Quantifying the Effects of Sea Level Rise on Estuarine Drainage Systems

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.