SDC R 90163 Final Design Manual Coastal Protection

5/24/2018 SDC R 90163 Final Design Manual Coastal Protection

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Manual for Design of CoastalProtection Works

November 2009

Sea Defence Consultants

Aceh and Nias Sea Defence, Flood Protection, Escapes andEarly Warning System Project


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Aceh Nias Sea Defence, Flood Protection, Escapesand Early Warning Project

BRR Concept Note / INFRA 300GI

Manual for Design of CoastalProtection Works

November 2009

SDC-R-90163

SEA DEFENCE CONSULTANTS

www.seadefenceconsultants.com


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Manual for Design of Coastal Protection Works, November 2009

Aceh Nias Sea Defence, Flood Protection, Escapes and Early Warning ProjectBRR Concept Note / INFRA 300GISea Defence Consultants

i

PREFACE

This document is prepared within the Sea Defence, Flood Protection, Escapes and Early Warning System

Project, further referred to as SDC, and is part of a framework of documents. This framework and the functionof the documents within are described in more detail in the Introduction and should lead to a national standard

in coastal protection along the Indonesian coasts.

The Design Manual lying before you offers a detailed elaboration of design rules and calculations for different

types of coastal protection measures. The manual aims at engineers who are responsible for the detailed design

of coastal works in Indonesia, and is accompanied by the Guidelines for Coastal Protection. The Guidelines

provides very useful background information for engineers especially on the functional design considerations of

coastal works. For full understanding, it is recommended to use both the guidelines and the manual.

SDC, November 2009


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TABLE OF CONTENTS

Preface .........................................................................................i

Table of Contents .............................................................................. iiDefinitions of coastal parameters ......................................................... ivList of Figures................................................................................... vList of Tables................................................................................... viList of Symbols.................................................................................viiList of Equations..............................................................................viii1 Introduction............................................................................. 11.1 Coastal protection in Indonesia ..................................................................................... 11.2 Purpose and use of guidelines and design manual................................................................ 21.3 Lay-out of the Manual................................................................................................. 3

2 Data acquisition ........................................................................42.1 Introduction ............................................................................................................ 42.2 Bathymetry ............................................................................................................. 42.3 Hydraulic conditions .................................................................................................. 4

2.3.1 Wind data .................................................................................................... 4

2.3.2

Wave data.................................................................................................... 4

2.3.3 Tidal data .................................................................................................... 52.3.4 Currents ...................................................................................................... 5

2.4 Results................................................................................................................... 5

3 soft measures ........................................................................... 73.1 Beach and dune rehabilitation ...................................................................................... 7

3.1.1 Introduction ................................................................................................. 73.1.2 Beach and dune vegetation ............................................................................... 73.1.3 Sand fencing/trapping ..................................................................................... 9

3.2 Mangroves..............................................................................................................103.2.1 Introduction ................................................................................................103.2.2 Storm wave reduction.....................................................................................103.2.3 Tsunami impact reduction ...............................................................................10

3.3 Beach and foreshore nourishment .................................................................................103.3.1 Introduction ................................................................................................103.3.2 Nourishment ................................................................................................10

4 hard measures (onshore) ........................................................... 104.1 Tidal wall ..............................................................................................................10

4.1.1 Introduction ................................................................................................104.1.2 Design considerations and process......................................................................104.1.3 Concrete tidal wall, without bed protection..........................................................104.1.4 Concrete tidal wall, with rubble mound bed protection ............................................10

4.2 Sea dike ................................................................................................................104.2.1 Introduction ................................................................................................104.2.2 Design considerations and process......................................................................104.2.3 Sea dike, with armour layer wave protection.........................................................10

4.2.4 Sea dike, with armour layer wave protection and special toe .....................................10


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4.2.5 Sea dike, without wave protection .....................................................................104.3 Sea wall ................................................................................................................10

4.3.1 Introduction ................................................................................................104.3.2 Design considerations and process......................................................................104.3.3 Rubble mound seawall ....................................................................................104.3.4 Rubble mound seawall, with special toe...............................................................10

5 Hard measures (offshore) .......................................................... 105.1 Groynes.................................................................................................................10

5.1.1 Introduction ................................................................................................105.1.2 Design considerations and process......................................................................105.1.3 Rubble mound groynes....................................................................................10

5.2 Detached offshore breakwaters ....................................................................................105.2.1 Introduction ................................................................................................105.2.2 Design considerations .....................................................................................105.2.3 Rubble mound detached breakwaters..................................................................10

List of References............................................................................ 10Appendix A Coastal conditions ........................................................... 10


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iv

DEFINITIONS OF COASTAL PARAMETERS

COASTAL AREA: The land and sea areas bordering the shoreline.

COAST: The strip of land that extends from the coastline inland to the first major change in the

terrain features. The main types of coast features are the following: Dune areas, Cliff areas and Low

lying areas, possibly protected by dikes or seawalls etc.

COASTAL HINTERLAND: The land that extends landward of the coast and which is not influenced by

coastal processes.

COASTLINE: Technically the line that forms the boundary between the COAST and the SHORE, i.e. the

foot of the cliff or the foot of the dunes. Commonly, the line that forms the boundary between the

land and the water.

SHORE or BEACH: The zone of unconsolidated material that extends from the low water-line to the

line of permanent vegetation (the effective limit of storm waves). The shore can be divided into the

foreshore and the backshore. The foreshore, also called the beach face, is the area between mean

low water spring and mean high water spring plus the up rush zone.

SHORELINE: The intersection between the mean high water-line and the shore.

SHOREFACE or LITTORAL ZONE: The zone extending seaward from the low water-line to some distance

beyond the breaker-zone. The littoral zone is the zone in which the littoral processes take place;

these are mainly the long-shore transport, also referred to as the littoral drift, and the cross-shore

transport. The width of the instantaneous littoral zone of course depends on the wave conditions.

BREAKER-ZONE or SURF-ZONE: There is no clear definition of the breaker-zone, but it can be defined

as the zone extending seaward from the shoreline that is exposed to depth-limited breaking waves.

The outer limit of the breaker-zone is called the BREAKER-LINE. The instantaneous width of the surf-

zone varies of course with the instantaneous wave conditions.

NEARSHORE ZONE: The zone extending seaward from the low water-line well beyond the breaker-

zone; it defines the area influenced by the nearshore currents. The nearshore zone extends somewhat

further seawards than the littoral zone.

CLOSURE DEPTH: The depth beyond which no significant longshore and cross-shore transports take

place due to littoral transport processes. The closure depth can thus be defined as the depth at the

seaward boundary of the littoral zone.


