Quantifying the Effects of Sea Level Rise on Estuarine Drainage Systems. Issue 6 (24th June 2022) - Waddington, K.
Abstract: Much of the development of the low elevation coastal zone has involved the reclamation of low‐lying floodplains and wetlands through the construction of flood mitigation and drainage systems. These systems function throughout the tidal range, protecting from high tides while draining excess catchment flows to the low tide. However, drainage can only be achieved under gravity when water levels in the catchment drains are higher than those in the estuary. Changes to the tidal range and to the duration of the rising and falling tides that occur throughout estuarine waters will result in dynamic variations in the window of opportunity for gravity discharge within and between different catchments and under sea level rise (SLR). Existing concerns regarding SLR impacts have focused on the acute effects of higher water levels, but SLR will affect the full tidal range, and drainage systems will be particularly vulnerable to changes in the low tide. This study introduces the concept of the drainage window to address this limitation by assessing how the present‐day and future SLR tidal regimes may influence the drainage of different estuarine floodplains. Applying the drainage window to two different estuaries indicated that SLR may substantially reduce the opportunity for discharging many estuarine floodplain drainage systems. Reduced drainage creates a host of chronic problems that may necessitate changes to existing land uses. A holistic assessment of future changes to all water levels (including low tide levels and extended flood recession periods) is required to inform strategic land use planning and estuarine management. Plain Language Summary: Estuaries are the tidal waters located where rivers meet the sea. The floodplains adjacent to estuaries are some of the most heavily developed areas in the world. Much of this development relies on integrated flood management and drainage schemes that use one‐way valves (floodgates) to protect the floodplains from inundation by high tides and floods, while allowing the floodplain drains to discharge when the water level in the estuary is lower than the water level in the drains. Tidal levels can vary along an estuary and may change under accelerating sea level rise (SLR). This study introduces the concept of the drainage window to quantify how much time is available to drain different floodplain catchments within an estuary and to identify how that window of opportunity may be affected by SLR. The drainage window was analyzed for two estuaries, with the results indicating that SLR may substantially reduce the time available to drain each system. Areas with less time to drain are more susceptible to chronic problems associated with prolonged inundation and waterlogging that may necessitate changes to existing land uses. These results could therefore be used to inform strategic land use planning and management in estuaries worldwide. Key Points: The drainage window is conceptualized and applied to two estuaries to quantify the effects of sea level rise on tidal drainage systems Areas that are protected from intermittent flooding may be vulnerable to chronic waterlogging due to impeded drainage Loss of function and amenity due to impeded drainage should be considered in future land use planning