Hydrology is the science of modeling water flow and volume to correctly size storm water features like retention ponds. It’s not as simple as picking a total maximum volume and using a calculator to determine the corresponding dimensions for the pond. Pond capacity is based around a 12-hour drain time, requiring careful planning for the dam and overflow features. Sizing and design of a retention pond begins with determining the water quality capture volume (WQCV) of the surrounding area. Since these ponds aren’t designed to dry up, they’ll also need to be placed near a year-round supply of baseflow from a spring or stream. With a few extra design considerations, you can design a healthy ecosystem that mimics the water purifying effects found in nature despite the pond being a man-made project.
Test the Soil
There’s no need to measure the nutrient levels of the soil like you would for agriculture, but soil stability and compression tests are a good idea. Soil engineers that provide these tests for construction companies will be able to take samples and tell you how the ground will perform under the load of your pond. Water weighs 8 pounds per gallon, so even a small 1,000 gallon retaining pond will weigh 8,000 pounds or more. This force, pressing against sandy or loose soil, will lead to voids and settling that cause leaks and water loss. Steep banks are also likely to collapse unless the soil has the structural capacity to withstand the tendency to slump. Thorough soil structural testing will help you plan a retention pond that holds up well under all conditions, with as little risk of collapse or leaks as possible.
Choose a Liner
Liners solve many of the potential problems faced by retention ponds, especially if the area’s soil is less than ideal for supporting a body of water. Concrete is a common choice for lining these ponds, but it’s far from the ideal material. It doesn’t allow for the growth of plant roots needed for vegetative banks that process contaminants from the water. In contrast, flexible geomembrane liners are easily covered in layers of dirt and mud to create the ideal rooting zone for water and edge plants. These impermeable liners are also better at preventing leaks and seepage from destabilizing the surrounding soil than concrete or packed clay liners. Seepage can lead to contamination of nearby natural waterways, so lining the pond is recommended to give the water time to settle and process before it reaches streams and aquifers.
Plan for 100 Year Floods
Many areas allow for the use of 25-year flood measurements, from the U.S. Geological Survey, to size the maximum capacity of a retention pond. However, 100-year flood records are a better basis for sizing ponds since these numbers represent water levels that have a 1% chance of occurring during any given year. This prepares the pond for multiple decades of reliable use without flooding damage or the need for multiple renovations to expand its size. Fluctuations in local climate patterns can always raise the flooding risk higher than what’s projected by national weather authorities, so making a long-term flood plan is essential. There’s no way to guess what the next year might bring.
Integrate Living Elements
The most successful retaining ponds all feature some amount of living elements to encourage the proper processing of unwanted contaminants. Bacteria do the majority of the water cleaning work, aside from the effect of gravity, to settle out particles and silt. Yet, bacteria require a medium to grow on, with water plant roots serving as an ideal source of cleaning colonies. Fish don’t directly contribute to the cleaning of the water, but they do offer the best option for controlling mosquitoes and other nuisance insects. Even crustaceans, birds, and other wildlife can contribute to the health of the pond, as they move in naturally, thanks to a planted and natural design.
Prepare for Regular Maintenance
Retaining ponds need cleaning at least once a year to remove muck that builds up along the bottom, in addition to seasonal cleaning of drainage pipes and overflow structures. Outlets consisting of short riser pipes and long culverts that direct water away from nearby structures can clog within just a few months, so these structures should be examined and tested every month. Make sure the pond is safe and easy to access by vehicle and on foot, so that maintenance workers don’t face any slip or drowning risks while handling routine maintenance and repairs.
Restrict Unauthorized Access
Fences don’t just keep pets and wildlife from trying to get a drink from the pond and slipping in. Security features also prevent people from accessing the pond, protecting you from injury lawsuits and illegal dumping. Retention ponds are designed for diluted runoff with relatively low levels of chemicals and nutrients from nearby surfaces. When someone attempts to dump manure sludge, leftover pesticides and paint, or other noxious liquids into the pond, this illegal action can create a chain reaction of environmental damage. Occasionally, people dump solid waste in the pond like unwanted furniture and appliances. Dumping this type of waste creates chemical spill risks and visual eyesores. Instead of trying to chase after the culprits, while paying out of pocket for pond recovery, prevent dumping from the beginning with fencing, alarms, and wireless cameras. At the very least, set up motion activated cameras to capture evidence of dumping if it’s unfeasible to fence in a large or steep-sided retention pond.
Deeper retention ponds are easier to maintain than shallow designs. Extra depth keeps water cool, to reduce algae growth, with no additional effort on your part. It’s also easier to keep fish in a deeper pond because they need room to hide and seek out the ideal water temperature. Dissolved oxygen rates also remain higher in deeper water since it escapes at the surface. These ponds only stay at the same depth when regularly dredged to remove silt and organic debris settling on the bottom. If cleaning chores are ignored for a few years, the accumulating inches of muck will dramatically reduce the total depth and volume of the pond. Adding six inches or so to your total measurement gives you a little extra breathing room if you miss a cleaning or two before risking overflow.
With all of these tips, you’re on your way to building the right kind of retention pond. Aside from sizing and placement concerns, start your project with the selection of the right pond liner. Liner selection will guide much of the design work; like maximum slope for the banks and surrounding berms.