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v

LIST OF FIGURES

Figure 1-1: Islands and provinces Indonesia................................................................................... 1

Figure 1-2: General set-up of the Design Manual and its broader context ............................................... 3

Figure 3-1: Illustrations of Ipomoea pes-carpae.............................................................................. 8

Figure 3-2: Illustrations of beach grass ........................................................................................ 8

Figure 3-3: Example of sand fencing with wooden slats (Indonesia) or sticks (Tunisia)................................ 9

Figure 3-4: Mangrove planting along the coast and a mangrove root system...........................................10

Figure 3-5: Wind wave transmission through 100 m mangroves (Schiereck and Booij, 1995) ........................10

Figure 3-6: Main design elements nourishment..............................................................................10

Figure 3-7: Illustration of active profile height and beach width grow after nourishment...........................10

Figure 3-8: Illustration options of placement in cross-profile (note: other options also possible) ..................10

Figure 3-9: Distribution of placed nourishment in longshore direction ..................................................10

Figure 4-1: The sheltered position of a tidal wall in the cross-profile ...................................................10

Figure 4-2: An example of a concrete tidal wall (Singkil, Sumatra)......................................................10

Figure 4-3: Typical cross-profile concrete tidal wall .......................................................................10

Figure 4-4: Main design elements concrete tidal wall ......................................................................10

Figure 4-5: Wind set-up increasing the water level.........................................................................10

Figure 4-6: Crest height concrete tidal wall .................................................................................10

Figure 4-7: Vertical equilibrium behind the structure......................................................................10

Figure 4-8: Piping length of a tidal wall ......................................................................................10

Figure 4-9: Forces on a concrete tidal wall ..................................................................................10

Figure 4-10: Failure mechanisms for an earth retaining tidal wall .......................................................10

Figure 4-11: Forces during HAT and LAT conditions ........................................................................10

Figure 4-12: Bed protection with geo-textile, gravel- and armour layer................................................10

Figure 4-13: The position of a robust sea dike in the cross-profile.......................................................10

Figure 4-14: A clay dike with rubble mound wave protection along the Dutch coast .................................10

Figure 4-15: Typical cross-profile sea dike (with armour layer wave protection)......................................10

Figure 4-16: Main design elements sea dike..................................................................................10

Figure 4-17: Design wave height at the structure depends on breaker plunge distance..............................10

Figure 4-18: breaker index at the toe of the structure (from SPM,1984)................................................10

Figure 4-19: Definition of stone dn50and layer thickness t ................................................................10

Figure 4-20: Different cross-sectional designs and their permeability parameters ....................................10

Figure 4-21: Geo-textile, gravel and filter layers ...........................................................................10

Figure 4-22: Scour development in short-term erosion.....................................................................10

Figure 4-23: Definition of Ltand ds ............................................................................................10

Figure 4-24: Scour development with long-term erosion...................................................................10

Figure 4-25: The position of a sea wall in the cross-profile ...............................................................10

Figure 4-26: An example of a rubble mound seawall (Pasi lhok, Sumatra)..............................................10

Figure 4-27: Typical cross-profile rubble mound seawall ..................................................................10

Figure 4-28: Main design elements rubble mound seawall.................................................................10

Figure 4-29: Definition of design water level hand freeboard Rcat a seawall.........................................10


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Figure 4-30: Illustration of threat wave overtopping a sea wall ..........................................................10

Figure 5-1: Lay-out scheme of a groyne field ................................................................................10

Figure 5-2: An example of a groyne system ..................................................................................10

Figure 5-3: Main design elements rubble mound groynes ..................................................................10

Figure 5-4: The trunk and head of the groyne ...............................................................................10

Figure 5-5: Layout scheme of a detached breakwater field ...............................................................10

Figure 5-6: An example of a detached breakwater system (Lakkopetra, Greece) .....................................10

Figure 5-7: Main design elements rubble mound detached breakwaters ................................................10

Figure 5-8: Trunk and head at a breakwater.................................................................................10

LIST OF TABLES

Table 4-1: Piping parameters for different soil materials .................................................................10

Table 4-2: Values for Sfor different damage levels and outer slopes ...................................................10


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vii

LIST OF SYMBOLS

All water levels and water depths used in this manual are relative to local MSL. If

the local benchmark Bakosurtanal is preferred, it should therefore betranslated to MSL per location according to the available data.

B : width of the structure [m]

Bprot : width of the bed protection [m]

Ccreep : piping coefficient [-]

d : water depth [m]

dn50 : nominal stone diameter [m]

The stone diameter dn is the diameter of a fictitious equivalent stone cube(note: not a sphere, which would results in ds). The nominal stone diameter dn50is exceeded by 50% of the stones in its grading.

ds : scouring depth [m]

g : gravitational acceleration [m/s2]

h : design water level [m +MSL]

H1/1 : wave height with frequency of occurrence 1/1 year [m]

H1/25 : wave height with frequency of occurrence 1/25 years [m]

HAT : Highest Astronomical tight [m]

hc : height of clay embankment [m]

hset-up : Wind set-up [m]

Hd : driving horizontal force [kN]

Hr : resisting horizontal force [kN]

Hs : local significant wave height [m]

L0 : deep water wave length [m]

LAT : Lowest Astronomical Tide (relative to MSL) [m]

Lp : piping length [m]LS : Land Subsidence [m]

Lt : width of the toe construction [m]

Md : driving resisting moment [kNm]

Mr : resisting turning moment [kNm]

MSL : Mean Sea Level:

N : number of waves [-]

P : permeability coefficient [-]

Pcvert : vertical clay pressure [N/m

2]

Pwvert : vertical water pressure [N/m

2]

Rc : freeboard [m]

Ru2% : wave run-up height [m]

S : damage number [-]

SLR : Sea Level Rise [m]

t : layer thickness [m]

Tp : wave peak period [s]

Vd : driving vertical force [kN]

Vr : resisting vertical force [kN]

W50 : nominal stone weight exceeded by 50% of stones in its grading [kg]

z : crest height [m]

: slope angle []

: relative density [-]

H : water head difference [m]

cr : critical surf similarity parameter [-]


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p : surf similarity parameter [-]

q : overtopping surf similarity [-]

c : volumetric density of clay [kg/m3]

s : armour stone density [kg/m3]

w : volumetric density of (sea)water [kg/m3]

LIST OF EQUATIONS

{parameter} based on {source} Modified?

[Eq. 1] : design water level SDC, Baseline Volume IV, 2009 N

[Eq. 2] : Wind set-up Rock Manual, 2007 N

[Eq. 3] : crest height SDC, Baseline Volume IV, 2009 N

[Eq. 4] : water pressure Verruijt, 2004 N

[Eq. 5] : soil pressure Verruijt, 2004 N

[Eq. 6] : safety to heave Van Baars, 2006 N

[Eq. 7] : piping length Schiereck, 2001 N

[Eq. 8] : piping length Schiereck, 2001 Y

[Eq. 9] : safety to uplifting Van Baars, 2006 N

[Eq. 10] : safety to sliding Van Baars, 2006 N

[Eq. 11] : safety to overturning Van Baars, 2006 N

[Eq. 12] : safety to soil foundation failure Van Baars, 2006 Y

[Eq. 13] : layer thickness Schiereck, 2001 N

[Eq. 14] : bed protection Best practice -

[Eq. 15] : crest height SDC, Baseline Volume IV, 2009 N

[Eq. 16] : surf similarity parameter Rock manual, 2007 N

[Eq. 17] : deep water wave length Schiereck, 2001 N

[Eq. 18] : local significant wave height Expert judgement -

[Eq. 19] : wave run-up Rock manual, 2007 N[Eq. 20] : wave run-up Rock manual, 2007 N

[Eq. 21] : maximal wave run-up Rock manual, 2007 N

[Eq. 22] : overtopping discharge Rock manual, 2007 Y

[Eq. 23] : maximal overtopping discharge Rock manual, 2007 Y

[Eq. 24] : stone stability, plunging (d


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1

1INTRODUCTION1.1 COASTAL PROTECTION IN INDONESIAIndonesia is an archipelagic country, with more than 17,000 islands and over 80,000 km of coastline (see Figure1-1). Along this extremely long coastline, coastal erosion and coastal flooding are significant and frequentlyoccurring natural threats. These threats are expected to increase further in the future due to climate changeand ongoing land subsidence. To manage the coastal threats, either by prevention or adaptation, coastalprotection programs are required in many places in Indonesia. A short description of coastal erosion issues andcoastal flooding issues in Indonesia is given in Box 1-1.

