Shore protection work for the coast of Periyathalai, Tamilnadu

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IIT Madras Shore protection work for the coast of Periyathalai, Tamilnadu Report Submitted to Public Works Department Tuticorin By, Prof. V. Sundar Prof. S.A. Sannasiraj Indian Institute of Technology Madras Chennai 6 36, India February 14

Shore protection for the coast of Periyathalai, Tamilnadu Indian Institute of Technology, Madras CONTENTS 1. INTRODUCTION 1. SCOPE AND OBJECTIVES 1 3. FIELD SURVEY 1 3.1 Location 1 3. Bathymetry survey 3..1 General 3.. Instruments used 3..3 Sea bed survey 3 3..4 Shoreline alignment 3 4. WAVE CHARACTERISTICS 4 4.1 General 4 4. Wave heights 4 4.3 Wave periods 4 4.4 Wave directions. LITTORAL DRIFT ESTIMATE 6.1 Distribution of sediment transport 6 6. PROPOSED COSTAL PROTECTION SCHEME 7 7. NUMERICAL MODEL FOR WAVE TRANSFORMATIONS 8 7.1 General 8 8. NUMERICAL MODELLING FOR SHORELINE EVOLUTION 9 8.1 General 9 8. Methodology 1 8.3 Input and output 11 9. RESULTS AND DISCUSSIONS 1 9.1 Shoreline evolution 1 1. DESIGN OF GROIN CROSS-SECTIONS 1 1.1 General 1 1. Design water level 13 1.3 Design of revetment section 13 i

Shore protection for the coast of Periyathalai, Tamilnadu Indian Institute of Technology, Madras 11. RECOMMENDATIONS 17 1. REFERENCES 17 FIGURES 1 Aerial view of Periyathalai in Tamilnadu state 18 Periyathalai coast bay area 19 3(a) A view of Ceeducer-Pro 3(b) A view of a GPS 1 3(c) A view of an Auto-Level 4 Grid on wave atlas for study area 3 Monthly distribution of wave heights (Jan-Mar) 4 6 Monthly distribution of wave heights (Apr June) 7 Monthly distribution of wave heights (July Sep) 6 8 Monthly distribution of wave heights (Oct Dec) 7 9 Monthly distribution of wave periods (Jan-Mar) 8 1 Monthly distribution of wave periods (Apr June) 9 11 Monthly distribution of wave periods (July Sep) 3 1 Monthly distribution of wave periods (Oct Dec) 31 13 Monthly distribution of wave directions (Jan Mar) 3 14 Monthly distribution of wave directions (Apr June) 33 1 Monthly distribution of wave directions (July Sep) 34 16 Monthly distribution of wave directions (Oct Dec) 3 17 Average breaker height at study area 36 18 Breaker Angle with respect to Shore Normal 37 19(a) Distribution of long shore current velocity over the surf width 38 19(b) Average alongshore current velocity for different months 39 19(c) Monthly sediment transport distribution along the surf width 4 Surf zone Width 41 1 Long shore Sediment Transport Rate 4 ii

Shore protection for the coast of Periyathalai, Tamilnadu Indian Institute of Technology, Madras Schematic diagram for finite difference scheme 43 3 Definition sketch of angles considered 44 4 Shore line evolution for the proposed groins field 4 TABLES 1 Wave characteristics for the tranquility study Proposed locations for the groins 7 3 Design details groin sections from shoreline upto the water depth of CD-.m 4 Design details breakwater head section in a water depth of CD-m 16 16 PLATE 1 Bathymetry 46 Plan showing up the proposed coastal protection structure 47 3 Cross section of the trunk portion 48 4 Cross section of the head portion 49 Top view and longitudinal section of groin iii

1. INTRODUCTION The executive engineer, Korampallam division, Water Resources department, Public Works Department (PWD), Government of Tamilnadu has requested the department of Ocean engineering, Indian Institute of Technology Madras to suggest suitable coastal protection measures that could also facilitate safe beach landing facilities for fishing boats in order to enhance the livelihood of the fishing community of the coastal hamlets at Periyathalai village (Fig. 1). The coastal stretch is located in Tuticorin district. In this regard, Prof.S.A.Sannasiraj visited the coastal stretch along with the officials of PWD, Er.Muthurani, AEE and Er.Uruvatti, AE and made a reconnaissance survey during December 13. The required field measurements were described. This report details the field measurements and subsequent desk studies that lead to the suitable coastal protection scheme.. SCOPE AND OBJECTIVES The objective of the project is to design a suitable coastal protection scheme along the fishing village of Periyathalai with special attention to coastal protection. As an additional benefit due to the proposed scheme the beach that is expected to form in between a pair of groins could serve as facility for safe landing of small crafts and vessels. The detailed scope of the project includes, o Field survey of sea bottom depth measurement, shoreline alignment and beach profiles. o Layout design of suitable Coastal protection scheme o Wave transformation study in the presence of the proposed scheme o Prediction of shoreline changes due to the construction of the proposed scheme o Design of cross-sections for the proposed coastal structures 3.FIELD SURVEY 3.1 Location The study area is shown in Fig.. The geo-coordinates centred on the study area are8 o 19 N and 78 o 4 E. The coast is located about 3 km south of Thiruchendur temple. This location is protected from northerly and easterly waves in the presence of the island country, Srilanka. However, this coastal stretch is exposed to waves from south and southwest directions. 1

