Slowing down the tidal flood wave is the key to reducing tidal flood risk in estuaries worldwide

Estuaries worldwide are expected to suffer increasing tidal flood risk due to climate change. Climate change is causing sea-level to rise and storm frequency and severity to increase, inducing more frequent tidal floods in estuaries, where most of the world’s largest cities are located. The key to mitigating this flood risk is by reducing tidal propagation from offshore to onshore. Flood alleviation in an estuary requires slowing down the passage of water coming into an estuary, rather than creating space for flood water. There is a nearly infinite supply of water flooding an estuary and thus, in contrast to terrestrial rivers, making space is not an effective solution for flood risk mitigation. To test this hypothesis, we investigate the effectiveness of natural tidal flood interventions for reducing water surface elevations along estuaries by performing sensitivity analyses on a 1D analytical model of tide propagation for estuaries worldwide, considering the impacts of future sea level rise. We find that increasing estuary bed roughness is the most efficient tidal flood intervention, whereas adding space for flood water has minimal impact on water surface elevations. A more focused  2D numerical model experiment simulating the hydrodynamics of the Clyde Estuary, Scotland, also reveals that roughening the estuary bed and banks significantly slows down the passage of water by absorbing tidal wave energy, delaying the tidal wave peak arriving from offshore and consequently reducing water surface elevation and tidal flood extent. However, the 2100 SLR projection consistently reduces the effectiveness of all such interventions, highlighting the challenges of implementing the most effective solutions for alleviating future tidal flood risk.