Quantitative analysis of erosion and accretion (1975–2017) using DSAS — A study on Indian Sundarbans

Abstract

Sundarbans, world’s largest mangrove forest is now struggling to maintain its existence in the face of severe erosion. Any management process to protect this vulnerable ecosystem and to rescue millions of people living here, require place specific erosion measurement. This paper attempted to assess point specific erosion/accretion rate between 1975–2017 using LANDSAT Images. Permanent waterline was demarcated from the rectified images using the algorithm Land Surface Water Index (LSWI) > Normalize Differential Vegetation Index (NDVI). 178 inhabited Mouzas (smallest revenue unit) and 48 islands located along the major estuaries from Hooghly to Raimangal are taken for predicting shore/bank movement using the Digital Shoreline Analysis System (DSAS). 7723 transects along the main river and 855 along the coast are generated at an interval of 100m for calculating End Point Rate (EPR), Net Shoreline Movement (NSM) and Land net Loss/Land Gain (LL/LG). From the EPR we found 62% and 85% transects are eroding along the estuarine channels and coast respectively. Multiplying mean EPR and transect interval (100m) between two transect, inter transects net land loss or gain were calculated. Among the channels intensive erosion were observed along the eastern bank of Muriganga in all successive years. Accretion increases landward from the coast in almost all channels. A sharp difference of erosion/accretion observed between reclaimed mouzas and islands covered by dense mangrove swamp. 54% mouza having embankments are retreating at an average rate of 6.14 metre/year, contrary to this only 35% mangrove islands are moving landward at a much slower rate (4.78 m/yr.) in comparison to habitat areas. Process of new land formation is also very steady in mangrove areas (4.78 hec./yr.) than habited areas (6.84 hec./yr.) which indicates channel stability and erosion resistance provided by Mangrove. Mangrove plantation can be a useful option for erosion management.