Almost all ponds, except for raised excavated designs on flat surfaces with high levees, intake some amount of runoff water. Even raised levee ponds still rise eventually from direct rainfall on the surface. Calculating just for holding the amount of water entering from a stream or wetland will leave you with a pond that floods dangerously during even a small seasonal storm. All ponds must feature two different spillways for processing excess water before it can flow over the dam or pond edges. Without spillways, embankment ponds are prone to washing out their dams. Since spillways experience water flow routinely or even daily, careful design is necessary to prevent erosion.
The first spillway needed by every farm pond is the standard drain. It’s designed to release extra water accumulating on a daily basis due to a constant flow from a stream or spring. For excavated ponds, these drains are only generally used during routine storms that add rain and runoff. There are many different designs for standard spillway drains, including trickle tubes, concrete monk outlets, and more.
Secondary spillways are emergency overflow management tools. These drains rely on higher placed valves and outlets so that the pond must rise substantially to begin draining. This second spillway should release into the same drainage area as the first, but the drainage basin should be sized and designed for the higher flow rather than just the standard daily overflow. Emergency spillway drainage zones need a width of 10 to 50 feet at minimum to handle the flow, so don’t underestimate the free space needed downstream from your farm pond.
Outlets, drains, and other spillways all need to release the excess water somewhere. These drainage areas must be sloped away from the pond, structures like homes and barns, and other bodies of water. Drainage areas require either a heavy vegetative cover or paving to prevent long-term erosion.