3. Bathymetry survey 3..1 General The seabed bathymetry dictates the layout design and construction of coastal structures. The hydrographical survey was conducted on7th January 14 by a project team under the supervision of Prof.V.Sundar and Prof.S.A.Sannasiraj. The entire team was divided into two groups. One group was responsible for the seabed survey. The second group performed the task of tracking shoreline, beach profiles and other significant morphological features on the shore. 3.. Instruments used The bathymetry survey using CEEDUCER PRO, sounding rope and hand held GPS with a beach landing boat was conducted. The CEEDUCER PRO is an integrated and highly portable hydrographic survey system based on single beam technology for the purpose of bathymetric surveying. The CEEDUCER PRO brings together a GPS receiver, echo sounder(s) with water temperature-velocity correction, and optional real time correction data such as Tide, Heave and GPS Differential inputs. It takes the real time data, along with static survey setup data, such as echo sounder drafts, GPS antenna height, water salinity, and transforms it into meaningful information relating to water depths at a continuous series of locations, and allows this concentration of data to be logged internally and/or transmitted to a PC running real time Hydrographic surveying software. The system is designed to run on internal power with basic peripherals connected. If required an external power supply of 1 volts DC (lead acid system) or 1-4 volts DC (for NiMH system) can be connected to run the system for an extended period. It is a very compact system, allowing the smallest vessels to be used for hydrographic survey work. Fig. 3a depicts the photograph of the CEEDUCER PRO used in the present survey. The GPS (Global Positioning System) is a space-based satellite navigation system that provides location in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. It is freely accessible to anyone with a GPS receiver. Three handheld GPS of Garmin made with an accuracy of m spatial resolution were used. A picture of GPS is shown in Fig.3b. One GPS was integrated to echosounder as a component of the seabed survey system. The second GPS was adopted to track the shoreline and the third one was used by the beach profiling team.

A Dumpy level along with two survey staff was used for beach profiling. A dumpy level or automatic level is an optical instrument used to establish or check points in the same horizontal plane. It is used in surveying to transfer, measure, or set horizontal levels. The level instrument is set up on a tripod and, depending on the type, either roughly or accurately set to a levelled condition using levelling screws. An operator looks through the eyepiece of the telescope while an assistant holds a graduated survey staff vertical at the point under measurement. The instrument and staff are used to gather and/or transfer levels during site surveys. A benchmark elevation is generally setup to transfer all the levels with reference to the site specific reference or standard benchmark available. This locally setup or global benchmark is a viable level during the construction stage. Fig.3c shows the picture of the auto-level used in the present survey. 3..3 Sea bed survey The profile routing into the sea to measure the water depth was done using Ceeducer. The survey was carried out for a length of km along thecoast and1. km into the Seaup to a water depth of about 1m. The readings from the instrument were then downloaded into a personal computer. Subsequently, tidal correction has been applied from the known tidal datum from Tutitcorin port. The SURFER software was used for gridding the data so that the contour plot of water depths can be obtained. Finally the bathymetry map was obtained. Plate 1 depicts the measured seabed bathymetry of Periyathalai coastal stretch. We can infer that the sea bed slope is steep near the shore up to 4m water depth. Further the slope is gentle with a shallow reef formation at about 8m from the shoreline. The waves break on the reef leading to a high tranquil near shore region close to the coast which is beneficial for the safe manoeuvring of fishing boats. However, the uniform wave breaking on the reef makes the manoeuvrability of small fishing boats vulnerable while crossing the reef. 3..4 Shoreline alignment The coastline is oriented at about 4 o clockwise from the North. About km stretch of shoreline has been mapped using GPS in a closer interval of m. 3

