Hydraulic Design

The issues of road crossings and their effects on animal movement came to prominence because of the impact of passage barriers on migratory fish. Initially, the focus was on upstream passage of migratory fish, such as salmon and sea run trout. To allow fish passage, a culvert needed to meet certain specific hydraulic conditions (water depth, velocity, and turbulence) for target fish species. In the Pacific Northwest , many species migrate upstream during high flow periods. Therefore, a culvert could be designed by engineering it to meet specific standards for velocity, water depth, and turbulence at high flows.

Hydraulic culvert design can be an effective approach when designing for a small number of target species with similar requirements, and when the hydraulic requirements of those species are known. Some migratory fish are strong swimmers and good jumpers. Others, like sturgeons may be capable of sustained movements, but are unable to leap over any but the smallest obstacles. Culverts that are designed by a hydraulic method to pass strong swimmers like salmon and trout are unlikely to accommodate smaller fish species, juvenile fish, and weak-swimming aquatic organisms that are important components of aquatic communities. Unfortunately, we have reasonably good information on swimming performance for only a handful of fish species.

Because we lack specific information on the requirements of most aquatic organisms, hydraulic design is not particularly effective for facilitating movement for a broad range of species. Even if more information on swimming ability were available, hydraulic design for a diverse array of species would be a significant challenge. Species – and even different life stages within species – move at different times of the year, during different flow conditions. It would be impractical to design culverts to meet specific depth, velocity and turbulence requirements during high, low, and intermediate flow conditions.