Bridges enable streams to maintain flow conveyance and to sustain aquatic life. Bridges include bridge structures and bridge-class culverts.

Stream crossings at highways often involve a constricted flow opening. The designer establishes a design storm frequency and other criteria and determines the size and type of opening. The design storm frequency is established considering factors such as functional classification of highway, size of the stream or by performing a risk assessment. Other criteria include allowable velocities, allowable backwater/headwater, FEMA rules and regulations, the NFIP, and any other local or state agency requirements.

FHWA requires a bridge scour evaluation as part of the hydraulic design process for span bridges across streams. The results of such an analysis may highlight the need for design adjustments such as increasing opening size, deeper foundations, pier and abutment protection, or other mitigation measures. Scour countermeasure design is approved by the Geotechnical Section of BRG. Include the bridge scour evaluation in the final drainage report (see ).

Bridge class culverts may require inlet and outlet protection for scour and debris control. Inlet scour results from the flow contraction as the culvert barrel constricts the natural channel. Scour at culvert outlets is a common occurrence; sediment and debris in a turbulent flow can be erosive; however, a formal scour design is not required for bridge class culverts.