4. WAVE CHARACTERISTICS 4.1 General The National Institute of Oceanography (199) published a wave atlas for Arabian Sea and Bay of Bengal (Latitude: N and Longitude: 6 9 E) compiling the ship observed data for 19 years from 1968 to 1986. The coastal region around India is divided into 1 grids, each of size latitude and longitudes as shown in Fig. 4. The grids 1 to 4 falls in the Bay of Bengal on the east coast, grids to 7 fall in Indian Ocean in the south and grids 8 to 1 all in the Arabian Sea on the west coast. The present study area comes under the grid located at 1 N and 7 8 E in the wave atlas, representing grid number six. The wave data (wave height, wave period and wave direction) from wave atlas given for deep-waters were used for the present study. 4. Wave height The monthly distribution of deep-water wave heights in terms of percentage of occurrence derived from wave rose diagrams are projected in Figs. to 8. The class interval adopted for the calculations is.m. It is observed from the results, that the most frequently occurring wave height is about 1.m with a percentage of occurrences of to 3% for the months January and November. From the above referred figures, it is also observed that the most frequently occurring wave height is 1.m with a percentage of occurrences of to 3% during the months of February, March, April, May, October and December. The most frequently occurring wave height is m with a percentage of occurrences between to 3% for the months June to September. 4.3 Wave periods The monthly distribution of wave periods in terms of percentage of occurrence derived from the wave atlas is projected in Figs. 9 to 1. The class interval adopted for the presentation is 1sec. From the above figure, it is observed that the maximum percentage of occurrence is the waves associated with periods ranging between and 6 seconds. 4

4.4 Wave direction The monthly distribution of wave directions with respect to geographic north in terms of percentage of occurrence obtained from the wave atlas is projected in Figs. 13 to 16. The class interval adopted for the presentation is. The average wave characteristics given as the input to the numerical model is as per given in Table 1. Table 1: Wave characteristics for the tranquility study Month Deep water wave direction (θ o ) w.r.t North Wave height, H(m) Wave period, T(sec) Wave direction (θ) from shore normal @d=7m January 36 1. -34 February 34 1. -17 March 18 1. 36 April 7 1. 1 May 7 1. -1 June 7. -46 July 7. 6-46 August 7. 6-46 September 7. -46 October 7 1. -46 November 18 1. 33 December 18 1. 33

. LITTORAL DRIFT ESTIMATE.1 Distribution of Sediment Transport The wave atlas data has been analyzed to obtain the monthly averaged wave height, wave period and wave direction. These are offshore wave climate and are transformed to the near shore location off Periyathalai coast using Snell's law. The average breaking wave characteristics were derived from the available wave data. The monthly distribution of mean breaker wave height for the study area is shown in Fig. 17. The results indicate that the mean breaker height varies from about 1.m to 1.8m. The breaker height is observed to be a maximum during south-west monsoon, in particular during June and July. The monthly distribution of the mean breaker wave angle with respect to shore normal is shown in Fig. 18.From the results it is seen that for the study area, the breaker angle with respect to shore normal and long shore current velocity are directed towards North during the months of January, February, May, June, July, August, September, October and is towards South during the other months. Further, the average long shore current velocity irrespective of its direction estimated for each of the months shows that it is high during the south west month, June to September as can be seen in Fig.19a. The surf width also is found to be high during the above months as stated earlier. The average long shore current velocity along with its direction for the different months shown in Fig. 19b clearly exhibits that during January and February, May to October is towards north, whereas, during the months March, April, November and december it is towards the south. The magnitude of the average long shore current velocity is found to be maximum during May to Septemper. Furthermore, the average monthly sediment transport rate during the different months shown in Fig.19c shows a trend in its variation similar to the variation in long shore current velocity. The average surf width in which the long shore drift is predominant is further estimated from the breaker wave height for the given bathymetry and is projected in Fig. for the different months. It shows that the maximum surf width of about 11m occurs during the months of June and July. However, it may be noted that the presence of submerged reef at a relatively offshore location makes the waves to break on it. Hence, the effective surf zone width is about 8m along this coastal stretch. Due to sufficient width between the reef and the shoreline, the waves might further raises up due to shoaling. Further, the derived wave characteristics were used to calculate the long shore sediment transport. Three different 6

methods CERC (1984), Komar (1976a), and by integrating the distribution across the surf zone (Komar, 1976b) have been adopted to calculate the alongshore sediment transport rate. The average sediment transport rate for the different months is shown in Fig.1.All the three methods have yielded similar order sediment transport rate. The net drift is found to be about 6933 m 3 per annum and directed towards the North. 6. PROPOSED COASTAL PROTECTION SCHEME The main objective of the scheme is to provide coastal protection to the Periyathalai coastal stretch such that the sufficient beach width is preserved for berthing of boats and to facilitate safe landing of smaller fishing vessels. It has been estimated that the net alongshore drift is towards north but of minor in nature. And, the risk of maneuvering the small fishing boats while crossing the shallow reef is very high. Hence, an 8m long groin (G1) on the southern side is envisaged by extending the groin up to the offshore reef. This would also ensure the protection from south-easterly waves during the south-west monsoon period from May to September. In addition, a groin of length m (G) is suggested north of G1 at a distance of 6m. In the presence of G1 and G, a highly tranquil basin would form which will act as coastal protection measure as well as facilitate safer berthing of fishing vessels. It is expected that the shoreline north of G might get eroded due to the predicted net northerly littoral sediment drift. Hence, two short groins of length 7m (G3) and m (G4) are proposed to tame the negative effect of groins G1 and G on the neighborhood shoreline. G3 is positioned at m from G and G4 is located about 1m from G3. Plate presents the proposed coastal protection scheme superposed on the bathymetry chart. The shore based locations of G1, G, G3 and G4 are given in Table. The proposed coastal protection scheme has been further examined from the wave penetration studies and through shoreline change numerical modeling, the shoreline behaviour in future is forecasted. These two tasks would verify the suitability of the proposed scheme. Table. Proposed locations for the groins Groin Starting point at the shore G1 8 19'9.6"N, 77 8'19.4"E G 8 '9.3"N, 77 8'3.71"E G3 8 '11.79"N, 77 8'4.84"E G4 8 '13.44"N, 77 8'44.13"E 7

