Report No FR/D0003

Mar 1991

1. Although dams in the UK are among the safest in the world and the provisions of the Reservoirs Act, 1975 ensure that that high standard will be maintained, it is prudent to consider the possible consequences of a dam failure, however remote. Considerable damage could ensue and one area in which information is lacking is in the assessment of potential damage caused by high velocity flow. There is considerable information on the effects and costs of damage caused by low-velocity flooding but clearly high velocity flow can cause much more serious structural damage.
2. This report, written in the form of guidelines for the application of the proposed methodology for the assessment of such damage, covers the research carried out to develop criteria, based on the characteristics of the flood wave, for the prediction of structural damage. The work was carried out as part of the ongoing program of Reservoir Safety Research for the Department of the Environment.
3. Very little detailed information was found from a review of case histories on damage in high-velocity floods and much use has been made of the detailed reports of the damage resulting from the failure of Dale Dyke dam in 1864. Criteria are proposed, based on the parameter of maximum depth of flow times the peak velocity of the flood wave at each location. These enable areas to be delineated in which inundation damage, partial structural damage or total structural damage are likely to occur.
4. The costs of structural damage are discussed in detail and some basic cost data (as at January 1988) presented. This has been derived from the extensive data base set up by the Flood Hazard Research Centre at Middlesex Polytechnic.
5. The major concern in any emergency involving a dam would be the risk of casualties. Analyses of past events carried out in the USA provide a methodology for assessing the potential casualties among the population at risk below the dam. These analyses suggest that with a warning time in excess of 1.5 hours it is possible to evacuate the area likely to be affected, if this is known, and limit the likely casualties to a very small number. Because of the excellent safety record in the UK comparable data does not exist and it is proposed, therefore, that the US approach should be applied in the UK but with appropriate caution.
6. The criteria and methods proposed in these guidelines are considered to be reasonable given the inherent accuracies in any prediction of failure. Although they could be further extended and refined that is not thought necessary at this time and no further Departmental research is recommended at present in this field.

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