The formation of an armouring layer can be observed on the base of morphological changs by direct measurements of granulometry (grain diameter generally grows after flood) or by computer simulation of changes of the grain sieve curve. This procedure is based on the Gessler’s analysis of the probability of grain movement, on the basis of the shear stress of the acting water. The critical shear stress is calculated using the Wang function developed for Carpathian streams. The armouring layer is formed by that part of non-cohesive bed material, which is not washed away during the flow. The increase in critical shear stress is due to the increase of the mean diameter dm as small fractions are washed out from the bed. The thickness of the armouring layer is approximately equal to the size of the largest stones of the layer. This process in rivers with natural flow is different from this one below reservoirs; in the former case it takes place at a significant distance below dams. In this case, the armouring layer formation causes bed scouring, linked with a change in the water slope and depth. This scouring may be assumed to be a stochastic process. According to Gessler, ...
