IHE.
DELFTHydraulic engineering department Coastal engineering branch
BASIC DESIGN OF SEA DIKES AND
REVETMENTS IN DEVELOPING COUNTRIES WITH
ILL - DEFINED INPUT BOUNDARY CONDITIONS
Report of Msc research Master of Science programme Prepared by:Thieu Quang Tuan
Sponsored by: RWSfDWW&llIE Supervised by:Ir.Hl Verhagen Mentor:Ir.Wout de Vries
The findings, interpretations and conclusions expressed inthis study do neither necessarily reflect the views of the International Institute for Infrastructural, Hydraulic and EnvironmentaI Engineering, nor of the individual members of the MSc-committee nor of their respective employers.
Acknowledgements
Within jive months, the study was completed under the subsidies of IHE and Hydraulic Engineering Division of Rijkswaterstaat. Apart from oneself efforts, the success is contributed by the heips of lecturers,mentors and friends atIHE.
I would express the deep gratitude to my supervisor and lecturer Ir.Henk Jan Verhagen for his invaluable and extensive guidance.
During the time in the Netherlands, I always got the supports and encouragement from A1r. Krystian Pilarczyk, who always regards me as hisfriend: I am indeed very gratejul to
him for what he has done for me, andI would like to give special thanks to him and his family's members.
I wou/d also thank Ir. Wout de Vriesfor his mentor's works as weil as his advtee and open discussions and especially hisfriendship.
I myself have enjoyed wonderful and unforgettable time in the Netherlands especially very warm atmosphere at IHE. I would give special thanks to all my friends and IHE staffs for their advice and creation of cosiness.
Finally, I would express my great gratitude to all members of my family and my colleagues in Vietnamfor theirfrequent encouragement.
IHE, March 1999 Author,
Table of contents, List of tables and figures
Chapter 1: INTRODUCTIONS 1.1
l.1. General introduetion . . 1.1
1.2.Scope,objectives and requirements 1.1
1.3.Methodology 1.2
1.4.Application limits of the guideline 1.2
1.5. Structure of the report 1.3
2.1 Chapter 2: HYDRAULIC BOUNDARY CONDITIONS ANALYSES..... 2.1
2.1. General 2.1
2.2. Water level estimation 2.1
2.2.1.Astronomical tide 2.2
2.2.2. Wind set - up (surge) 2.3
2.3. Waves estimation 2.3
2.3.1.General 2.3
2.3.2. Wind generated waves - fetch limited 2.5
2.3.2.Depth -limited wave height 2.6
2.3.3.Mangroves - damping wave height 2.7
2.4. Currents 2.7
2.4.1.Tide driven current (tidal current) 2.7
2.4.2. Wind driven current 2.8
2.4.3. Wave driven current 2.8
2.4.5.Ocean currents 2.8
2.4.6.River current .
3.1
Chapter 3: DIKE BODY....... 3.1
3.1. General 3.1
3.2. Materials and structures of dike body.. 3.1
3.2.1.Materials usedfor dike body 3.1
3.2.2.Structures of dike body... 3.2
3.3. Dike geometry (appearance) 3.2
3.3.1.Outer and inner slopes selection . 3.3
3.3.2.Berm and itsfunctions .
4.1
Chapter 4: DIKE HEIGHT CALCULATION 4.1
Table ofcontents, List oftables and figures
4.1. Design water level........................ .. 4.1
4.2. Wave run - up estimate 4.1
4.2.1.Wave run - up formulae 4.2
4.2.2.Reduction factors................... 4.3 4.2.3.Calculation of wave run- up.......... 4.3
4.3. Other alignments 4.3.
4.3.1.Sea level rise 4.4
4.3.2. Sub - soil settlement consideration 4.4
4.3.3.Gust bump and seiches .
5.1
Chapter 5: DESIGN OF REVETMENT 5.1
5.1. General 5.1
5.2. Revetment systems and the selections 5.1
5.3. Revetment thickness and stone size calculations 5.2
5.3.1. Criticalloading 5.2
5.3.2.Required stone mass (size)for rip - rap 5.4
5.3.3.Required thickness oftwo layers rip - rap .
6.1
Chapter 6: FILTER STRUCTURES 6.1
6.1. Functions and structural requirements 6.2
6.2. Granular filter and geotextile 6.3
6.3. Design of granular filters 6.3
6.3.1.Required size offilter material 6.4
6.3.2.Required layer thicknesses offilter .
7.1
Chapter 7: TRANSITION STRUCTURES 7.1
7.1. General 7.1
7.2. Toe structures 7.1
7.2.1.Functional specijications 7.1
7.2.2.Material size usedfor toe. 7.3
7.2.3.Toe prototypes (forms) 7.5
7.3. Upper edge of revetment . 7.5
7.4.Someother types oftransition structure .
Table of contents, List of tab/es and figures
CONCLUSIONS AND RECOMMENDATIONS
LIST OF REFERENCES APPENDIX M
Table ofcontents, List oftables andfigures
List of tables and figures
(the background report)Table 3.1. Considered parameters for the choice of dike body structure Table 5.1. Critical failure mode of rip - rap structure
Table 5.2a. Required mass (size) of stone used for rip - rap, slopes 1:3-ê-1:8
Table 5.2b. Required mass (size) of stone used for rip - rap, slopes 1:9 -i-1:14
Table 6.1.Required grainsizesfor two layers of granular filter Table 7.1.Calculation of required rock size for toe
Graph 2.1. The shallow water wave height forecasting in a standard wind field Figure 2.2. Determination of the local design water depth
Figure 2.3. Wave damping in mangroves, plants emerge above water Figure 3.1. Illustration of dike body structures
Figure 6.1. Sequence of grain sizes in a revetment cross - section
Figure 7.1. Toeform 1,low scour potential, constructed in dry condition
Figure 7.2. Toe form 2, low to moderate scour potential, dry execution condition Figure 7.3. Toe form 3,severe scour potential, dry execution condition
Figure 7.4. Toe form 4,free dumping under water condition
Abstract
Abstract
The coastal structures are of great variety both in type and function. Among those, dike in association with revetrnent is being regarded as the most typical measure for shore proteetion because of its relative importance and economical feasibility. Recently, a broad knowledge on the design concept of these types of structure has been developedinseveral developed countries like The Netherlands, Japan, America, etc. However, it is found uneasy to deploy this achievement to the situation of developing countries simply because of the differences in design circumstance.
Generally, in order to have a proper design of dike and revetment, a sufficiency of observed data on boundary conditions is required. However, that is not the case one can usually encounter in present situation of developing counties. Mostly, in provincial level, relevant data for design purposes are not available or in uncertain quality and therefore the term " ill - defined input boundary conditions " is known for that reason.
