A rock weir can be used to seal a gully overfall or prevent gully formation. The overfall of a gully is usually the point where the most active erosion and gully growth occur. Many gullies have more than one overfall. Sealing the overfall(s) usually stabilizes a gully. For a weir to be effective, the entire overfall should be covered with rock. Rock should be sloped from the bottom of the gully up to and across the top of the overfall.
The rock should be arranged so it is higher on the sides than in the center. When finished, the rock surface should be cup or saucer shaped to direct water flow to the center of the weir. The weir should be shaped so water can not flow around the edges. Sometimes this involves creating small rock or earthen terraces to guide water into and over the rock weir. If water flows around the rock, it will probably create a new overfall, defeating the purpose of the weir.
The size of rock necessary to construct a weir depends on the volume of water flow. As the watershed size increases, the size of the rock should increase. A weir with a small watershed up to a few acres can usually be constructed with 4- to 8-inch diameter rock. A weir with a larger watershed, such as 10-20 acres, usually requires 12- to 18- inch diameter rock (riprap) due to the increased water flow and turbulence.
A rock weir is a reasonable approach to stabilizing a small to medium sized overfall. Correctly placed rock can provide long-term erosion control. The cost of a rock weir significantly increases as the height and width of the overfall increases.
The practicality of constructing a rock weir should be compared to other alternatives. Other alternatives for stabilizing a gully overfall include diverting water with a terrace to a different location or constructing a dam impounding water at the overfall and releasing water through a pipe.
Although useful for stabilizing an existing gully, a rock weir is also appropriate for preventing gully formation. An excessively sloped location that will receive concentrated water flow such as a terrace outlet or a pond spillway is a candidate for rock weir construction to prevent erosion.