Dependence between extreme sea surge, river flow and precipitation in eastern Britain



Flooding in estuaries may be caused by both high river flows and by high sea levels. In order to investigate whether these tend to occur simultaneously in eastern Britain, the dependence between high sea surge (observed sea level minus predicted astronomical tide), river flow and precipitation was studied using a measure of dependence specially developed for extremal dependence. Extreme events were interpreted using meteorological maps. This new analysis found that the strongest flow–surge dependence occurs between river flow on the north shore of the Firth of Forth and sea surge at Aberdeen, Wick and Lerwick. In contrast to most other catchments in eastern Britain, the area to the north of the Firth of Forth is not sheltered from south-westerly winds by any major topographical barrier. Therefore, precipitation from this direction may be orographically enhanced as it encounters the hills on the northern side of the firth, and high river flows may ensue. Events resulting in both high river flow and surge in the northern part of the study area were found to be caused by cyclones travelling north-eastward to the north of Scotland. High surge events, only, were associated with similar storm tracks, but without much precipitation from the fronts. High river flows, only, were associated with rain-bearing east–west-directed fronts over northern Britain, with slow-moving depressions located over or to the west of the British Isles where they are unable to generate a strong surge in the North Sea. The dependence between river flow and surge was found to be stronger during winter than summer, and a lagged analysis revealed that the dependence is strongest when flow and surge occur on the same day, but was also strong for lags of plus and minus 1 day. For precipitation, the dependence with both flow and surge is strongest when precipitation precedes them by 1 day. Copyright © 2002 Royal Meteorological Society.