Influence of groundwater in compound flooding in UK estuaries. 

Low-lying estuaries, deltas, and bays are especially prone to flooding from multiple sources of high river discharge, coastal flooding from waves and storm surges, and pluvial flooding from intense rainfall – with groundwater levels a lesser researched flood driver. When these drivers occur simultaneously or sequentially, they create a greater impact, and are referred to as Compound Flooding. Recent compound events such as Hurricane Katrina (New Orleans in 2005), Cyclone Nargis (Myanmar in 2008) or Storm Xynthia (French Atlantic coast in 2010) have been shown to result in significant loss of lives and properties in coastal lowlands. Globally, 2.15 billion people reside in near-coastal areas, with 898 million in low-elevation coastal zones. The UK has a long history of estuarine flooding from compound events. UK climate projections indicate that there will be hotter and drier summers and prolonged wet winter periods, with an increase in the frequency and intensity of extreme storm surge and rainfall events that are also more likely to co-occur. Climate projections also indicate sea level rise at most locations around the UK which will make the coastal areas increasingly vulnerable. Groundwater is an important and dynamic component of the coastal environment. Coastal aquifers are vital fresh groundwater resources that are frequently subjected to coastal flooding due to increased runoff, storm surge and sea-level rise. Despite its lesser volumetric contribution in comparison with fluvial inputs, recent studies have found the presence and movement of groundwater may be both volumetrically and chemically important in river dominated coastal environments and requires future attention in view of climate change. Through our study we aim to investigate the different drivers influencing compound flooding in UK estuaries. Our focus is on the Conwy estuary in North Wales, which is a flashy catchment that floods several times per season. A serious recent compound flood event was due to Storm Ciara (February 2020) where river gauges hit record levels and combined with intense rainfall and high storm tide, impacting 172 properties. River Conwy drains a catchment of nearly 600 km2 and includes large mountains with high annual precipitation of around 1700mm per year and a baseflow contribution of 27%. Baseflow, which is the contribution of groundwater to surface water components, is notably influenced by topography, geology, vegetation, land use, and climatic factors. In this study we will develop a coupled catchment and groundwater model in Caesar Lisflood to understand how groundwater processes in the form of the baseflow can influence compound flood events in the estuary. Model simulations are calibrated against past fluvial and tidal flows to show how the river discharge, groundwater and associated drivers are likely to influence the magnitude, behaviour, and timings of compound flooding in the future.