The importance of estuary shape in evaluating the flood risk in estuaries

Floods are a constant threat to communities within and around estuaries worldwide. This is because several drivers may occur there at the same time which leads to the compound flooding (e.g. high river discharge + storm surge). In this study LISFLOOD-FP hydrodynamic model is applied to examine the efficiency of different management strategies on flooding in the Dyfi Estuary, in Mid Wales. Five different bathymetries, including the current one, were developed in ArcGIS and included in the analysis: a) “hold the line”, b) “remove the line”, c) “advance the line”, d) “retreat the line”, e) “breach in the line”. Modified November 2020 compound flood event boundary conditions were forced from the coast and from the Dyfi bridge. Model results shown, among other things, that nature-based solutions in the lower estuary, represented by salt marshes and floodplain restoration measures, have a great potential in reducing water elevations across the estuary, unlike the advance the line scenario or the current bathymetry.

Additionally, results obtained from different management scenarios are analyzed and compared against results from the selected design storm events. These events were based on modifying parameters such as 1) relative timing of flood peaks, 2) storm duration and 3) climate change sensitivity (SLR and increase in discharge) which provided set of different compound flood events that were forced by LISFLOOD, in combination with the current estuary shape, unlike when modelling different management scenarios. This approach enabled us to perform an effective comparative analysis which addressed the key hypothesis of the research stating that changes in estuary shape will have bigger effect on flooding than changes in flood event itself. Indeed, it is shown that variability in water elevations caused by different management scenarios is bigger compared to variability caused by changing the boundary conditions only, although not always leading to higher water elevations along the estuary.