Applying Stormwater Control Measures in Series

Many sources, including the US Environmental Protection Agency, National Research Council, and state stormwater manuals, recommend the use of stormwater control measures (SCMs) in series to form a treatment train. The concept of using multiple treatment processes to improve results is a long-standing remediation strategy in the water and wastewater fields. As for stormwater management, the reasons for using a series of treatments are to maximize and diversify performance by increasing the volume removed, improve outflow quality, increase longevity, and reduce maintenance requirements. However, there are few studies to support these assertions.

Pennsylvania Department of Environmental Protection Growing Greener Grant, includes a vegetated swale, followed by two rain gardens in series, and finally an infiltration trench. The swale and rain gardens act as pretreatment to the infiltration trench so that less flow annually reaches the infiltration trench and the flow that does enter the infiltration trench has a lower sediment load to preserve the infiltration capacity over a longer design life. The aerial photo in Figure 2, adapted from Google Earth, displays the collection area and layout of the SCMs. The swale, rain gardens, and infiltration trench are noted by the green, yellow, and red arrows, respectively. The six monitoring and sample collection sites are represented by white arrows.

The swale is approximately 40 meters long, has a 1-meter bottom width and 15-centimeter ponding depth, and is filled with about 23 centimeters of engineered media; side slopes are at a 2:1 ratio. There is a weir midway through the swale to help slow flow and to serve as a monitoring location. The rain gardens, which are approximately 1 by 2 meters in area with 45 centimeters of media, also have 45 centimeters of ponding depth and 2:1 side slopes. The final SCM, the infiltration trench, is about 2.1 meters by 2.7 meters in area and 1.2 meters deep. It is contains 20 R-Tanks, pictured in Figure 3, which were donated by ACF Environmental. The crates allow for easy maintenance and have 95% porosity to store a larger volume of water than traditional stone.

Table 1 summarizes each SCM, its intended use in the system, and the design capture amounts in terms of rainfall and volume of storage. Values in

Table 1 are expected rainfall capture amounts and volumes for the individual SCMs (or pair of rain gardens), and when accumulated, the system is expected to capture a 1-inch storm event or volume of 22.45 cubic meters. These values are predicted from design and will be compared to field data once monitoring of the site begins.

Construction of the site took place from September 30, 2011, to November 4, 2011. All three SCMs were excavated, and the swale and rain gardens were then filled with a media composed of 85% sand, 10% fines, and 5% organics. The weir plates located halfway along the swale, at the end of the swale, and following each rain garden were also installed during this phase. Each weir plate, mounted to wooden posts, is 1.37 meters wide with a 0.3-meter v-notch depth. The media was placed on the upstream side of each weir plate to the tip of the v-notch with a 15- to 23-centimeter drop on the downstream side to allow for flow measurements. Upon completion of the weir installation, backfill of the swale and rain gardens was also finished, and some of the plants were planted during this time. Construction timing was not ideal from a planting perspective, but allowing some of the plants to stabilize and settle over a few months was beneficial. The remaining plants, mainly in the vegetated swale, were planted in early June. During this planting phase, erosion control matting, topsoil, and grass seed were also put down to mitigate erosion during plant establishment.

Construction of the infiltration trench began with a layer of geotextile, followed by a layer of stone to level the bottom of the trench. Then another layer of geotextile was placed followed by the R-Tanks; the second layer of geotextile was then wrapped around the R-Tanks. The R-Tank installation guide recommended at least 0.3 meter of stone around the tanks, and at this site there was between 0.3 and 1 meter of stone on all four sides, as shown in Figure 4. Topsoil and Xeripave pervious pavers were placed at the surface. The system is not connected to the stormwater sewer system, and instead the pervious pavers at the surface are intended to serve as the overflow. Figure 5 shows the completed infiltration trench with the pervious pavers. An access door, also shown in Figure 5, was installed for research purposes to establish and maintain monitoring equipment. The door also provides access to the 12-inch-diameter pipe, which allows a vacuum to enter the infiltration trench if clogging were to occur. Pictures of the completed system from October, 2011, and June, 2012, are included as Figures 6a through 6d. Research is intended to examine the benefits of pretreatment through the swale and rain gardens, which should reduce the need for maintenance of the infiltration trench.

To analyze water-quality and -quantity changes throughout storm events, there are six monitoring points along the system. The five weir plates each serve as a monitoring point, along with the infiltration trench. Pressure transducers will continuously measure the height of flow, which will be converted to a flow measurement. Grab samples will be taken during storm events to analyze water-quality parameters, which include but are not limited to pH, conductivity, chlorides, nitrogen (NO_x), total Kjeldahl nitrogen (TKN), total phosphorus (TP), orthophosphates, total suspended solids (TSS), and total dissolved solids (TDS). Temperature will also be collected with each pressure transducer. It is important to note that while this particular treatment train contains weir plates and a box at the beginning, these devices are for research purposes and are not required for standard implementation.

Because the detailed effects of constructing SCMs in a series is not fully understood, this research site at Villanova University hopes to answer a few questions. In terms of quality, there are questions revolving around the individual systems. What are the water-quality benefits of the vegetated swale? What water-quality benefits are there of two rain gardens in series? What are the water-quality effects of an infiltration trench with pretreatment? Finally, what are the water-quality benefits through the system as a whole? Research questions also focus on water quantity. What are the effects of SCMs in series on stormwater volume and peak flows? Will the total volume removed exceed the sum of the parts? The final research questions are related to maintenance and longevity of the system. Will the benefits of applying SCMs in a series decrease maintenance needs and increase the longevity of each individual SCM? By focusing on these research questions, an initial analysis of the Villanova treatment train is expected to be completed in the spring of 2013.