{ship collision}

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PROTECTION

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sliding 9 SCOUR ~IIZZZ' ~ ~ \.._.../"

-tilting _{erosion fore shore} 1)SETTLEMENT

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_{AJ DIKE}

BJ DAM

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fig

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OVERVIEW Of THE FA/LURE HECHANISHS

-HUMAN FAILURE EXPLOSION SABOTAGE 'ACTS OF GOD'

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GENERALLY• 'WATER PRESSURE SLOPE > STABILITY

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SIHPL IFIED FA UL T TREE FOR A DIKE

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DAMAGE

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OF FAILURE

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TEST :

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,FUNCTIONS I : MODEL :

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BOUNDARYj

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CONDITIONS

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MATERlALS_GEOMETRY

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Fig.

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THE CONCEPT OF THE UL TIHATE LlHIT STA TE

OF FAILURE HECHANISH

-. 11.-.

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ETC.

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For all these modes of failure, the situation where the forces ac-ting are just balanced by the strength of the construction is Con-sidered (the ultimate limit-state). In the adapted concept of the ultimate limit-state (fig. 5), the probability-density function of th e ft po ten t i a I th rea tft (I 0 ad s ) a nd th e ftres i sta n ce ft (dik est ren 9 th )

are combined. The category "potential threat" contains basic vari a-bles th at can be defined as threatening boundary conditions for the construction e.g. extreme wind velocity (or wave height and period)water levels, and a ship's impact (colission) • The resistan-ce of the construction is derived from the basic variables by means of theoreticalor physical models (e.g. theoreticalof semi-empiri-cal stability-model of grains). The relations that are used to de-rive the potential threat from boundary conditions are called transfer functions (i.g. to transform waves or tides into forces on grains or other structural elements).

The probability of occurence of this situation (balance) for each technical failure mechanism can be found by applying mathematical and statistical techniques. The safety margin between "potential threat" and "resistance" must guarantee a sufficient low probabili-ty of failure. The different philosophies are currently available in construction practice:

1. deterministic, 2. quasi-probabilistic and 3. probabilistic.

For fully probabilistic approach more knowledge must still be acqu-ired concerning the complete problems associated with the use of theoretical models relating loads and strengthi improved knowledge of the theoretical relation between wave attack (induced pressures) and the strength of the revetment, of the probability of slope

(in-)stability related to the various soil parameters, and also of the theory of internal erosion is urgently needed. Studies on all these topics are still g01ng on in the Netherlands. The present Dutch guidelines for dike and dune design follow a philosophy, that lies between the deterministic and the quasi- probabilistic ap-proach (13) ,(31) ,(35).

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The ultimate potential threat for the Dutch dikes is derived from extreme storm surge levels with a very low probability of exceedan-ce (1% per century for sea-dikes and 10% for river dikes) and equa-ted with the average resistance of the dike without any apparent safety margin.

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Besides the ultimate limit-state, there are situations, where the ever continuing presence of a (frequent) load causes a detoriation of constructional resistance in time, without any imminent danger of failure (e.g. fatique of concrete and steel, creep or erosion of clay under the revetment, clogging or U.V. detoriation of geotex-tile, corrosion of cabling, un-equal settiements o r deformations, etc.). However, this detoriation of constructional resistance can cause an unexpected failure in extreme conditions. These are, so called, the serviceability- and fatique limit states which can also be considered as inspection and maintenance criteria.

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As already mentioned, the fully probabilistic approach for dikes based on the limit-state concept is rather cumbersome because a

theoretical description for various failure modes is not available yet. To overcome this problem a scheme to simulate nearly all pos-sible combinations of natural boundary conditions in a scale model

of the construction and to correlate the damage done to the boun-dary

conditions can be developed (black box approach). Of data of boundary conditions, resistance parameters

course, field and damage are

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- 13 -

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preferredcient amount.as base for correlation, if they are available ln suffi

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It has to be also stressed that having quantified (even roughly)

the fault tree, it is possible to pay extra attention to those mechanisms which contribute most to the overall probability of fai -lure. Thus, this approach is an important element in the attempt to the total quality con trol of the dike design and dike execution.Mo-reover, the probabilistic approach can be applied to some important parts of the total defence structure (e.g. revetments) where the necessary input is already available from the recent investigations in the Netherlands (9) ,(12) ,(31).

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The fully description of probabilistic approach for dike design

lies to far beyond the scope of this report. However, the detailed

information can be found in the Dutch reports and publications (1), (9), (31),(35). Taking knowledge of these recent developments can be rather profitable especially for estimation of possible risks involved rn the realized projects and for finding the optimum be-tween the risks and the investment.

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14

-design water level

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mean high water :

norma[- water level

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~----====---"":'=_"=":'____:~:::::=:::::::::;:::>"""7

atceptable lopding mean löw water 1 I 1 I , I 1 I III I TIl -I- -,- _, IJ

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6 LDADING ZONES ON A DIKE

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3.13. SHAPELoadingANO zonesHEIGHT(26OF) A OIKE

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The degree of wave attack on a dike or other defence structure du-ring a storm surge depends on the orientation in relation to the direction of the storm, the duration and strength of the wind, the extent of the water surface fronting the sea-wall and the bot tom topography of the area involved. For coastal areas there is mostly a certain correlation between the water level (tide plus storm sur-ge and wind set-up) and the height of the waves, because storm sur-ge and waves are both caused by wind. Therefore, the joined fre-quency distribution of water levels and waves seems to be the most appropriate for the design purposed (also from the economical point of view).

