A box type parallel bar barrier over a pond's overflow pipe protects the overflow pipe, the emergency spillway, and the fishery. It functions as a trash guard that prevents large debris from flowing into and plugging the pipe. It deters beavers from plugging the overflow pipe. By allowing the overflow pipe to function properly, it minimizes erosion in the emergency spillway caused by excessive water overflows. It prevents harvestable size fish from leaving the pond through the pipe. It does not allow grass carp over 8 inches long to escape through the pipe. It prevents adults of undesirable fish species from coming in through the pipe.
A parallel bar barrier should be constructed with round metal rods. We use 1/2 to 3/4 inch diameter rods for corners and braces. A 4- to 5-foot wide barrier needs only one brace rod in the center of each side. If the barrier is in a location near the shore where people are likely to get on it, we generally use 1/2 inch rods for most of the construction. If it is covering a standpipe away from the shore, we generally use 3/8-inch rods for most of the construction.
All horizontal rods should be leveled, spaced with 1-inch gaps, and welded between each rod. The level rods with 1-inch gaps minimize clogging by debris and prevent passage of most fish over 8 inches long.
Water and debris should not be able to enter the pipe without passing through the 1-inch gaps in the barrier's walls. A box type parallel bar barrier should be three-dimensional with a top, sides, and when necessary, a bottom. A barrier covering a riser (standpipe) of a barrel and riser overflow pipe needs a bottom on the portion of the barrier outside the pipe (Figure 1). The sides of a barrier covering a hooded inlet pipe can be buried at least 6 inches into the pond bank avoiding the need for a bottom (Figure 2).
Each side of the barrier should be substantially wider than the diameter of the overflow pipe. Where beavers have previously plugged an overflow pipe, I believe a box type barrier should be at least 4 feet wide on each side. Barriers 4 feet wide prevented the plugging and damming of overflow pipes in 4 of 5 Noble Research Institute ponds where beavers had been causing such problems.
At the pond where beaver dammed around the barrier, we overcame the problem by piping water into the barrier through a perforated pipe lying on the pond bottom. We also installed a flapper gate on the back of this overflow pipe to prevent beaver crawling through the pipe and damming along the inside of the barrier.
I do not know the optimum height for a box type barrier. Each of our barriers generally rise a distance above the overflow pipe approximately equivalent to the overflow pipe's diameter. Our barriers rise 14-22 inches above the tops of the pipes and have functioned well.
Barriers should last several decades with very little maintenance. We installed them several years ago at the Pasture Demonstration Farm and are satisfied with them.