ABSTRACT
Peak discharges from major runoff events in the UK have been assembled by engineers in relation to reservoir practice (ICE, 1933) and a revised analysis with records updated (ICE, 1960). More recently (NERC, 1975), with wider terms of reference, a more comprehensive study of floods has been made involving the collation of nationwide records for the whole of the British Isles. From a combination of flood discharge measurements and post-event discharge calculations taken from the ICE and NERC published records, a graph of peak discharge against catchment area has been drawn (Fig. 12.4-(a)). Twenty peak discharges for areas ranging from 4 km2 to nearly 1800 km2 are plotted, and an enveloping curve is drawn. Most ofthe points represented by catchments less than 25 km2 pertain to the tributary flows contributing to the disastrous Lynmouth flood in August 1952, where the peak flow was estimated at 650 m3s·1 from 101 km2 (Dobbie and Wolf, 1953). The highest measured flow in the UK is the 2402 m 3s·1 on the River Findhorn at Forres in Scotland in 1969 from a catchment area of 782 km2 • There are other historic records of flood levels on many rivers in the country, and some of these could possibly provide peak discharges above the curve in Fig. 12.4(a). However changes in the river profiles and cross-sections over the ensuing years makes the conversion of level data into discharges unreliable. Comparable data for the USA are also shown in Fig. 12.4-(a). The drainage areas range from 2 km2 to over 8000 km2, with corresponding peak discharges from 144 m3s·1 to 21 000 m 3s-1 (Crippen and Bue, 1977). These extreme events in the USA come from a wide range of different climatic regions, some of which experience tropical rainfall intensities. From the enveloping curves of the two plots, a 5 km2 catchment somewhere in the USA may well produce a peak of 600 m 3s·1, but in the UK only a 150 m 3s·1 peak may be expected from the same drainage area. (The Findhorn record in Scotland would not be worth plotting on the USA graph!) Thus, when making a general appraisal of peak records, such a relationship of peak discharge to catchment area should be made according to climatic region. The USA has been divided into 17 flood regions (Crippen and Bue, 1977), and such subdivisions in the UK have been made for more detailed studies of flood events (NERC, 1975). A world-wide relationship between maximum floods and catchment area has been published by Rodier and Roche, 1984 (Fig. 12.4-(b)).
