A scour analysis is required for new bridges, replacements, and widenings. Where a scour analysis indicates high depths of potential contraction scour, a structure larger than that required by the basic velocity and backwater criteria may be more cost effective than to designing foundations and armoring to withstand the scour. The potential for deep local scour can be reduced by enlarging the structure, but designing foundations and armoring to withstand local scour depths may be more cost-effective. Generally, a multi-disciplined team should assess the validity of calculated scour depths.

Stream stability issues such as potential vertical and horizontal degradation may warrant accommodations in the bridge design. If the channel is vertically degrading, it is likely that, as the channel deepens, the banks will slough resulting in a widening. Also, where significant meandering is occurring, meanders tend to migrate downstream and increase in amplitude. Structural options to accommodate either of these cases can include longer structures with deep enough foundations to accommodate anticipated degradation or deep enough foundations with abutment foundations designed to act as interior bents to allow future lengthening of the bridge.

Refer to the and the Bridge Division Geotechnical Section for further information on bridge scour calculations and protection and for stream stability issues.