Fish Passage Solutions on the West Coast: Culverts, Tidegates, and Fish Screens
Construction and development activities affect aquatic habitats.
Construction and development activities affect aquatic habitats. They impact the hydraulic conditions of a natural waterway and can block fish from migrating to and from the ocean. We work to minimize these impacts by using innovative engineering designs that facilitate safe, timely, and effective fish passage in estuaries and inland watersheds.
Culverts are large pipes that pass water under roads to protect them from erosion or flooding. Culverts are especially popular for small streams or in remote areas where building a bridge would be too expensive or impractical. Though most culverts do a pretty good job of passing water under the road, they often do not allow fish to pass. The downstream end of the culvert may be too far above the water’s surface for upstream migrating fish to enter. Water in the culvert may be moving too swiftly, or be too shallow for fish to pass in either direction. Debris may also collect in the culvert, not only blocking fish passage, but water as well. During floods, blocked culverts are responsible for many road failures.
Because there are so many road crossings, culverts may be a very significant barrier to fish habitat in a watershed. Culvert improvements typically involve lowering the downstream end of the culvert to the water’s surface, reducing the steepness of the culvert, and making design modifications to reduce water velocity and increase water depth in the culvert.
Tide Gates serve to drain tidelands (areas that incoming tides regularly cover) for agricultural or other uses. First a dike is built to isolate the area to be drained. A large pipe or culvert passes through the dike. On tidewater side of the pipe there is a hinged door which opens outwards towards the bay or estuary. When water levels are higher on the side of the pipe towards the drained area, the weight of the water holds the door open, allowing water to flow out into the bay or estuary. When the tide rises, the level of water on the tidewater side becomes higher than on the drained area side, holding the door closed so water does not flow back into the drained area.
Drained areas are typically not totally dry and have small streams or marshes that are potential habitat for small fish. With traditional tide gates, passage of fish and water between the tidewater and the drained area is limited. This leads to stagnant water and fish being excluded from the habitat or trapped on the drained side when they wish to leave.
Modified tide gates use floats or other devices that hold the gate open until the water on the drained side reaches a particular level, and then it closes. This allows a longer period when the gate is open so water can be exchanged and fish may enter or leave the habitat in the drained area.
Fish screens physically exclude fish from pipes, canals, or other structures that collect water from their habitat. To serve its purpose the screen must have holes small enough to prevent fish from passing through the screen, or becoming stuck in the holes, while allowing enough water to pass to serve its purpose.
Water current passing through the screen must be slow enough that the fish can avoid it and not be stuck against the screen. There should also be flow along the face of the screen which guides fish away from the screen and to an exit that leads back to the river. Dead ends where fish cannot find their way back to the river are almost as bad as no screen at all.
Screens are often angled to the incoming current to guide fish along the face of the screen to an exit at the downstream end which returns them to the river. Some screens are mounted on drums that are turned by paddlewheels. As these screens turn, they pass by a brush that removes debris.