As a solution to that problem, in a narrower scope, the study aims to produce a guideline on the use of dike and riprap revetrnent that would be capable for local consultants in developing countries. To fulfil this intention, in spite of lacking data, determination of the input boundary conditions is carried out based on several available physical data. The following sequence is the structural design processes, which has taken into consideration the actual design circumstances like lack of input data, material availability, and labour-based construction. In addition, the revetrnent structure is analyzed as a whole to limit structural failures. The guideline is built in a systematic way and tries to minimize the misusing in design and execution. Moreover, the design procedure is provided rather straightforwardly in association with a helpfui table system to facilitate the users.
The study holds a special significanee in term of helping local consultants in developing countries to solve their most comrnonly encountered coastal problems. And hopefully, it might have a small contribution to the transfer of knowledge between countries.
Introductions Chapter 1
Chapter 1
INTRODUCTIONS
1.1. General introduetion
In many parts of the world,especiallyin some developing countries like Bangladesh, Indonesia,Vietnam, etc. the local govemments have difficulties in dealing with daily coastal problems.In general, sea defense systems are complex in which the small and medium size structures are predominant. These structures are mostly being designed and managed by local consultants and authorities (provinces,districts ... ). Due to the lack of knowledge or information, those local designers and contractors have not been able to integrate properly the failure mechanism of the structures or simply provide structures in mechanical way without considering the facts of their regions.
This MSc study aims to help the local consultants in developing countries in design and execution of dikes and revetments. The outcome is a design guideline for particular structures,which is compatible for local consultants capacities
1.2. Scope, objectives and requirements
1.2.1. Study objectives and scopes
The coastal structures are of great variety both in type and in functioning. Among those, dike and revetment are the most popular type of shore proteetion structures because of its relative importance and feasibility. In general,to have a proper design of dike and revetment, a sufficiency of observed data on boundary conditions is required. However, that is not the case one can often encounter in present situation of developing counties. Mostly, in provinciallevel, relevant data for design purposes are not available and the term " ill - defined input boundary conditions " is known for
that reason. To solve this problem, the study tries to formulate the input boundary conditions based on several available physical data.The following sequence is the structural design processes, which also consider the given constrains of labours, materials, equipment, etc. in developing countries.
The study aims to establish a design guideline for shore proteetion by using dikes and revetments. The guideline is built in systematic way and tries to minimize misuse in design and execution. The revetment structure is analyzed as a whole, limited to two-layer riprap.The result of study is presented in a design guideline and in a background study report. The guideline has its classification, which can enable the users to identify problems and find answers. The study report gives detailed processes for establishment of the design guideline.
1.2.2. Requirements
Because users are local consultants, the outcome therefore should be performed in simple and easily used formats. Since simple calculation methods are preferabie, it is not possible to perform a detailed study on boundary conditions in the design
Introductions Chapter 1
guideline. Result of each design part is tabulated in classes in accordance with some other parameters, so that the designer can easily quantify what he needs.
1.3. Methodology
To fulfil the above goal, the following steps are proposed:
o Define the input boundary conditions, which are relevant for the design of dike and revetment. Those conditions consist ofhydraulics as well as physics.
o For each input parameter, a physical analysis is essential to find out what are the predominant effects on its magnitude and characteristics. In addition, special attention should be paid to the physicallimits like: water depth, fetch length, etc.
.0 In association with several speciallimits defined in the previous step, using the
available literature and mathematical models to quantify the input parameters. For this purpose, relevant classifications on hydraulic boundary conditions, bottom topography and geology or even ecology are needed
o The design processes will be treated in such a way that the relation between
failure modes is much more relevant than the accuracy of the calculation methods. This allows the designers to anticipate on the most common failures of the structure.
o Construction methods will be discussed in order to make the designs more
feasible to actual situation of applied regions (constrain of labors, materiaIs,
equipment, etc.)
1.4. Application limits of the guideline
The design guideline is specially made for local consultants therefore it has some certain limitations. As a user,Itis quite important to know what application limits are to avoid the misusing in design structures.The following gives limits to the use of the design guideline
o In many cases the input parameters of hydraulic boundary conditions are
ill-defined i.e. significant wave height, storm surge etc. However, these parameters some time can be estimated that they will not exceed certain values based on physical conditions like depth - limited, fetch - limited ... The estimations will be used as the input for the design purposes. That means the designs are theoretically conservative. When the input data are well determined, it is recommended to use more advanced design guidelines (i.e. CURIC/RIA 1991,CURITAW 1992).
o The design guideline is aimed for small and provincial level structures with simple but commonly found boundary conditions.
o In the typhoon I heavy storm areas (extreme conditions), this design guideline is inapplicable. For this purpose, one should refer to much higher level of consultants aswell as the design standards.
Introductions Chapter 1
1.5. Structure of the report
The report consists of two main parts: the background report as the main and the design manual as the appendix. The main report shows the establishment for the design guideline.
The background report is comprised seven chapters, the main contents of each are summarized asbelow:
Chapter 1: introduetion to the study: study objectives, methodology, design circumstances,etc.
Chapter 2: this is an essential starting point in the design procedure: determination of the most relevant input parameters for the design purposes of dikes and riprap revetments such as surge levels,wave parameters, tides, etc. The outputs of this chapter are mostlytabulated to the design manual for the users. Chapter 3: first step in the dike design:the dike body,conceming with the use of local materiais, selection of dike structures, outer and inner slopes consideration, etc.
Chapter 4: dike height calculation, discussion on determination of factors defining the dike height: design water level, wave runup, alignments. The wave runup height is calculated accordingly to Van der Meer formulae and several assumptions taking into consideration the lack of input data.
Chapter 5: important step in design of the main riprap slope. The approaches for calculation of required rock size and riprap thickness are given in this chapter. Moreover, using the standard stone classes and several crucial assumptions, the needed parameters for the riprap revetment are determined and tabulated as well in the appendixM(design manual).
Chapter 6: in this chapter, the filter rules are discussed to select a suitable approach for fitter design. Subsequently, the design of filter is emphasized on functions, the use between geotextile and granular filter, dimensioning of filter layersincase of lacking data,etc.
Chapter 7: the riprap revetment is integrated as a whoie. Hence, design of the remaining support elements for the revetment is treated in this chapter i.e. toe design and several transition structures.The toe is designed so that it can prevent the revetment from collapse due to impact of waves and currents. The related aspects are selection of a capable toe form,determination of rock for the toe. The design manual, in fact, is the output of the study.lts structure is provided rather identically to the background report. However, for a complete design procedure, chapters of construction and maintenance aspects and an illustration example are included.
--~~----
---Hydraulic boundary condition analyses Chapter 2
Chapter 2
HYDRAULIC BOUNDARY CONDITIONS ANALYSES
2.1. General
In this chapter, all parameters needed for the design purposes will be treated. As mentioned in chapter l,the measured data for those parameters are not available or in low quality. However, they can be estimated accordingly to the physical conditions. The results of this estimation will be shown in several classes referred to some certain conditions.
