Objective of this study was to examine the interaction of flow regimen and scouring at round piers and to obtain simple relationships in order to give the planning engineer a clear and practice oriented method of calculation. The experiments were preceded by an intensive study of the literature and the theoretical treatment of the problem. The present study proved that, in contrast to former belief, short duration floods with intense sediment transport are most the predominant cause for the scour, the reason being, that in this case the scour is always refilled through the continuous bed-load. The biggest and most dangerous scours are formed when, upstream of the pier the river bed is stable . The pier diameter, the stream velocity, the sediment diameter and, most of all, the elapsed time were recognised being important parameters for the scour and its dimensions. The latter is of great importance, especially during stable bed.