For sea-walls in the tidal region, fronting deep water, the follo-wing approximate zones can be distinguished (fig. 6)

I the zone permanently submerged (not present in the case of a high level "foreshore"):

11 the zone between MLW and MHW: the ever-present wave-loading of low intens ity is of i m po r tance for the long-term behaviour of structure:

111 the zone between MHW and the design level, this zone can be heavily attacked by waves but the frequency of such attack re-duces as one goes higher up the slope:

IV the zone above design level, where there should only be wave run-up.

A bank slope revetment in principle functions no differently under normal circumstances than under extreme conditions. The accent is, however, more on the persistent character of the wave-attack rather than on its size. The quality of the sea-ward slope can, prior to the occurrence of the extreme situation, already be damaged during relatively normal conditions to such a degree that its strength lS

no longer sufficient to provide protection during the extreme storm.

The division of the slope into loading zones has not only direct connection with the safety against failure of the revetment and the dike as a whole, but also with different application of materials and execution- and maintenance methods for each zone (fig. 7).

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3.2 Oike shape (21)

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The shape of the dike needs to be observed as longitudinally.

in cross-section as well

Cross-section

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The gradient of the bank may not be so steep that the whole slope o r the revetment can lose stability (through sliding). These crite-ria give, therefore, the maximum slope angle. More gentle (flatter) slope leads to a reduced wave-force on the revetment and less wave run-up; wave energy is dissipated over a greater length. By using the wave run-up approach for calculations of the crest height of a trapezoidal profile of a dike for different slope gradients, the minimum volume of the embankment can be obtained.

However, this does not necessarily imply that minimum earth-volume coincides with minimum c o sts , An expensive part of the embankment comprises the revetment of the waterside slope and the slope sur-face (area) r n c r e e se s as the slope angle decreases. The optimum

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BLOCK PAVEMENT

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W.L

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PALE FENCE

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FASCINE I REED IGEOTEXTILE

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....00

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~Of

concr.te bIockl Q~O Jl(0.~ Jl(0.20 SUbla

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Subloyqrconsistsof clay 0.80 thickorminqstona

0.70 thlck unc!arCruShqdstona0.10 thick d_,.",.i",.",_"ions_""atedin m_{to NA"}

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g. 7 EXAHPLE OF DIKE PROTECT/ONS

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cross-section (based on costs) can be determined when the costs of earth works per m3 and those of revetment per m2 are known. Careful

attention is, however, needed because the revetment costs are not always independent of the slope angle, e.g. for steep slopes the heavy protection is necessary while for the mild slopes the ( cheap-er) grass-mat can provide a sufficient protection. Another point of economie optimalisation can be the available space for dike con-stuction or improvement.

The common Dutch practice is to apply the slope 1 6n 3 on the inner slope and between 1 on 3 and 1 on 5 on the outer (seaward) slope. The minimum cr est width is 2 m. The original (old) Dutch dikes were made of local clay and as steep as possible to minimize the

quanti-ty of soil. The steep outer slopes were protected against wave at-tack by all kinds of materials like wood, stone, bricks, mattresses of willow twigs balasted with stones, grass, etc. The core of a modern dike is made of great quantities of sand, brought into place mostly as hydraulic fill. This sand is covered mostly with a clay layer of thickness up to 1 m. In some recent works the clay layer have been replaced by the layer of mine-stone. In both cases the dikes have been protected by a revetment of pitched stones (basalt) or placed concrete blocks. The need to repair great lengths of sea dikes in a short time af ter the 1953 flood-disaster in the ~ether-lands, led to the introduction of asphalt revetments. This has ne-cessitated entirely new dike construction with asphalt revetments overlying directly the sand core. Depending on the type of asphalt mixture the special requirements and restrictions can be formulated on the steepness of the slopes and the zone of application (under water of dry), (27).

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The water-side berm is a common element in the Dutch dike construc-tion. It could in the past lead to a reduction in the expenditure on stone revetments (on a very gently sloping berm a good grass-mat can be maintained) and it produced an appreciable reduction in wave runup.

Present practice in order to obtain a substantial reduction in wave run-up, is to place the outer berm at (or close tol water level of the design storm flood. If the berm lies too much below that level, the highest storm flood waves would not break beneath or on the berm and the run-up will be inadequately affected, and the grass-mat on the upper slope too heavily loaded by waves leading to

pos-sible erosion.

For the storm flood berms at high design levels as in the Nether-lands (freq. 10-4) there are in general no problems with the growth of grass on the berm and the upper slope. However, there can be circumstances which require also the application of a hard revet-ment on the berm and even on a part of the upper slope I.e. when higher frequency of water level is applied leading to more frequent overwashing of the upper part by salt water due to the run-up or wave-spray (a comon grass-matt can survive only a few salty events a year). An important function of the berm can be its use as an ac-ce ss road for dike maintenance.

In gener al care should be taken to prevent erosion of the grass-mat at the junction with the revetment. The abrupt change in roughness may lead to increase of bottom turbulence and more local erosion. It is advisable to create a transition zone by applying the cell-blocks, geogrids or other systems allowing vegetation.

