Abstract

Flood risk in low-elevation coastal zones (LECZs) is traditionally considered mainly in terms of land elevation, sea-level rise, subsidence, tides, storm surge, and how these conditions and forces interact with artificial coastal-defense systems. Here we examine the ability of coastal morphodynamic processes to mitigate future flood risk, focusing primarily on large LECZs including deltas. Coastal wetlands (marshes, swamps, and mangroves) that are common in LECZs are widely believed to respond to increased flooding by increasing vertical accretion rates, as is well documented in salt marshes. An extensive dataset of present-day wetland change is available from coastal Louisiana by means of almost 300 monitoring stations that record rates of surface-elevation change and vertical accretion, enabling rates of shallow subsidence and relative sea-level rise to be determined at each site. This dataset demonstrates the strong correlation between vertical accretion rates and rates of relative sea-level rise. However, there is a limit to this correlation: >40% of the sites exhibit an accretion deficit, putting these wetlands in danger of drowning. This condition is reached relatively quickly in coastal Louisiana due to the lack of elevation capital, which is due, in turn, to the microtidal regime. River deltas, mainly large deltas with high fluvial sediment supply, are uniquely suited to gain land elevation and thus mitigate future flood risk. Here we consider the morphological changes in deltas due to environmental forcing, notably changes in sediment supply. Drainage basins that experience increased sediment fluxes (usually due to deforestation and agricultural activity) drive transitions to more river-dominated delta morphologies that are typically associated with progradation and delta-plain growth. On the other hand, we find that reduced riverine sediment fluxes that are common in drainage basins with dam construction can lead to increased tide and wave influence. Counterintuitively, this can be associated with tidal import of sediment and land gain through the infilling of fluvial channels. On the other hand, increased wave influence is more likely to lead to coastal erosion and net land loss.
Original languageEnglish
Publication statusPublished - 1 Dec 2019

Keywords

  • 1821 Floods
  • HYDROLOGY
  • 1824 Geomorphology: general
  • 1862 Sediment transport

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