Detention Pond Calculations

Dry Pond Design

• A dry detention pond should be at least 10 acres in size, three times longer than it is wide and drain within 24 hours. Size requirements vary by state. For example, Washington state requires that a dry pond should provide 1 foot of space above the projected peak of a 25-year storm. The pond should be at least 100 feet from any septic tank or sewage drainage and 5 feet from any property line or vegetation. You should provide a way to release overflow in an emergency to accommodate a 100-year storm or in case the primary overflow gets plugged. Slowing the flow of water entering a dry pond will reduce bank erosion. Mosquitoes may breed in shallow pools of water left standing for more than a week.

Wet Pond Design

• A wet pond should be three to five times longer than it is wide with widely separated inlets and outlets. To increase wintertime fish survival and discourage the growth of rooted aquatic plants, the pond should be 3 to 6 feet deep. To improve the water quality, flush it more often during extended dry periods using ground water if possible. Any underwater shelf near the pond edge should be covered by rooted plants. This prevents children from getting to deep water and makes the pond more attractive and helps remove pollutants. If you have waterfowl on the pond, do not heavily plant more than one-half of the pond edge. Put deep sediment traps or underwater dams near the entrance to reduce the accumulation of sediments in the pond. You should be able to completely drain the pond so you can maintain it. Make sure your inlets and outlets have safety gratings, and you need an emergency spillway.

Pollutants

• If you design a wet pond properly, you can remove 70 percent to 90 percent of suspended solids, 60 percent to 95 percent of heavy metals, 70 percent of nutrients and 70 percent of oxygen consumed by bacteria. Expect a six-year wait to gain an ecological balance in a wet pond. During this time, the pond may be plagued by bad smells, excessive growth of algae and fish kills. You should not use the water for fishing or swimming. Dry ponds can help eliminate 61 percent of suspended solids and 26 percent to 54 percent of heavy metals.

Maintenance

• Drain and dredge a wet pond periodically to remove contaminants on the bottom. If you don’t maintain it, the quality of the water won’t improve. About 1 inch of sediment will collect each year. Deeper ponds need to be dredged less often than shallow ponds. You should dredge your pond if the accumulated sediment is more than 25 percent of the original depth. Remove excessive aquatic plants and algae to prevent them from rotting.

Plans and Cost

• To design embankments, foundations, gratings, emergency spillways and anti-seep collars, check plans and manuals provided by the Bureau of Reclamation, the U.S. Natural Resources Conservation Service and the Army Corps of Engineers. In 2004, a wet pond cost about $40,000 per acre of pond surface to construct and $1,500 a year to maintain.