Figure 1-1: Islands and provinces Indonesia

Coastal protection measures may provide insufficient protection or can even have adverse effects ifinappropriately planned or improperly designed, built and maintained. Knowledge of coastal processes and thefunctioning of different types of coastal protection measures are therefore of key importance in planning anddesigning viable and sustainable coastal protection programs.

Once implemented in a coastal protection program, the measures should be properly designed to fulfil theadopted functional- and structural requirements. This Manual aims to provide guidance to coastal engineersresponsible for the design and addresses subjects as data acquisition, design considerations (e.g. materials,allocation, etc.), calculation formulae and the pre-set order of design steps.

It should be noted that appropriate coastal zone management and functional design considerations shouldalways precede the design process from this manual. Therefore, accompanying this national Design Manual, aGuideline for coastal protection was drafted. Concluding, the following documents have been prepared:

Guidelines for Coastal Protection

Manual for Design of Coastal Protection(document in front of you)


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Box 1-1: Erosion and flooding issues in Indonesia

1.2 PURPOSE AND USE OF GUIDELINES AND DESIGN MANUALWhere the guidelines provide specific guidance to managers of local RR authorities (e.g. SDA, Bappeda etc.)and e.g. NGOs or private development companies, this manual aims at engineers who are responsible for thedetailed design of coastal works in RR.

It should be noted however, that engineers will find useful background information in the Guidelines forCoastal Protection: especially the functional design considerations of coastal works. For full understanding, itis recommended to use both the guidelines and this manual.

Set -up of Manual f or Design of Coasta l Prot ecti on

The set-up of this design manual is straightforward: the four basic approaches determine the overall groupingof every coastal protection measure. Protection measures from three of these approaches are discussed in this

manual: soft measures, hard measures (onshore) and hard measures (offshore). Adaptation measures arediscussed in the Guidelines (see Figure 1-2).

The set-up of design is very uniform: the main elements of each measure are designed in a pre-set order,clearly presented in well structured steps. Every step towards a proper structural design is being described,including detailed design rules and formulae for each step. Throughout each chapter one calculation exampleis shown, each design step goes accompanied by an example calculation in a greenly coloured box. Wherenecessary the assumptions are being treated with optimisation options (other use of material, variation inlocation, variations in dimensions or considerations relating to costs, maintenance or construction) in sidesteps and yellowish coloured variation boxes.

Coastal erosion issues

Coastal erosion has increased in Indonesia since the 1970s, due to the convertion of mangrove forests into shrimp ponds

and other aquaculture activities. Other causes for increased erosion have been unmanaged coastal development,

diversion of upland freshwater and damming of rivers. According to different sources, coastal erosion has been reported

throughout many provinces in Indonesia, including amongst others Lampung, Northeast Sumatra, Kalimantan, West

Sumatra (Padang), Nusa Tenggara, Papua, South Sulawesi, northern Java and Bali.

In Bali, several coastal protection schemes were planned and implemented to protect the valuable coastal tourism asset.

The implemented schemes included breakwaters, jetties and revetments but also nourishments. These structures were

effective in stopping coastal erosion to some extent; however the structures were considered a major sight disturbance

on the beautiful and touristic beaches of Bali.

Coastal flooding issues

Two types of coastal flooding can be identified; tidal flooding and tsunami flooding. Regular tidal flooding usually occurs

on a relatively small scale in specific low-lying certain areas (below normal high tide levels). Tidal flooding on a larger

scale can occur during extreme events and is expected to increase in the future due to land subsidence. For example in

Jakarta and Semarang, significant land subsidence is ongoing due to groundwater extraction. Large-scaled tidal flooding

has already occurred during extreme spring tide in 2007 and 2008, however due to ongoing land subsidence in a few

decades a normal springtide may cause large-scaled flooding as well. Land subsidence and increased tidal flooding can

also be caused by earthquakes, which occur frequently in Indonesia.

Tsunami flooding usually has a low frequency of occurrence but a major impact. The tsunami hazard therefore requires

a different approach. It was concluded from several studies that structural protection against large tsunamis is

technically difficult and very expensive. Moreover, the chance of occurrence for a certain tsunami wave height is very

difficult to predict and will always be surrounded by uncertainties. If a larger tsunami wave occurs than the design event

taken into account, a protective wall or breakwater may only increase the impact (damage and mainly loss of life)

because of a false sense of security behind the protective wall or breakwater. Concluding, for tsunami flooding,

implementation of adaption measures are recommended to minimize the loss of life. Large tsunami protection structures

are not considered to be economically feasible nor are they considered recommendable from a safety point of view.


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Figure 1-2: General set-up of the Design Manual and its broader context

Decision for interventionCoastal protectionRegional strategy

Design Manual

Rehabilitation,Nourishment, etc.

Tidal wall, Sea dike,Sea wall, etc.

Detached breakwater,Groyne system, etc.

Hard measures(onshore)

Hard measures(offshore)

Soft measures

Warning System, Raisinghouses, Relocation, etc.

Adaptation.

Data collection forspecific measure

Guidelines

Database

Databooks

Framework of the Design ManualThe national Design Manual for Coastal Protection is set up as a part of a framework of documents. Based onthe regional strategy and the guidelines for coastal protection a decision is made for intervention in thecoastal system. Data collection is required to acquire sufficiently accurate data about the local physicalproperties (tides, waves, water levels etc.), depending on the measure opted for. A database in combinationwith the Databooks can provide such information. If not available, local data acquisition is discussed in thisManual. Depending on the approach following from the regional strategy, the protection measure is eitherdiscussed in the guidelines (Adaptation) or in this manual.

Guidelines for Coastal ProtectionThe Design Manual is accompanied by the Guidelines for Coastal Protection. While the Manual offers a moredetailed elaboration of design rules and calculations for different types of coastal protection measures, theGuidelines provides very useful background information for engineers especially on the functional design

considerations of coastal works. For full understanding, it is recommended to use both the guidelines and themanual.

1.3LAY-OUT OF THE MANUALLocal coastal conditions are indispensable for the design of protection measures. Data acquisition (if no- orinsufficient data is available from the Databooks) is therefore the first subject to be addressed.

Chapter 2: Data Acquisition

From that point forward, the chapters are organized based on the three basic approaches discussed in thismanual:

Chapter 3: Soft measures Chapter 4: Hard measures (onshore)

Chapter 5: Hard measures (offshore)


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2DATA ACQUISITION2.1 INTRODUCTIONThe design of coastal protection works is based on site-specific data of the coastal conditions. The availabilityand quality of data for the project location can be evaluated with an online geo-based database, wherein 10km stretches of Indonesian coastline are specified. If no data (or database) is available one can obtain thenecessary data as indicated below.

2.2 BATHYMETRYThe local bathymetry influences hydraulic conditions such as wave parameters, tidal propagation, sedimenttransports and water level set-up. These hydraulic conditions subsequently determine the design conditionssuch as wave attack and design water levels used for coastal protection works. Reliable bathymetric data istherefore of key importance for designing coastal protection works.

The bathymetry can be obtained either by digitizing nautical charts (available at the Tentara NasionalIndonesia Angkatan Laut or Dinas Hidro-Oseanografi) or by conducting detailed bathymetric surveys.Offshore bathymetry based on satellite data can be obtained from international institutions such as NOAA.

Because the coastal zone is a dynamic system, the bathymetry might change considerably during differentseasons and/or after (extreme) events with morphological consequences (storms, tsunamis etc.). Note thatnautical charts are based on historical observations and might therefore differ from the current bathymetryand/or the results from bathymetric surveys, which are based on instantaneous observations.

