(10) The Development and Changes of Coastal Landscape Morphology of the Ganges-Brahmaputra-Meghna Mega Delta in Bangladesh
Nobuhide Nishikawa and Tetsuji Muto
Graduate School of Fisheries Science and Environmental Studies, Nagasaki University
A series of 2D tank experiments was conducted to explore how a river delta being fed bimodal grain-size sediment responds to non-uniform, basin water depths in the transverse direction. During each run, upstream water discharge, rate of sediment supply and base level were all kept constant. Sediment used was a 50:50 mixture of 0.1 mm and 0.9 mm quartz grains. A quite similar experiment was done previously but with 0.1 mm uniform sediment. Our experimental observations imply the presence of a particular feedback mechanism of delta distributary channels in response to differential bathymetry. When an active distributary channel empties into shallow water, a delta lobe rapidly extends in the offshore direction, but then in a relatively short period becomes inactive as the feeder channel migrates or avulses to another location. When an otherwise similar distributary channel empties into deep water, on the other hand, delta lobe progradation takes place only slowly, and it takes longer for the feeder channel to migrate to another location, whereby the delta can develop an isotropic shoreline configuration as a whole. Differential basement bathymetry can thus affect local residence time and avulsion frequency of active feeder channels. However, differential bathymetry does not function by itself, but always in combination with the profound effect of alluvial aggradation. These observations are substantially consistent with the existing notion, which was obtained from the uniform sediment experiment, suggesting that the compensational behavior of active distributary channels holds regardless of grain-size distribution of supplied sediment.