Impact of intertidal habitats on storm-induced flooding in estuaries: Application to the Thames Estuary, UK

Intertidal habitats have been rapidly reduced worldwide due to land reclamation. To mitigate risks related to flooding, erosion, and biodiversity loss, intertidal restoration and creation are increasingly implemented worldwide. Nevertheless, the mechanisms through which these habitats affect estuarine flooding are still underexplored. This study developed a three-dimensional, idealized semi-analytical model to systematically examine intertidal effects on extreme high water levels (EWL) in estuaries. As a case study, the model was applied to the idealized Thames Estuary (UK) during the storm Xaver, the strongest North Sea surge in the last seven decades. Our results show that semi-diurnal tides and lower-frequency surges (with periods greater than ∼24 h) are the main drivers of EWL in the idealized Thames Estuary during this storm. The semi-diurnal tides and surges in the estuary are strongly amplified and delayed by intertidal habitats, and these phase delays increase with increasing tide/surge frequency. Intertidal effects on lower-frequency tides and surges are minor. Intertidal habitats increase EWL by more than 0.1 m during the storm Xaver, which is mostly controlled by habitat-induced changes in the semi-diurnal tidal amplitudes and surge phases. Although the Thames Barrier closure effectively reduces flood risks upstream during this storm, it increases EWL by ∼0.2 m in the outer estuary. Also, channel shallowing can reverse intertidal effects from raising to lowering EWL. Our findings reveal that intertidal habitats play a variable role across different tide and surge constituents, providing a critical scientific basis for understanding uncertainties associated with their effectiveness in estuarine flood risk mitigation.