Structures built in or near rivers and other channels can be vulnerable to scour around their foundations. If the depth of the scour becomes significant, the stability of the foundations may be endangered, with a consequent risk of damage or failure of the structure. In the past 15 years, there have been several bridge failures, resulting in transport disruption, economic loss and, on occasion, loss of life. Some of these failures are included in the manual's summary case studies. The factors influencing scour are complex and vary according to the type of structure. Protection works for preventing scour need to be designed to withstand the flow forces imposed on them and have to be practicable to build and install, while minimising adverse environmental effects. The manual therefore considers scour processes, estimating and assessing scour, protective measures, monitoring, environmental factors, risk assessments, and cost and benefit analyses. This manual is intended for engineers engaged in the design, construction, operation and maintenance of structures in the water environment that may be subject to scour of erodible beds or banks. It is equally relevant to UK and worldwide scour problems, and to new as well as existing structures.