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Sedimentation Patterns in the Ganges-Brahmaputra Delta System

Abstract

The monsoon-driven Ganges-Brahmaputra river system transports large amounts of sediment from the Himalayas to the delta. The two rivers combined bring ∼1 billion ton into Bangladesh as reconstructed from observations at Hardinge Bridge and Bahabadur gauging stations from the late 1950's onwards. Downstream spatial distribution of sediment flux into the deltaic distributary channel network and deposition rates onto the floodplain and delta plain are remarkably unconstrained, yet of critical importance to the understanding of the overall delta sediment budget. We numerically model daily incoming sediment flux with a distributed hydrological basin model, WBM-SED. The model uses re-analysis climatology to calculate the water balance and routes water and sediment throughout the Ganges and Brahmaputra drainage basins. The estimated flux provides a boundary condition to the lowland sedimentary system. We present a simple approach to sediment routing over the delta distributaries and into tidal channels; we use channel network characteristics to distinguish between three orders of channels and route suspended load according to their planview dimensions. In the tidal delta, we reverse our simple routing scheme with sediment flux coming from the nearshore water. Characteristics of associated islands, such as nearest-edge to water distance, are determined for each of the characteristic channel order classes. We then use two cross-sectional process models, AquaTellUS and FV-SED, to calculate cross-channel sediment flux deposited on delta islands during river flooding and tidal flooding. Monsoonal flooding and a high tidal range are highly efficient mechanisms to re-deposit sediment onto the delta plain, especially in areas of high channel connectivity in the coastal zone. This finding is corroborated by our field data on sedimentation rates in the coastal zone, which highlighted that over a single monsoonal season as much as 1 cm/yr of sediment is deposited widespread in the tidally-controlled areas of the abandoned Western delta. Preliminary field data in the fluvial-dominated reach shows higher sedimentation rates locally (∼5 cm/yr), but exhibits a more spatially varied sedimentation pattern. These results are comparable to rapid near-channel sedimentation as indicated by the numerical modeling. Our simplified concept helps highlight unknowns in the deltaplain storage term in the source to sink system of the Ganges-Brahmaputra delta.