Drainpipe, Dam and Spillway
Drainpipes should be utilized in all ponds. At some point, the pond will need to be lowered or drained for reasons of management or repair. The drainpipe, with the use of a sleeve or trash rack, can also enhance fertilization by drawing water from the bottom of the pond and not from the productive upper layer. Several types of drainpipes and valves are available, and the NRCS should be consulted regarding the type, size, and placement of the pipe.
Professional assistance should be obtained when constructing dams. The above-ground portion of a dam, which impounds water less than 15 feet deep, should be constructed with good quality, well-compacted clay. Dams that impound water more than 15 feet deep must have porous material in the downstream slope to provide drainage; otherwise, water pressure could build-up and cause sloughing. Adequate freeboard (distance between water level and the top of the dam) should be incorporated during construction to ensure that water does not flow over the dam during periods of heavy rainfall. Freeboard should be at least three feet or higher if the drainage area is large.
In large ponds or lakes, wind may create waves that could severely erode the face of a dam. Under such circumstances, the dam should be armored by laying filter fabric and covering with riprap (rock or concrete rubble) about 2 feet above and below water level on the pond side, particularly if the dam is over 300 feet long. On most small ponds a good sod on the face of the dam will be sufficient to prevent erosion by wave action.
The width, length, and type of spillway best suited for a pond is determined by watershed area, average annual rainfall, topography, vegetative cover, and soil type. Consult a NRCS representative to determine the proper spillway for your pond. The spillway should be wide enough so that the overflow will not exceed a depth of 6 inches, which will prevent excessive numbers of fish from being lost during excessive rainfall. The spillway should be constructed about 6 inches above the level of the overflow pipe.
A barrier, as well as a 3-foot vertical drop, should be placed in the spillway to prevent the escape of harvestable fish from the pond and the entrance of wild fish from downstream. The barrier should not be constructed of hardware cloth or screen, which could become clogged and endanger the dam during floods. In general, the barrier should be 16 to 20 inches high, but never more than half the depth of the spillway. A horizontal pipe through the dam is not a sufficient spillway for watershed ponds!