The historic evolution of coastal flood exposure in the UK

Abstract

Coastal flooding is a serious and growing threat, with 200 to 300 million people estimated to live within the coastal floodplain worldwide today. This exposure is not static and it is increasing globally due to rising populations and sea level rise. While there have been scenario-based investigations of future exposure, there is a knowledge gap concerning historic analysis of how exposure to coastal flooding has evolved. Understanding what drives changes in exposure can help us to better predict how exposure may evolve in the future under the combined pressures of climate change induced sea level rise, growing populations and increasing development within coastal floodplains. This thesis developed a quantitative methodology to evaluate the detailed historic evolution of exposure with regards to changing coastal population and other drivers of exposure. This includes formal definitions of exposure with and without defences. The occurrence of damaging coastal and river flooding over a >100 year period in the UK was evaluated which showed that reported flooding has been increasing significantly. Subsequently a framework was developed for quantification of exposure with and with- out defences: new GIS methods were developed to spatially distribute census population data across the indicative floodplain based on residential development patterns observed on historic maps and a rapid inundation model. A time series dataset on changes in defence heights was compiled from historic and contemporary records. A computational FAE (Fraction of Attributable Exposure) technique was used to evaluate the effect of flood drivers on the changing exposure. As a demonstration of the methodology, population exposure to a range of flood events was evaluated at 10 year intervals between 1801 and 2011 for Portsea and Hayling islands in the UK’s Solent region, representing a city with significant flood risk, and a more rural location, respectively. 1801 represents the first UK Census, while sea level data is available at Portsmouth since 1960 and this is extrapolated back to 1801. The results show that exposure has grown significantly at both sites. Annual average people exposure (averaged across a range of recurrence intervals) increased from 176 and 27 to 6,911 and 692 over the study period in Portsea and Hayling, respectively. Most of the exposure in Hayling developed after 1931 when residential areas started to encroach on the coastal floodplain. In Portsea, the exposure grew until 1931 and then decreased until 1981 and is now growing again, following changes in the Portsea population. Population growth and residential development have been much bigger drivers of increased exposure to coastal flooding than sea level rise in the region studied, accounting for 71% and 85.5% of the growth in exposure in Portsea and Hayling, respectively, with sea level rise explaining the balance. The methods presented are generic and could be readily extended to a national level analysis. It could also be repeated elsewhere in the world where the necessary data on population and flood characteristics (land elevation, flood levels, sea level change) are available. By understanding historic changes in exposure, an improved understanding of changes in flood risk can be developed, including a reality check on scenarios to inform future flood risk management.

More information

Identifiers

ORCID for Derek Clarke: orcid.org/0000-0002-5433-5258

Catalogue record

Export record