Road Ecology Center

Pennsylvania contains approximately 83,000 miles (133,547 km) of streams and 119,000 miles (191,471 km) of public roadways. Fish movement can be impeded by highway culvert designs that create sheet flow or increased current velocity within the culvert barrel, and/or perched outlet conditions. The Pennsylvania Fish and Boat Commission and the Pennsylvania Department of Transportation reviewed performance measures of existing culvert designs and conducted a literature review to develop culvert designs that enhance fish passage. Design guidelines were established for pipe culverts and statewide design standards have been adopted for single cell and twin cell box culverts. Pipe culverts can be depressed at varying depths below streambed elevation depending upon the upstream drainage area and the diameter of the pipe. Single and twin cell box culverts are depressed twelve inches (305 mm) below streambed elevation. Box culverts installed in waterways with a stream slope less than four percent are constructed with a different baffle design than those installed with stream slopes greater than four percent. Stream flows are directed to the primary cell of the twin cell box culvert structure while the secondary cell is designed only to accept storm flows. All culvert structures are installed parallel to stream gradient and riprap used to protect the inlet and outlet is placed to match the invert elevation of the structure.