Stormwater management is essentially the process of trying to mimic pre-development hydrology with post-development site planning and engineering. Early attempts focused on managing the rate of precipitation leaving a site, but now we take a more integrated approach that incorporates rate, volume, and water quality. It’s not surprising that a stormwater management approach that focuses on infiltrating water back into the soil has gained popularity. However, with all good things there can be the potential for problems, and stormwater infiltration is not immune to those problems. Listed below are 3 common problems that arise when infiltrating stormwater and some easy ways to correct them.
1. The Soils
When designing a site plan it’s tempting to immediately pencil in the stormwater BMPs in the low spots and to rely on the NRCS soil map and the hydrologic soil groups to assume that it will all work out when things get constructed. Unfortunately, although soil maps are a nice place to start, and it would make life so simple to send water to those low areas, believing in either or both of those assumptions is likely leading you to some future construction problems. Soils can change dramatically over a small extent and knowing what is there before you commit to a site layout will save you headaches further into the project.
You’ll want to get a look at the soil through a soil test pit to determine any limitations to stormwater renovation. Assuming the soil looks suitable, you’ll want to conduct an infiltration test. Most test procedures are based on measuring the saturated hydraulic conductivity. The point is to measure the rate of water moving into the soil. Once you know the soil is suitable for renovating stormwater and you have a measured rate at which it can infiltrate water, you can properly site and size your infiltration BMP.
You have the perfect soil, in the exact location you want to place a BMP..… until you destroy the soil during construction. Now your perfect soil is the functional equivalent of concrete. The most frustrating thing about compaction is that it can be minimized or avoided with just some simple construction and project planning techniques.
First, limit vehicle and foot traffic in areas that will be used for infiltration. Second, excavate out most of the soil you need to remove, but leave 6 to 12 inches in place. Then, as you are ready to finalize the BMP, remove the remaining soil as you back the equipment out of the area.
During any land development project, the use of erosion and sediment controls are common. How many times has there been a proposed infiltration BMP used as a sediment trap during construction, but when converted to an infiltration basin all the erosion controls are taken away? What you’re left with is a vulnerable BMP just waiting for a precipitation event to funnel all that upslope soil into the basin before vegetation has been established. If the silt and clay get into the basin before significant vegetation establishment occurs, the infiltration surface “silts over” and the infiltration rate plummets. Now you’ve effectively turned your infiltration basin into a stagnant retention pond. Stopping this from happening is easy, just make sure proper erosion and sediment control measures are in place around the BMP. Things like Silt Soxx or silt fence work great to accomplish this.
As discussed, fixing problems is always harder and more costly than avoiding them. Take the time upfront during planning to understand the soil conditions of your site. Think out construction sequences and equipment work flow and spend a little more time prepping the infiltration BMP to avoid compaction. Finally, protect the infiltration BMP like the valuable piece of infrastructure it is. Although they are often simple in regards to design and construction, infiltration BMPs need some attention during and post construction to assure their designed function is realized.