Assessing Morphological Changes due to Hydrometeorologic Influences in Mehendiganj Island, Meghna Estuary, Bangladesh

Abstract:

The Ganges-Brahmaputra-Meghna river system is the largest river system in the world with massive discharge rates and sediment loads (annually over one billion tons). Sediment from these rivers has formed one of the largest and most densely populated deltas in the world. The combined rivers discharge through the Meghna estuary into the Bay of Bengal. The study area, Mehendiganj Island, is located in the morphologically dynamic Meghna estuary region of the delta and is characterized by rapid accretion and erosion. The net effect of erosion-accretion processes between the years 1987-2012 was analyzed using Landsat satellite imagery. Time-lapse series were generated over consecutive monsoon periods to estimate net erosion, and reveal that morphological changes are influenced by hydrological conditions (e.g. areal extent of flooding surface, hydrometeorology) driven by high river and sediment discharge, mainly during the seasonal monsoon (wet) period. The hydrological conditions and, consequently, the morphological changes exhibit a skewed pattern in annual distribution on account of high-energy condition prevailing during the monsoon. Total erosion and accretion within the study area was estimated to be about 5997 hectares and 2922 hectares, respectively. The measured annual erosion rates were as high as 1493 hectares, which were about 15% of the existing land surface within the study area. Discharge rates and sediment loads over the course of the study period were calculated using a numerical model (WBMsed) and was validated by comparisons with field-measured values. Moreover, hydrological parameters were analyzed in the context of statistical hydrology in order to obtain trends and were correlated with annual accretion and erosion rates attained from the satellite image analysis. Anomalies in the patterns of annual accretion and erosion rates were detected during extreme hydrometeorological events such as high floodwater years and cyclones. The morphological changes are associated with changes in different hydrological parameters. These massive changes caused social hazards with loss of life and property within the densely populated locality.