Impact of Groundwater in Compound Flooding: A Case Study of the Conwy Estuary in Wales.

Compound flooding refers to the co-occurrence of multiple flooding mechanisms, leading to more severe and complex flood events. Low-lying coastal deltas and estuaries are especially prone to compound flooding as they face multiple drivers such as storm surges, high river discharge, intense rainfall, tides, and sea-level rise. These combined sources amplify the impacts, resulting in significant loss of life and property, as seen in Hurricane Katrina (New Orleans, 2005), Cyclone Nargis (Myanmar, 2008), and Storm Xynthia (French Atlantic coast, 2010). Globally, 2.15 billion people live in near-coastal areas, including 898 million in low-elevation coastal zones. The UK has a long history of estuarine flooding caused by compound events. Climate projections suggest hotter, drier summers and wetter winters, accompanied by more frequent and intense extreme events. Sea-level rise is expected to exacerbate vulnerabilities in the UK's coastal regions (UK Met Office, IPCC 2014). Coastal aquifers are frequently affected by flooding, making groundwater a critical factor in estuarine geomorphology. Recent studies have highlighted the significant volumetric and chemical importance of groundwater in river-dominated coastal systems, warranting further investigation under climate change scenarios.

In this study we have developed a coupled catchment and groundwater model using Caesar Lisflood to assess groundwater’s contribution to compound flood events. The model is calibrated using historical fluvial and tidal flow data to evaluate how river discharge, groundwater, and associated drivers shape flood magnitude, timing, and behavior. Additionally, the study analyzes the sensitivity of the estuary to changes in hydrogeological parameters by observing variations in modeled groundwater heads and simulated discharge in response to changes in aquifer properties. Our research focuses on the Conwy estuary in North Wales, a flashy catchment that experiences frequent flooding events. A notable compound flood occurred during Storm Ciara (February 2020), when record river levels, intense rainfall, and high storm tides combined to affect 172 properties. The Conwy River drains a 600 km² catchment with annual precipitation averaging 1,700 mm and a baseflow contribution of 27%. Baseflow, the component of streamflow discharged from groundwater storage, reacts slowly to rainfall and is notably influenced by topography, geology, vegetation, land use, and climatic factors. This research delves into the lesser-studied role of groundwater in estuarine hydrology, providing insights into its potential impact on compound flood dynamics under future climate scenarios.