The following sections,one can easilyrealise that the wind parameters are often used to predict some another important parameters such as wave height, surge, etc. Therefore,information on the wind is very essential for this chapter. Fortunately,most of the countries in the world, the long - term wind observations can be found in permanent meteorological stations located in harbours or near airports.In case that the wind observation data are not found for a particular location, then the use of wind synoptic charts is reasonable.
2.2. Water level estimation
2.2.1. Astronomical tide
The basic driving forces of tidal movements are astronomical and therefore entirely deterministic. Tide is one of the important factors defining water level. Tide differs from place to place and has local characteristics. Practically, Tide can be predicted by using harmonie analysis in combination with measured signals, Hence, it is unlikely that a simple estimation method can be carried out without any availability of data. Fortunately,Tides have been observed throughout the world (developing countries as well).The tidal constants therefore are usuallyknown in many places.
For the purpose of this design guideline,the accuracy of tidal predictions are not of importance. Therefore, it is acceptable to use the typical tidal data of the regions. In case tidal constants are available, one can use the following equation derived from tidal analysis theory:
n
HMax
=
Ho +If
H;1
Inwhich,
HMax- maximum tidal level estimated at an arbitrary location.
f -nodal factor for amplitude i, which can be determined from astronomical analysis Ho- mean water level
H,- tidal amplitude of componenti n- number of important tidal components
Hydraulic boundary condition analyses Chapter 2
Depending on location, several components considerably contribute to the tidal amplitude, for the rest are neglected. Mostly,the semi - diurnallunar and solar tides are important (M2 and S2).Hence,we can derive a simple equation as below:
HMax =Ho +HS2 +O.977HM2
Where,
HS2- semi-Iunar tidal amplitude HM2- semi-solar tidal amplitude
Apart from that, the following attentions need to be paid when one determines the maximum water level in his region:
o Identify the local tidal characteristics: order of magnitude,which components are
considerable to simplify the calculation. For this purpose, the references are
available in local authorities.
o The amplification of tidal amplitude or tidal resonance problem in a long bay.
This phenomenon can increase the tidal amplitude by several times. Therefore, it is needed to check the possibility of occurrence of the resonance problem in the bays. The condition for resonance take place is that the length of the bay approximates to an odd number of a quarter of the tidallength: I:::::l;4L,%L, etc.
2.2.2. Wind set - up (surg~)
The surge is a result of the interaction between wind and water surface. During a
heavy storm, surges can be developed up to several meters. The surge level is
dependent on water depth, wind speed, fetch length, geometry of the basin,etc.
The best way to estimate the surge levels is extract them from water level observations recorded during storm conditions. To do that,a long - term observation data are required and it is not matched the idea of this study as weU as the capacity of the users. The other way remaining is that estimate surge levels using the present available formulae. However,those formulae are not very accurate. In order to have good estimation, they should be calibrated by taking into account the local characteristics. On the other hand, for this design guideline, results of boundary conditions determination will be tabulated in several classes to enable the users. This means that highly accurate formulae are unnecessary.
Depend on the type of geometry,there are three main solutions as follows: • Lakes /reservoirs and nearlyenclosed water bodies (constantwater depth)
A {" _ 1 C Pair
*
U2F Á>LlU - - W-- --COSy
2 Pwarer gh
(2.1)
In which,
AS' - wind set-up (surge)height(m)
C, - friction coefficient,ranges from O.8xl0-3 to 3xIO-3 Pair - density of air (kg/rrr')
pwater - density ofwater (kg/rrr')
Hydraulic boundary condition analyses Chapter2
u
-
wind speed(mis) F - fetch length(m)h- water depth (m)
4> - angle betweenwind directionandnormal ofthe coast (degrees)
• Continental shelf (constant water depth)
• Semi - enc/osed bay
. U2F
!:lS
=
Cw PaIr *--cosç) Pwa/er gh(2.3)
Due to the uncertainty of the input parameters as well as the importance level of the design, the calculations are therefore limited to some certain conditions such as low wind speeds (high wind speeds U> 25 mis in extreme conditions are not relevant to the design probabilities of these specific structures), relative short length of the wind fetches, etc.
The above formulae have been incorporated in Cress - package (Coastal Engineering Support System developed by IHE). The surge levels estimate is done by using Cress, the results are c1assified and tabulated in the tables ofthe design manual (fromM.2 to M.9, the fetch length is chosen as a main c1assification).
In fact, the differences in result of calculation between formula 2.2 and 2.3 are negligible. Therefore, for semi - enclosed bays / estuaries condition one can use the results tabulated for the continental shelf from tableM 4 toM.7
2.3. Waves estimation
2.3.1. General
In this sector, the simple methods of determination for the wave inputs will be treated.
For most ofthe cases,ifthe wind statistics are available then one can predict the wave heights by using locally generated wind waves formulae. In very shallow water areas,
where the waves are mostly broken, we can know the maximum wave height can exist in that special condition. Moreover, the waves heights are limited by the fetches. The fetch - limited conditions can be very helpful as well in predictions of the wave characteristics.
2.3.2. Wind generated waves - fetch limited
To predict the wind - waves, empirical wave growth formulae are based on the relation between the characteristic wave in the standard win field.This wind field is given through the average wind speed Uw, the fetch F and the duration of the wind
Hydraulic boundary condition analyses Chapter 2
field. An additional characteristic parameter is the water depth h, which is usually
assumed constant.
The following method is valid when the condition of locally generated waves is true. This means that in location where swell has been observed then the methods are no longer applicable.
Based on the empirical establishment, the locally generated wind waves formulae were proposed by Bretschneider in 1954 (ref.to ShoreProteetion Manual 1984, UiS.
Army Corps of Engineers). The idea of this formula is to forecast the shallow water wave in a standard wind field.
For estimate of significant wave height Hs
[
0
.
7
5
0
]
0.012S
(
g~
)0
.
42
gHs (gd) u
~
=
0.283tanh 0.53077
tanh [ d0
.
750
]
(2.4) tanh0
.
S30
(
!
2 )For estimate of peak waveperiod Tp
( gF)0.25 gT [(gd)0.375] 0.077
7
-;; =
2JZ".1.2 tanh 0.83377
tanh [ d0.3
7
5
]
(2.5) tanh 0.833(~2 ) In which,U- wind speed at 10 m above surface(mis)
d - water depth of the area, where the wind blows over(m) F - fetch length of the wind(km)
This method is applied when the wind blows over a water surface at a constant velocity, with a required duration of time and length of fetch to establish steady state generation. The approach has been programmed in Cress - package. However, for this study and design purpose the graph derived for shallow water condition (Graph2.1) is more practical (refer to CIRIA/CUR report 154).The derivations from that graph are shown in the tables ofthe design manual from M10a to M.15b: (the wave periods are estimated by using Cress)
2.3.2. Depth -limited wave height
In many cases, there are beaches and foreshoreswith slope less than 1:30,are defined as shallow foreshores. Due to the relative shallow water depth and high wave steepness,the associated deepwaterwaveswill break fewtimes on the foreshores.The local wave height in front of the dikes in these conditions can be determined in accordance with the local water depth d and so called depth - limited wave heights
rules.
