Pool Simulation Culvert Design for Fish Passage

Geomorphic simulation culverts are a commonly preferred design for crossings that need to accommodate fish passage. Because these culverts are typically designed to replicate conditions found in a channel riffle, their effectiveness as fish passage conduits depends on the continuity of sediment transport characteristics between the channel and the culvert. Thus, where flow contraction would create high flow velocities and scour conditions within the culvert, stream simulation culverts may be rendered ineffective in two ways: scoured material might not be replaced, and scour of the material could lead to upstream channel degradation. These problems could potentially be avoided in a culvert designed to simulate the riffle-pool-riffle reach-scale morphology found in a natural low-gradient valley contraction. A prototype pool simulation culvert has been designed and installed in a 1%-gradient stream in eastern Kentucky. Like the stream simulation culvert, the pool simulation culvert was designed to simulate natural stream processes within the culvert, including sediment transport, debris and flood conveyance, and fish passage. The design differs from stream simulation culvert designs, however, in several ways. It relies on a grade control and flow throttle at the culvert inlet and the formation of an energy dissipation pool within the culvert that extends through the outlet to a downstream riffle. The culvert slope is based on maximizing the flow area within the barrel at the outlet of the culvert, which minimizes the exit velocity. Backwater to the culvert outlet is controlled by the downstream riffle crest elevation and floodplain elevation. Anticipated benefits of the culvert design include (1) elimination of all barriers that would impede fish passage, (2) reduction in outlet velocity, (3) reduction in the required length of the downstream scour pool, and (4) integration of a permanent grade control at the culvert inlet.