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DAM SAFETY, FLOODPLAIN MANAGEMENT

Rodent control on earthfill dams

For earthfill dams, the three most destructive rodents are the groundhog (aka woodchuck), muskrat and beaver. These animals are attracted to the environment created by a dam and reservoir. The burrowing habits of the groundhog and muskrat may threaten the integrity of an earthfill structure. Beavers attempt to plug up principal and emergency spillways and may even try to raise the elevation of the top of dam.

The groundhog is the largest member of the squirrel family. They will burrow in from the downstream side of the dam staying above the phreatic line (the upper surface of seepage) to remain dry. The burrow will have multiple tunnels and chambers and always has more than one entrance. Occupied burrows are easily spotted as the groundhog continually removes dirt from the den so fresh dirt will be visible on the surface of the dam.

Eradication of a groundhog infestation should commence in early spring when burrows are active and easy to find. If control measures are delayed, other game animals may cause further damage as during the fall and winter they may take refuge in the groundhog burrow. Fumigation is one of the most effective methods of controlling a groundhog population, however, trapping or shooting may be a better approach around buildings or in a high fire hazard area. It has also been observed that a well cleared and mowed dam discourages inhabitation.

The muskrat is a small, stocky rodent with rich dark brown fur and is mostly nocturnal. Muskrat dens are constructed by burrowing into the banks of lakes and streams. These burrows generally start six to eighteen inches below the water surface and then rise to above the phreatic surface (the upper surface of seepage). The burrow starting from the reservoir side offers a path for water flow, and can seriously weaken the embankment. The hazard increases when a muskrat den on the upstream side is close to a groundhog den on the downstream side. If the earth partition between these dens should fail, there is a conduit completely through the dam.

One successful method for eliminating a muskrat burrow is to place rip-rap on the upstream face of the dam. The rip-rap should start about three feet below the water surface. As the muskrat attempts to burrow, the rip-rap will collapse into the excavation, discouraging the attempt. Heavy wire fencing lain in a similar manner will yield the same result, however, rip-rap slows wave erosion on the upstream face of the dam and thus has a two-fold value. Also, removal of vegetation along the shore line will discourage habitation. Trapping may also be considered as a method of eliminating muskrats.

Beavers will plug spillways and have even been known to try to raise the elevation of the top of dam. This kind of activity can lead to overtopping and possible failure of the dam. Removal of the cuttings and woody vegetation from the dam and surrounding areas (shore line) will help correct the problem, but only temporarily. Beavers are not easily discouraged and they will continue to build. Placement of electrically charged wires in the spillways has achieved some success, however, if the dam is frequented by the general public this might not be a desired method. Trapping and removal of the beavers is always an alternative.

One method for back filling the burrow is called mud-packing. Place one or two lengths of metal stove or vent pipe vertically over the burrow. When the pipe is properly sealed, pour a slurry (90 percent earth, 10 percent concrete, and enough water to make the slurry flow) into the pipe and allow it to flow into the burrow. Fill the burrow with slurry to within six inches of the surrounding ground. Fill the last six inches with dirt suitable for grass.

For more information see: Dam Owner’s Guide to Animal Impacts on Earthen Dams (FEMA pub# L-264)