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. The rate of change in gauged river level and discharge can be used to assess and compare the severity of AWF events within and between catchments. We use several metrics of discharge severity to investigate AWFs on 260 Scottish gauged catchments. We use the full flow record for each station and map the occurrence of maximum 15 min change in river levels and discharge. We map a further three measures to compare risk between catchments including the multiple of the 15 min flow increase from the initial to terminal discharge. The concurrent increase in velocity is difficult to measure but wave celerity can be assessed where there are observations of the time of wave onset at more than one point on a channel. We investigate several such events on the River Findhorn in northeast Scotland. Such events need better monitoring forecasting and warning, particularly as extreme downpours are becoming more frequent with global warming.