Spatial Variability of Mud Bank along the Southwest Coast of India and Its Impacts

Abstract

The mud bank at Alleppey along the southwest coast of India is a persistent one that forms in June and dissipates by September during the southwest monsoon (SWM).A calm patch of turbid water with a thick fluid mud at the near bottom helps in dampening the large waves.This holds a high societal and economic importance as a large number of fishing boats access the shore using this mud bank as a gateway.This study explores the spatial variability of the Alleppey mud bank at the southwest coast of India through Synthetic Aperture Radar (SAR) remote sensing techniques and its effect on the coastline.The presence of the Alleppey mud bank at Punnapra beach was evident during 2007, 2015 and 2016 through the SAR signatures.However, since 2017, the mud bank has been forming at the Vandanam coast, which is around 3~km south of the Punnapra coast, i.e.at a different location from its regular region of the formation.The observed beach erosion at Punnapra (~ 30~m/yr from 2015 to 2020) confirms the absence of a calm region, which was due to the migration of the mud bank.Conversely, the formation of a mud bank at the Vandanam coast makes the region calm and conducive for accretion (~ 8~m/yr).The shift of the spatial location of the mud bank in 2016 is attributed to the Southern Ocean swell which struck the coast in Aug 2016.We hypothesize that the shift in the mud banks formation region after 2016 in the Kerala coast was influenced by the swells, generated in the Southern Ocean during the last week of July 2016.Geospatial variations of Alleppey mud banks are not only influenced by the regional forcings, but also by remote forcings such as the swells generated far away.The mud bank area was observed to have reduced in 2018 as compared to other years.The formation, existence and dissipation of mud bank of the southwest coast of India are socially linked as it provides a conducive environment for the calm landing of the fishing boats during rough monsoon season.It provides livelihood to thousands of fishermen in that time.It also protects the coast from monsoonal erosion.This study explored the importance of the presence of mud bank and its influence on the adjoining coastal region.