Anatomy of a Wet Detention Pond
by Jason Abert, Vice President of Service Delivery
Did you know that oftentimes the ponds you see on the side of the road, beside a grocery store, next to a school, or behind a house serve far more purpose than just being a nice, aesthetically pleasing water feature? These wet detention ponds are more than meet the eye; they’re a powerful tool for holding stormwater and improving watershed health. Wet detention ponds can be used to mimic natural ecosystems and can provide equal or additional benefits if maintained properly. However, if a wet pond is not well maintained, you may experience infrastructure failure and ultimately a notice of violation (NOV) from your local regulators.
Wet detention ponds are designed to manage the quantity and quality of runoff leaving your system, which is why they are one of the most common stormwater control measures (SCMs) used by landowners and businesses1. Water quantity and quality are important to control and measure, because flooding and pollutants can affect the ecosystems and watersheds downstream. When a rain event occurs, wet ponds are designed to capture and store runoff in order to treat it and release it at a controlled rate, mimicking the natural flow rate before there were parking lots and buildings. Wet ponds are designed to permanently hold the first inch of rain that is common among most rain events, and temporarily store rainwater and runoff occurring from much larger storms, such as a 25- or 100-year storm. A wet detention pond is commonly constructed with a forebay, which is designed to capture the “first flush,” or first inch of a rain event. Since the first flush typically contains the highest concentration of pollutants, and to increase the ease of maintenance, forebays are an effective component of a wet pond.
The longer residence time (the amount of time each droplet of water spends in the SCM) will allow for more biological processes within the soil and plants to occur. Much like a natural pond, biological processes, from sunlight and plant nutrient cycling, can assist in pollutant uptake1. Once the pond has filled, the outlet structure will slowly discharge water at a designed rate and reduce velocities downstream. Controlling the amount of water coming in and out of the system will help with flood control, limit streambank erosion, and will prevent system and property damage that could result from flooding.
One of the most important aspects of a wet detention pond is the routine maintenance and preventative care that will ensure that your system is working properly for the life of the system. A central element in your stormwater pond is the outlet structure, because it controls the water being discharged from your system. If a pond is holding water above the outlet structure longer than 24 hours, you may have a problem with your SCM. Common issues that can affect your SCM’s outlet structure can include sinkholes around your structure, sediment build-up, a cracked or clogged low-flow or outlet pipe, or overgrown vegetation. A clogged outlet system may need to be cleared or dredged depending on the severity of the clogging, and sediment or vegetation will need to be removed1. When these issues arise, your system may not function the way it was designed, and a stormwater professional will need to perform maintenance or possible repairs to make your system functional again. Commonly, ponds are designed with an emergency overflow, if the outlet control structure were to fail, which is typically a depression in the dam slope designed to prevent complete dam failure.
It isn’t just about making your system functional, though. In many properties, wet detention ponds dominate what your customers see when they drive up to your property. You need to maintain this large green space just like you need to maintain the landscaping on traffic islands and your perimeter. Many property managers hire landscapers to provide this work, and that comes with drawbacks: often, these ponds are designed with beneficial plant species in mind that will die if they are mowed too low or otherwise treated improperly. Their landscaping equipment may get bogged down, literally, as the conditions and water levels in a wet detention pond are ever-changing. Also, equipment used for landscaping services are not designed to traverse steep pond slopes or cut turf at a desired height, and commonly cause destabilization of soils. Lastly, they don’t have the expertise to inspect the structures and deficiencies that are red flags for regulators. Fortunately, companies like R+R are well-versed in the art of pond aesthetics and national laws and regulations.
Wet ponds that go un-maintained will experience damages that could be costly to your business if not addressed quickly and correctly. Flooding can arise when your system no longer infiltrates or discharges water; this could result in flooding parking lots and property damage. A wet pond that has been neglected can ultimately need to be restored, and this will be where your business will experience substantial costs. Alongside the repairs that must be made to a failing system, your local municipality may distribute an NOV for not complying with post-construction stormwater laws. Once these fines are issued, charges will accrue until the issue is resolved. Working with a stormwater professional will ensure that your business will be stormwater compliant and aware of any preventative maintenance needed.
Wet detention ponds are designed to have a maintenance plan to avoid any design failures. Properly managed SCM’s can have an extensive lifespan and are less likely to need costly repairs than a system that goes un-maintained. Indeed, they often provide a visually pleasing element that significantly increases the value of the property and the impact on the community’s environment. A stormwater maintenance plan will make sure you and your business are aware of any repairs and housekeeping that needs to be made before they become large and costly, or prior to an annual regulatory certification. Talk with a stormwater professional today to learn more about your businesses SCM’s and respective stormwater maintenance programs.