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_DueLongitud_to i_irregularnal prof_itiesile _{in the longitudinal profile of an embankment,}

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... s: .2' QJ s:_.... QJ.

i5 settlement _{"execution level __}

t--finol crest height _ -- - - __

~.-

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_

,:n4

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seicheIsquollosciUotlon+gust / /

/

/. '\~?

- - - dike af ter construction

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-- finol dike shope revetment

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Fig.

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DETERf1/NA T/ON OF D/KE HEIGHT

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construction

Ie stage lij _{LOG time-}

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i.e.30 yeors \

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i.e. sond-fill primory settlement (execution stage)

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--

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-

-

--

-

---

--

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---

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--I

settiement secundary settlement

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Fig

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9 SETTLEf1ENT

AS FUNCT/ON OF T/f1E

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the1n connectionbank, somewithreachesthe topographyof the slopesof thecouldterraiben insubjectedfront ortobehmoreind than normal wave or current attack. Not all revetments are equally suitable for use on a curved longitudinal profile, e.g. some (rec-tangular) block systems may leave gaping joints going around cu r-ved.AIso, the mechanical methods for placing of blocks is in prac-ti ce limited mainly to straight lines or to large radius bends with

sufficiently large areas.

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3.3 Dike height and wave run-up

3 .3 • 1 _{General consideration on the height of a dike (16), (21)}

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The height of a di ke was for many centur ies based on the highest known flood level that could be remembered. It is evident th at in this way the real risk of damage or the probability of flooding we-re unknown. Little was known about the relation between the cost to prevent flooding and the cost of the damage that might re sult from flooding. In the 20th century it was found that the occurrence of extremely high water levels and wave heights could be described adequately in term of frequency in accordance with the laws of pro-bability calculus. However the curves of extreme values, based on a relatively short period of obsevations, have to mostly be extrapo-lated into regions far beyond the field of observations with the risk for some uncertainties.

Af ter the 1953 disaster, the frequency of the risk of flooding was studied in the Netherlands in relation to the economie aspects. Fi-nally it was decided to base the design of all sea dikes fundamen-tallyon a water level with a probability of exceedance of 10-4 per annum. In the Netherlands the storm-surge is mostly incorporated in the estimated water level. If it is not a case, the storm-surge should be calculated separately and added to design water level. Besides the design flood level several other elements also play a role in determining the design crest level of a dike (fig. 8).

- Wave run-up (2% of exceedance is applied in the Netherlands) de-pending on wave height and period, angle of approach, roughness and permeability of the slope, and profile shape (gradients, berm) •

- An extra margin to the dike height to take into account seiches (oscillations) and gust bumps (single waves resulting from a sud-den violent rush of wind)~ this margin in the Netherlands varies

(depends on location) from 0 to 0.3 m for the seiches and 0 to 0.5 m for the gust bumps.

- A change in chart datum (NAP) or a rise in the mean sea level (assumed roughly 0.25 m).

- Settlement of the subsoil and the dike-body during its lifetime (at least 30 years), (see also fig. 9).

The combination of all these factors mentioned above defines the freeboard of the dike (called in Dutch as wake-height). The recom-mended minimum freeboard is 0.5 m.

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3 .3 .2 Wave run-up (15, (18)

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The effective run-up (R), on an inclined structure can be defined as R = Rn'YR YB Y~

where Rn = run-up on smooth plane slopes, defined as the vertical

-IRREGULAR

WAVES

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4

I

₂

Ru max/Hs

3

up

1

~

2

Ru max/Hs= 0

.

9 ~p

0~~~2

~4~ __ ~6~ __ ~8 __ ~1

~~~

!

"'"

~p

=tonaJ-(2TtHs

/gTJ

•

~~~~

!

/rip -rap

,

~ ~

~;

i

::::

::

::

"...

'~::::

L~

:~.l'!!.

Rd mox/Hs =0.31~p-0.17

smooth slopes

Rd 2% IHs

=

0.33

gp

.

down

1

Rd

max/Hs

3

rip-rap

:

085/015

=

2

.

25

050

=

20

;

30

;

40

m m

F

i

g

.

10 RUN-UP AND RUN-DOWN FOR SMOOTH AND

RIP-RAP

SL OPES

-I

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height above still water level, YR= reduction factor roughness and permeability, YB = reduction factor due Y~ = reduction factor due to oblique wave attack and index.

For random waves Rn can be expressed by

due to slope to berm and ~ = break er

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2.5 Cn ~p where

I

where Cn ceedance and

a

= by 2% of

= constant depending on the type of wave spectrum and ex-percentage, Hs = significant wave height, Tp = top period angle of slope. The values for Cn = C2% (run-up exceeded

waves) estimated from the measurements are roughly equal

I

C2%to: = 0.55 à 0.60 for a small spectrum and C2% = 0.70 for a

wide spectrum.

Using C2% = 0.70 and wave steepness of about 5% (typical storm value for the North Sea Coast) one obtains the so called "Old Delft Formula" commonly used in the past for calculation of 2% run-up

(R2%) on the Dutch sea dikes, viz.

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R2% = 8 Hs tanU

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which is valid for ctgU ~ 3 and relatively smooth revetments.

