Shoreline Dynamics In Response To River Sediment: A Case Study

Abstract

The coastal zone of Karwar is acquiring increasing importance due to its rich ocean resources and favorable conditions for the development of port-based industries, defense activity, tourism, fisheries, and small scale industries. Rivers and its networks are the major sources of the sediments, which supplies to the coast. These sediments are responsible for beach nourishment and shoreline configuration. The present study is carried out with a view to study the long-term shoreline configuration with the response to pre-construction dam and post-construction of the dam, to study the seasonal variation on shoreline configuration, to investigate the change in Kali estuary to assess the impact of the dam on sediment yield, To understand the sediment dynamics of beach face sand using granulometric method and To quantify the seasonal coastal process in terms of beach sand volume. These objectives are addressed using various conventional data, related tools, and freely available satellite data. Kali river basin, Aghanashini river basin, Karwar coast and Aghanashini coast along the west coast of India is the study area. There are five dams constructed across the Kali river basin for hydel power purposes. The presence of these reservoirs regulates stream flow and thus sediment load in the basin. However, the free flow of water across the catchment of the Aghanashini river leads to the unobstructed or natural passage of sediments and sediment budget to the downstream and the river mouth, as the catchment is not disturbed by the reservoir. Survey of India toposheet was used to prepare the base map. A conceptual, continuoustime and semi-distributed, SWAT2012 (Soil and Water Assessment Tool) model was selected for the sediment yield analysis. Dam location and dam discharge data were one of the major inputs for the model to estimate the sediment yield. Simulated and observed values of runoff are compared, and calibration and validation were done for the basins using SWAT CUP. The long-term shoreline configuration was carried out using LANDSAT satellite products only. The predominant direction of sediment transport was determined by drawing sediment trend matrices based on the statisticalviii parameters of beach face sediments. To understand the change in Kali estuary a portion of Devabag beach, satellite data were used for the duration from 1975 to 2018. The Total Station survey was carried for Ravindranath Tagore beach and Devabagh beach of Karwar coast, including a seasonal wise profile survey and cross-section survey during 2017. Based on the present analysis of sediment yield, it is concluded that the Sediment yield obtained at the catchment outlet was 1.39t/ha/year and 4.58t/ha/year for the Kali river and Aghanasini river basins respectively. It was observed that the decline in sediment load in the Kali river basin compared to the Aghanashini river basin indicates that the influence of reservoir operation on streamflow and sediment yield. The analysis to study the long-term shoreline configuration with the response to the pre-construction dam and post-construction of the dam, shows that shoreline of the Karwar coast was having accretion and later, it is turned in to erosion zone due to postconstruction of the dam. It shows the importance of natural river flow. Shoreline change analysis on the Aghanashini coast shows the accretion zone due to the natural flow of the Aghanashini river to the coast. The study estuary change shows that for the period of 1975 to 2018 northern part of the estuary has lost area and construction of seawall was revealed from ground truth data. Beach profile studies by total station on Ravindranath Tagore beach reveals that beach profile changes according to the season. The volume of sand is decreased during the pre-monsoon season and increased during the post-monsoon season. The accuracy of all the results can be increased by an increase in a number of inputs in the case of SWAT tool, the accuracy of results obtained from the satellite can be increased by higher resolution data. From the study, it is concluded that the river and its network are the major sources for the sediment supply to the coast. Sediment is one of the major factors for beach nourishment. Dam influences the flow of the river and its network and reduces sediment supply to the coast. A natural flow of the river and natural supply of river sediment enhances the beach nourishment and maintains the equilibrium of sediment budget to the coast.