In an estuary or a river, a sill may be the initial foundation or the lower part of a structure that has to be constructed on a bed of alluvial material. Sometimes a sill is used to reduce the mixing of different types of water in an estuary. In an estuary a sill has to be designed for flow in two directions: flood flow and ebb flow. The chapter explains relations used to predict the maximum scour depth for sills with horizontal bed protection. It deals with additional measures to reduce the risk of failure. The chapter describes some field experiments with loosely packed material. J. W. Dietz performed extensive research on two-dimensional scour downstream of horizontal beds and low sills. A reverse flow develops with a flow direction opposite to the main flow direction. In both the mixing layer and the recirculation zone the turbulence energy is relatively high compared to uniform flow conditions.