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As a safe approach it is mining the run-up due to

recommended to use C2% the wind-waves (smooth

= 0.70 for deter-slopes) • In this

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case !.2.%= 1.75 p o r R2% = 0.7 Tp Vg Hs tanU for ~p < 2 à 2.5

HS"

and !.2.%= 3.5 or R2% = 3.5 Hs for ~p ~ 2.5

HS"

Some experimental results for smooth and rip-rap slopes are summa-rized in fig. 10.

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The reduction factors for surface roughness YRcan be roughly estimated as follows:

and permeability,

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Covering layer

asphalt, smooth concrete

concrete blocks, geotextile-mats, open stone-asphalt, grass-mat pitched stone, basalton

rough, permeable block mats gravel, gabions

rip-rap (min. thickness 2XD50)

...YL

1 0.95

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o •

0.9080 0.70 0.60

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In a case of slopes with a berm the run-up will be reduced by a factor B. The effect of a berm with a constant width (Bl is ma-ximum when the berm is situated approximately at the average water level (dB < 0.5 H, see definition scheme in fig. 11).

It has furthermore been found that the run-up diminishes with in-creasing berm-width although the reduction rapidly falls off once a certain minimum width is exceeded, i.e. B = 0.25 Lo for non- and weak breaking waves, and B = 4H for strong breaking waves, H/Lo > 0.03. The reduction factors YB for the berm width equal o r larger than the minimum width mentioned above, may be roughly esti-mated as follows:

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Q

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FIGA DEFINITIONS

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~

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c>

a:: a::

I

(ctgex

=

5)

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o~ __ ~

~~

__ ~~

__ ~~

___

-10 -0.8 -0.6 -04 -0.2 0 05 10 15 2.0 25 dB _

E..e..

ro-

H

FIG

B

REDUCTION OF WAVE RUN-UP DUE TO BERM

AS FUNCTION OF ~ AND ~~

I

I

I

,

,

,

V

-... ~ ./

[\

\

~

\

1\

\

"

Î\

'\

\

1\

\

\

I

I

',0 0,9 0,8 I(~ 0,7

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1

0,60,5

I

0,4

I

0,3 0,2

I

0,1 10 20 30 40 50 60 70 eOn 90 -_.~ I-'

I

I(~ : ~ : cos

f3

.

~

2-cos3 2

f3'

"'e·O

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I

FIG C REDUCT!ON OF RUN-UP DUE TO OeL/aUE WAVE APPROACH

Fig.

11

REDUCT/ON OF WA VE RUN-UP

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slope, ctga 5 to 7 4 3 YB (at dB < 0,5 H) 0.75 à 0.80 0.60 à 0.70 0.50 à 0.60

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Oblique wave attack, under an angle ~ can be roughly taken into ac-count by Yj3:

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Yj3 = cos

(13 -

10·),

13 ~

65· For

13

> 65°, Rn

=

Hs (not less than Hsl)

(N.B. ~ is reduced by '0° on account of variation of

13).

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Note ,.: a recent investigation in Germany *) on the oblique wave attack indicates that in the range 0<B<35°, instead of reduction, there 1.S even a slight increase of the runup (see Fig. 1tc ) ; For this reason (at this moment), it seems bet ter to assume no reduc-tion of runup in this range or (more safely) to follow the Fig. 11

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c ,

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Note 2·: depending on the wave spectrum, i.e. the ximum wave height and the type and permeability of of subgrade, the run-up can vary reasonably and protection has to be more or less extended. For model investigation may give a proper answer.

anticipated ma-revetment, type thus, the slope particular cases

I

The lowe r

protection limitis necessary)of slope canareabe roughlyattack bydefinedwaves by (where Rd ( d0wn ) = (0, 8

g

+ O. 5 ) f0r

g

< 2. 5 Hs a primary

I

and

I

=

2.5 for

g

> 2.5

I

Below this limit, if necessary,

on the base of occurring return longshore current or (orbital-)

slope protection has to be designed flow (shipwaves) or on the base of veloeities of wind waves.

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*) Tautenahin, E, Kohlhase, S. and Partenscky, H.W., Wave run-up at

sea dikes under obligue wave approach. Proc. 18th Conf. on Coastal Engineering, 1982.

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24

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FILTER

GRAVEL STATIC EQUILIBRIUM ~ ~

TOPLAYER ~ STONES .:::THICKNESS I WEIGHT ?

I

",/ GRANULAIR SYNTHETIC

BLOCKS DYNAMIC EQUILIBRIUM

f:JtIL(JRE

I

DESIGN CRITERIA 8

CRrrE,

C O(/~ FLOW PATTERN

RiJt

""

~

HOW DO I HAVE

O.4,t~/~6"~

SCOUR

~V~~~~,,"\'O

TO DESIGN IT ? ~8 PROCESS

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MEASURES

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CRITICAL SAND-PACKING ? FLOW SLIDE? 2S

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a = farces àJe tecb.m-rush

b = uplift pressures dle to water in filter

c = uplift pressures àJe to approachingwave front dK change in voelocity field

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/ / /

/

/' / / / / _/

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I

e '"' wave iIIpact

f '"' uplift pressures àJe to mass of water falling on slope

9 .. lew pressures 00 slope àJe teAir entraiment hK farces àJe to ~rush

/ I / ./

I

/

// /'

/

,/

_L_

/

----L'_

/

_L

/

Fig

.

