Building footprint layers show that flooding risk increased more due to greater building exposure than to greater peak discharge with urbanisation in SE France

Abstract

Study region

South-east Mediterranean France

Study focus

Urbanisation and climate change are producing unprecedented flood risks. Urbanisation increases storm peak discharge and flooded area. Building in the flood plain exposes more people and infrastructure to floods. This study examines the impact of urbanisation over 3 decades (1988/90–2020) on flood risk in SE France. Building footprint layers were used to quantify urban growth, and peak discharge and flooded area were modeled for 3 12-hr rainfall events: 80 mm, 140 mm, and 200 mm. Building growth ranged from 28 % to 65 %. Total imperviousness grew at a rate that was equal to or greater than built area. The relative increase in peak discharge was greatest for the 80-mm event (+12.2 %) and diminished to its lowest value for the 200-mm event (+2.4 %).

New hydrological insights for the region

Flooded area increased proportionately to changes in peak discharge, but mean growth in flooded building footprint area was almost 43 times greater than growth in flooded area. New buildings within the flooded perimeter contributed more to flood risk than changes in peak discharge and flooded area. Planning legislation limited growth close to channels, but flood risk grew rapidly beyond in less restricted areas. Building footprint data provide an accurate approach to mapping changes in flood risk with urbanisation.