Design Overview: The concept of this project was to create a stormwater wetland out of an existing detention basin, creating a water quality treatment facility.

The existing stormwater detention basin was originally designed to reduce the increased peak flows caused by the construction of the university campus. As shown below, runoff enters the basin through sheet flow from a large parking lot (left), and through two major pipes (48” and 36”) entering the site (lower left corner of Figure 1.) There is also a steady baseflow entering the site. Originally the site had a 12” underdrain that kept the basin dry except during storm events. In the past the university had kept the basin mowed.

Figure 1: Original stormwater detention basin.

Water quality considerations were not part of the original design. The baseflow entering the basin represents the very headwaters of a small first order stream that eventually drains to the Schuylkill River. While the basin remained mostly dry, there was always some flow through the underdrain, even during a severe drought during the summer of 1999. This observation supported the concept that the site was ideal for the creation of a stormwater wetland.

The design focus required for water quality is very different then that used traditionally to create a peak flow detention basin. The first part of the rain event or the “first flush” is critical. Traditionally the "first flush" is defined as the first inch of runoff. What is not usually understood is that designing for this volume includes the majority of the annual rainfall for this region. As per Pennsylvania’s BMP manual (PACD 1998) the 75% rainfall event by volume is 1.25 inches. Assuming .25” is lost due to depression storage and infiltration, designing for 1 inch of runoff captures 75% of the rainfall volume falling on the site in an average year. This fact along with the theory that the first inch is the most polluted makes this initial small volume the focus for water quality design.

The site is designed to control both water quality low flows and extreme event peak flows. The sediment forebay and berm were placed to insure all runoff from the parking lot and piping went through the forebay. The forebay is placed to the side for maintenance access and to prevent the sediment in the forebay from being washed out during larger storm events. A series of small berms and meanders were created and the underdrain was removed. Lastly the outlet structure was modified to hold back low flows, maintain the water quality wetland, and still control the 2 – 100 year storms.

Energy Gradient: The most important design component of the project was to insure that the energy gradient of the system enables a slow meandering path for smaller storms as shown below in Figure 2. This dictated the elevation of the outlet weirs and that of the sediment forebay. The devices were originally designed so the difference in elevation between the outlet weir and that of the sediment forebay was only 0.5 feet. The outlet weir was also designed to be adjustable and and control the water surface elevation and energy gradient in the wetland.

Sediment Forebay: The sediment forebay was originally designed to hold .1 inches of rain over the entire watershed. In the original design, this resulted in a 40’ x 50’ structure 4.5’ deep. This was reduced to 40’ by 40’ and 4’ deep by assuming that the gradually sloping approach could also be included in the volume. To construct the forebay, the site was excavated leaving the underdrain intact to avoid flooding. Next, a 5” reinforced concrete pad was poured. The purpose of the concrete pad was to support maintenance of the site. After the pad was in place, gabions were placed along the lower side, using geotextiles and an earthen berm to make it impervious. The gabions merged with the earthen berm extending across the basin. All materials used were from the excavation of the forebay, so as not to change the available storage.

Figure 2: Concrete pad under the sediment forebay.

The forebay outlet weir was created from the gabions, with a low weir passing the 2-year storm with a larger step passing the 10+ year storms. The contractor added a design element by placing small riprap around the edges. This change enhanced safety, allowing someone to walk out of the basin in case they fell in. The permanent pool of the sediment forebay has remained stable and is three feet deep.

Figure 3: Completed sediment forebay.

Outlet Structure: The design focus for the outlet was to maintain the existing flood control functionality and still sustain the wetland by restricting low flows. First the area in front of the outlet structure was excavated, removing the old underdrain. Next a series of gabions were placed in front of the existing structure, with the top at a level above the previous underground 12” orifice but below the first weir in the outlet structure. The gabions were wrapped in geotextiles to make them impervious and reinforced with an earthen berm. Then an adjustable wooden weir designed for the two year storm was placed between the concrete structure and the gabions. Flow during smaller events goes along and around the outside of the gabions, then over the weir through the original 12” orifice. Larger storms have unimpeded access to the original T-shaped weir designed to control flood events.

Figure 4: Modified outlet structure during a storm event.

Meander Design: The design of the meanders was simply to construct earthen berms and sculpt out a series of turns extending the flow path with as flat a bottom as possible. See Figure 5. The grading was intentionally rough, to allow for multiple micro-habitats. The chief concern was to insure the energy gradient was as flat as possible to force a meander pattern and to minimize high velocities and channelization.

Figure 5: Meander Detail.

The planting scheme was designed by Chuck Leeds, the chief horticulturist of Villanova, and is detailed in Figures 6 and 7. Plants were selected based upon their ability to thrive at different inundation levels, and for diversity. It is understood that natural competition may change this mix rapidly. Before planting, the side was sprayed with a herbicide to kill off the phragmites that had previously dominated the site. The University Arboretum is providing maintenance for the site.

Figure 6: Planting Scheme.

Figure 7: Planting List.