Interpolation software provides tools to transform the depth samples from surveys or nautical charts into acovering map of the sea bottom. Cross-profiles perpendicular to the coast normal are either deducted from

this sea map, or are surveyed separately.

2.3 HYDRAULIC CONDITIONS2.3.1Wind dataWind data is important in coastal engineering for its influence on local generated waves and the calculation ofwater level set-up. Because Indonesia is located around the equator, the influence from the earth rotation(Coriolis-effect) on the atmospheric wind systems is negligibly small. Strong winds are therefore quiteuncommon and their influence is relatively small.

Offshore wind time series at 10 m above the water surface - are available from international institutes such

as the ECMWF (European Centre for the Medium-range Weather Forecast) or based on satellite observations(e.g. from Argoss). With a statistical analysis of the wind data (i.e. Extreme Value Analysis), extreme windspeeds and directions for certain return periods can be determined. Winds blowing in the direction away fromthe project location will not induce water level set-up (it might actually induce water level set-down) andshould not be taken into account.

The maximum wind speed U10for a chosen return period is used in this manual to calculate the wind set-up.

2.3.2Wave dataLocal wave conditions depend heavily on the offshore bathymetric features such as land boundaries, offshoreislands and water depth. These waves can be generated by local winds (sea-state) or by distant storms (swell)and change their characteristics as they propagate into shallower water. The wave climate in the nearshore

area (see Definitions of Coastal Parameters) is thus determined by the offshore wave climate.


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The most reliable nearshore wave climate is determined from measurements over a longer period of time(~several years) that includes the seasonal variety during monsoons etc. With the help of statistical analysis ofthe wave data (i.e. Extreme Value Analysis), extreme wave heights and wave periods can be determined fordifferent wave directions. The maximum wave conditions should be used for the extreme nearshore waveconditions.

If no wave measurements in the nearshore area are available, 2D wave modelling should be applied to transferoffshore wave parameters to nearshore wave parameters. Offshore time series based on hindcast models areavailable from international institutes such as the ECMWF (European Centre for the Medium-range WeatherForecast) or based on satellite observations (e.g. from Argoss). As for the measured wave data, statisticalanalysis is used to determine the offshore extreme wave parameters for different wave directions. Theseparameters are transferred to the nearshore location with the 2D model. The maximum wave conditions at thenearshore location are used as extreme nearshore wave conditions.

In this manual, use is made of the wave height Hsand wave period Tpwith a 1/year return period, as well as Hsand Tpwith a return period of the chosen design safety level.

2.3.3Tidal dataLong term tidal observations (~30 years) provide the most reliable and accurate data on water levelfluctuations including monthly-, seasonal- and yearly variations. Less accurate tidal data might be available inthe form of collected sets of tidal constituents from e.g. Indonesian Tide Tables (ITT), IHO database or theAdmiralty Tide Tables (ATT). However, these sets of tidal constituents contain only the major constituentsthat contribute to the tidal signal and are only available for specific locations such as major harbours andlarger cities along the coast. In general, seasonal and yearly fluctuations of the tidal signal can not be derivedfrom these constituents alone. For certain locations, monthly- and daily variations of the water level mightalso be available from the nautical charts. This data will not include seasonal and yearly variations and willonly contain the maximum and minimum water levels from which no time series or tidal constituents can bederived.

If no reliable data is available (either because no constituents for the specific location can be obtained or noobservations have been carried out), the best way to obtain reliable tidal data is setting up a measuringprogram, preferably over a longer period (~several years). Tidal constituents can be derived from the

measured water levels with harmonic analysis software such as Delft3D-TIDE.

Because in most coastal protection works the available time is limited, setting up long term observationprograms is in general unviable. Tidal modelling is in these cases the best option to determine the tidal data.These models provide data with the minimum accuracy level that is required. Boundary conditions for suchtidal models should be offshore tidal constituents (based on global satellite observations, e.g. from theTpxo6.2 model) and/or tidal constituents at locations nearby. Measured tidal constituents and water levels canbe used to validate the model results.

To include the yearly cyclic effects, in this manual use is made of the HAT(= Highest Astronomical Tide) andLAT (= Lowest Astronomical Tide). These values represent the highest and lowest water levels respectivelythat occur during a full year.

2.3.4CurrentsCurrents in the coastal zone can be induced by monsoon winds, wave action, tidal action and large scale oceancurrents. Especially in areas where large water level gradients are present, related to the tide as well as large-scale water level variations, relatively strong currents can be induced. This is often the case along islands orsuch which act as a barrier for the propagation of the tidal wave. Information on the ocean currents can beobtained from tidal stream atlases or determined with a 2D tidal model.

2.4 RESULTSThe results from the data acquisition should be properly organised to be readily implemented in the designprocess. As an example the results are shown here for the fictitious location X2.The coastal conditions used inthe examples are appended in Appendix A.


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BathymetryThe coastal profile (in [m +MSL]) is drawn from the sea map. Below, only the part up to 100 m from thestructure toe is shown. In practice, the profile with a distance up to the nearshore area (where the waveparameters are determined) has to be established.

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

-20 0 20 40 60 80 100 120

MSL

Coastal profile: location X2

Hydraulic conditionsThe hydraulic conditions determined from the measurement program or model results are tabulated as below.In this case, a return period of 25 years is adopted as safety level.

Wind Wave Tide CurrentU10[m] H1/1[m] T1/1[s] H1/25[m] T1/25[m] LAT[m +MSL] HAT [m +MSL] vmax[m/s]

15 1.95 15.3 3.2 16.9 -0.57 +0.58 0.07


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3SOFT MEASURES3.1 BEACH AND DUNE REHABILITATION3.1.1IntroductionCoastal beaches and dunes have a value well beyond that of habitat, serving as coastal protection andpreservation in several ways. Continuous barrier dunes serve as flexible barriers to storm surges and waves andare of particular value in affording protection to low-lying backshore areas and in helping to preserve theintegrity of low barrier islands. Beaches and dunes provide protection more effectively and at a lower costthan a seawall. For example, the most densely populated areas of the Netherlands, which are largely situatedbelow sea level, are protected against coastal flooding only by sand dunes.

Dunes depend on beach sand for their formation and beaches need the reservoir of dune sand during storms.Consequently, the beach and dune should be managed as a single unit in terms of sand balance. There shouldbe a large area of dry-sand beach over which the wind can blow and pick up the sand grains to build up thedunes. Unless a dry-sand beach is present, dune formation is unlikely to take place.

Vegetation can play an important role in holding sediment together and thereby decreasing beach and duneerosion rates. Dune restoration refers to the process that aims to return the shoreline system to a dune systemthat existed before (whether or not this was pristine). The goal of this process is to emulate the structure,functioning, diversity and dynamics of the dune ecosystem using reference dune systems as models. Dunes canprovide protection against flooding and erosion in storm events and tidal inundation of the hinterland. Re-greening of the coastal area consists of artificially planting vegetation on sandy dunes and higher beaches,with the objective to trap the sand and to make it available for the dynamic process of the beach and forcoastal protection. Dune vegetation promotes large-scale trapping of sand.

3.1.2Beach and dune vegetationVegetation on beach and dunes has the following functions in coastal protection:

The vegetation reduces wind speed in the contact area between wind and sand. The sand cannot bepicked up by the reduced wind and will remain in placed (sand trap).

The roots of plants form a kind of natural geo-textile in the top layer of the sand. This increases thestrength and erosion resistance.

Vegetation will prevent sand transport further inland, decreasing potential hinder to roads or urbanareas in the hinterland.