7. NUMERICAL MODEL FOR WAVE TRANSFORMATION 7.1 General The, IIT Madras has developed numerical models on the diffraction/refraction of waves due to the presence of near-shore structures. The model is developed using the mild slope equation because of its generality in dealing with complex wave fields. The mild slope equation is solved by generalized conjugate gradient method as it has a fast convergence rate. The combined refraction-diffraction equation that describes the propagation of periodic, small amplitude surface gravity waves over an arbitrarily varying mild sloped sea bed according to (Berkhoff, 197) is, Where. CCg Cg C - Complex velocity potential - Angular wave frequency C - Phase celerity and Cg - Group celerity. The above equation is transformed into a Helmholtz equation of the form, K ( x, y) Using the following relations ( ). ( CCg). CCg and K k ( CCg). Where k = wave number K and are modified wave number and Wave potential function Finite difference scheme is employed for the numerical discretization of Helmholtz equation. The derivatives are approximated using centred difference scheme. Writing the discretized form of Eq. for each grid in the domain and applying suitable boundary conditions, the system of resulting algebraic equations can be written in matrix form as, A = f 8

Where A is the coefficient matrix, is the nodal values of velocity potential and f is a vector obtained from the boundary conditions. The numerical solution of the above system of equations is arrived using generalized conjugate gradient method. The method successively estimates new approximations to the solution, considering the direction of residual error vector, till the prescribed accuracy is achieved. The offshore boundary is modelled as an open boundary in which case only incident waves and reflected waves are allowed to propagate. The lateral boundary as well as the shore is considered to absorb the wave energy. The groins or any other obstruction is treated as partially reflecting boundaries by prescribing the reflecting coefficients. The model requires the wave characteristics (viz. wave height, wave period and its direction) and the water depths at all the grid points. It also requires the location of the groins. The model gives the wave characteristics inside the domain. 8. NUMERICAL MODELLING FOR SHORELINE EVOLUTION 8.1 General Structures in the near shore environment are built for different purposes. These may be for the formation of artificial harbors, shore protection measures, seawater intake systems, disposal of effluent, etc. There are several configurations of such structures with respect to the shoreline, among which, structures normal to the shore is most common. The construction of a shore-connected structure often leads to changes in the shoreline. This warrants a study on the shoreline due to presence of the shore-connected structures. Such a study is very much essential in planning stage; so as to assess the impact of shore connected structures on the adjacent shoreline. Numerical models offer the capability to study the effect of the wave characteristics, structure dimensions and other associated parameters in providing reasonable estimates of the shoreline response. As the ocean waves approaches the near shore it undergoes transformations like shoaling, refraction, diffraction and breaking. The phenomena of wave 9

breaking throw sediments to the surface due to the turbulence generated. The sediments in suspension are then driven by the wave-induced currents. Since the direction of waves in the near shore is oblique, the currents induced by them have two components. One along the shore called long shore current mainly responsible for the long shore sediment transport, which plays an important role in the shoreline changes especially due to the shore connected structures. The other component is in the direction normal to the shore, in which case, the mode of sediment transport is called onshore-offshore sediment transport. When a structure normal to the shoreline is constructed, it will intercept the free passage of long shore sediment transport, which results an imbalance in the quantity of sediment in the near shore especially near the structure. This leads to accretion on the up drift side and erosion on the down drift side of the structure. 8. Methodology Kraus and Harikai (1983) proposed a numerical scheme to solve the one line model using Crank Nicholson implicit finite difference method. The non-dimensional equation of shoreline y n, t 1 B Q n, t 1 Q n 1, t 1 C n where B t and C B Q n n, t n 1, t n, t n, t x Q x q y The non-dimensional shoreline is divided into n grid points at equal nondimensional interval, x *. Then shoreline changes over a non-dimensional time, t * is calculated using Crank-Nicholson finite difference scheme. The schematic diagram for finite difference scheme is shown in Fig.. In this method, Q * at the time interval (t * +1) is expressed in terms of the shoreline coordinate of y*, first isolating the term involving sp (angle of shoreline normal to x-axis) using trigonometric identities. One of the term involving sp is then expressed as first order quantities in y* at time step (t*+1). Q K D cos o sin b Where, o = - sp and is wave direction with respect to x-axis. The definition sketch showing the angles is shown in Fig. 3. 1

