Characteristics of gauged abrupt wave fronts (walls of water) in flash floods in Scotland

Abstract

Extremely rapid rates of rise in river level and discharge are a subset of flash floods ("abrupt wave front floods", AWFs) and are separate hazards from peak river level. They pose a danger to life to river users and occur mainly in the summer. Using level and discharge records from 260 Scottish gauged catchments, we present the spatial distribution of annual maximum 15 min rises in river level and discharge, along with derived metrics to assess the severity of AWF events. These include normalised and proportional measures of flow change, as well as ratios that characterise the intensity of AWF events. We estimate wave celerity by analysing the time difference in wave onset recorded by successive gauging stations along a river channel. This approach is applied to several AWF events on the river Findhorn in northeast Scotland, allowing for detailed examination of their dynamics. Our findings suggest that flood forecasting models with outputs of peak discharge and river level may not adequately represent the risk posed by rapidly rising flows, especially at national scales where hydroclimatic and geomorphological variability trigger different AWF metrics. We show that AWFs may intensify downstream, with wave fronts steepening as they travel through lowland river reaches, as observed in multiple events on the river Findhorn, showing a necessity of more accurate and frequent river measurements. We conclude that AWFs need better monitoring forecasting and warning, particularly as extreme downpours are becoming more frequent with global warming.