Suitable species for regreening in saline conditions in Indonesia are (for example): Ipomoea pes-carpae andbeach grass. The main considerations to take into account are described below for both types.

Ipomoea pes-carpae Ipomoea pes-carpae (beach morning glory) is a fast growing creeper on sand in saline conditions that

is abundant in the Indonesian coastal zone. Wide areas can be covered in Ipomea. Cuttings can be harvested in areas where Ipomea grows abundantly; there is no need for preparing

seedlings in nurseries. Cuttings must be planted in the wet sand under the top layer of dry sand. The top of the cutting

should be higher than the dry sand top surface of the beach, to prevent it from being covered by thedry sand before rooting properly.


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Figure 3-1: Illustrations of Ipomoea pes-carpae

Ipomoea pes-caprae Creeping forward, covering large areas

Uprooted branches Cutting of Ipomea

Beach grass Beach grass is a grass that follows behind the Ipomea in the race of covering up the sand. Its a grass

with fine stems that from its roots form new branches, growing out of the surface of the sand. Itmultiplies by growing forward by roots, thus forming a perfect network of roots in the sand,preventing erosion.

Beach grass can be found abundantly along the Indonesian coast, allowing uprooting of plants insufficient quantity. There is no need for preparing seedlings in nurseries.

Figure 3-2: Illustrations of beach grass

Beach grass front, following the Ipomea Uprooted beach grass stems


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3.1.3Sand fencing/trappingSand fencing on beach and dunes has the following functions in coastal protection:

Sand fences reduce the wind speed, and in this way stop the sand from being transported by the

wind. The sand is trapped at the very place of the screens of sticks etc. In this way, sand fencesspeed up the process of sand dune build-up.

Sand fencing can be executed on a community level. Generally locally available materials that can be acquiredat low costs are used. Sand fences can consist of sticks, leaves, branches, reed or other natural availablematerials.

Figure 3-3: Example of sand fencing with wooden slats (Indonesia) or sticks (Tunisia)

According to (GTZ, 2007) the following design considerations apply:

Fences should be placed near the natural vegetation line or dune line Sand fences usually consist of vertical wooden slats joined by wire and supported at 1 m intervals by

fence posts which are hammered deep into the sand.

The space between the slats should be about the same width as the slats themselves so the fence has

a porosity of 50%

A 1 m high fence with 50% porosity will usually fill to capacity within 1-2 years. The dune that builds

up against the fences will be about as high as the fence itself.

Deposition of sand at screens will result in the screens being covered up. To sustain the development

of the dunes beginning to develop, new screens have to be installed


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3.2 MANGROVES3.2.1IntroductionMangrove (re)forestation increases the safety of the hinterland. The aerial roots and tree trunks play animportant role in wave damping and their flow resistance dissipates the energy of floodwaters and tsunamis.Thiscauses much of the sediment load which was transported by the water to settle. The erosion protection isfurther increased by the mangrove root system which serves as a sand trap and increases soil strength anderosion resistance.

Important criteria for mangrove planting are:

The hydrological conditions should be satisfied: no strong wave attack and the area should be under

some influence of tidal fluctuations.

Type of mangroves selected should be the same as the one which originally existed on the project

location.

State of soil; the soil conditions such as soil salinity, pH balance, and other soil parameters should be

fulfilled. Usually sandy beaches are not suitable for mangrove planting, muddy coasts and river

mouths provide much more fertile soils for mangrove planting.

The density of the plantation in relation of the effectiveness of wave attenuation.

The width of the mangrove greenbelt to be planted

The caring of the re-planting process.

Figure 3-4: Mangrove planting along the coast and a mangrove root system

3.2.2Storm wave reductionMazda, et al. (1997) stated that the provided protection by mangroves is strongly dependent on the tree heightand the vegetation density of the trees (submerged trunks, branches and leaves). The typical forest density ofa mature mangrove is about 1,000 trees per hectare (1 tree per 10 m2), and a sparser density will result in alower wave damping capacity and lower resistance to water movement.

The wave damping capacity of a mangrove forest for storm waves could be expressed in terms of the wavetransmission coefficient KT, which is the transmitted wave height divided by the incoming wave height.Schiereck and Booij (1995) found a relationship between KT , the density and the waterdepth as shown inFigure 3-5. These values of KT are likely to decrease by increasing the width of the greenbelt, thedesignated strip of land for the mangrove forest.


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Figure 3-5: Wind wave transmission through 100 m mangroves (Schiereck and Booij, 1995)

waterdepth [m]

0.2

0.5 1.0 1.5 2.0

0.0

0.0

0.4

0.6

0.8

1.0

KT[-]

sparse

average

dense

3.2.3Tsunami impact reductionAccording to Tri, et al. (1998), a mangrove is efficiently implemented if the width of the greenbelt isproportional to the wavelength of the incoming waves. This indicates that the greenbelt should be as wide asthe wavelength of the 1/year wave to significantly reduce the yearly maximum incoming wave. If thegreenbelt is designed reduce the impact of tsunamis however, the width of the greenbelt should be in theorder of kilometres. Calculation of the wave length used to determine the width of the greenbelt is furthertreated in section 4.2.3.


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3.3 BEACH AND FORESHORE NOURISHMENT3.3.1IntroductionBeach or dune replenishment, also known by the term nourishment, is the artificial addition of sand or gravelto the coast to improve beach or dune conditions. Within the context of the Indonesian coastal defencestrategy, the condition refers to the capacity of a beach or dune to act as a buffer against storm erosion,coastal retreat or tidal inundation to protect the land behind.

The main design elements are shown in the flow diagram in Figure 3-6.

Figure 3-6: Main design elements nourishment

Nourishment

1. Required nourishment volume

Net long-shore transport

Availability of sediment2. Borrow area

3. Placement and shape Local conditions

Required cross-profile

Function of nourishment

The included box in the design is a variation wherein the nourishment is combined with hard structures whichare discussed further in this Manual:

Box 1. Nourishment combined with hard structures


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3.3.2NourishmentRequired nourishment volume

The required nourishment volume depends on the function that the nourishment should fulfil. The requirednourishment volume is described for different functions below.

1. Flood protectionThe required nourishment volume can be determined by assessing the required beach and dune profile. Thebeach or dune should be higher than the beach run-up level and provide sufficient buffer to allow for erosionduring an extreme storm event. Cross-sediment transport modelling should be executed to obtain the requiredstorm profile.

2. Create a new beachIf a certain beach width is required, the required volume of beach fill can be calculated by multiplying therequired beach width with the total active profile height (high water level- closure depth). The active profilein seaward direction is limited by the closure depth, which is defined as the depth beyond which no significantsediment transport due to waves or currents occurs (see Figure 3-7 and Section 4.2.4). In this method, it isassumed that the new equilibrium profile slope (after redistribution of the nourishment in the cross-profile) is

equal to the existing beach slope. This assumption is only true when the nourishment sediment is similar to theoriginal sediment. It is always recommended to use similar or coarser sediment for the nourishment.

Figure 3-7: Illustration of active profile height and beach width grow after nourishment

3. Mitigate long-term coastal erosionThe required nourishment volume to mitigate the effects of long-term coastal erosion can be determined by

computing the erosion rates per year with detailed sediment transport models. The erosion volume over theplanned lifetime of the nourishment should be replaced by nourishment material. The growth in beach widthdue to a certain nourishment volume can be calculated by dividing the total volume by the total active profileheight (high water level- closure depth). Note that added to the calculated required nourishment volumes, 10to 20 percent losses during realization should be taken into account when determining the actual nourishmentvolume.