Hydraulic boundary condition analyses Chapter 2
1.0 - H~ Equivalentdeepwater j_11
0.8 - significant height f----+-+-t-+t+i-t--+---+--++--t,-1I-t"H
0.6f- U."
=
Windspeed . 1I
0..4t--F =Fetch1ef1gth '_+--!-H1t+i+---+--+-+-+i-t-lrH
O.3l--h - Watl/ifdepthatend offetch_LOeep_limit
9 Acceletatioo of gravity r- .-0.2 ~ f
=
0.01, bottom friction factorI
!
100000 50000 6000~,'g
0.1 0.08 gF U2wGraph 2.1: The shallow water wave heightforecasting in a standard windfield
Approximately, arelation between the maximum significant wave height and the design water depth d was derived accordingly to the design graphs of Van Der Meer
(1990). The phrase "ru/es of thumb" is known as a principle for this phenomenon: (Hs) max =0.5 d
(H2
'YJ
max=
0.6 dIn which:
(Hs) max - the maximum significant wave height which can exist in shallow foreshore
condition
(H2
'Y
J
max - the maximum wave height in shallow water which is exceeded by 2 % ofthe waves
d - the design water depth in front of the structure at a seaward distance of
half of the wavelength in deep water ( 'l'2Lap)
The deepwater wavelength is defined by relationLop =gT2 =1.56T2
2Jr
The wave height(Hs) max can be seen as an upper limit (conservative estimate) for the
significant wave height for a particular value of d.
In previous section, it is likely that the most commonly occurred wave period ranges from 4 to 6 seconds (peak period). The wavelength in deep water Lap therefore varies between 25m and 55m. Consequently,it is recommended that the local design water depth in front of the dike d can be taken at distance of 15 - 25 m seawards (see also
Figure 2.2).
Hydraulic boundary condition analyses Chapter 2
~: .- .' _. .vv.'_...
L DeAL DES1GN VATER DE PT H d
I
"
15 -112 Lap2D {'Id
Figure 2.2:Determination ofthe local design water depth
2.3.3. Mangroves - damping wave height
In many tropical coasts, one can find that some species oftimber are living against the
waves and salt water. They are so - called mangroves. Mangroves play significant
roles in environment as well as the proteetion for the coasts/structures against erosion, damages, etc. The mechanism of mangrove with respect to the resistance against the
waves is not fully understood. It has been tried to formulate by many researches.
Because of its local characteristics, the results are different from place to another. However, in muddy coasts where mangrove is found, the waves are expected not to be so high because of biological reason (less than 1.00 m is the living condition).
Furthermore, for wide mangrove foreshore, the waves are mostly broken when they
come near the dikes.The wave height in front of the dikes in this case is considerably reduced.
In principle, wave damping in mangrove forest is due to the following causes: waves breaking when they approach shallow water areas,
wave damping due to the roots, trunks and branches of mangrove,
bottom friction,etc.
Recently, a formula with respect to the wave damping in reed stands has been proposed (2.6). It is assumed that the formula is applicable for mangroves (lecture note of Revetments, Sea - dikes and River -levees):
H
B(
-eB
J
-=exp
--H; cosj] (2.6)
Where:
c - coefficient depends on number of sterns per unit area
H,- the incoming wave height (m)
HB - the wave height in thevegetation at B meters from the wave entrance Design of sea dikes and revetments with ill- defined input boundary conditions 2.6
Hydraulic boundary condition analyses Chapter 2
B - width of the vegetation
jJ- angleof wave incidence
The formula is valid when the plants emerge above water, and the water depth between the plants upto 1.0m.
Because of differences in numberofsterns per unit area andstem thickness, thevalue of c for mangroves is still in controversial. However, it has been researched that the hydraulic roughness of mangroves, or inversely via the Chezy- coefficient, is lower than the one for reeds. For large mangrove forests Schiereek & Brooij (1995) suggested a Cmangroveof20mO.5Is,aslarge astwiceCreed.
In order to estimate the dampingcapacity of mangroves,the evalues of 0.02 for low density of sterns and 0.05 for high stem density are used (halfvalues compared to the values ofreed). The result is shownin Figure 2.3.
Finally, we can conclude that for a mangrove forest with a minimum width of 150 -200m and the plants are emerged above water during storm condition, the remaining wave height in front of the structure can be neglected.
Wave damping in mangroves (estimate) I--LOWstemdensity-High stemdensityI
:f 100 ii; ::J: ;? 80 ~ c .2 60 'ti ::::I "0 Q)
...
40 1: Ol ~ ..r: 20 Q) > RI 3: 0 _.--
I I -i
~--
~ !/'
/'"
->
I /
V//
V ~ 0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00 Width of vegetation B (m)Figure 2.3: Wave damping in mangroves, plants emerge above water
For more complexsituations (particular characteristics of mangrove trees, variation of water depth among trees in forest, etc.), one should use more advanced approaches like wave energy decay over propagation in combination with consideration of mangrove resistance against thewaves.
Hydraulic boundary condition analyses Chapter 2
2.4. Currents
Currents are of significant importance with respect to many coastal phenomena such as coastline evolution, construction, transport, etc. In a more direct sense of this chapter, currents are considered as the cause of failures of the structures through erosion of the seabed. Especially the toe should be protected against scouring by currents. The current pattems in general can be determined using hydraulic computations.
Currents are the result of several driving forces.The most important currents are:
2.4.1. Tide driven current (tidal current)
The astronomical water level variations along the coast induce tidal currents. The amplitude of the tidal current is proportional to the vertical tidal amplitude, tidal period, and the shape of the bathymetry. Since the bottom topography is complex, analytical solutions for tidal currents are impossible, but technically possible by using mathematical (numerical methods) or physical models with a lot of efforts and out of capacity of this study. However, a few types of currents permit derivation of useful analytical expressions. Where such derivations are possible, the solutions require a simplification of certain terms in goveming equations and a suitable schematisation of geometry. Besides that, sometime empirical inputs are ofnecessity.
Where tidal current is known as predominance, to have an idea about its order of magnitude it is recommended to take some samples of measurement.
2.4.2. Wind driven current
In locations where monsoon winds are prevailed, there will be a current pattem as a result of interaction between the winds and water surface. This type of currents is significant in surface layer and can be measured without any complication. In shaIlow areas,a special attention should be paid to this type of current. Because of its relative constant in amplitude as weIl as direction (depend on wind characteristics) one can refer to current synoptic or admiralty charts, observed in the vicinity of main shipping routes, river mouths and estuaries.
The uses of these data are very helpful but with caution,because they are measured as surface veloeities and these amplitudes differ significantly from surface to bottom.
2.4.3. Wave driven current
Part of the energy of the breaking waves is transformed into the generation of the longshore current, especially in surf zone this current plays an important role in transportation of sediment along the coast. This current is indirectly taken into account in determination of sediment transport capacity. And therefore,it is relative not relevant for design of revetments.
