T-Culverts

T-Culverts can prevent beavers from plugging road culverts (figure 15). The following prerequisites should be considered when determining whether these devices might be effective at a particular site:

• Streamflow needs to be moderate.
  
  
• Culverts should be 3 feet in diameter or smaller.
  
  
• The normal flow through the culvert should fill about one-fourth of the culvert's diameter.
  
  
• Both ends of the T-culvert need to rest in relatively still water, about 4 to 6 feet deep, for best results.
  
  
• The substrate should be solid, to prevent material from sloughing into the culvert and filling it. The substrate is particularly important if a trench is dug to install the T-culvert.

Figure 15—This T-culvert is installed in front of a road culvert .
—Drawing by Dr. Jeanne Jones

T-culverts can be constructed from any metal culvert by cutting a hole in the side to install a smaller connecting culvert. The larger culvert is used for the T. A 4-foot-diameter T-culvert is recommended for road culverts between 6 and 18 inches in diameter. Larger road culverts, from 18 inches to 3 feet in diameter, require a T-culvert that is at least 5 to 6 feet in diameter. These large inlets enable water to enter at a low velocity, which is less noticeable to beaver. The T-culvert should be 8 to 12 feet long—longer is better.

A smaller culvert connects the T-culvert to the road culvert. The connecting culvert is inserted into a hole cut to fit about one-third the distance from the bottom of the T-culvert. When the T-culvert is set in place, this hole must be below the road culvert, so the connecting culvert angles upward. The connecting culvert should be slightly smaller than the road culvert so it can slide about 6 inches into the road culvert at a slight upward angle. If the connecting culvert is much smaller than the road culvert, waterflow will be restricted.

The length of the connecting culvert depends on site conditions, but 6 to 8 feet usually is adequate. The ends of the T-culvert are covered with number 6-gauge reinforcing wire mesh. Small holes drilled around the perimeter permit the mesh to be wired into place or the protruding ends of the mesh can be threaded through the holes and bent over.

Forest Service Experience—No respondents had used this method.

Culvert Blocks

Another approach is to slow waterflow through the culvert. The culvert outlet is blocked with boards, plywood, or metal sheeting across the culvert opening. Wire mesh can be added for support if necessary. Water is allowed to seep through cracks in the boards or through small holes drilled in the covering. The concept is to limit the sound of running water, reducing the likelihood that beavers will dam the culvert. Such an approach can be used only during low water and the blockage must be removed before high water is anticipated. Although this approach might be feasible in some situations, it would require close monitoring.

Forest Service Experience—No respondents used this exact method. However, some persons have plugged culverts themselves during low water to keep beavers from damming them. A low-water ford can be created in the road by hardening a dip near the culvert, allowing water to flow across the road. The culvert is unplugged during high water. This method is safe for low-volume roads.

Three-Log Drains

This simple drain is made from three or more hardwood logs. Beavers rarely chew through hardwood logs (such as oak logs) when they are underwater. The structure is placed perpendicularly through the dam as described for the pipe drain. Two logs are laid on top of a board or sheet of iron with their upstream ends slightly apart. A third log is placed on top. Water runs through the funnel formed by the logs and seeps along their edges.

A similar approach might be used to prevent beavers from damming a stream. A blind drain constructed from stones, logs, tiling, perforated drain pipe, or similar materials laid along a streambed will allow water to flow beneath a dam. If beavers build a dam, the streambed below the dam will remain permeable, preventing water from collecting behind it. Generally, beavers will move to a more suitable site.

Another adaptation of the three-log drain reduces opportunities for beavers to plug openings created by uneven logs (figure 16). Sheet metal (30 inches wide and 6 to 8 feet long) is laid along the bottom of an opening cut through the dam. Two green or waterlogged poles, 6 to 9 inches in diameter and 10 to 16 feet long, are placed on top of the sheet metal. The upstream end of these logs needs to be at least a foot lower than the downstream end. Green sticks (1 to 2 inches in diameter) are placed across these logs to support a third log, which is laid across the sticks and centered above the bottom logs.

Sheet metal, about the size of the sheet placed on the bottom, is wrapped over the top log and along the sides of both bottom logs. The sheet metal is nailed along the sides of both bottom logs and the top of the top log to hold the drain together. The intake end of the drain can be anchored by driving a forked stick into the pond bottom with the fork hooking the top log to hold it down.

Forest Service Experience—No respondent had used this method.

Figure 16—This drawing shows a modified
three-log drain could be used to partially
drain ponds impounded by beavers.
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