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

Abstract

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 ~1.7 × 106 m3 of sediment infill. Using conservative values, we estimate that the dredge spoils could be used to elevate ~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, ~547 000 houses could be raised above the flood levels and ~500 km of embankments could be elevated. With a mean household population of 4.3 people, repurposing dredge spoils can positively impact ~2.4 million people within the vulnerable southwest region of the GBM Delta.

Lowland rivers: Geomorphology, human impacts, and management

“…And flood basin inundation can be exacerbated by ground subsidence, particularly when soft organic-rich deposits undergo oxidation due to embankment and drainage. Local-scale changes in flood basin hydrology have implications to patterns of sediment dispersal, aquatic habitat, vegetation change, and as we are frequently reminded, human safety (Valentine et al, 2022). Key types of hydraulic infrastructure used to manage flood basins includes levees (dikes), flood gates, weirs, diversion channels, sluice gates, canals and channels, ditches, and pumps (Hudson, 2021;Singer et al, 2008, tables 6.1, 7.1).…”

“…And flood basin inundation can be exacerbated by ground subsidence, particularly when soft organic‐rich deposits undergo oxidation due to embankment and drainage. Local‐scale changes in flood basin hydrology have implications to patterns of sediment dispersal, aquatic habitat, vegetation change, and as we are frequently reminded, human safety (Valentine et al, 2022).…”

“…The issue is recognized by the Intergovernmental Panel on Climate Change (IPCC) as a major concern, especially for Asian mega deltas (Cruz, 2007). The intricacies of this process are well illustrated in the Ganges–Brahmaputra–Meghna (GBM) Delta, where tidal channel embankment and flood basin land‐use change has driven oxidation and subsidence of organic rich surface soil (Valentine et al, 2022). Deltaic tidal channels have largely infilled due to the levees (dikes) trapping sediment within a smaller embanked floodplain, resulting in storage of fine‐grained sediment within degrading channels.…”

“…Combined with subsidence, the difference between embanked floodplain and polder floodplains range between 1.0 and 1.5 m. Dredging of tidal channels has exacerbated the problem, as much dredge spoil is dumped along channel bank tops, further reducing hydrologic connectivity between channels and flood basins. An important management issue, therefore, concerns what to do with the dredged sediment, argues Valentine et al (2022). In the case of a sinking delta with a high population density the sediment can be repurposed (recycled) to heighten dikes or build up lands to remove human settlements from flood risk.…”

“…The 10 special issue contributions herein reviewed concern important fundamental and applied topics within lowland river science, specifically highlighting three key themes. These include ‘natural’ river channel dynamics within settings that vary from the standard body of knowledge (Guha et al, 2022; Rogério et al, 2022; Sinha et al, 2022), geomorphic response to channel engineering (Mossa & Chen, 2022; Marteau et al, 2022; Tang et al, 2022; Wu et al, 2022), and flood basin management in response to environmental change (Keim et al, 2022; Phillips, 2022; Valentine et al, 2022). The categories are not mutually exclusive, and most contributions have linkages across multiple themes.…”