Scour and Scour Protection

Scour (the removal or redistribution of erodible material) may occur on the boundaries of unlined channels or adjacent to concrete structures in rivers. Criteria are presented for predicting the likelihood of scour in cohesionless material, and for designing loose protective linings (riprap). Application of the Shields critical shear stress criterion is described for channel beds and sloping banks. The HEC-11 procedure (a reformulation of the Shields criterion) enables the required riprap size to be related to the flow velocity. Guidelines for the protection of the material underlying riprap are also presented. The complex flow pattern around bridge piers causes intense local scouring which progresses with time and stabilizes at an equilibrium depth. Methods are presented for predicting the equilibrium scour depth and its evolution with time under clear-water and live-bed conditions. If the anticipated scour depth is unacceptable, scour can be prevented or restricted by protecting the bed around the pier base or by installing features that modify the local flow pattern to reduce its scouring action. Methods are presented for designing riprap protection, and some flow modification devices are described.