Classification of Indian Coastal Tidel Inlets and Some Aspects of Morphology Changes Under Cyclone Event

Abstract

Coastal inlets are the openings along the coastline which connects wide-spread oceans and the adjacent water bodies. These coastal inlets vary in nature depending on many parameters including waves and tides. Coastal inlets play a major role in regulating the inland flow and in feeding the coast with hinterland sediments to maintain the coastal morphology. The studies for inlet dynamics are essential to understanding the coastal and inland waters interaction like physical processes as well as the exchange of nutrients or pollutants between the two water bodies. To study the inlet dynamics, understanding its behaviour in the form of type of dominance (i.e. tide-dominated or wave-dominated or river dominated) along the coastline and morphology of the tidal inlets are very essential. Although many methods are available in the literature to classify the tidal inlets worldwide, comprehensive information on inlet classification and river discharge at majority of the inlets is not available for the Indian coast. This study identified 471 tidal inlets along the nine maritime coastal states of India, and focuses on the classification of these inlets. The classification is based on various parameters viz., tidal range, wave exposure conditions, morphology, hydrodynamics and non-dimensional parameterization of wave, tide and river discharge forces with and without mean wave period. All these 471 tidal inlets have an inlet width greater than or equal to 5m at the confluence of river with the sea. The inlet morphology depends on the tidal range, significant wave height, mean wave period and river discharge. Therefore, these parameters were determined in the vicinity of each of the inlets. The tidal range for each of the inlet is obtained by simulating the water level variations for about 30 days along the coast using a state of the art hydrodynamic model of MIKE by DHI, i.e., MIKE-HD flow model. The wave data, viz., significant wave height (H s ) and mean wave period (T z ) are obtained from the validated MIKE21 spectral wave model simulations for the year 2010 using NCEP/NCAR wind as input data. iThe Central Water Commission (CWC) and State’s Water Resource Departments maintain gauging stations to measure the stage, discharge and water quality in the major streams and rivers across the country. The data maintained by CWC is available through online portal Water Resources Information System (India-WRIS). Out of 471 tidal inlets, only 36 inlets have the gauging stations within 1km from the inlet entrance (Source: Karnataka State Natural Disaster Monitoring Centre), and therefore, the available discharge is directly taken from the database for the classification by assuming no losses between the gauging station and the sea. Due to the limited information available on the measured river discharge, discharge at ungauged stations along Konkan and Malabar coast has been estimated using catchment characteristics rather than rainfall-runoff data. The catchment characteristics viz., catchment area, length of the main stream, a distance of the centroid from basin outlet and equivalent stream slope are determined using basin maps in ArcGIS® software. The Synthetic Unit Hydrograph (SUH) methods like CWC method, Snyder method, Soil Conservation Service (SCS) method, Gray method, Croley method, Transmutation approach method, two parameter Gamma distribution method and Hybrid model are used to estimate the flood discharge. Since there is no measured data available, the discharge estimation at the tidal inlets has been done in two stages. Firstly, for the CWC measured data, the above said SUH methods are applied and validated with the measured data. Later, the discharge for ungauged basins is simulated based on the fitted parameters for selected methods. The results showed that even though the CWC, Snyder and SCS methods are used widely in practical engineering problems, the manual fitting of Unit Hydrograph with limited characteristics points needs a high degree of precaution as well as trial and error. Alternatively, the probability distribution functions (pdfs ) based SUH methods used in the study give the full shape of unit hydrograph and also satisfies the prerequisite for UH criterion (i.e., area under the UH is unity). A new non-dimensional UH is formulated which is applicable for Konkan and Malabar coastline of India. The classification of the Indian coastal tidal inlets based on the tidal range has been carried out based on the values given by Davies (1964) and Hayes (1975). Wave iiexposure condition classification is carried out using the basic measure of wave energy which is quantified with the parameter H 2 T 2 , where, H is the annual average significant wave height, and T is the mean wave period. Geomorphological classification of the tidal inlets is carried out visually using the satellite images. Hydrodynamic classification of tidal inlets is carried out according to the relative strength of tide and waves with tidal range serving as a surrogate for the tidal prism or tidal current. In the Hydrodynamic classification, wave heights during non-monsoon, southwest monsoon and northeast monsoon periods are considered to identify the variations in the classification of the inlets in addition to annual wave heights. Non-dimensional classification is carried out for 75 inlets for which the river discharge information is available. In addition to the three main forcing functions viz. tide, waves, and river discharge, the non-dimensional classification considers the mean wave period. Thus, the tidal inlets classified by hydrodynamics and non-dimensional methods are compared with the geomorphological classification. A new range is identified for the non-dimensional classification by incorporating the mean wave period. Majority of the inlets along the Indian coastline fall under the category of micro- to meso-tidal environment. Hence for many river mouths and tidal inlets along the east coast and southern part of the west coast, inlet migration depends upon the magnitude of littoral drift, tidal current and depth of the inlet at the entrance. Tidal inlets along the west coast of India (219 in number) are subjected to high wave energy environments, whereas east coast inlets are subjected to moderate wave energy environments. None of the inlets fall under the category of mildly exposed wave environments. Majority of the tidal inlets on the east coast of India and the southern regions on the west coast (Kerala, Karnataka, and Goa) are observed to be wave-dominated inlets, whereas, the inlets towards the north on the west coast (Maharashtra and Gujarat) are tide- dominated. Intermittently Closed and Open Lakes and Lagoons (ICOLLs) are present in the coasts of southern states: Andhra Pradesh, Tamil Nadu, and Kerala. The blockage of tidal inlets may lead to flooding due to excess rainfall in the upper reaches of the watershed. The blocked inlets will impact significantly the sediment discharge system in the coastal region. On the other hand, excessive alongshore iiisediment transport could result in sediment deposition at the inlet mouths and lead to inlet closure. The coastal wave climate has a significant influence on the coastal sediment transport system as well as in the breaching of the closed tidal inlets. These interlinked phenomena are still in the interest of research community as the underlying physical processes are temporally and spatially variable. Also, the formation of flood and ebb deltas is yet to be understood. Moreover, the passing of storms in the vicinity of tidal inlets poses very different problems both in terms of increased river discharge due to storm associated rainfall or due to increased wave activity and storm surges. The morphological changes of the tidal inlets associated with the storm activity is a challenging research area on which models have not yet fully mimicked all the processes. Therefore, to study the morphology changes in tidal inlet vicinity due to a cyclone, numerical modeling study is undertaken at a specific location on the east coast of India, where some of the measured data is available. To study morphological changes near the vicinity of a tidal inlet, a case study of cyclone Phailin has been taken which had a landfall near Gopalpur in Odisha state. The reasons to take up this study area are: (1) availability of measured wind and wave data, (2) the landfall point is closer to the coast where a tidal inlet exists and (3) satellite images showing a breach of the berm after the cyclone, which was not there before the cyclone. Using the MIKE21 numerical model, the conditions for breaching of the berm has been successfully reproduced, and thereon morphology around the inlet and in the near shore region is studied.