Scale model tests show that the natural tendency of scour holes in heterogeneous subsoil and unidirectional flow is to grow in downstream direction, with a stable upstream slope and undermining of the downstream edge. The initial depth development is fast, gradually decreasing towards an equilibrium depth. Field data of two recently developed scour holes show for one case a similar development. For the other case, scour hole growth mainly proceeds in the dominant upstream direction. Differences may be linked to the presence of tide, horizontal heterogeneity of the poorly erodible layer, the influence of a nearby confluence or scaling effects. This will be further investigated@en