Estuaries and low-lying coastal areas are at high threat for compound flooding with multiple concurrent or consecutive drivers, e.g., heavy storms, coastal flooding, high tides, and sea level rise. 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. As the effects of climate change become more prominent, extreme weather events and natural disasters are predicted to become more frequent. As per the Environment Agency, one in six properties in England are at risk of fluvial and coastal flooding from rivers and the sea with many more properties at risk from pluvial flooding. Coastal areas around the world are particularly vulnerable to increase in magnitude and frequency of future flood events in response to climate change (IPCC 2014). As per the UK Met Office, coastal flood risk in the UK is also predicted to increase over the 21st century with an increase in the frequency and magnitude of extreme water levels around the UK coastline.

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. 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. 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.