Predicting land subsidence and cascading flood hazards on deltas in the twenty-first century 

Densely populated coastal deltas worldwide face cascading flood hazards associated with sea-level rise, storm surges, dwindling sediment supplies, and land subsidence. One of the greatest hurdles to hazard prediction stems from this last component—land subsidence—which can vary drastically in space and time for a given delta. Here we constrain subsidence variations on the Ganges Brahmaputra Delta, using a state-of-the-art 1D compaction model based upon fundamental principles of porous-media mechanics and groundwater flow; as well as constitutive relations for porosity and edaphic factors (e.g., plant roots, animal burrows). The model accurately reproduces field observations (GNSS, RSET-MH, optical-fiber compaction meters, auger cores), showing compaction-induced subsidence rates of 1–30 mm/y depending upon local thickness and lithology of underlying Holocene deposits, forest tree density, and sedimentation rate. Sedimentation drives a dynamic compaction response over timescales of 10–100 years, such that floodplains cut off from sediment after embankment construction in the 1960s have undergone significant elevation loss, but are now experiencing a gradual subsidence slowdown. Some of the fastest subsidence rates can be attributed to buried Pleistocene paleovalleys infilled with thick Holocene sediments, portending a legacy of ancient sea-level changes on future flood hazards. Updated coastal flooding estimates informed by our model indicate that compaction-induced subsidence will be responsible for up to 50% of twenty-first-century relative-sea-level rise, and exert a first-order control on flooding hotspots. This predictive subsidence model can improve assessments of coastal flood risk on the Ganges-Brahmaputra and other deltas worldwide; and help inform ongoing billion-dollar restoration efforts facing crucial decisions as to where and when coastal barriers, sediment diversions, and settlement relocations should be implemented in the coming century.