Borrow area

The following design considerations are applicable for determining the optimal borrow area:

The availability of nourishment material is a very important factor for the feasibility of a nourishmentscheme. If sediment is abundant anywhere close to the nourishment area, the costs can be reducedsignificantly. Furthermore, the abundance of sediment might also be a problem in some areas, thus

solving both problems with one solution. An example of this is a silted up river mouth, which might be


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closed off or too shallow for boats to enter. This is an often seen problem after a tsunami or extremeflood event where the morphological activity and cross-shore transports are high. Another option issedimentation at a structure upstream; if long-term erosion is caused by the presence of thisstructure, by-passing of sediment past the structure might be a solution.

If nourishment material is not abundantly available at river mouths or accreting coastlines nearby, thematerial should be taken from offshore locations. The location should be chosen well outside of theactive coastal zone (beyond the breaker zone in extreme conditions) to prevent negative effects on

the coastline and adjacent coastline. Another important consideration for the borrow area are the environmental aspects. An assessment of

(negative) environmental effects due to taking sand at the proposed borrow area should be executed.

Placement and shape

The placement in the cross-profile as well as the placement alongshore should be considered. The main designconsiderations for both are described below.

A. Placement in the cross-profileThere are different options for the placement in the cross-profile. The options are indicated in Figure 3-8 anddescribed shortly below.

Figure 3-8: Illustration options of placement in cross-profile (note: other options also possible)

1. Placement in the dunesIn the dunes the fill sand is not attacked by waves unless a large part of the dunes is eroded during a stormsurges. This type of beach fill may be applied for strengthening the dunes system against a breakthrough inextreme conditions. Placement in the dunes however is the most expensive type because transport by trucks isrequired and the sand has to be placed exactly in the required shape.

2. Placement on the beachWith this type of nourishment a widening of the beach is obtained. This method is cheaper than placement inthe dunes because the sediment can be placed by rainbowing it straight from a boat or by pipelines ending onthe beach. After placement, the sand will be reshaped by waves and currents and thus redistributed over theactive cross-profile. Part of the sediment placed on the beach will be transported in offshore directionbecause the entire active profile up to the closure depth will move seaward, see Figure 3-7. The redistributionafter placement might lead to the misunderstanding amongst involved parties that the nourishment is noteffective.

3. Placement in the offshore zoneThe sand is continuously subject to the forces of waves and currents. After placement, the sand will bereshaped by waves and currents and thus redistributed over the active cross-profile. Part of the sedimentplaced offshore will be transported towards the beach, see Figure 3-7. This method is the cheapest because

the sediment can be dumped directly from the boat. Applicability however depends on whether boats can


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reach the active zone where it is reasonably sure that dumped sand will benefit to the active cross-profile.The risk of this method is loss of the sand to deeper offshore locations before it can be picked up andredistributed onshore.

B. Placement alongshoreDepending on the placement in the cross-profile, there are also different options for the longshore placement.If the sediment is placed in the active beach profile (on the beach or offshore, not in the dunes), the sediment

will be redistributed by natural forces after placement. This provides different options for placementalongshore:

1. Direct placement where neededThe sediment is placed exactly where it is needed. Usually a coast has to be protected over some distance andtherefore the beach fill will have an elongated form.

2. Placement of a stockpileA stockpile is placed in a certain location, from the stockpile the sand is transported and distributed along thecoast by the natural hydraulic forces. The distribution of a stockpile is shown in the figure below, where thecoloured lines show the longshore coastline position after 1 and 5 years. The expected redistributionalongshore can clearly be seen in this figure.

3. Continuous nourishment

At one or two points continuous nourishment is executed. From these points, the sand is distributed along thecoast, as in the preceding case.

Figure 3-9: Distribution of placed nourishment in longshore direction


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NourishmentBox 1: Nourishment combined with hard structures

As discussed in Section 3.3.2 and shown in Figure 3-9, the nourished sediment will be redistributed in thecross- and longshore direction. After several years, most of the nourished sediment is redistributed to

adjacent stretches of coast and the shoreline will tend to reposition itself towards its original equilibriumprofile again. In order to maintain a certain position of the shoreline, maintenance nourishment programsshould be carried out.

To reduce the necessity and required volume of a maintenance nourishment program, it is possible tocombine the (initial) nourishment with a groyne system or with offshore breakwaters (treated in Section5.1 and 5.2). The groynes will trap the nourished sediment, thereby preventing it from beingredistributed in the longshore direction. Breakwaters will provide shelter from wave action at theshoreline which decreases the transport capacity (hence redistribution) in longshore direction. Anotheroption might be to construct a submerged breakwater as a sill to reduce the transport in crossshoredirection from the shore towards deeper water.

These combined solutions will not be designed in further detail in this manual, but an example of thecombination of nourishment and a groyne system / detached breakwaters is Bali, Indonesia. Groynes andoffshore breakwaters were constructed prior to a nourishment program. Below a picture is shown from thegroyne before the nourishment was carried out on the left side, while on the right side a picture is shownfter the nourishment. The groynes will help to keep the sand within the groyne bays and reduce themaintenance volume required.


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4HARD MEASURES (ONSHORE)4.1 TIDAL WALL4.1.1IntroductionConcrete tidal walls aim to reduce/stop flooding. The tidal wall should be placed at least landward of thebeach and preferably in wave sheltered areas (see Figure 4-1). Because of this placement, significant waveattack leading to wave overtopping is not expected in front of tidal walls. Minor wave attack might occur,possibly leading to a requirement to add some bed protection in front of the wall. In locations exposed tosignificant wave attack, other solutions should be chosen (e.g. a dike or seawall).

Figure 4-1: The sheltered position of a tidal wall in the cross-profile

Figure 4-2 shows an example of a concrete tidal wall in Singkil.

Figure 4-2: An example of a concrete tidal wall (Singkil, Sumatra)


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Standard design

The tidal wall dimensions in this manual are standardized, only the crest height is calculated based on localhydraulic conditions. The basic design for the concrete tidal wall used is shown in Figure 4-3.

Figure 4-3: Typical cross-profile concrete tidal wall

clay embankment

concrete

wooden piles

1.80 m

1.50 m

1.50m

1:1.5

0.3m

Whether a pile foundation is required, depends on the soil parameters such as shear and bearing capacity. Thishas to be determined by a geotechnical specialist because it cannot be expressed in standardized design rules.The same applies for their spacing and length. If constructed, note that the pile foundation should at least belocated below LAT,to prevent deterioration due to the presence of oxygen (rotting processes).


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4.1.2Design considerations and processDesign example and variations

The main design elements are shown in the flow diagram in Figure 4-4. These elements are worked outconsecutively in this chapter. One calculation example is given in this chapter, shown in green boxes

throughout the text. In addition, some possible variations (based on other conditions or other requirements) tothe example are shown in yellow boxes.

Figure 4-4: Main design elements concrete tidal wall

Concrete tidal wall

Erosion inspection4. Bed protection

1. Crest height

Freeboard

vertical equilibrium2. Clay embankment

piping length

3. Geotechnical stability Failure mechanisms

design water level


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Following the considerations above, depending on the expected wave attack, two types of concrete tidal wallsare discussed in this chapter:

Concrete tidal wall, without bed protection (no wave attack expected), see Section 4.1.3; Concrete tidal wall, with rubble mound bed protection (some wave attack expected), see Section

4.1.4.

