AMES, Iowa -- We cannot turn off the rains, but proper construction can maintain pond integrity during heavy rain events. Recent rains have saturated the soil and filled ponds; additional precipitation may test the construction of ponds.
When ponds are full and the outflow cannot keep up with the water inflow, the emergency overflow must handle the additional water. Ponds with a grassed or riprapped area that the excess water can flow over without causing erosion damage to the dam are ideal. Another advantage is a dam shaped to make maintenance easy and safe, while discouraging burrowing animals and deep-rooted trees from perforating the dam.
In addition to shaping the shoreline, an integrated management plan that takes into account all uses of the pond enhances aquatic plant management. Prevention of erosion and weak spots in the dam through regular maintenance will greatly reduce the risk of a dam failure, and lengthen the life of a pond.
Rainy periods like this fall test the integrity of ponds and there isn’t much you can do solve problems in the middle of a downpour. The best course of action is proper construction.
Guidelines for Pond Construction
Quality ponds are designed to store the cleanest water possible for a given location. In new construction, site selection plays an important role in the quality of water available for storage in a pond. One of the primary functions of a pond is to control water flow, and to regulate water flow rates coming off the watershed to reduce erosion. Sediment control is best achieved by preventing erosion in the watershed, backed up with a sediment basin to trap incoming silts and sands. An existing pond can be renovated or upgraded to a quality pond by using the same criteria.
Watershed and pond size
The size of the watershed area determines the volume of water that will enter a pond during a storm. A ratio of watershed area to pond surface area of 10–to-1 to 20-to-1 is ideal in most Midwestern states. This means that a 10-20-acre watershed will supply enough water for a one-to-two-acre aesthetically pleasing pond. It also will be a nice size for swimming and fishing.
A sediment basin may be justified when the watershed contains cultivated land, construction sites, gravel roads or other sources of sediment. A small basin, one-quarter acre in size, can trap a large percentage of the sediment that otherwise would end up in the main pond.
Soils for dam construction
A location may have a good watershed, but it also needs the right type of soil to build a dam. Soils that can be compacted into a low permeability dam usually are made of at least 20 percent clay and are low in fibrous organic matter. That means topsoil, tree roots and drain tile must be removed from the construction site and from the location from which the fill is to be obtained. County soil survey maps published by the three National Resource Conservation Services (NRCS) offer a general guide of soils in a given location.
Construction Dam: slopes, berms, safety for mowing
The dam should be constructed not only to impound water, but also to be easily and safely maintained. Mowing the dam to prevent trees from perforating the dam with their roots is recommended. The dam should be constructed with slopes flatter than 3:1 (slopes three feet horizontally to one foot vertically) for safety and to prevent wheel slippage or rollover with common mowers. To prevent the mower (and pedestrians) from sliding into the water, construct an eight-foot wide berm on the pond side of the dam, one foot above water level. This berm also discourages muskrats from burrowing into the dam and provides extra protection against wave action while making placing riprap easier.
Information on building quality ponds in Iowa can be found in ISU Extension publication ‘Building Quality Ponds – Managing Iowa Fisheries’ (PM 1352K).
For additional information about pond construction and management, contact Allen Pattillo, Fisheries Specialist with ISU Extension and Outreach, at 515-294-8616 or firstname.lastname@example.org.