The flow caused by a dam breaking across its entire length can be approximated by a one-dimensional, unsteady flow model in form of the St. Venant equations. In this model, the flow is governed by the river geometry and the river roughness, which is quantified by Manning's coefficient. The roughness characteristics are generally difficult to estimate under natural conditions. Thus, the estimates of the Manning's coefficient will in general be subject to uncertainty. In this paper, the uncertainty in the discharge and depth hydrographs due to the uncertainty in estimating the roughness characteristics of a river, is investigated. A specific case of the Noppikoski dam in Sweden that failed in 1985 is used to illustrate the sensitivity of the flow simulation on the roughness coefficient. The analysis shows that the uncertainty in the dam break flow simulation due to the uncertainty in estimating Manning's coefficient, is significant. The uncertainty is larger at greater distances from the dam, and is greater for the discharge hydrograph than for the depth hydrograph.

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