The elliptical form of mild slope equation, which deals with combined refraction-diffraction, Q K cos sin (4) D sp b Q K sin cos sin cot sin sin () D b sp sp sp Q E y y F (6) n n 1, t 1 n, t 1 n Where E n K D cos sin sp,t sin b,t / x and F n K D sin sp,t sin b,t By substituting above equations, give the final equation as given below BE Q n 1 BEn Q BEn Q En Cn Cn 1 n 1, t 1 n, t 1 n 1, t 1 n F The above equations represent a set of (N-1) linear equation for (N-1) unknowns. The end values are specified as boundary conditions, that is, Q * 1 = and Q * N+1 = Q * N. The above equation results into a tridiagonal form which is solved for Q *. This process is repeated for the entire duration and non-dimensional quantity is converted into real quantities using the corresponding scale factors. The program has been validated with published results. 8.3 Input and Output The coastal line is discretized into number of grids with an equal spacing of 1m. The co-ordinates of the existing shoreline were provided. The length of the structure and grain size of the sediments required for the calculation of active depth of the sediment transport and water depth at the tip of the structure are the inputs given to the model. In addition to these, the monthly wave characteristics and the number of years over which the shoreline change is desired to be mentioned. The output shows the predicted shoreline changes after a period of 1,, 1, 1, & years. The upstream of the structures shows advancement of the shoreline position, while, the downstream end shows the erosion. 11

9. RESULTS AND DISCUSSIONS 9.1 Shoreline evolution The numerical model to predict the shoreline evolution due to the shore-connected structures has been used to predict the shoreline changes due to the proposed groins. The wave characteristics given as the input to the numerical model is as per given Table 1. The length of the groins, water depth at the end of the groins and the present status of the shore are to be given as the input to the numerical model. The numerical model was executed for the most frequently occurring wave characteristics for the different months as stated earlier. The result on the predicted shoreline variations over years are projected in Fig. 4. The shoreline prediction has been made at the end of 1 year, years, 1years, 1 years, years and years after the construction of the groins and has been presented by superimposing the shoreline patterns. It is showing the advancement of shore for a maximum distance of about 7m on the southern side of Southern breakwater, and about 3-4m on the upstream sides of the groin field. 1. DESIGN OF GROIN CROSS-SECTIONS 1.1. General The design of groin section is mainly being carried out for the stability of armour units and the overall hydraulic stability of the section formed by rubble stones. It is to be noted that the geotechnical stability of the groin sections has to be ensured such that seabed profiling is mainly coarse sand to fine silt and without any major clay content beneath the seabed. If the clay content is higher, the stability of the cross section will become questionable. Hence this warrants additional design requirements in the event the soil beneath the sea bed is of clay which is not considered in the present report. The section is designed to be safe up to a water depth of -m CD (Chart Datum). It is decided to adopt naturally available rubble stones of sufficient weight since this coastal stretch is partially protected by the submerged reef, thus avoiding larger waves into the near shore region. The following section presents the design of a typical rubble mound groin section at -3m CD. 1

1. Design water level Following design data has been adopted for the design of rubble mound groin section. The mean high water spring (MHWS) is +1.1m above CD. For the design of the section, MHWS is adopted as maximum water level. Maximum high water spring, H wmax = 1.1m The design water level for the breakwaters can thus be set as the sum of MHWS and the water depth with reference to CD, i.e., d = 1.1 + 3.= 4.1m 1.3 Design of groin section Armour Layer A typical design of groin cross section is given here. The size of the armour stones for the groin section is calculated by using the Hudson formula, which is recommended by CERC (1984). 3 WH r D W K 3 ( S 1) cot Where, D r W=weight of an individual armour unit in the primary cover layer. Tetrapods are recommended for the present site conditions. W r =Unit weight of rubble stones,.6t/m 3. H D =Design wave height at the structure site in meters, S r =Specific weight of armour unit relating to water at the structure Sr ( Wr / W w) W w =Unit weight of seawater = 1 kg/m 3 =Angle of structure slope measured with the horizontal in degrees =1:(chosen) K D =Stability coefficient, for rubble stones in breaking wave condition is for random placement. 13

