Revised approach for the calculation of groundwater flooding annual average damages: establishing a probability-based relationship for groundwater flooding

Project Groundwater Greater Lincolnshire, as part of the Department for Environment, Food & Rural Affairs (Defra) and the Environment Agency (EA) Flood and Coastal Resilience Innovation Programme, seeks to enhance understanding and resilience to groundwater flooding. Existing economic appraisal methodologies and datasets within the Multi-Coloured Manual (MCM) are predominantly tailored to fluvial, pluvial, coastal flooding. The Flood Hazard Research Centre (FHRC) delivered a scoping report to Lincolnshire County Council (LCC) in July 2023, highlighting the need for groundwater-specific appraisal methodologies. A significant gap was identified regarding methods for estimating Expected Annual Damages from groundwater flooding due to the different flood mechanisms and impacts compared to other flood types.
This report outlines key considerations for developing a probability-based relationship for groundwater flooding, focusing on different types of groundwater flooding relevant to proposed Lincolnshire case study areas (Grimsby, Scopwick, Barton/Barrow-upon-Humber). Traditional methods such as the development of a probability-discharge relationship, as done for fluvial flooding, are not applicable due to the lack of discharge record data for groundwater events. Instead, the use of groundwater borehole levels (annual maximum level) has been identified as a more suitable dependent variable, though difficulties such as limited data and non-stationary time-series data due to changing abstraction rates exist.
Flood duration has been identified as a critical variable for evaluation of economic damages. Groundwater flood frequency analysis must therefore consider the likelihood of prolonged periods of high groundwater levels. This report considers that this may be addressed through the use of a Standardized Groundwater Flood Index (SGFI) or using maximum winter effective rainfall data to develop discrete event probability curves.
The final proposed approach for estimation of the frequency of groundwater flooding involves defining a benefitting area, identifying a relevant borehole and establishing the relationship between the receptors at risk and the borehole, and conducting a frequency analysis to establish event probabilities. The feasibility of this proposed approach will be further investigated and discussed for the selected case study areas within the second phase of this project. As groundwater flooding contexts and mechanisms vary, the approaches used for economic appraisal are also likely to be adapted to the context. Groundwater appraisal will likely require significant local survey and expertise to fully define the most suitable approach for each study area.