12

FA/LURE HECHAN/SHS OF SLOPE REVETHENT

-I

I

F/G GfOTECHN/CAL FA/LURf I100ES

suffeston

I

~

____.

-

~

-

--_-~

e::

~

kink ."dls

dulruCllOn loploy@r or locol ."dlng du@10"'ave impocl

I

plplng under lilt bloek. ~ ~'

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I

- - - '.rosion pallern -'dlrfClion lrOundwaler flow

I

pipingunder cloy-Ioy.r r---~

I

I

bllllOcrO-l!Itehonlsms lifllng up af prot@etlweunlh ~ ~.- \.. . . <" \;;.0'"... ...

-:J

.

Int.rnal fllt@r transport

a) IIIlero-I!Itchanlms

I

cyellC eOlllpQetlOlldut to waw. illlpClct

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flo", shd.

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F

i

g

.

13 REVIEW OF GEOTECHNICAL FAILURE HODES OF A DIKE

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- 27

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4 • 14. STRENGTHGeneral OFapproachREVETMENTS

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Once the hydraulic design conditions have been established, actual design loads has to be formulated. For a given structure many dif-ferent modes of failure may be distinguished, each with a different critical loading condition. Schematically, this is shown in fig.

12 and fig. 13.

For the dike as a whoIe, instability may occur due to failure of subsoil, front or rear slope. Each of these failure modes may be induced by geotechnical or hydrodynamical phenomena.

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A modern (good) engineering pratice requires that attention should be given to all possible modes of failure of the construction under design.

I

A brief overview of the failure mechanisms of dikes, dams or banks is given below (35):

An overflow and/or wave over topping at high water-levels is a weIl known mechanism, which leads to water entering the polder and to soaking of the dike. The dangerous consequences result from the soaking of the body of the dike and erosion of the inner slope.

I

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Micro-instability of the soil material at the inner slope may re-sult due to seepage and a high phreatic plane.

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Ahighslipphreaticcircle planeat innerin aslopedike.mayThisbe willcausedbe amongthe caseotherwhenthingsthe dura-by a tion of the high water level is long or permanent.

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A slip circle in the outer slope may Occur when a low water follows an extreme high water (or sudden draw-down). The body of the dike is heavy with water and slides down.

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A slip circle in the waterway bank may obstruct the fairway. This instability can be caused by a rapid draw-down of the water table in the waterway or the presence of weaker or impermeable layers in the subsoil.

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Alocal shear failure (sliding of a revetment) parallel to the slo-pe mayalso be the consequence of a rapid draw-down or hydraulic gradients perpendicular to the slope.

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bedErosionmay be(removalcaused ofby particles)wave or currentof theinduceddike/bankshearprotectionforces sometimesor the assisted by hydraulic gradient forces.

I

Piping (internal erosion) may occur ive , the gradual format ion of a material entraining flow under an impermeable revetment or through alocal concentration of permeable material in the dike body/foun-dation. When the "pipe" eventually reaches the high waterside the process of internal erosion will accelerate.

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Migration indicates the transport of material behind the revet-ment. The transport may be parallel to the bank causing local slum-ping of the revetment or vertical resulting in an S-shaped pro-file. Material mayalso be lost through the revetment when filter requirements are not met.

I

-waves

water-

_structure

L..

..-level

I

""" external

I

"'" geometry ~Ir

external

r-pressures

_.. _internol geometry

black

~,

internal

_box

f-pressures

Ji-f~

~D-resultant

L.t

_load

_stable

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L

Z

I

_~

_L

I

<

S

~

strength S I I I j

no

"

deformat

i

on

_..

_...

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F

i

g

.

14

SET UP OF BAS

I

C RESEARCH AND

STABILITY

COf1PUTA TlON

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A liquefaction may Occur in loosely packed sands under influence of a shock or a sudden draw down. In this case the sudden increase of pore pressure reduces the shear strength pratically to zero and the soil behaves as a liquide

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Pumping is seen when the and th us generates a flow partieles of the soil. revetment of water bends under underneath. external The flow pressure entrains

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Settlements are due to consolidation, compression, migration, oxi -dation of organic material (i.e. peat layers).

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Horizontal earth dam,

sliding however, o r

tilting is mostly unlikely for a dike

for rigid structures it is of paramount _impor-or an tance.

lee may severely attack the revetment during wintertime.

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Heave of the soil may be caused by the format ion of ice crystals within the grain skeleton of the soil during the winter.

I

Ship collision against the dike/bank may cause considerable damage.

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In the design process, one is most interested in the ultimate limit state (U.L.S.) of a failure mechanisme This state is reached when the acting extreme loads are just balanced by the strenght of the structure. If the ultimate limit state is exceeded, the structure will col lapse or fail. The concept of the ultimate limit state is given in fig. 5.

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The present section is restricted to the stability of the front slope, moreover only instability as a result of hydrodynamical pro-cesses is taken into account.

The set-up of the studies and stability computation is shown sche

-matically in fig. 14.

Starting with the hydraulic input data (waves, water levels) and the description of the structure, external pressures on the seaward slope are determined. Together with the internal characteristics of the structure (porosity of revetment and secondary layers) these pressures result 1n an internal flow field with corresponding internal pressures.