2.4.4. Ocean currents
The ocean currents are caused by geographical- driving forces:Coriolis acceleration, trade winds, differences in water temperature between continents, etc.
Hydraulic boundary condition analyses Chapter 2
The dominant location of ocean currents is offshore and therefore for shore proteetion structures they are not of importance.
2.4.5. River current
Inthe adjacent to rivers (estuaries,river mouths) the effect ofriver discharges/currents is noticeable. Especially during peak discharges, river currents can cause severe damages (erosion) to the structures.
Dike body Chapter3
Chapter 3
DIKE BODY
3.1. General
In this chapter,the problems in relation to the dike bodies such as dike geometry,dike structures,use of materials,etc.will be discussed.
The dike geometry is not onlytechnical selection but also finance - related issue. The structural stability is partially affected by the geometry. In addition, when the dike slopes are too stable it will result in a costly design. In general, the dike geometry is chosen in accordance with the geotechnical condition and after that is the optimal consideration. In the section of dike geometry,the selection of outer and inner slopes will be treated. Furthermore, a berm placed on outer slope is also technically interesting as a measure for reduction of dike crest height.
3.2. Materials and structures of dike body
3.2.1. Materials usedfor dike body
The dike bodies are usually made of earth. Economically, the best way to construct is to utilise the local available materials such as clay, sand, etc. The homogenous or inhomogeneous materials are both usable.
3.2.2. Structures of dike body
The dike body can be made from one to several layers of material. The most commonly used structures are(Figure 3.1):
sand layer as a core, clay partly covered outside (top and inner slope): type 1 sand layer as a core,clay fully covered outside: type 2
single layer of clay or sand, or etc: type3
The choice of those above structure types may be based on some information given in the following table (3.1):
Table3.1. Consideredparameters for the choiceof dikebody structure
Type 1 Type 2 Tvpe3
•
rarity of clay material•
rarity of clay material•
availability of claynear site near site material near site
•
heavy filter/geotextile•
simple or without filter•
simple or without filter structures for revetment structures for revetment structures for revetmentDike body Chapter 3
Apart from that, some important factors have effect on the choice of the dike body
structure are the total cost of the dike and cost optimisation: the cost of clay
transportation to the site,the reduction cost due to simple/without filter layer,the cost variation among the structure types, etc.
The main functions of the clay layer can be: Reduce the uplift pressure on revetment
Prevent dike body materials from washing away due to wave,CUITentimpacts
Proteetion layer for inner slope against erosion due to wave overtopping, overflow,etc.
Environmentally friendly: grass protection, etc.
Lower the phreatic line inside the dike body. Consequently, may avoid seepage
and some other geotechnical failures.
~~~
O
~.
:
.
':.e---.: : ' .
:"..-... "~""': : ~:"'.
;@
~
Sal1d eore, c10y poTtty c.Qverea
Figure 3.1: Illustration of dike body structures
The thickness of the clay layer should be large enough (usually larger 0.80 m) to
facilitate the execution and survive temperature fluctuations (avoid cracks, etc). The
disadvantage of the clay layer is the difficulty of placing revetment on it especially
underwater condition.
3.3.
Dikegeometry (appearance)
3.3.1. Outer and inner slopes selection
The outer slope has a significant effect on the height of the dike as well as the stability
of revetment. The criteria for the choice of the slope are slope stability, which is
related to the geotechnical properties of material, method and condition of
construction, stability of revetment,and cost optimisation.
Dikebody Chapter3
The milder outer slope, the less wave run - up and wave forces on the revetment. Consequently, the required dike height will be less but it does not mean that the
volume of the dike body materials and the length of the revetment are reduced.
The inner slope is usually taken as the natural angle of material. If the low crested dikes (allowing some overtopping) are considered, the inner slope need to be chosen in accordance with the amount of wave overtopping.
Depend on the type of body materiaIs,some commonly used slopes are proposed as shown in the appendicesM (the design manual, tableMI6a). However,this choice of outer slopes is the first guess. The other decisive criterion for slopes selection is based on the fact of labour - based execution. This can be understood that the steeper slope or relatively harder wave, the higher required mass of rock on revetment to fulfil the structural stability. Thus sometimes results in very heavy required weight of stone i.e. greater 60kg, and obviously incapable for a normal man. Therefore, tabIe/diagram
M 16b in the manual, which is derived from the results of chapter 5 (Design of revetment) might decide the outer slope angle of the dike.
3.3.2. Berm and its functions
Berm is a plain strip placed between slopes and often adapted to the conventional type of structure. In the Netherlands, the outer berm is usually found as a typical element in the dike cross - section. The main functions and advantages of a berm are:
facilitate the execution and maintenance works by providing a space for workers, equipment, etc.
considerable reduction of wave run - up and the height of the dike therefore will be less than with out berm.
increase the structural stability because of increase in width of the dike.
The berm width is chosen in accordance with the execution conditions and cost optimisation. Since the situation of developing countries and small structures are considered, the executions are conducted preferably with limited mechanical equipment. The berm is therefore provided as a catwalk for labours and normally taken as a minimum width of1.50m.
Dike height calculation Chapter4
Chapter 4
DIKE HEIGHT CALCULATION
4.1. Design water level
The design water level is the still water level during design storm condition. Depend on how it is defined, there are several contributed components. Commonly, the design water level consists of two most important issues: the maximum astronomical tidal level and storm surge (the determination of these two components has been treated in chapter 2). However, in locations where the tidal differences are small, the design water level can be derived directly from the observed water level records without any separation of tide and surge.
4.2. Wave run - up estimate
The freeboard above the design water level is decided by wave run - up or overtopping calculation. Itis obvious that too low dikes will lead to flooding: either by overflow, or by dike breaching because of too much wave overtopping. The height of the dike therefore should meet a certain criterion, which is defined as 2 % wave runup or admissible wave overtopping rate. In fact, there is no physical exponent for the use of 2% wave runup. However, it is widely used for all design purposes.
The admissible rate depends on various conditions and usually gives higher standard compared to the 2 % wave run - up criterion. In the context of these particularly considered structures, the 2 % wave run - up criterion is much more practical and it will be chosen in the calculation of the dike height.
The 2 % run - up means that the freeboard provided by this criterion will be exceeded by 2 % of the coming waves.
In general, wave run - up height is proportional to the wave scenarios i.e. wave height and wave period, and revetment properties i.e. slope angle, slope roughness and permeability, and etc.
