Q&A Drainage: Mounds or Constant Grade?
By: John George, Agricultural Engineering Associates
Reader Question: “I am planning some new feedlot pens. Should I design for mounds in the pens or for a constant drainage, grade from the front to the back of the pen?”
John George’s Answer: Designing for good pen drainage in wet conditions can utilize either of these approaches. A constant uniform slope (typically 3% to 5%) from the bunk pad to a debris channel at the bottom is sometimes utilized to convert flat land that has limited drainage grades (typified by land leveled for flood irrigation). Pens of this design rely upon sheet flow of runoff from the pens which can be obstructed by the presence of too much manure and mud on the lot surface. Impeded drainage function then results in the generation of more mud so keeping pens clean and in good shape is critical.
An alternative design approach for such flat sites utilizes mounds running perpendicular to the bunk-line with a graded dip between the mounds to serve as a drainage channel with a more gentle drainage slope to the back of the pen. This approach allows for improved drainage functionality due to higher slopes on most of the pen surface (sides of the mounds) with the efficiency of defined drainage in the drainage swales between the mounds. Defined drainage channels which collect sheet flow from adjacent slopes require much lower slopes (perhaps 1% or less) for efficient removal of runoff from the pen area with non-erosive flow velocities. Feedlot pens of this design may require less than half the earth moving as a constant slope design.
Experienced feedlot design engineers consider management preferences and other factors including land slopes and site geometry in developing options to provide functionally efficient pen conditions for good cattle performance under the full range of weather conditions. A key parameter to maintaining good pen conditions in either design is the ease with which pens can be accessed for scraping of manure and/or snow and its removal for stockpiling or direct land application.