The resultant load on the revetment has to be compared with the structural strength, which can be mobilized to resist these loads. If this strenght is inadequate the revetment will deform and may

ultimately fail.

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In many cases, the various processes cannot be described as yet. Therefore a "black box" approach is followed in which the relation between critical strength parameters, structural characteristics and hydraulic parameters are obtained empirically.

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The types of revetments which are presently being studied are shown in fig. 15. In this figure the critical mode of failure, the cor-responding determinant loads and the required strength are summari -zed qualitatively. Results obtained for rip-rap, placed block re-vetments, asphalt and grass are discussed in more detail in the

following sections.

I

-critical

determinant

strength

failure

mode

wave loading

I

sand I gravel

_{• inition of}

_{• velocity field}

_{• weight,}

!

motion

In

waves

friction

I

_{• transport}

of

_{• dynamic}

I

material

'stability'

I ~

• profile

I

formation

! i

:

clay I grass

_{• erosion}

_{• max}

_.

_velocity

_{• cohaesion}

I

• deformation

• impact

• grass-roots

I I

I

~

• quality of

I

clay

np-rop

_{• inition of}

_{• max. velocity}

_{• weight,}

I

motion

_{• seepage}

_friction

• deformation

• permeability

~

of sublayer I

core

I

gabions I

_{• inition of}

_{• max. veloc}

_i

_ty

• weight

(sand-, stone-,

motion

_{• wave impact}

_{• blocking}

cement- )

_{• deformation}

_{• climate}

_{• wlres}

mattresses

_{• rocking}

_{• vandalism}

_{• large unit}

incl. geotextiles

_{• abrasion I}

_{• permeability}

eerroston

of

~

incl. sublayer

wlres

_.»:

• U

.

v

.

placed blocks

_{• lifting}

_{• overpressure}

_{• th}

_i

_ckness

_,

incl. block mats • bending

• impact

friction,

• deformation

.»:

interlocking

• sliding

• permeabil

i

ty

incl. sublayer I

geotextile

• cabling/pins

aspha

l

t

_{• eros}

_i

_on

_{• max}

_.

_veloc

_i

_ty

_{• mechan}

_i

_ca

_l

• deformation

• impact

streng th

• l

i

ft

i

ng

• overpressure

• we

i

ght

r

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Fig

.

15 REVIEW OF SLOPE REVETMENTS WITH CRIT/CAL

MODES OF FAILURE

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4.2 Failure modes and determinant wave load

I

Classical slope revetments may be divided in different categories

(see fig. 15) e.g.

- Natural material (sand, clay and grass)

- Protected by loose units (gravel, rip-rap)

- Protected by interlocking units (concrete blocks and mats)

- Protected by concrete and asphalt slabs.

I

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In this order the resistance of the friction, cohesion, weight of the units, interlocking and mechanical difference of strength properties, also different.

protection is derived units, friction between strength. As a result of critical loading conditions

from

the the are

I

Maximum velocities will be determined for clay/grass dikes and gra-vel/rip-rap, as they cause displacement of the material while up-lift pressures and impacts, however, are of more importance for pa-ved revetments and slabs, as they tend to lift the protection.

As these phenomena vary both in space and in time, critical loading conditions vary both with respect to the position along the slope and the time during the passage of a wave. Instability for grassl

clay and gravel/rip-rap will occur around the waterlevel, where ve-locities are highest during up and down rush. Moreover , wave 1m-pacts are more intense in the area just below the still water level.

Instability of paved revetments without too much interlock occurs at the pink of maximum down rush, where uplift forces are higher, just before the arrival of the next wave front.

If the protection is pervious uplift forces are strongly reduced.

Instability will have occurred due to the combined effect of up-lift- and impact forces, just af ter wave breaking.

Concrete slabs and asphalt will mainly respond to uplift forces at maximum set-down. Due to the internal strength of the protection wave loads are distributed more evenly over a layer area, th us cau-sing a higher resistance against uplift, compared with loose block pavement.

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4.3 Wave loading and wave structure - interaction

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The interaction between waves and slopes is dependent on the local wave height and period, the external structure geometry (waterdepth at the toe, slope with/without berm, the crest elevation and the internal structural geometry (types, size and grading of revetments and secondary layers). The type of structure wave interaction is conveniently characterized by the so called breaker parameter

defined as (see also fig. 16):

I

I

tg

c

where H

₌

incident wave height

g=

-

Lo

=

wave leng

2

h.at deep water ~

(=

1.56 T 1n metr1c un1ts)

T

=

wave period

a.

=

slope angle of the front face

I

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For large values of the wave length or for large values of OC

(steep slopes) , the wave behaves like a long wave, which reflects against the structure without breaking - a so cal led surging wave. For shorter waves and medium slopes waves will short and break, causing plunging breakers for

g

values in the range of 1-:-3. This figure is common along the Dutch coasts with slope angles of 1 to 3 + 1 to 5, wave periods 6-8 5 and wave heights of 3+5 metres.

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-ACCRETION

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STABILITY

(STATIC EQUILIBRIUM)

NO MOVEMENT

OR

INITIAL MOVEMENT

AND

PROFILE DEVELOPMENT

(DYNAMIC EQUILIBRIUM)

OF COARSE MATERlALS

.•••••

_

AND

THEIR APPLICATION

IN

COASTAL ENGINEERING

- 33

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For mild slopes wave breaking becomes a

resulting in a more gradual dissipation

of breaking is called "spilling".

