On 07 August 2010 a disaster occurred at the barrage Niedów located on the river Witka as a result of unprecedented rainfall in the Nysa Luzycka catchment area. The flood wave swept through the dam crest destroying almost the whole structure within a few dozen minutes. The only element that did not vanish was the concrete structure of the spillway and water-power station. The process of dam breach is not so far completely resolved and reliable predictive models of dam breaching are still challenging problem. Modelling of the process requires an evaluation of parameters affecting this phenomenon, namely, the breach size and the timescales for the breach formation. The latter is particularly important for characterizing the outflow hydrograph and, consequently, for the assessment of the damage risk of areas below the dam. The paper presents a mechanism of breaching of the Niedów dam which consists of three stages: the downstream slope wash out, the breaking down of the dam crest and the damaging of upstream slope revetments. The comparison of breach parameters with that evaluated based on several empirical formulas and numerical modelling is also presented.

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