Session C: Discussion

Mr J.F. KEERS (Paper 4)

The Paper refers only briefly to the flood studies work carried out by the Meterological Office. It is therefore an opportune time to introduce a basic problem concerning the variability of storm rainfall. This is the great variability of the rate of change of rainfall intensity with time on occasions of rainfall of extremely short duration. For example, Fig. Gi shows that the 5 h storm observed at Bridgewater, Somerset, on 10 July, 1968, actually contained a number of separate peaks of rainfall intensity. When these peaks are considered as part of separate contained storms their return period varies between 5 years and 100 years. However, the return period of the 5 h rainfall event is estimated to be 2000 years. Fig. G2 shows a very different kind of actual storm.