For the design of structures, surqin q and

more continuous process,

of wave energy. This type

main importance.

The area which suffers from wave-loading is bounded by the highers

uprush and the lowest downrush point. Obviously this zone is

vary-ing with the tide. The value of maximum up and downrush is shown in

fig. 10, both for impervious and pervious slopes. If the uprush

ex-ceeds the crest level, figures are no longer applicable.

I

I

plunging breaker are of

No reliable formula are available to predict the maximum veloeities

during uprush and downrush. For surging and spilling breaker,

nume-rical solutions have been obtained, which are, however, not yet

operational. A solution for the plunging breaker has not yet been

obtained. Thus, the wave loading on grass/clay dikes and gravel

(rip-rap protection) cannot yet be computed properly.

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4.4 Stability of loosely materials

An extensive research program has been performed recently in the

Netherlands on statie and dynamic stability of rubble mound

revet-ments, breakwaters and gravel beaches (12), (17), (18), (19). These

type of protection were studied experimentally to determine the

re-lationship between the critical streogth parameter, Hs/~Dn (H

=

wa-ve height, On

=

nominal grain/stone diameter and ~

=

specific

den-sity Ps - Pw I Pw)' and the parameter ~ describing the type of wave

attack. Using Hsl On parameter, the rough classification of

protec-tive applications is given in figs. 17 and 18.

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Fig.

16

BREAKER TYPES

I

b1"QOk

-borm dynomicolly

prof"q stoblq

S -shcpQ rock SIopqs sond brochels wotqr5

COllopsing

I

1 2 5 10 20 50 100 200 500 1000 2000 !JOOO

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I

F

·

17 TYPE OF STRUCTURE AS FUNCT/ON OF H

5 /

LJDn50

fg

.

I

New stability formula have been determined for different appli

ca-tions. An example of the general stability criterion involving all

design variables is presented fo r rubble mound revetments in fig.

