Dhanya, S.; Mohan, R.; Mullai Vendhan, K.; Ramana Murthy, M.V., and Sajeev, R., 2020. Assessment of performance of a groin constructed on Puducherry coast - A case study. In: Sheela Nair, L.; Prakash, T.N.; Padmalal, D., and Kumar Seelam, J. (eds.), Oceanic and Coastal Processes of the Indian Seas. Journal of Coastal Research, Special Issue No. 89, pp. 84-91. Coconut Creek (Florida), ISSN 0749-0208.
India, having coastline of about 7500 kms, is facing severe problems of erosion, threatening millions of lives. Construction of planned ports, harbours, and industries accelerate the erosion and degrade the environment along India's coastline. The reduced rainfall, due to changes in the monsoon pattern over the years, has in turn lessened the sediment inflow into the oceans. This reduction in the sediment flow leads to considerable erosion along the coastlines. Groins are typically built to stabilize a number of natural or manmade beaches against erosion, mainly due to a net loss of beach material. The influence of a groin is accretion of the beach material on the updrift side and erosion on the downdrift side. Puducherry, a Union Territory, is facing severe coastal erosion problems along some of the stretches, especially to the north of Puducherry harbour. In this study, the status and impact of a 60 m long groin constructed at Vaithikuppam, on Puducherry coastline is studied. A coupled and fully integrated 2D model for waves, currents and sediment transport is used for the assessment. The hydrodynamic processes and morphological stability near the groin are studied using field observations and bathymetric data. The numerical model results were validated and found to be in good agreement with the field data. The hydrodynamic conditions and sediment transport patterns are predicted using the model results and the impact of the groin on the morphological characteristics are established and presented.
The dynamics of coastal landscapes are mainly controlled by coastal processes, the morphology of the sea and human activities. Unusual natural disasters near coastal areas and ongoing manmade activities are rapidly changing the coastal landscapes. As a result, most coasts become inherently dynamic. Natural forces such as wind, waves and currents move the unconsolidated sand, which shifts the position of the shoreline. As waves approach the shoreline, they are affected by processes such as refraction, soil friction, wave breaking and shoaling, due to the undulations in the seabed.
Coastal structures are generally built at locations where beach and dune erosion cause serious problems. The decision to build a coastal structure should be based on a thorough analysis of the shoreline developments in the past and estimated developments in the future (Leo, 2013). Artificial structures can influence sediment transport, reduce the ability of the shoreline to respond to natural forcing factors and fragment the coastal space. The need for more coastal defence structures arises directly from the increasing coastal erosion that affects many coasts (OSPAR Commission, 2009). In an effort to protect and preserve vital resources, hard engineering structures such as revetments, seawalls, breakwaters, and groins have been the predominant techniques.
Groins are built to stabilize a stretch of natural or artificially nourished beach against erosion that is due primarily to a net longshore loss of beach material. These are narrow structures, either inclined or perpendicular to the shoreline. The effect of a single groin is accretion of beach material on the updrift side and erosion on the downdrift side; both effects extend some distance from the structure (Burcharth and Hughes, 2003). The groin length, relative to the surf zone width, may be expected to be an important parameter in relation to the morphological response around groins (Kristensen et al., 2016). It was found that groins offer better solution for coastline erosion of Alappad coast with respect to sediment trapping efficiency as well as shoreline development when compared to breakwaters (Lalu, Anitha, and Tilba, 2014). When the longshore sediment transport threatens to cause a problem such as siltation of harbour entrance etc., a long groin can be constructed just slightly updrift from the harbour entrance or river mouth. Though, it prevents sediment movement, it can cause erosion on the other side (Sundar and Sundaravadivelu, 2005). Groins are designed in such a way that longshore sediment transport is locally influenced in order to increase the persistence of sand at natural or artificially nourished beaches (Weichbrodt, 2008).
Sedimentary dynamic, tidal current, wave action and estuarine and tidal channel displacements have played an important role in controlling coastal changes (Hari and Nandyala, 2014). Shoreline evolution can be natural or can be caused by the side effects of marine constructions, designed/artificial beaches and shoreline protection structures (DHI, 2013). Erosion corresponds to increasing longshore transport, whereas accretion corresponds to decreasing longshore transport, i.e. the shoreline evolution is connected to gradients in the longshore transport rates (Kristensen et al., 2016). The integration of remote sensing and GIS technology is very useful for long-term reasonably accruable shoreline change studies (Mageswaran et al., 2015). Both natural (littoral drift, tidal action, near shore bathymetry) and anthropogenic processes (construction of seawalls, groins or breakwaters) along the coast modify the shoreline configuration and control the erosion and accretion of the coastal zones (Kannan, Kanungo, and Murthy, 2016; Mujabir, 2011; Mukhopadhyay et al., 2012). Prediction of shoreline oscillation due to the presence of structures is extremely important prior to their construction (Sundar and Suresh, 2011).
