By Bryce Sundbo, Regional Engineer, Saskatoon
To prevent negative impact to surface water, livestock operations must contain runoff from pens, silage pits, manure piles or composting sites on their property. Control works consisting of diversion ditches/dykes and holding ponds can help contain the runoff. Holding ponds should be included as part of the operation design, which should incorporate proper pen slopes that promote drainage and pen drying.
All sites require pre-screening and an initial soils evaluation. For some sites, this work will identify the need for a detailed site investigation. The geotechnical nature of the site and regulatory requirements will influence the required commitment from the developer.
A topographical survey will define the contributing area for runoff. This allows the design of pens and control works to potentially reduce the contributing area and take advantage of natural slopes and depressions. The survey will also facilitate the layout of the site for construction.
Holding pond design
Holding ponds require capacity for at least 7.5 cm (3 in.) of runoff from the contributing area. This design volume is defined as the full supply level (FSL). Many jurisdictions use a one in 25-year/24-hour rainfall event to determine the holding pond volume. For Saskatchewan, this corresponds very closely to 7.5 cm (3 in.). To provide a factor of safety against overflow, the ponds must be designed with 30 cm of freeboard (vertical depth above the FSL).
Holding ponds are often designed as evaporation (shallow) ponds that allow natural evaporation to remove some, or all, of the water that collects. This requires a large surface area in relationship to the liquid depth, which is usually about 1 m. Holding ponds are often located adjacent to the pens to reduce the contributing area. Where this is not practical, ditches can be used to direct runoff to a holding pond from another location on the site. Deep holding ponds are commonly used in large feedlots. They require less space, but must be emptied to allow for the next rain event or spring runoff.
Sloughs contained entirely on the operator's property may serve as holding ponds, provided they have sufficient capacity and the site investigation confirms that infiltration will be minimal.
Safety and maintenance
Cattle should not have access to holding ponds. If access is unavoidable, lower side slopes will lessen the impact of hoof action, which causes a reduction of the pond’s volume. Deep holding ponds should be fenced as a safety precaution to prevent access by children and livestock.
Apply effluent from holding ponds on the land according to normally accepted agricultural practice. Liquid manure handling equipment or irrigation equipment can be used for this operation.