The included boxes in this chapter for the supporting design example concrete tidal wall are:

Concrete tidal wallDesign considerations

The design elements shown in Figure 4-4 are influenced by design considerations and boundary conditionsbased on e.g. the location of the structure, material availability and/or maintenance possibilities. Because

all these design considerations influence one another, they are an integrated part of the design. Someimportant considerations are discussed below.

Location and use of materialThe location of the structure determines specific variables like soil characteristics, the water head overthe structure and the amount of wave action. In this way it influences the dimensions (and necessity) ofthe clay embankment, the geotechnical stability and the necessity of a bed protection. The choice of thelocation therefore partly determines the ultimate dimensions of the design. In general, the location forthe tidal wall should be chosen as high as possible, with as less wave action as possible and on (sandy) soilwith sufficient bearing capacity.

Also the construction space depends on the location, which influences the use of material and theconstruction method (e.g. no space for the embankment so a cutoff-wall is required, use of concreteinstead of a large clay body and/or the necessity of a pile foundation). For the use of concrete in aconcrete tidal wall the saline conditions are very important, because it requires specific concrete quality.

Construction costs, life time and maintenanceWithout maintenance, the strength decreases and the failure probability of the structure will increase.Proper maintenance programs for the structure will thus increase the life time and safety provided. This isshown in the figure below where the vertical increase in strength in the graph represents maintenance andrepairs. The strength of the properly maintained structure is sufficient during extreme loading (designconditions) at all times, while the strength of the structure without maintenance will decrease until itultimately fails, possibly not even during severe conditions.

Normal load

Extreme load

Strength with maintenance

Strength without maintenance

S

trength/safety

Time

If quick and sufficient repair facilities are available, some damage during extreme events can be allowedas long as it will not induce direct failure. The strength of the structure will then be restored bymaintenance and repair works afterwards. This reduces the strength requirements and construction costs.Note that to determine the overall costs, the costs for (regular) maintenance and repair works should alsobe taken into account.

A regular inspected and repaired structure thus allows for a different construction and design than onewhich is required to last its life time without maintenance (see figure above). All these considerationsshould be taken into account in the design to reduce costs and optimize the durability and performance ofthe structure.


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Box 1: Design assumptions for example calculation (see next page) Box 2. Example calculation crest height Box 3. Example calculation clay embankment, vertical equilibrium Box 4. Example calculation clay embankment, piping length Box 5: Variation with construction cutoff-wall Box 6: Example calculation bed protection

* Whether this is required for a certain location, should be determined by a geotechnical specialist.

Concrete tidal wallBox 1. Design assumptions for example calculation

The example of calculation illustrating the design rules is based on the following assumptions:

The following cross-shore profile with depth relative to MSL is representative for the location:

-1,6

-1,2

-0,8

-0,4

0,0

0,4

0,8

-20 30 80 130 180

The fictitious location is called Location X1. Hydraulic conditions can be found in Appendix A.

The structure is founded on a sand layer with sufficient bearing capacity. Therefore, no pile

foundation is needed in this example*.

The structure foundation level is at LAT.

The standardized concrete wall dimensions are shown below. Only the crest height is calculated

based on the local conditions.

MSL +0.4 m

MSL +0.3 m

MSL

1.50 m

1.80 m

1.50m

1:1.5

0.3m

crest heightclay embankment height


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4.1.3Concrete tidal wall, without bed protectionCrest height

The design water level and freeboard Rc determine the crest height of the concrete structure.

The design water level (with a safety level of 25 years) is:

with h Design water level, relative to MSL [m]HAT Highest astronomical tide, relative to MSL [m]SLR Sea level rise estimate for 25 years [m]LS Land subsidence estimate for 25 years [m]hset-up Wind set-up [m]

The estimate for SLRand LSare in this manual both assumed uniform in Indonesia as 0.1 m in the coming 25years. For further information on this subject one is referred to the Guidelines of Coastal Protection.

Wind set-upThe wind set-up is an additional rise of the water level due to the shear stresses exerted on the water surfaceby wind forces. In some locations along the Indonesian coast the wind set-up might be negligible small due tothe absence of high wind speeds and/or the presence of a steep foreshore. However, this is not necessarily thecase and wind set-up therefore has to be taken into account.

The total wind set-up hset-up is shown in Figure 4-5, with the fetch as the distance between the landboundaries.

Figure 4-5: Wind set-up increasing the water level

The wind set-up is:

with friction value between air and (sea) water [-] (=3.10-6)u Average wind speed during storm conditions [m/s]

g gravitational acceleration [m/s2] = 9.8 m/s2F Fetch length [m Wave direction related to the coastline [](perpendicular = 0)h Water depth [m]

upsethLSSLRHATh [Eq. 1]

cos2

1 2F

gh

uh upset [Eq. 2]


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FreeboardNote that wave attack is always expected to be relatively limited for concrete tidal walls (because of theirplacement in wave sheltered areas). If required, the rubble mound bed protection aims only at reducing thescour in front of the structure. Wave overtopping and run-up is not expected to be significant. For thefreeboard Rcthe minimum value of 0.5 m is therefore adopted (see Figure 4-6):

where Rc freeboard of 0.50 mz Crest height above MSL [m]

Figure 4-6: Crest height concrete tidal wall

Rc

z

h

MSL

cRhz [Eq. 3]


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Clay embankment

The clay embankment has the purpose to make the structure watertight. Its dimensions are determined by thecriteria of:

Uplifting; Piping.

UpliftingThe clay embankment behind the concrete structure must have a minimal height to prevent heave. Heaveis aprocess in which the water under the clay body lifts the body upwards. The downward pressure exerted bythe weight of the clay should be greater than the upward pressure from the water caused by the water headover the structure. This is shown in Figure 4-7.

Concrete tidal wallBox 2. Example calculation crest height

The following parameters are assumed for LocationX1:

F = 20 kmh = 40 m = 0

Using the hydraulic conditions from Appendix A, the wind set-up is calculated with [Eq. 2]:

05.010.20408.9

2510.3

2

1cos

2

1 32

62

F

gh

uh upset m

With SLR= LS= 0.1, the design water level is calculated with [Eq. 1]:

25.105.01.01.00.1 upsethLSSLRHATh m +MSL

Adding a freeboard of 0.50 m, the design crest height zis:

75.15.025.1 cRhz m +MSL

MSL

z = MSL+ 1.75 m

h = MSL+ 1.25 m


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Figure 4-7: Vertical equilibrium behind the structure

Hclay pressure

water pressure

The (upward) vertical water pressure is determined by the difference in water level over the structure:

with Pw(vert) Vertical water pressure [N/m2]w volumetric density of water [kg/m

3] (=1025 kg/m3)g Gravitational acceleration [m/s2] (=9.8 m/s2)H water head [m]

The (downward) vertical pressure of the clay is determined by its weight:

with Pc(vert) Vertical clay pressure [N/m2]

c volumetric density of clay [kg/m3] (= approx. 1600 kg/m3)

hc the height of the clay body [m]

Combining [Eq. 4] and [Eq. 5] and adding a safety factor of 1.5, the minimal height of the clay embankment toprevent heave is given by:

HgP wvert

w )( [Eq. 4]

ccvert

c ghP )( [Eq. 5]

g

Hgh

c

wc

5.1 [Eq. 6]


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PipingThe length of the clay body behind the structure is determined by the piping requirement: the length of theflow path below the structure should be sufficiently large to prevent the formation of micro-channels (pipes).These pipes can undermine the structure. Furthermore, if the length is too short, seepage water will still beable to induce flooding of the hinterland.

Bligh developed the following formula for the piping length:

with Ccreep Creep coefficient [-]Lp required piping length [m]

Table 4-1 shows the value of Ccreepfor different soil materials.