A numerical model for studying the impact of long jetties on shoreline evolution on eastern coast of Bandar Abbas was formulated using MIKE 21/3 Coupled Model. In the coupled model, HD, SW and Sediment Transport modules were run concurrently and the wave-wave and wave-current interaction were studied. The results were in good agreement and it was observed that the direction of sediment transport was along dominant waves in the area. Mike 21 Spectral Wave Module was used for simulation of wave climate in the nearshore area and Sediment Transport Module was used for the calculation of sediment transport in the south-central Kerala Coast and the study revealed that apart from natural processes, anthropogenic factors also play an important role in the erosion/accretion process of the coast. The littoral sediment transport and shoreline changes along the Ennore coast, located on the southeast coast of India, was studied using LITDRIFT module of LITPACK and wave hindcasting was simulated using MIKE 21 SW model (Ranga Rao et al., 2009). Numerical simulation model can reproduce the features of shoreline change due to coastal structures such as groin and detached breakwater, and thus is a powerful tool for functional design of coastal structure system (Aditee, Santosh, and Kudale, 2015).
Located at the southern tip of the union territory, the Puducherry Port is planned to accommodate barges, steamboats and large vessels; however, today it serves only as a fishing harbour. The construction of two breakwaters is the principal cause of Puducherry's coastal erosion. These two breakwaters obstruct the natural south to north drift of sand during the monsoons (IOM and NCSCM, 2011). In an attempt to limit the effects of this beach erosion, there has been a continuous process of construction, reinforcement and maintenance of seawalls along the shoreline north of the harbour. During the last decade, the Government of Puducherry has, on several occasions, proposed to build other structures along the Puducherry coast, such as groins for the increased armouring of the coast, breakwaters asmitigation against the effects of a new fishing harbour at Pudukuppam, and a deep-sea water port at Puducherry.
Puducherry city is the capital of Union Territory of Puducherry with main economic activities like small-scale industries. It is situated in the Coromandel Coast between 11°45′ and 12°03′N latitudes and 79°37′ and 79°53′E with an area of 293 sq. km. The existing port of Puducherry is situated between two major ports namely Chennai and Tuticorin. Puducherry has hot and humid summer, cool winter, and two distinct monsoons (southwesterly and northeasterly). The maximum and minimum temperature recorded are 35.7° in the month of June and 20.9° C in January respectively. The region experiences two different monsoons, North East and South West, annually. During SouthWest monsoon, the waves approach the coast from the S-E direction and during the North East monsoon, the wave direction is from N-E and E. The normal wave climate in the Bay of Bengal is mild with significant wave height variation from 1m to 1.5m and peak period variation from 7 s to 9 s, cyclones result in a severe wave climate with significant wave heights ranging from 4m to 6m and peak periods ranging from 10 s to 18 s. Vaithikuppam in Puducherry is located 11°56′46.10″N latitudes and 79°50′17.24″E longitudes and about 4.5 km north of Puducherry harbour.
Need for Study
Considerable local efforts made by the government to protect the coastline from natural and manmade activities failed to resolve the problem which has now shifted to the coastal villages in the north of Puducherry harbour.
Vaithikuppam is one such coastal village severely affected due to sea erosion. Due to erosion, the fishermen have lost the nearby berthing place and have to anchor their boats in the fishing harbour located far away from their village. During July 2015, Puducherry Public Works Department constructed a 60 m long groin with loosely packed boulders available along the site between Kuruchikuppam and Vaithikuppam, to form a temporary berthing facility to the fishermen at the seashore. To prevent an adverse impact on the coast it has become imperative to study the effect of groin at Vaithikuppam.
A thorough understanding of the coastal processes near the groin is required to assess the status of the existing groin in Vaithikuppam village. The following objectives were formed to achieve the purpose of the study:
Analysis of the near shore wave dynamics with the presence of groin.
Numerical modelling study to analyse the sediment pattern before and after the construction of the groin.
The numerical modelling simulations of the Vaithikuppam groin were carried out using MIKE 21/3 Coupled Flexible Mesh (FM) module developed by the Danish Hydraulic Institute (DHI), Denmark. Flexible Mesh was selected as it uses a finite volume approach, which combines the numerical stability of the Finite Difference method (regular rectangular mesh) with the ability of the Finite Element method to create complex geometry (flexible triangular or rectangular grids). Investigations were undertaken by numerical computer modelling and included assessment of wave climate, currents and coastal processes. Potential changes considering the construction of groin at Vaithikuppam were assessed.
Regional Wind to Wave Model
Indian Ocean Bathymetry
For simulation of waves in the Indian Ocean, a large domain, ranging from 6°S to 25.46°N (Latitude) and 60°E to 100°E (Longitude) was selected. An unstructured triangular mesh is generated with varying sizes of triangles (elements). C-MAP data for deep-water regions along Indian Ocean and bathymetry data sets of measured data are applied to shallow water regions in the study area, by interpolating them to each element in the flexible mesh bathymetry.
Wind is the basic input parameter for wave simulation. Successful wave hind cast and forecast depend on accurate wind fields deduced from meteorological models and analysis. The wind data was collected from European Centre for Medium-Range Weather Forecasts (ECMWF). The wind data was obtained for a period from January 2012 to December 2012 with spatial resolution of 0.25°×0.25° and temporal resolution of 3 hours. The data for wind was obtained as u and v components of wind velocity (m/s) at 10 m height.
