Devices that Prevent Beavers from Damming Culverts
Various devices can be used to prevent beavers from entering and damming culverts. Some of these devices prevent the beaver from damming the end of the culvert. Others keep the beaver from building a dam inside the culvert. Maintenance is much easier if beavers build their dam outside the culvert rather than inside.
These devices prevent beavers from constructing dams inside culverts. They do not prevent beavers from damming a culvert. They should always be installed on the upstream end of a culvert; otherwise, beavers are likely to fill the culvert with debris, using the device to help them start their dam. Wire mesh mounted to a metal frame can be installed across the front of a culvert.
A rod grill can be constructed by welding ½- to ¾-inch steel rods (rebar) to a plate at 6-inch intervals. The vertical rods are driven into the streambed. Pressure from the current will hold them in place. A slightly more complicated design uses a plate as wide as the culvert that is welded between two posts or angle iron mounted on either side of the culvert. The rods are not welded to the plate, but are inserted through holes drilled every 6 inches (figure 5). This approach allows individual rods to be pulled when the grate is cleaned. Once most of the debris in front of the grate has been removed, the rods can be lifted, enabling smaller material and mud to wash through the culvert.
Another design uses rods welded about every 6 inches to links of chains fastened to the upper ridge of the culvert and stretched in both directions along its outside perimeter (figure 6). The rods and chains continue for several feet or more on the ground in front of the culvert. A collecting chain fastened to the bottom rod is looped back above the culvert. When debris plugs the culvert, this chain is hooked to a vehicle so the whole contraption, along with the debris that has collected, can be pulled back over the culvert.
Forest Service Experience—The Eastern and Alaska Regions have used culvert guards effectively. Some employees have built grates of rebar or tubing that mount on the culvert. Others have drilled holes through the end of the culvert that allow rebar to slide through. This method allows the rods to be removed easily for maintenance. The problem with this approach is that the end of the culvert breaks down over time. Everyone who has used culvert guards says that although the guards do not prevent beavers from building dams, they do allow dams to be removed more easily.
Many designs have been prepared for installing fencing in front of culverts (figures 7 and 8). Some designs are called deep water fences or beaver deceivers. No matter what the fences are called, they are designed to keep beavers away from culverts. Waterflow and its associated noise are directed away from a culvert, reducing the cues that cause beavers to build dams. If beavers do attempt to halt waterflow, the area they must block is considerably larger. The deeper the water, the more difficult it is for beaver to pile up enough materials to restrict waterflow.
Fences can be constructed from a variety of materials, but 6- by 6-inch reinforced steel mesh held in place with steel posts works well. A variety of sizes and configurations (rectangular, triangular, trapezoidal) have been proposed for beaver fences. Site condition plays an important role in fence designs. The farther the fence perimeter is from the culvert, the more effective the fence is likely to be. Sites with rapid waterflow may require larger exclosures. Generally, designs that fence off an area 10 to 20 feet on each side of the culvert are adequate. Make sure that the fence blocks access from the shore and across the top of the culvert.
The fence's bottom must be tight against the stream or pond bottom. Burying the fence can help prevent beavers from digging underneath. A floor, made from the same mesh, can be added where the bottom is uneven or if the substrate is unstable. Beavers do not climb, so extending the wire a couple feet above the high water mark is adequate. Wire mesh across the top of the enclosure may be beneficial. Some designs incorporate a deck across the top, allowing humans to access the impounded water, while blocking beavers.
A device can be installed to allow waterflow into the fenced area. If beavers block the fence, the device can serve as an emergency spillway during high water. Additional fenced areas established farther from the culvert, perhaps in deeper water, can be connected to the first fence by laying pipe between them (figure 9). If shallow water or other conditions prevent fences from being built close to culverts, fences built farther out can serve as an intake point for a modified waterflow device. The culvert can be extended to the fenced area or connected with a pipe. The pipe would need to fit snugly inside the culvert and be of similar size to pass the desired waterflow.
Forest Service Experience—Fencing culverts is the most widespread method Forest Service personnel have used to keep beavers from damming culverts. Respondents said that culvert fences have been used in the Northern, Rocky Mountain, Intermountain, Pacific Southwest, Pacific North-west, Southern, and Eastern Regions. Almost everyone had some success with fencing, but respondents pointed out some shortfalls. In some cases, the beavers did not build dams. In other cases, they built dams on the fence, which required periodic maintenance. It is especially important to extend the fence below the ground level or the beavers will dig under it. In northern climates, ice flow sometimes destroys the installations.
This method extends the culvert with heavy-gauge wire mesh that would be nearly impossible for the beaver to plug. A form was made from 6-gauge concrete reinforcing mesh panels (8 feet long) covered with galvanized welded wire mesh (14 gauge with a 1-by 2-inch mesh, figure 10). The form was rolled into a cylinder that was held in place with number 3 hog rings. Hog rings also attached the wire mesh to the larger reinforcing mesh. These panels can be transported fairly easily to problem sites and connected to make longer extensions. The wire culvert can be placed in a notch cut in a beaver dam or attached to the opening of a road culvert. If the road culvert is larger than the wire mesh culvert, any remaining opening needs to be covered with additional mesh or beavers will fill the culvert with debris. Normally, at least three sections are connected to form an extension at least 24 feet upstream from the culvert.
More complex models have been proposed. The heavy-gauge wire mesh is rolled into a cylinder to fit the inside diameter of the culvert to be protected. Hog rings hold the shape of the cylinder. The cylinder slides inside the culvert where it forms a tight fit. Afterward, the cylinder is covered with a lighter and smaller mesh wire (1- by 2-inch mesh).
A larger cylinder is formed using heavy-gauge wire mesh around the inner cylinder, with a diameter about 12 to 20 inches larger. Fencing wire or other lightweight wire (9 gauge) strands are tied to the inner cylinder and connected to the outer cylinder so that the inner section forms a central core about 6 to 10 inches from the outer cylinder. The outer cylinder is mounted over the outside of the protected culvert with its ends bent to fit tightly around the culvert. Metal fence posts are driven through or on either side of the outer wire to hold the apparatus in place. Additional sections can be added as necessary.
Forest Service Experience—No respondent had used wire mesh culvert extensions. The Forest Engineering Research Institute of Canada (FERIC, http://www.ferric.ca/en/) describes the successful installation of a wire mesh culvert extension in Preventing Beaver Dams from Blocking Culverts (M. Partington 2002). That installation cost about $1,600 Canadian.
Enlarged cylinders are similar to the wire mesh culvert extensions, but are much larger. Wire mesh panels used to reinforce concrete, or similar mesh materials, are rolled into cylinders (figure 11). These cylinders are considerably larger than the culvert they protect, perhaps 10 feet in diameter. The longer the cylinder, the better. The intent is to keep beavers as far away from the culvert as possible. Both ends of the cylinder may be closed with mesh panels. The front panel is cut to form a tight fit around the protected culvert.
Metal fence posts may be able to anchor smaller cylinders, but 2- to 4-inch steel pipe may be needed for large cylinders. Wooden posts are not advised for obvious reasons. The area in front of the culvert needs to be dug out as deep as possible before the cylinders are installed. The large mesh cylinder is not covered with smaller mesh, as is the case in some other designs. Larger sticks are caught by the large mesh, but smaller debris washes on through the culvert.
Forest Service Experience—No respondents had used enlarged wire cylinders.