Flood risk of embanked areas and potential use of dredge spoils as mitigation measures in the southwest region of the Ganges-Brahmaputra-Meghna Delta, Bangladesh

In the 1960s, earthen embankments, locally known as polders, were first constructed in the Ganges-Brahmaputra-Meghna (GBM) Delta with the intention of protecting agricultural lands from salinity; however, unintended consequences of the barriers have negatively impacted the region. Once-active distributary channels have morphed into terminal, headless tidal channels as the sediment-laden water that was once dispersed across the tidal delta plain is now restricted by embankments. Elevation loss within embankments and channel siltation adjacent to embankments are deleterious consequences of hard engineering and pose significant flood risk when effective sea level rise is considered. The purpose of this study is to assess the current and future flood risks of the embanked areas in the southwest region of the GBM Delta and to expand on the potential for local-based solutions (e.g. mud plinths and embankments). We propose a novel source of material for these structures-repurposed dredge spoils. Project sites are along terminal tidal channels, proximal to coastal towns in the southwest region of the delta. We calculate here that as the effective sea level rise will continue to increase in the years to come, the flood risk will correspondingly increase (>67%). Geospatial analyses reveal that several prominent tidal channels have reduced in width over the last 30+ years, and recent dredging operations removed similar to 1.7 x 10(6) m(3) of sediment infill. Using conservative values, we estimate that the dredge spoils could be used to elevate similar to 20 000 houses on mud plinths or raise 18 km of existing embankments. On a delta-wide scale, if all infilled tidal channels were dredged to 10% of their original widths, similar to 547 000 houses could be raised above the flood levels and similar to 500 km of embankments could be elevated. With a mean household population of 4.3 people, repurposing dredge spoils can positively impact similar to 2.4 million people within the vulnerable southwest region of the GBM Delta.