Dam break floods and floating debris: A preliminary assessment

A qualitative appraisal of the effects of floating debris on dam break flood waves is presented. Reference is made to the Johnstown flood of 1879 which killed 23,000 people, and which seems to have been aggravated by large amounts of debris in the flood wave. Water velocities and depths needed for debris generation are estimated by comparison with wind velocities needed to uproot trees, and these velocities are compared with the velocities and depths in typical dambreak flood waves, confirming the debris production potential of such waves. Preliminary experiments were carried out with a horizontal flume and simulated debris. It was found that debris accumulations of the magnitude to be expected following a dam break in a forested or urbanized valley can have a major influence on the nature of the dam break surge. The magnitude of the effect depends mainly on the debris generated or collected by the flood wave and on friction forces between the debris accumulation and valley floor and walls. The debris can decrease the speed of the surge and increase its height relative to the simple water surge, and constrictions can allow the surge to build up again. 5 refs., 2 figs., 1 tab