77-Assessing Spilway Design Flood and the Collapse of Gasper Dam, June 29,1917.

Abstract

The spillway design flood of some UK dams has recently been under review. Although there have been no fatalities caused by dam failure in recent years, it is now realised that some darns will have to be strengthened in order to avoid a future disaster. Gasper dam in Wiltshire, UK was built in the late Victorian era and was destroyed during the great storm of 28-29 June 1917. It was rebuilt in 1920. The dambreak flood discharge has been estimated at three locations downstream of the dam. They are much higher than estimates using current methods such as Hughes et al., (2000) showing that the hazard posed by dam-break floods is greater than previously supposed. Since 1978 the dam has been inspected five times. The most recent inspection (2004) noted that it is presently classified as a category B dam in the UK dams are graded A-D — which means that some overtopping during the maximum flood is permissible. However, since 1920 the total number of houses at risk from a dam breach exceeds 10. This means that the dam should be upgraded to category A, such that the dam should safely pass the probable maximum flood (PMF). The current inspector's report estimated the PMF as 59 m3 s-1. However, five new estimates of the PMF have been produced. These range from 71-150 m3 s-1. These new results were obtained by using: 1. The ICE (1996) rapid method. 2. The FEH UH (1999) approach. 3. Transposition of the Martinstown storm of 1955 which took place 45 km to the south. 4. Transposition and maximisation of the same storm. 5. Analysis of two historic floods and an estimate of bankfull discharge. When runoff rates from other floods in the UK are applied to the Gasper dam catchment a PMF of at least 100 m3 s-1 is suggested. Therefore unless Gasper dam is modified to safely pass the higher estimate of the PMF then it remains a serious threat to both people and property downstream. These results have serious implications for the safety of other dams in the UK and perhaps elsewhere.