1 9 •

-sandy

beaches

(nouri

shment

)

~~~5~==1~O====~=C2*OJ_

~~

_L3

~

O~

LL

J_LlJ40

-

...

~

H/~Dn

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I

I

I

I

I

I

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I

proteet

i

on

layer

~

rubble-mound

breakwaters

__

_

/

homogeneous

rock-fill

profile format ion

~rength.ned

part

~

,

"self adjusted"

or berm

profile

homogeneous

gravel

profi Ie format

i

on

-

s

t

a

ti

e stab

ilit

y

roc

k-

f

i

ll

dynam

i

c stab

ili

ty

rock-f

ill

I

I

I

dyna

mi

c stab

il

i

t

y

g

r

a

v

e

l

Fig. 18 APPL/CA TlON OF COARSE HA TERlALS

IN COASTAL ENGINEERING

-0.7r---r---r ~

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I

I!I impermeable core

o

6 • permeable core Thompson (1975)

o

5 e permeable core .. homogeneousstructure

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I

0.4 5= AID~ SWL

.r:

0.3 SIVN

_•

02

•

••

o

1

I

I

0°10 20 30 40 5.0 HsIDn50

«z

P ff- 0.18 --- formula for plungmg waves

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I

I!I

..

S

=

On , S 0

=

WSO SO% mass A/Dn2S0 (Wsol ps) 1/3 value of the distribution 60 70 N = number of waves Hs

=

significant wave height T2

=

average wave period ~z < 3(breaking waves) p = permeability coefficient

Fig

.

19A GENERAL STABILITY

FORHULA FOR RUBBLE HOUND REVETHENTS

A physical description for S is the number of cubical stones with a side of 1 x 0nSO, eroded over a width of 1 x 0nSO. The "no damage" criterion is taken generally to be when S is between 1 and 3 stones eroded.

I

In order to obtain stability formula including the permeability of the structure (revetment) , a permeability coefficient p is introdu-ced. This permeability in the stability formula e.g. P = 0.1 for the impermeable core and P

=

0.6 for the homogeneons (per.meable) structure tested. In fig. 20 four structures are shown with diffe-rent estimated values for P. For the time being it is left to the engineers judgement to choose the correct value for the structure to be designed. The final formula for plunging waves (breaker index

gz

< 3) is:

I

I

I

. ~

=

6.2 pO. 18 (S/vN) 0.2

I

°nSO with ~z = t e nü (2TIHs/g Tz2)-0.S

(Tz = average wave period)

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-I

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FOR cota ~3

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I

I

7 H 60n50 H / ( / ,0 0.22 ç-0.54 6t---.';:"__+--+--I • A0 n 50 - 4.4 S2 VN ) ~ z

t

r:

.

,0 1/6 0.1 5t---+-+--+--I -- -- H./60n50 - 1.25

v

a

(S2

I

v

N) ~z Ij:3000 WAVE$

~.

P""

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I

I

I

I

1~~-~~~~---~-__~__

L__~~_~~-L~ 0.5 0.6 0.7 0.8 0.9 1 2 3 _{4 5 6 7 8 9 10}

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Fig

.

19B RIP-RAP STAB/L1TY FORHULAE FOR N=3000 WAVES

AND AN IHPERHEABLE CORE

37

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p=

0.1

@

~~

p=

0

.4

®

1.~

~S~~

~

.

c,\>\e

\~~ec:.~e;

Dn50A/Dn50F =4

.

5

OnSOAlDnSOF = 2 OnSOFIOnSOC = 4

P=0

.

5

0

~

P= 0

.

6

@

1.~

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Dn50A/Dn50C= 3

.

2

no f

il

ter

no core

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OnSOA= nom

i

nal d

i

ameter armour

Dn50 F = nominal d

i

ameter filter

Dn50C

=

nominal diameter core

Structures

on

@

,

®

and

®

have been tested

.

The value of P for

@

has been assumed

.

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_Fig

_.

_{20 THE PERMEABILITY COEFFICIENT"P}

_"

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The methods of improvement of stability of rip-rap protection and the conceptional transition from the rip-rap into the block revet-ments are illustrated in figure 21 (19) ,(25).

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Current attack. In some cases, especially regarding the toe and bottom-protection in front of the structure, it can be necessary to con trol the stability of loosely materials against attack of the current. For this purpose the formula developed by Pilarczyk (16)

can be applied:

U 2 5

On 5 0 = ( cr) • B1Vk 4Jcr 9 llhI

where: 0n50 = grain diametre; (W50/Ps) 1/3 > 1 mm, h = water depth, Ucr = critical velocity, 4Jcr = critical Shields parameter, ll=relative density k = slope reduction factor = (1-sin2a/sin2~)O.5,

~ = angle of internal friction of material , a = slope angle Bl = stability coefficient. The values of 4Jcr and Bl can be estimated from tables 1 and 2 below:

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A. RIPRAP

D.AU STONES ARE PlACED

WITH THEIR LONGEST SIDE PERPENOIClLAR TO THE

SlOPE

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B. STOOE OVERLAY

(ONE TOP. LAYER I

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c.BINDERS ARE PlACED PERPf:NDICULAR TC THE SLOPE

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TOP VIEW

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F BASALT ON

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Fig

.

21

RIPRAP DESIGN AND IMPROV/NG MEASURES

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Table 1

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state of partieles _4Jcr absolute rest 0.03 start of instability 0.04 movement 0.06

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Table 2 flow conditions _Bl

major turbulent flow incl. local 5-6

disturbances and constrictions;

also outer bends of rivers

normal turbulence of rivers and

channels _7-8

minor turbulence; uniform flow

smooth bed conditions 8-10

It has to be stressed that, whatever method is adapted, the

experi-enc~ and sound engineering judgement play a large part in a proper

design of protective structure.

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4.5 Uplift forces. Block- and impervious revetments

The uplift forces are of importance as weIl for the impervious

(as-phalt, concrete) as for the pervious (block-) revetments. However

the calculation methods of uplift are quite different for the both

cases.

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The quality of concrete blocks was gradually improving in the last

decades and the cost diminishing (a.o. due to mechanical placing) s

that, at present concrete blocks of various sizes and shape are

used satisfactorily in coastal (dike) protection under a variety of

conditios (especially in countries with shortage of natural

materi-ais) • Many different kinds of, of ten patented, revetment blocks

ha-ve actually been used. The fact that design rules are still limited

in quantity has stimulated investigations in this area.

In respect to the block revetments a distinction can be made

be-tween:

1. Free blocks of different design.

2. Flexible interlocked blocks, i.e. due to grouting, cabling, etc.

(i.e. Basalton blocks, Armorflex-mats).

3. "Rigid" interlocked blocks(i.e. ship-Iap, tongue- and groove,

etc.) •

The first two systems (see also figure 22) have been recently teste

in the Delta Wave Flume at the Delft Hydraulics Laboratory (DHL) 1n

co-operation with Delft Soil Mechanics Laboratory (DSML).

The free placed blocks were tested for both, permeable as weIl as

umpermeable (clay) sublayers. The large scale tests have shown that

rectangular closed blocks placed directlyon clay form a very

strong revetment. When there was "good" quality clay (no erosion

of the sublayer) it was impossible to create damage conditions

within the range of possibilities of the wave generator

(H s

=

2. 0 m, Hm a x

=

2. 6 m, blo c k s D

=

O.1 0 - O.1 5 m th i c k). Bes ides

the requirement of good and homogeneous clay a very important

execution requirement lS the smooting of the slope before the

placing of the blocks. If the blocks are to perform properly, they

must adhere to the clay without the presence of too many

interstices and cavities. In the case of poor clay (sandy clay) or

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40

-PLACEO STONES STONE ':':':'..'~':_ PITCHING . ','.:: (BASALT) TONGU E-ANO ~ GROOVETY~

O

SHIP LAP TYPE ~ ~TEPPED WAFFLE TYPE TYPE

interlock

i

ng

bloeks

~

ARMORFLEX BLOCK ANO MAT

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PLACEO BLOCKS _{TYPE 'HARINGMAN'}

~

_W~

_BUILDING ~ BLOCKS GOBI BLOCK MOOIFIEO TONGUE ANO GROOVE TYPE 'A BLOCK'

tj

/ll/)!J'h.!.ASAL TON ~

Ji};

'

·::SA

LTON

REVETMENT

F

i

g.

22A EXAMPLES OF BLOCKS TESTED ON LARGE SCALE