In the water depth of 4.1m, the maximum possible significant wave height that can incident on the groin section is.m. From Hudson's formula, the weight of rubble stones is worked out to be 1.4Tin two layers to withstand the design wave height of m. Accordingly, it is recommended layers of rubble stones in the range of 1.T to.t. Core layer The size of stone in core layer is taken as W /1 to W /1 (as per CERC, 1984). Rough angular quarry stones are suggested for under layer for which W =6 kg / m r 3 Toe Mound The size of stone in toe mound is taken as W /1 to W /1 (as per CERC, 1984). Rough angular quarry stones are suggested for toe layer for whichw =6 kg / m r 3 Crest width Crest width, r is arrived from the formula r nk W W r 1 3 Where, n= number of stones on the crest = 3 K =Layer coefficient Thickness of armour layer The thickness of the armour layer is calculated by following, t nk w w r 1 3 Where, n is the number of armour layers. Here, n =. Crest elevation The crest elevation of the groin is given by, Crest elevation = R + free board + Design Water Level 14

Where, R = wave run up estimated as per CEM (). Free board may be adopted in calculating the design elevation to give free height for exceptional cases of storms and cyclone waves that hit the toe of the structure to avoid dangers. Here, a free board of.m has been adopted. However, to avoid inundation due to long period swells, the crest elevation of armour layer is further elevated so that the proposed elevation is kept at +4.m level Filter Layer The filter layer is recommended for a thickness of 3mm with 1kg to kg rubble stones following the suggestions of SPM of the order of W/ to W/6. However, in the present case, it may not be required. Accordingly, trunk sections have been designed at 1m, 3m, 4m and m water depth as well as head section at m water depth. Plate 3 depicts the trunk sections at 1m, 3m, 4m, m and Plate 4 depicts the head section at m respectively. There will be anchoring of groin into the beach for a length of m beyond the HTL into the land. A geotextile mat is recommended to be laid below the entire stretch of groins so that the settlement of groins would be minimized by uniform distribution of groin weight into subgrade soil. The details of materials at different layers of trunk section in the water depths of 1m, 3m, 4m and m have been presented in Table 3. Further, the details of the head section are presented along with Table 4. 1

Table 3. Design details groin sections from shoreline upto the water depth of CD-m. Trunk Section Up to 1m water depth 1m to 3m water depth 3m to 4m water depth 4m to m water depth Crest (+)3.m (+)4m (+)4.m (+)4.8m elevation Crest width 4m 4m 4m 4m Side Slope 1: 1: 1: (both Side) 1: Bedding Layer.3m thick with 1mm to 1kg quarry stones Toe Mound - Core Material Armour Layer 3kg to kg rubble stones of 7% above 4kg -.3m thick with 1mm to 1kg quarry stones 1.m thick filled with 3kg to kg quarry stones kg to kg quarry stones of % above 1kg 1.m thick filled with 1.t to t stones with 7% stones above 1.t Under Layer - - Geotextile.3m thick with 1mm to 1kg quarry stones m thick filled with 3kg to kg quarry stones kg to kg quarry stones of % above 1kg.m thick filled with.t to 3.t stones 1.m thick filled with 3kg- kg Table 4. Design details breakwater head section in a water depth of CD-m. Trunk section Crest elevation Crest width m water depth (+)4.8m 4m Side Slope 1:. Bedding Layer Toe Mound Core Material.3m thick with 1mm to 1kg quarry stones m thick filled with 3kg to kg quarry stones kg to kg quarry stones of % above 1kg.3m thick with 1mm to 1kg quarry stones m thick filled with 3kg to kg quarry stones kg to kg quarry stones of % above 1kg.6m thick filled with 4t to 6t stones 1.m thick filled with 3kg- kg Armour Layer Under Layer.6m thick filled with 4t to 6t stones 1.m thick filled with 3kg- kg 16

11. RECOMMENDATIONS Plate presents the proposed coastal protection scheme. It is advised to start the construction of groins G1 and G at the first. During the construction stage, the shoreline orientation has to be closely monitored and if there is any significant erosion observed on the northern side of G, the groins G3 and G4 must be constructed. It is strongly advised to do borehole test to ensure that the seabed (silt/ sand) is capable of supporting the rubble mound sections without large settlements. Plate 3, Plate 4 and Plate represents the cross section and longitudinal section of groins. Prof. S.A. Sannasiraj Prof. V. Sundar 1. REFERENCES 1. Berkhoff(197)Computation of combined Refraction and Diffraction.. CERC (1984) Shore Protection Manual. Coastal Engineering Research Centre, US. 3. Goda,Y. (198) Random seas and design of maritime structures. 4. Kressener(198)Tests with scale models to determine the effect of currents and breakers upon a sandy beach, and the advantageous installation of groins.. Sundar, V. () Behaviour of groins. Fifth international symposium on wave measurement and analysis, WAVES, Madrid, Spain, 3-7 July. 6. US Army Engineer District, Willington (1973) Hurricane-Wave protection-beach- Erosion control Brunswick country, N.C., Beach projects, Yaupon beach and long beach segments. 7. Komar, P.D.(1976a),Beach process and sedimentation, Prentice Hall Englewood Cliffs, N.J., 49p. 8. Komar,P.D.(1976b),Longshore currents and sand transport on ocean Eng. III, ASCE,333-34. 17