4.2.1. Wave run - up formulae
The wave run - up Ru2%is vertically measured distance with respect to the still water
level(SWL).Practically, the wave set - up in front ofthe dike is included in Ru2%. Van der Meer and Janssen, based on their experimental works, proposed the design formulae for wave run - up on a sloping structure in 1995 as seen follows:
valid for ~..5'2 (4.1)
valid for 2 < ~..5'4 (4.2)
Dike height calculation Chapter4
Where,
Ru2%- the 2 % wave run - up on a slope
Hs - significant wave height in front of the structure
YR- reduction factor due to slope roughness and permeability YB - reduction factor due to berm
1/J- reduction factor due to oblique wave attack
y,. - reduction factor due to shallow water
çp -
breaker parameter (surf similarity) for peak period,çp
= tan a~Hs /Lo a- outer slope angle of the dike
La- wave length in deep water 4.2.2. Reduction factors
• Reduction due to s/ope roughness and permeability
The roughness and permeability of the dike slope has a considerable effect on the reduction of wave run - up, especially rip - rap structures. Since the study is limited to the riprap revetment consisting of two layers of stones, the roughness reduction factor in this case YRcan be chosen as a value of 0.60 (refer to Delft hydraulic report, H 638, 1993).
• Reduction due to berm structure
The berm is usually situated at the elevation of the design water level, where it gives highest reduction factor to the wave run - up.
Van der Meer (1993) proposed a formula for berm width reduction, which is valid for a horizontal berm:
B/Hs rb =
---___::_--2cota+ B/
n,
In which deis the water depth of the berm with respect to design water level, B is the
berm width, cotais the slope of the dike.
The wider berm the larger reduction to the wave run-up. However, it is found that the most effective berm reduction might be achieved when its width is exceeded 0.25 La
for non-breaking waves and about 4Hs for strong breaking waves.
• Reduction factors due to oblique wave attack andshallow water
The reduction due to oblique angle of wave attack can not be taken into account properly in these given ill- defined input boundary conditions and therefore the value of 1.0is nominated.
Reduction for depth limited situation is given by the following formulae (lecture note of Revetments, Sea - dikes and river -/evees):
Dike height calculation Chapter4 r,
=
H2%=1-0.03
(
4-l!_
J
2
l.4Hs Hs h for -< 4H
s h for -è4 Hs Jh = 1.0However, when the local waveHs (at the toe of structure) is used, value ofJh is very uncertain.As a firstapproximation, this can be assumed equal to l.O.
4.2.3. Calculation of wave run- up
Thewave run - up phenomenon was completely incorporated Cress routine#241. Using Cress, the wave run - up height calculations are carried out for rip - rap revetment with different slopes, with and without berm reductions. Additionally, the calculation isvalid for conditions as described below:
o The wave height ranges from 0.50 m to 2.00 m (higher wave heights are not relevant as mentioned in chapter ofhydraulic boundary condition)
o The wave periods are adopted in such a way that the wave steepness Sop ranges from 0.03 to 0.05. In fact,this range ofwave steepness is usually found when the waves are locally wind - generated and therefore it matches the limit of the studied boundary conditions.
o The chosen dike slopes are varied between 1:3 to 1:8.The protective armor istwo layers of rip - rap structure.
o In order to have a considerable effect on the wave run - up, the berm width is chosen as minimum width of1.50 m
The results are tabulated in appendicesM from tableM.17a toM.18b
4.3. Other alignments
4.3.1. Sea level rise
Ithas been observed that the sea level isrising.The order of magnitude of sea level rise differs from place to another and unpredictablewithout sufficient data. There are some reasons for the cause of sea level rise, in general it could be the Greenhouse effect. Practically the relative sea level rise, is the rise of sea level itself plus land subsidence,is commonlyused.
In order to have a good estimate on sea level rise,long term observation data of mean sea level are required (the time record of thosedata is in order of fewcenturies as the minimum).
However,focusing on the particularly studied structures,their service life is expected to beshort enough.Thereby the relative sea level rise in those cases isnot necessarily
Dike height calculation Chapter4
taken into account in the design processes but it wi11be handled by means of maintenance or dike height improvement works.
Nevertheless, to be on safe side, the relative sea level rise is aligned and one can take a value in order of 5 - 10cmextra in the dike height calculation.
4.3.2. Sub - soil settlement consideration
The upper soil layers are settled down due to the weight of the dike structure. This story is concerned with geotechnical aspects.To predict accurately soil settlement is not of difficulty with the help of computational models since sufficient geotechnical data is provided.
However, due to the lack of information as mentioned in previous chapters. One can only roughly estimate something, which are based on certain limited conditions. The sandy subsoil in general has a very quick creep and the final settlement is considerably small with respect to another types of subsoil. For a long lasted sandy foundation, the settlement is negligible.
In the locations where considerable settlements have been found in short duration of time, special attention needs to paid to geotechnical survey. The consideration of settlement in this case should be done by specialists with computational models.
Apart from that, some characteristics of clayey or mixed clayey subsoil needs to be considered. The creep process of clay subsoil happens slowly, it takes years or even decades to reach final settlement. For the design lifetime ofthe particularly considered structures (10 - 20 years) and for good quality of clay subsoil, a settlement of
0.20-0.30 mis expected.
4.3.3. Gust bump and seiches
Gust bumps and seiches can cause an additional elevation up to some decimetres.
When one design a dike situated at the coast, the maximum gust bump must be taken into account.
Seiches are periodic long waves of relative long periods (compared to wind waves), ranging from a few minutes to hours. Seiches are caused by meteorological phenomena and can be observed in water level records. For the dike located in a harbour basin, seiches are of relevanee in determination of the dike height. Especially for relative short basin (with a length of 10 - 20 km), the resonance may take place and seiches can develop up to its maximum amplitude.
Design of revetment Chapter 5
Chapter 5
DESIGN OF REVETMENT
5.1. General
Revetment is a slope proteetion structure. The revetment is under attack of waves and currents. Therefore, the design of revetment has to fulfil the requirement of sustainability to the loads.Recently,a broad knowledge has been developed about the stability of open slope revetments under wave and current loads. The study aims to utilize those approaches in dimensioning and integrate the failure mechanism of revetment. The results will be summarized and tabulated in the design manual for uses.
5.2. Revetment systems and the selections
Slope revetments are of greatvariety in structure.They can be divided into different categories asseen below:
Natural materials: sand,clay,grass Loose units:gravel,rip- rap
Interlocking units: concrete blocks,mattresses Concrete and asphalt slabs
In principle, the choice of a revetment system isbased on the physical and hydraulic boundary conditions of where the revetment presents i.e. waves, currents, constrains of labours and materials, equipment,etc. For example, in location where the wave is so high one can not use slope revetment of gravel or grass but concrete blocks or rip -rap is sustainable.However,ifwe consider the availability of material and equipment the concrete blocks seem to be infeasible for remote areas. Whereas riprap revetment is so costly solution as in the Netherlands, simplybecause of unavailability of stone quarries,and therefore concrete blocks become preferable.
Focusing on more practical detail,riprap and concrete blocks are chosen as the most feasible systems for these specific studied boundary conditions (both physical and hydraulic). Due to time constrain, the study is limited to rip - rap revetment with considering the actual situation in developing countries such as labours - based execution,lack of input data,etc.
The rip - rap revetment will be designed in such a way that the maximum stone size can be lifted by a normal man (less than 60 kg) but the revetment still fulfils its stability against the impacts ofwaves and currents.
