Bridge: Stream Crossings
This guidance covers bridges and small structures that carry streams or other features that have been labeled as Waters of the U.S . Bridges are defined by Federal Highway Administration (FHWA) as structures with clear spans of 20 ft. or greater. Small structures are defined as having a clear span length or diameter of 5 ft. up to 20 ft. Small Structures are also included in this guidance if they have a clear span length or diameter of 3 ft. up to 5 ft. with the fill over the structure being less than the clear span of the structure. For smaller culverts or culverts that carry drainage, refer to Culverts in the Roadway section.
Where possible, bridges should cross a stream at a straight section, instead of at a bend. When a bridge is built over a stream bend, the substructure is susceptible to attack as flood flows can be directed at an abutment or pier, increasing scour potential, and the foundation costs. The bridge may also need to be lengthened to move the substructure units out of the way of the flood flows. In general, a bridge that crosses a straight section of a stream will result in a shorter and less costly bridge.
When possible, box culverts should be used for small stream crossings, since they can be constructed quickly and are virtually maintenance free. Often environmental entities object to box culverts because they do not provide a natural stream bottom, but lowering the bottom of the culvert to allow for 2 ft. or 3 ft. of siltation can address this concern. Consider precast box culverts when feasible, in order to speed up construction.
Pipe culverts for stream crossings should consist of concrete pipes, not metal pipes, since the metal pipes have a history of deteriorating and will require major maintenance such as installing liners or paving the inverts. Since these culverts are carrying streams or Waters of the U.S, stream diversions will be necessary to construct them, requiring permitting, which adds cost to a future replacement project. Concrete pipes also have the added advantage of needing minimal cover as compared to metal pipes.
For structures with short spans, piers in a stream or other waterway should consider piles with protective jackets and concrete caps above the water line. This will eliminate the need for a cofferdam or dewatering to construct the pier. This may not be possible for longer span bridges, which will have piers that experience larger longitudinal forces, requiring footings with multiple rows of piles and cofferdams for construction.