The regional model results were validated with the wave rider buoy (WRB) wave data at 35 m water depth deployed off Puducherry coast for the year 2012 for August, September and October.
Figure 5a, b and c show the comparison of mean wave direction, θ (degree), peak wave period, Tp (s) and significant wave height, Hs (m) of the simulated wave and observed wave at Wave Rider Buoy location. The modelled wave heights are found to be in good agreement with the observed wave heights with a correlation co-efficient of 0.88 (Figure 6). This proves the reliability of the model for extraction of boundary conditions to the local wave-to-wave model.
Local Wave to Wave Model
Study Area Bathymetry
The survey was carried out for a stretch of 8 km along the coast, covering a length of about 3 km north and about 5 km to the south of the Vaithikuppam groin location. The local domain was set between 1315893 mN to 1323715 mN (Northing) and 372560 mE to 375953 mE (Easting). The bathymetry and topography data collected from field survey was applied to local model bathymetry. For the larger domain in the mesh, a resolution of 80 m was used progressively reducing to 30 m at the applied site of interest.
The offshore boundary conditions like significant wave height (Hs), mean wave direction (θ) and peak wave period (Tm), extracted from the regional model are used as the forcing function in the local model.
Validation of Local Model
Figure 8a, b and c show the comparative study of the mean wave direction, θ (degree), peak wave period, Tp (s) and significant wave height, Hs (m) of the simulated wave and observed wave at Directional wave rider buoy location, for 15 days in January 2015. The modelled wave heights of local wave to wave model are in good agreement with the observed wave height at water depth of 6.5 m with a correlation coefficient of 0.84 (Figure 9).
Validation of Water Levels
The water surface elevation replicated from the model is validated with the observed water level data at the water depth of 6.5 m, for the month of January, 2012. The simulated surface elevation is in close match with the observed data as shown in Figure 10 with the correlation co-efficient of 0.91 (Figure 11). This high correlation value indicates that the model is able to reproduce water level and currents accurately in the entire model domain of the respective study areas (on the particular study area).
Coupled Model Setup
The model was simulated for combined wave and current. The forcing parameters from hydrodynamics and spectral wave model were incorporated in ST model for creating a sediment transport model which includes the effect of both waves and currents. A sediment transport table is generated with combinations of bathymetry, coastal current, wave and sediment conditions appearing in the simulation sediment properties.The wave field from SW model simulation was given as forcing. The feedback on hydrodynamic, wave and sand transport calculation was included. Hence water flow and current variation from HD simulation is also included.
RESULTS AND DISCUSSIONS
With the overall aim of study the effects of the groin on waves and sediment transporting, simulations were conducted with and without the presence of groin.
The wave rose plots were made in the absence of and with the help of groin (Figure 12) and the maximum significant wave height observed in both the cases remained the same with a value of 1.96 m, since the length of the groin is 60 m.
The current speed and direction were plotted using rose plots at two points, one about 120 m to the north and the other south of the groin as shown in the Figure 13. Without the groin, the maximum current speed in the north was 1.29 m/s, and that in the south was 1.36 m/s, and the current speed decreased with the groin viz. 0.74 m/s in the north and 0.68 m/s in the south. In addition, the current direction also showed a change, which is mainly due to the alignment of the groin.
It is observed that, during the month of August, to the north and south, magnitude of current speed is relatively less with the presence of groin initiating the sediment deposition, whereas the magnitude of current speed at the tip of groin is high which in turn indicates the localized erosion due to tip scouring.
In the month of August, before the construction of groin, there was a net erosion of 550 m3/s in the sediment cell around the area but after the construction, about 1559.61 m3/s was seen to be deposited in the area.
For December, the current vector plots without and with the groin is shown in Figure 17. During December, the rate of accretion in the sediment cell increased from 1418.3 m3/s to 2672.1 m3/s after the construction of the groin.
A systematic study was carried out to assess the groin construction at Vaithikuppam. The depth averaged and fully integrated 2D model study for waves, currents and sediment transport is carried out using MIKE 21/3 FM coupled model and it has been established to simulate the scenario with and without the groin. To ensure the accuracy of the model, the results were calibrated and validated with measured data.
The correlation co-efficient for significant wave height is 0.88 and 0.84 for regional and local spectral wave model and that for the water level is 0.91.
The maximum current speed is found to decrease after the groin construction from 1.29 m/s to 0.74 m/s to the north of groin. Similarly, for the south it decreased from 1.36 m/s to 0.68 m/s while there was an increase in the current speed near the tip.
For the SW monsoon, the sediment transport for the month of August was analysed and it was observed that after the construction of groin, the contour lines shifted 20 m towards the offshore bed level and remained the same after the 5 m contour lines. It was noted that a sediment is being deposited with the groin whereas without the groin, it was being eroded.
The sediment pattern for the month of December was analysed and it observed that an accretion had increased with the presence of groin.
One year with and without the groin, indicates erosion and accretion according to the seasonal pattern. There is no significant impact observed on the coastline with the presence of groin, groin length being 60 m.