5.3. Revetment thickness and stone size calculations
Design of revetment Chapter 5
5.3.1. Criticalloading
Because of differences in structure, the failure mechanism of one revetment system is differed from the other. And therefore, the design approach may vary among the revetment systems.
The critical load for rip - rap is the current along the slope, which is the cause of material displacement. The instability of rip - rap will occur around the still water level, where the maximum veloeities during wave runup and downrush take place. Because of relative permeable structure, the uplift pressure on stones is not of relevanee with respect to ofrip - rap instability.
The following table gives an overview of critical mode of failure of rip - rap in regards to wave impacts:
Table 5.1.Critical failure mode ofrip - rap structure
Critical failure modes Determinant wave loading Structural strength
•
initiation of motion•
maximum velo city•
weight•
friction•
deformation•
seepage•
permeability of sublayer5.3.2. Required stone mass (size) for rip - rap
Via large - scale model tests, an empirical approach was derived by Krystian W.Pilarczyk in1990 (lecture note ofRevetments, Sea dikes and River levees)
(5.1)
withctga è2.0 and ~ 53.0 in which:
~ - surf similarity parameter corresponding to peak wave period Tp [-]
Vu- system - determined (empirical) stability upgrading factor, Vu= 1 for rip - rap
r/J - stability function for incipient motion, which is defined at ~ = 1.0. For rocks with some allowed movements, r/J =2.25 is applied.
H,- significant wave height (m)
D - specific size or required thickness of protective unit (m), D
=
(M5r/Ps)1/3=
Dn50a - outer slope angle [0] LJm- relative density of rock
b - exponential factor related to the interaction process between waves and revetment type, thevalue of0.50 is usabie for rip - rap [-]
M50 - average required stone mass (kg), which is defined as the value at 50 % of the grain size distribution curve
ps - solid mass density ofrock (2650 kg/m3)
For the cases when ~ >3.0,the size calculated at ~
=
3.0 can be appliedDesign of revetment Chapter5
The riprap slope is a loose material system, which has a self-healing effect of the loose rocks after initial movement. Due to this effect, a certain displacement of rock units can be acceptable and therefore the stability factor rjJfor this case might have a highervalue compared to alternative systems(up to rjJ5'3.0).
The above Pilarczyk approach can be found in Cress routine 513.The following table (table 5.2) is the result of calculation using Cress. However,this result cannot be used directly for the design manual. One should consider the fact that in developing countries labour - based execution is much preferable. Therefore, the stone mass should not exceed 60.0 kg. Additionally, the calculated stone sizes need to be transferred to the standard classes in accordance with the Manual on the use of rock in
coastal and shoreline engineering, CIRIA/CUR 154.
Table 5.2a.Required mass (size)of stone usedfor rip - rap,slopes 1:3-ê-1:8
Wave Dike slopes
height 1:3 1:4 1:5 1:6 1:7 1:8 B,(m) 0.50 8..dl 5+7 3+5 2+4 2+3 2 0.14-0.16 0.12-0.14 0.11- 0.12 0.10- 0.11 0.09- 0.10 0.08 - 0.10 0.75 26+38 16+23 11+16 8+12 6+9 5+8 0.21-0.24 0.18 - 0.21 0.16 - 0.18 0.15 - 0.17 0.13-0.15 0.13-0.14 1.00 - 38+55 26+38 20+28 15+22 12-i-18 - 0.24-0.27 0.21- 0.24 0.19-0.22 0.18- 0.20 0.17 - 0.19 1.25 - - - 38+56 30+44 24+36 - - - 0.24- 0.28 0.22 - 0.26 0.21 - 0.24 1.50 - - - 42+60 - - - 0.25 - 0.29 1.75 - - - -2.00 - - -
-Table 5.2b. Required mass (size) ofstone usedfor rip - rap,slopes 1:9-ê- 1:14
Wave Dike slopes
height 1:9 1:10 1:11 1:12 1:13 1:14 H,(m) 0.50 1+2 1+2 1 1 1 1 0.08 - 0.10 0.08 - 0.10 0.08 0.08 0.08 0.08 0.75 4+6 4+5 3+5 3+4 2+4 2+3 0.12-0.13 0.11 - 0.13 0.11-0.12 0.10-0.12 0.10-0.11 0.09- 0.11 1.00 10+15 9+ 13 8+ 11 7+JO 6+9 5+8 0.16-0.18 0.15 - 0.17 0.14-0.16 0.14-0.15 0.13-0.15 0.13-0.14 1.25 20+30 17+25 15+22 13+19 11-i-17 10+ 15 0.20-0.22 0.19- 0.21 0.18-0.20 0.17-0.19 0.16- 0.19 0.16-0.18 1.50 35+52 30+44 26+38 23+33 20+29 18+26 0.24-0.27 0.22- 0.25 0.21- 0.24 0.20-0.23 0.20- 0.22 0.19-0.21 1.75 - - 40+59 35+52 31+46 28+41 - - 0.25- 0.28 0.24- 0.27 0.23 - 0.26 0.22 - 0.25 2.00 - - - - - 43+60 - - - 0.25 - 0.28
Note: The dash (-) marks stand for the calculated stone masses are too heavy
(greater 60.0 kg) and not applicable for labour - based execution. In each cell, the
Design of revetment Chapter5
above row (in italic) expresses the required stone mass in kg whereas the lower one
shows the required stone size inmeters.
5.3.3. Required thickness of two layers rip - rap
The revetment needs a sufficient thickness to attain its structural stability.
Additionally, porosity and layer thickness of the revetment should be calculated in order to estimate the volumetrie requirement of rocks.
The perpendicular layer thickness of arevetment and its porosity depend upon several factors,which are mostly summarized as seen below: (C/RIAICUR Report 154)
Weight and grading of rock(W50 and W851W15)
Mass density of rock(Pa)
Method of placement Shapes of rock
For narrow grading, the following equation can give a good approximation of revetment thickness:
ta
=
nktDn50 (5.1)Where,
n - number of layers making the total thickness of revetment,
kt- layer thickness coefficient,
D50n - required rock size for revetment.
Practically, rip- rap revetment is usuallymade oftwo layers of rocks as the minimum requirement to achieve a good working condition.Thus,the factor n in above equation is set to 2. The value of kt can be referred to the CURIC/RIAReport 154, which is
based on the classification of rock shape and method of placement. Consequently,for two layers rip- rap with irregular shape and narrow grading of rocks,thevalue of 1.0
is derived for kt.
The stone masses tabulated in tables5.2a and 5.2b are transformed into standard stone classes in accordance with the table 19 in C/RIAICUR Report 154. Whereas the revetment thicknesses are derived from the relation (5.1)andthe given calculated rock sizesDn50. The whole result of these calculations is shown in the design manual table
M.19a andM.19b.
Filter structures Chapter 6
Chapter 6
F
I
LTER STRUC
T
URES
6.