Fig.1 Aerial view of Periyathalai in Tamil Nadu state. 18

Fig. Periyathalai coast bay area 19

Fig.3(a). A view of Ceeducer-Pro

Fig.3(b). A view of a GPS 1

Fig.3c. A view of an Auto-Level

Fig.4. Grid on wave atlas for study area 3

% of occurance % of occurance % of occurance Shore protection work for the coast 3 JANUARY 1 1. 1 1.. 3 3. 4 4. Wave Height (m) 3 1 1 FEBRUARY. 1 1.. 3 3. 4 4. Wave Height (m) 4 3 3 1 1 MARCH. 1 1.. 3 3. 4 4. Wave Height (m) Fig. Monthly distribution of wave heights (Jan-Mar) 4

% of occurance % of occurance % of occurance Shore protection work for the coast 4 3 3 1 1 APRIL. 1 1.. 3 3. 4 4. Wave Height (m) 1 1 MAY. 1 1.. 3 3. 4 4. Wave Height (m) 1 1 JUNE. 1 1.. 3 3. 4 4. Wave Height (m) Fig.6 Monthly distribution of wave heights (Apr June)

% of occurance % of occurance % of occurance Shore protection work for the coast JULY 1 1. 1 1.. 3 3. 4 4. Wave Height (m) AUGUST 1 1. 1 1.. 3 3. 4 4. Wave Height (m) SEPTEMBER 1 1. 1 1.. 3 3. 4 4. Wave Height (m) Fig.7 Monthly distribution of wave heights (July Sep) 6

% of occurance % of occurance % of occurance Shore protection work for the coast 3 OCTOBER 1 1. 1 1.. 3 3. 4 4. Wave Height (m) 3 NOVEMBER 1 1. 1 1.. 3 3. 4 4. Wave Height (m) 3 3 1 1 DECEMBER. 1 1.. 3 3. 4 4. Wave Height (m) Fig.8 Monthly distribution of wave heights (Oct Dec) 7

% of occurance % of occurance % of occurance Shore protection work for the coast 4 3 3 1 1 JANUARY 6 7 8 9 1 11 1 13 14 Wave period (sec) 3 3 1 1 FEBRUARY 6 7 8 9 1 11 1 13 14 Wave period (sec) 3 1 1 MARCH 6 7 8 9 1 11 1 13 14 Wave period (sec) Fig.9 Monthly distribution of wave periods (Jan- Mar) 8

% of occurance % of occurance % of occurance Shore protection work for the coast APRIL 3 1 1 6 7 8 9 1 11 1 13 14 Wave period (sec) MAY 1 1 6 7 8 9 1 11 1 13 14 Wave period (sec) JUNE 18 16 14 1 1 8 6 4 6 7 8 9 1 11 1 13 14 Wave period (sec) Fig.1 Monthly distribution of wave periods (Apr- June) 9

% of occurance % of occurance % of occurance Shore protection work for the coast JULY 18 16 14 1 1 8 6 4 6 7 8 9 1 11 1 13 14 Wave period (sec) AUGUST 1 1 6 7 8 9 1 11 1 13 14 Wave period (sec) SEPTEMBER 3 1 1 6 7 8 9 1 11 1 13 14 Wave period (sec) Fig.11 Monthly distribution of wave periods (July Sep) 3

% of occurance % of occurance % of occurance Shore protection work for the coast OCTOBER 3 1 1 6 7 8 9 1 11 1 13 14 Wave period (sec) NOVEMBER 3 1 1 6 7 8 9 1 11 1 13 14 Wave period (sec) DECEMBER 3 3 1 1 6 7 8 9 1 11 1 13 14 Wave period (sec) Fig.1 Monthly distribution of wave periods (Oct Dec) 31

% of occurance % of occurance % of occurance Shore protection work for the coast 9 8 7 6 4 3 1 JANUARY 1 3 7 9 11 13 1 17 19 1 3 7 9 31 33 3 Wave direction 8 7 6 4 3 1 FEBRUARY 1 3 7 9 11 13 1 17 19 1 3 7 9 31 33 3 Wave direction 9 8 7 6 4 3 1 MARCH 1 3 7 9 11 13 1 17 19 1 3 7 9 31 33 3 Wave direction Fig.13 Monthly distribution of wave directions (Jan Mar) 3

