GLOBAL ASSESSMENT OF THE IMPACT OF SEDIMENT RETENTION BY RESERVOIRS ON MAJOR DELTAS

A significant global impact of dam construction on river systems is the trapping of sediment by reservoirs. It is estimated that approximately 30% of the global sediment load is trapped behind reservoirs. This trapping has significantly changed the input of fluvial sediment to coastal areas and deltas. Many rivers that had been choked with sediment resulting from changes in land use during the nineteenth and early twentieth centuries are now transporting minimal sediment to the coast. Deltas are the interface between terrestrial sediment and the coastal zone and are particularly sensitive to these perturbations in sediment input. The primary objective of this study is to assess the global impact of sediment trapping and the resulting decrease in sediment flux on the potential erosion and inundation of major deltas. A global database of basin characteristics, anthropogenic influence on sediment loads and coastal zone characteristics has been compiled for 30 major deltas globally. The relative coastal change risk of the 30 major deltas is evaluated using a coastal vulnerability index (CVI) developed from the database. The CVI is based on natural riverine sediment load, sediment trapping efficiency of the regulated basin, local relative sea level change, rate of delta subsidence, significant wave height in the coastal zone, tidal range, coastal slope and sedimentology of the delta. A coastal risk model has also been produced based on regression relationships between measured delta erosion rates and the above variables for select deltas. CVI and model results are validated using documented studies and satellite and aerial imagery. The CVI and risk model will be used to predict deltas particularly vulnerable to future reservoir construction and changes in fluvial input.