1. Functi
o
ns and structural requirements
The cover layer of revetment is usually laid on a filter structure, which can be a geotextile layer, one or two layers of granular filter, or combination.The filter is an important transitional element between the base (core)and cover Iayers.Inaddition to functional aspects, filter structure plays roles as seen follows:
• Transfer of hydraulic loads from cover layers and redistribute more regularly to the base or core.Thus helping structure gains its geotechnical stability (sliding and liquefaction).
• Preventing dike structure as a whole from geotechnical failures due to pressure build - up inside the dike body.
• Preventing of base material from washing away due to impacts of waves and currents.
To achieve fully above functions,for granular structure,the filter should be designed so that the filter materials must not be flushed out through the spaces between rocks of the cover layers. Secondly, the so-called intemal stability is required for loose grained
granular filter. The intemal stability implies that among filter itself the finer fractionllayer cannot be flushed out through the pores between the larger fractionllayer. The design sequence of a filter can be started outwards from the base layer. Within arevetment cross-section, the size of material is successively coarser from the base layer/core to the cover layer (Figure6.1).
For granular filter against erosion, more attention is paid to stability criteria rather than the permeability.
rip - rop loyprs
r:'ilt€>r'" loy€>r'"S
Figure 6.1.Sequence of grain sizes in a revetment cross - section
Filter structures Chapter 6
Design of geotextile structure has its own requirements such as soil retention,
permeability, anti - clogging, survivability, durability, etc. A geotextile is widely used to reduce the filter height, and because of easiness of placement.
6.2. Granular filter and geotextile
As a filter function, granular filter and geotextile are both usable. However, depend on actual situation each type has its advantages as well as restrictions. In term of design aspects, a comparison between the uses of granular filters and fibre filters (geotextiles) is summarized as seen below: (CIRIA/CUR Report 154)
Advantages
Granularfilters Geotextiles
Sometimes self - healing Durable elements
Smaller hydraulic gradients Good support and load spreading Flexibility to subsoil settlement
Diversity
Small construction height Tensile strength
Relatively cheap Easiness of placement Disadvantages
Granularfilters Geotextiles
Larger construction height Preparing and mixing gradation Spreading composition
Unclear porosity
Difficulty in accurate placement Relative expensive
Unclear long term behaviour Gaps occur when base settles Vulnerability
Larger hydraulic gradients Survivability
Geotextiles work as a separator, which prevents washing out of base materials,
Sometimes, geotextile alone cannot fulfil completely the requirements of a filter structure. Hence, a thin granular layer on top might be required as ballast or undemeath as smoothing base.
As already mentioned above, one cannot anticipate the long-term behaviour of geotextile. Due to the weathering or another impacts of environment, its mechanical strength and intentional functions can be depleted in time. The most unwanted degradation of geotextile is the clogging which may occur by silt (pollution) or biological growths (bacteria). Clogging reduces the filter permeability considerably and become the cause of hydraulic failures of revetment (high uplift pressure ).
Itis experienced that with relative heavy boundary conditions (high wave, H,è1.Om), the use of geotextile as a filter is not acceptable and granular filters are recommended instead. However,the filter structure is uneasy to be placed under water condition and consequently the quality cannot always be assured properly. Hence, altematively,
combination of granular sublayers (functioning as cushion or filler layers) and geotextile can be applied.
Filter structures Chapter 6
In addition to construction aspect, when rip - rap proteetion is placed directlyon
geotextile can cause potentially damage to it by sharp edges of rocks. Thus more attention must be paid to the design criteria ofsurvivabilityof geotextile.
Since given situation that in developing countries the understanding of behaviour of geotextile as well as the use of it is limited, this report only emphasizes on design aspects of granular filters, which are considered as the most applicable uses.
6.3. Design of granular filters
6.3.1. Required size offilter material
Design of a granular filter can be based either on static or on dynamic filter rules, The dynamic filter rules are sophisticated and give more proper design of filter compared with the others. However, to do that more information on hydraulics such as flow veloeities at the interface between filter and rocks, gradient in the subsoil, etc. are required. Since the static filter rules are much simpIer and the results show reasonably, this approach therefore is widely employed for design purposes.
Terzaghi in the 30's derived the static approach for filters. The principle of this is geometrically closed structure, which create filter stability based on geometrical properties of materiais. The filter material sizes are chose in relation to the base material size so that the materials in the base and in the filter itself cannot be moved. For this purpose, grain size distribution curves of the base and filer materials are basically required. The commonly characterized grain diameters, which are used to present the design criteria, aredis, d50, andd85. These partiele diameters are equivalent
to the sieve sizes, which have weight passages of 15%,50%, and 85% respectively. The following relations (6.1 +6.4) characterize for design criteria of a granular filter in association with a given base layer material.
d Stability criterion: ~ <5 d85B (6.1) d Permeability criterion: ~ > 5 dl5B (6.2) d
Internal stability criterion: ___22f_<10 dlOF (6.3) d Segregation criterion: ~ <25 dSOB (6.4)
The stability and permeability criteria are contradictory. With these rules,a stabie and permeable filter can be designed. The design process can preferably start from the base layer outwards.This usually leads to two or more layers in filter.
Since crucial data on material properties are not available, however, in order to design the filters the following assumptions have been proposed:
Filter structures Chapter 6
o The base layer is sand, which ranges from fine to coarse. For another soil base layers,more detailed information on soil properties are required.
o The materials used for the base as well as the filter are narrow gradation. Thus the diameter ratio Dss/Dls is less than 1.50 (CURIC/RIA Report 154)
o Within a grain size distribution curve, the value of Dso is the mean whereas DIs
and Dss are standard deviations.
Based on those above assumptions, one can derive relations for the characterized diameters DIS, Dso, and Dss as follows:
DIS zO.80 Dso (6.5) Dss z1.20 Dso (6.6)
In addition to the properties of the base layer material, the following sand standard classification is used in calculation (/HE Lecture note of Soil Mechanics, Lubbking)
Grain size 60um - 200 um 200um - 600 um 600um - 2 mm Sandtype Fine Medium Coarse
The results of calculation are shown in table 6.1. In which, mostly two layers of filter are required to provide a smooth transition from the base to the cover.
6.3.2. Required layer thicknesses of filter
There have been no exact formulae for filter thickness calculation. However, because of executional reasons, each layer of filter has to meet its minimal requirement of thickness in association with the size ofmaterial (lecture note ofSea dikes).
Material size Sand
Gravel
Coarser than gravel
Minimal requirement of thickness 0.10m
0.20m (2 +3) Dso
Apart from that, the total thickness of the filter should not be less than 0.5Dso of the rip - rap cover. In addition to the filter thickness, reference is made to the chapter of construction and maintenance aspects.
To facilitate the users,table 6.1 is simplified and adapted to the design guideline table
M.20. The table shows the required grain sizes and thicknesses of the filter layers in accordance with the given sizes ofthe base layer and the cover.