% of occurance % of occurance % of occurance Shore protection work for the coast 14 1 1 8 6 4 APRIL 1 3 7 9 11 13 1 17 19 1 3 7 9 31 33 3 Wave direction 18 16 14 1 1 8 6 4 MAY 1 3 7 9 11 13 1 17 19 1 3 7 9 31 33 3 Wave direction 18 16 14 1 1 8 6 4 JUNE 1 3 7 9 11 13 1 17 19 1 3 7 9 31 33 3 Wave direction Fig.14 Monthly distribution of wave directions (Apr June) 33

% of occurance % of occurance % of occurance Shore protection work for the coast JULY 1 1 1 3 7 9 11 13 1 17 19 1 3 7 9 31 33 3 Wave direction 18 16 14 1 1 8 6 4 AUGUST 1 3 7 9 11 13 1 17 19 1 3 7 9 31 33 3 Wave direction 18 16 14 1 1 8 6 4 SEPTEMBER 1 3 7 9 11 13 1 17 19 1 3 7 9 31 33 3 Wave direction Fig.1 Monthly distribution of wave directions (July Sep) 34

% of occurance % of occurance % of occurance Shore protection work for the coast 1 OCTOBER 1 8 6 4 1 3 7 9 11 13 1 17 19 1 3 7 9 31 33 3 Wave direction 9 8 7 6 4 3 1 NOVEMBER 1 3 7 9 11 13 1 17 19 1 3 7 9 31 33 3 Wave direction 6 DECEMBER 4 3 1 1 3 7 9 11 13 1 17 19 1 3 7 9 31 33 3 Wave direction Fig.16 Monthly distribution of wave directions (Oct Dec) 3

Max. Hb Shore protection work for the coast 1.8 1.6 1.4 1. 1.8.6.4. Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Months Fig.17. Average breaker height at study area 36

Apha(in deg) Shore protection work for the coast 6.E-1.E-1 4.E-1 3.E-1.E-1 1.E-1.E+ -1.E-1 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec -.E-1-3.E-1-4.E-1 -.E-1 Months Fig.18. Breaker Angle with respect to Shore Normal 37

Along shore current velocity (cm/sec) Along shore current velocity (cm/sec) Along shore current velocity (cm/sec) Shore protection work for the coast.7.6 Jan..4 Feb.3. Mar.1 Apr. 1 3 4 6 7 8 9 Surf zone width (m) 1. 1. May.8 June.6.4 July. Aug. 4 6 8 1 1 Surf zone width (m) 1. 1. Sep.8 Oct.6.4 Nov. Dec. 4 6 8 1 1 Surf zone width (m) Fig. 19(a)Distribution of long shore current velocity over the surf width 38

Mid surf zone velocity (cm/sec) Shore protection work for the coast 4 3 1 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec -1 - -3 Months Fig. 19(b). Average alongshore current velocity for different months 39

Volume (m 3 /sec) Volume (m 3 /sec) Volume (m 3 /sec) Shore protection work for the coast 6.E+ 4.E+.E+ Jan Feb Mar Apr.E+ 1 3 4 6 7 8 9 -.E+ -4.E+ -6.E+ Surf zone width (m) 1.E+1 1.E+1 8.E+ May June July Aug 6.E+ 4.E+.E+.E+ 4 6 8 1 1 Surf zone width (m) 1.E+1 1.E+1 8.E+ 6.E+ 4.E+.E+ Sep Oct Nov Dec.E+ -.E+ 4 6 8 1 1-4.E+ -6.E+ -8.E+ Surf zone width (m) Fig. 19(c). Monthly sediment transport distribution along the surf width 4

Width in(m) Shore protection work for the coast 1 1 8 6 4 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Months Fig.. Surf zone Width 41

Rate(m*3/month) Shore protection work for the coast Method N(cu.m/y) S(cu.m/y) Net(cu.m/y) Komar 867.86-183.1 684.73 CERC 9173.83-193.4 7868.41 Distribution 8683.77-18613. 6819.9 1 Komer CERC Distribution 1 4 6 8 1 1 14 - -1 Months Fig.1. Long shore Sediment Transport Rate 4

Y Q1, Q = Longshore sediment transport rate at grid point y1, y = Shoreline co-ordinate at mid point of grid q1, q = Nourishment quantity at mid point of grid q1 q qi qi+1 q3 y1 y y3 yi yi+1 Q1 Q Q3 Qi Qi+1 X grid Fig. Schematic diagram for finite difference scheme 43

y Normal to Shore Line Parallel to shore Line Deep water Wave Direction sp Parallel to X -axis Shore Line x Fig.3. Definition sketch of angles considered 44

After 1 Year After Year After 1 Year After 1 Year After Year After Year 4 3 1 1 4 6 8 1 Periyathazhi 1 Fig.4 Shore line evolution for the proposed groins field 4