Table 4-1: Piping parameters for different soil materials

Soil material Ccreep[-]

Silt 18

Fine Sand (150 200 m) 15

Coarse Sand (300 1000 m) 12

Fine gravel (2-6 mm) 9

Coarse gravel (> 16 mm) 4

The piping length is determined by

with a,b,c,d piping lengths as indicated in Figure 4-8 [m]

creepp HCL 5.1 [Eq. 7]

dcbaLpiping [Eq. 8]

Concrete tidal wallBox 3. Example calculation clay embankment, vertical equilibrium

The water head His the difference between the design water level h on the sea-side and the land levelon the land-side hsand (in this example: MSL +0.4 m, see Box 1) on the land-side:

85.04.025.1 sandhhH m

With w =1025 kg/m3and c =1600 kg/m

3, the minimal clay height hcis determined by:

8.08.91600

85.08.910255.15.1

g

Hgh

c

wc

m

MSL

h = 1.25 m +MSL

H

0.8 m0.4 m

MSL +0.4 m


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Figure 4-8: Piping length of a tidal wall

H

a

b

c

d

Concrete tidal wall

Box 4. Example calculation clay embankment, piping length

It is assumed that in this example the soil material is coarse sand (Ccreep= 12).

According to [Eq. 8], the piping length is:

151285.05.15.1 creepp HCL m

The foundation level of the concrete structure is at LAT = -0.9 m, this leads to:a= 0.3 + 0.9 = 1.2 mb= 1.80 mc= 0.4 + 0.9 = 1.3 m

7.103.18.12.115 cbaLd p m

The length of the clay embankment should at least be 10.7 + 0.3 (width of the heel) = 11 m.

c = 1.3 m

MSL +0.3 m

MSL

1.80 m

11 m

b =1.8 m

a= 1.2 m

d = 10.7 m

0.3 m

MSL +0.4 m


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Geotechnical stability

Geotechnical stability is determined by the horizontal and vertical forces of both soil and water. Thecalculation of these forces is beyond the scope of this manual. Reference is made to a theoretical descriptionof soil mechanics for these calculations (e.g. Verruijt, 2004). Note that these types of calculations should beexecuted by a geotechnical specialist. In this manual only a qualitative description of the geotechnical aspectsis given. This is described first for the situation without pile foundation; a short description of the situationwith piles is also added. A pile foundation is applied if the underground does not provide sufficient bearingcapacity and/or if other geotechnical stability requirements are not met without the pile foundation.

The main forces induced by both water and soil are illustrated in Figure 4-9. These forces can induce multiplefailure mechanisms, which are described below.

Concrete tidal wallBox 5: Variation with construction cutoff-wall

If the required length of the clay embankment takes up too much space in the hinterland, an option toreduce this is to construct a cutoff-wall under the concrete structure. This will increase the flow path with

twice the construction depth of the cutoff-wall (see the figure below). A cutoff-wall is a simple screenthat can easily be placed.

11 m - 2L

L L

If we for example assume a screen with a length of 2 m, the clay embankment dimensions change:

7.63.38.12.315 cbaLd piping m

The required length of the clay body becomes 6.7 + 0.3 = 7 m

Another approach is to replace the entire clay embankment by a cutoff-wall. In that case, the requiredscreen length can be calculated as:

L= 11 m(required length clay embankment without cutoff-wall) / 2= 5.5 m


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Figure 4-9: Forces on a concrete tidal wall

Failure mechanismsWith geotechnical calculations it should be tested whether the structure meets the stability requirements.

This is determined by its resistance against the following failure mechanisms (see also Figure 4-10):1. uplifting2. sliding3. overturning4. eccentricity

Figure 4-10: Failure mechanisms for an earth retaining tidal wall

4.3.

1. 2.

In case of a structure with pile foundation, additional terms such as pile resistance and additional bearingcapacity should be taken into account in the calculation of the forces. It is noted that the driving- andresisting forces can change with different conditions. This is illustrated below during LAT (left) and during HAT(right). Another aspect is the sediment density, which is different in both dry and wet conditions.


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Figure 4-11: Forces during HAT and LAT conditions

1600 kg/m3

1800 kg/m3 2000 kg/m

1600 kg/m

1025 kg/m3

During LAT,the driving force is from right to left. The overturning point is therefore the toe of the structure.During HAT,the situation is the other way around due to the water pressure. The turning point is now the heelof the structure and the driving force from left to right.

The resistance to the mentioned failure mechanisms is determined as follows:

1. Uplifting:The water pressure below the structure induces an upward pressure. This upward pressure is the driving forcein lifting the structure upwards. Without pile foundation, the resisting force is only the weight of the structureitself:

with Vr resisting vertical force [kN]Vd driving vertical force [kN]

2. Sliding:

During LAT conditions the higher ground on one side is pushing the structure to the other side. This soilpressure is thereby the driving force for sliding along the base. The soil pressure on the other side and theshear between the structure and the soil are the resisting force. The presence of a pile foundation enhancesthe resistance against sliding:

with Hr resisting horizontal force [kN]Hd driving horizontal force [kN]

3. Overturning:The pressures on the sides cause the structure to have a tendency to rotate around the toe of the structure.

The driving turning moment is caused by the soil pressure on one side, while the vertical pressures on thestructure and soil pressures on the other side induce a resisting moment.

with Mr resisting turning moment around the structure toe [kNm]Md driving turning moment around the structure toe [kNm]

4. Eccentricity:The soil below the structure can fail due to eccentricity of the residual force, as the bearing capacity can

become lower than zero at the toe. In this case it has become an upward pressure, lifting one side of the

5.1

d

ruplifting

V

VF [Eq. 9]

5.1

d

r

slidingH

HF [Eq. 10]

0.2

d

r

overturnM

MF [Eq. 11]


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structure. As a first estimate for the safety against this failure, the residual force should have its point ofaction within the centre of the structure:

with B width of the structure [m]

As indicated these mechanisms should be calculated by a geotechnical specialist. If it turns out that thestructure does not meet the stability requirements, additional measures should be considered:

Add a pile foundation (to increase the bearing capacity of the underground, or to increase theresisting forces);

Increase the dimensions of the concrete wall (to increase the resisting forces).

In case of a pile foundation, the structure is founded on a deeper sand layer. The different types of resistingforces increase by the presence of the poles, which increases the overall geotechnical stability.

4.1.4Concrete tidal wall, with rubble mound bed protectionIf the structure is subject to moderate waves, a bed protection in front of the structure might be necessary.Whether this is necessary can be predicted based on wave calculations, however inspection at regular intervalscan also indicate the necessity. Note that significant wave attack is not expected at tidal walls, consideringtheir preferred wave-sheltered location inland.

The design of a concrete tidal wall with rubble mound wave protection is similar to the normal concrete tidalwall as described in Section 4.1.3. Additional a rubble mound bed protection should be placed in front of thestructure. The design rules for this protection are based on the following assumptions:

The structure does not need the bed protection for geotechnical stability. Failure of the protectiontherefore doesnt lead to direct failure of the complete structure;

The structure is not placed on the waterline and is only designed to prevent flooding (not erosion).Possible waves in front of the structure are therefore expected to be moderate. If wave attack is

significant, a different type of protection measure must be considered. The bed protection is inspected on a regular basis and extra stones can always be added if this turns

out to be necessary later. The bed protection consists of a geo-textile, a gravel layer and an armour layer (see Figure 4-12).

Figure 4-12: Bed protection with geo-textile, gravel- and ar

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