Site-Specific Thresholds for Storm-Driven Compound Flooding in UK Estuaries

Estuarine flooding is driven by extreme sea-levels and river discharge, either occurring independently or at the same time, or in close succession to exacerbate the hazard, known as compound events. Estuaries have their own dynamics, and different behavior means flooding will occur under different conditions. Recent UK storms, including Storm Desmond (2015) and Ciara (2020), have highlighted the vulnerability of mountainous Atlantic-facing catchments to the impacts of compound flooding including risk to life and short- and long-term socioeconomic damages. There is a need to identify site-specific thresholds for flooding in estuaries, which represent the magnitude of key drivers over which flooding occurs, to improve prediction and early-warning of compound flooding.

In this study, observational data and numerical modelling were used to reconstruct the historic flood record of an estuary particularly vulnerable to compound flooding (Conwy, North Wales). The record was used to develop a method for identifying combined sea level and river discharge thresholds for flooding using idealised simulations and joint-probability analyses. Only 6 records of known flooding are identified in the official record. The key limitation of using historic records of flooding is that not all flooding events have been documented, and there are gaps in the record. Therefore, this research also identified the top 50 extreme sea-level and river discharge events in the historic gauge measurements in the estuary, and cross-checked these against online sources using web scraping to establish if these additional 100 extreme events also led to flooding. A more comprehensive historic record of flooding allows more accurate thresholds for flooding to set in each estuary.

Caesar-LISFLOOD, a hydrodynamic flow and morphological evolution model, is used in a sensitivity test to simulate inundation under different idealized sea-level and river discharge conditions to further isolate accurate thresholds. The variation in flooded area from a baseline scenario is used to capture flood magnitude associated with each scenario. The results show how flooding extent responds to increasing total water level and river discharge, with notable amplification in flood extent due to the compounding drivers in some circumstances, and sensitivity due to a 3-hour time-lag between the drivers. Joint probability analysis is important for establishing compound flood risk behaviour. Elsewhere in the estuary, either sea state (lower-estuary) or river flow (upper-estuary) dominated the hazard, and single value probability analysis is sufficient. These methods can be applied to estuaries worldwide to identify site-specific thresholds for flooding to support emergency response and long-term coastal management plans.