Bio-Infiltration Rain Garden

Rain Garden

Summary Handout

Urban stormwater has the potential to deliver large pollutant loads and increased flows to receiving waters over short intervals. The increased runoff has been shown to accelerate stream bank erosion and reduce infiltration thereby increasing drought effects on the headwaters of streams. Best management practices are currently recommended by PaDEP to control stormwater runoff. Detailed information on their performance is still emerging as recognized by Pennsylvania’s Chesapeake Bay Nutrient Reduction Strategy (PaDEP 1996a) and the Pennsylvania Handbook of Best Management Practices for Developing Areas (PACD 1998).

An existing traffic island was retrofitted to become a bio-infiltration BMP. The facility intercepts flows that would normally be captured by inlets and delivered through culverts to a dry detention basin without a chance to cleanse or infiltrate the stormwater runoff. Designed for the smaller storms (0 – 1.5 inches), the runoff infiltrates thereby reducing downstream stormwater volumes, stream bank erosion, and non-point source pollution to the headwaters of the Darby Creek. The advantage of this type of facility is the capture and infiltration of over 90% of the yearly rainfall. This will significantly reduce stream bank erosion and protect low baseflows during times of drought. As all the stormwater for these rainfalls is infiltrated, there is NO pollutant input to the headwaters for smaller storms.

During larger storm events (2 – 100 year storms) a significant portion of the rainfall, as well as the first flush of pollutants is infiltrated. Flows over the capacity of the BMP go through the original culvert system to a dry detention pond.

The bio-infiltration pond is incorporated within the “Best Management Practice Demonstration Park" on Villanova’s campus, and joins the retrofitted stormwater wetlands, infiltration trench, porous asphalt and porous concrete site. To validate the performance of the device, rainfall and groundwater depth are monitored. In addition to those displayed on this website, results are included in the final report to the PaDEP. 


This project was funded by the PADEP Growing Greener Program. Villanova University contributed matching funds to the project. A table outlining the total construction costs is provided below.

Table 1: Total Construction Cost

Activity Cost Activity Cost
Excavation $8,912 Emplacement $6,933
Dumping $2,400 Pipe Installation $450
Making soil $6,248 Curb Cuts $554
Sand $2,755 Observation Wells $547
Total Construction Cost                        $28,799

The bio-infiltration area is located in a grassy traffic island in the parking lot of Villanova's West Campus. The West Campus parking lot serves a dormitory complex and a recreation area. The traffic island was retrofitted into the shape of a shallow bowl, allowing the inflow of water to accumulate within.

The drainage area is approximately 1.21 acres , with 52 percent impervious cover.

Drainage Area Site Plan
As Built Contour Drawing

Bio-infiltration: Bio-infiltration is accomplished by using a combination of porous soils and vegetation. The area consists of a 4-foot layer of sand mixed with the existing soil located at the traffic island. The species of vegetation planted at the site can survive in both dry and wet conditions over long periods of time. The area also has a grass buffer zone extending from the curb to the sand.  

Essential Features of a Bio-Infiltration Basin
Vegetation Growth

Design Calculations: The total volume of the excavation, as well as the volume of the sand/soil fill are calculated in the table below. Using a range of porosities, the volume of void spaces in the fill was calculated.

Volumes and Porosity

Inflow: Inflow was controlled by making two cuts in the curbs as shown in Figure 1 and 2. Placement of these cuts was determined by observing the flow of runoff on the parking lot. The inlet for the storm sewer on the north end of the pond was filled with gravel and then sealed with asphalt. A second curb cut was made where the now-filled inlet is located. The curb cuts were connected to the pond via channels lined with rip-rap. A 12" corrugated black PVC pipe was installed in the south inlet. It diverts flow in the storm sewer into the bio-infiltration pond. A diversion weir was placed in the inlet. The elevation of the weir determines the maximum water surface elevation in the pond. When this elevation is reached all additional incoming flows will bypass the pond and continue downstream to an existing detention basin.

Profile of Bio-Infiltration Basin
Figure 4: Profile of Bio-Infiltration Basin

Weir Construction: To better monitor the amount of runoff captured, a V-notch weir was inside the south inlet. The weir was machined from an 18-inch square aluminum plate. The top width of the weir is 9 inches and the maximum head on the weir is 9 inches. The weir design was based on ASTM standards.

Excavation of original soil. The placement of curb cuts was determined by flow of water across the pavement.
Pipe Installation
A 6-in pipe was installed that connects the storm sewer to the traffic island.
Storm Drain
Storm drain at the end of the north inlet channel was filled and covered with asphalt.
Installation of monitoring wells within traffic island.
Installation of monitoring wells within traffic island.
Traffic Island
Completed traffic island with vegetation and monitoring equipment.
Completed Excavation
Excavation is Complete.
The original soil was mixed 50/50 with sand to provide fill.
Mixed soil added into traffic island.
Mixed soil added into traffic island.
Mulch placed over soil.
Mulch is placed over soil to protect vegetation. Inlet channels are lined with rip rap.

Figures 1-3 illustrate the performance of the Traffic Island during a storm event over 48 hrs.

The basin fills up during a storm event.
Figure 1: The basin fills up during a storm event.
Significant infiltration within the basin after 24 hrs.
Figure 2: Significant infiltration within the basin after 24 hrs.
Figure 3: Almost complete infiltration after 48 hrs. of infiltration.
Figure 3: Almost complete infiltration after 48 hrs. of infiltration.

Figures 4-6 illustrate the performance of the Bio-Infiltration Traffic Island during colder seasons. 

Graphical representation of storm events in January 2005.

Snow Cover
Figure 4: Snow cover in the Traffic Island after snow storm.
Figure 5: Snow surrounding the Traffic Island with melting water in the basin.
Figure 6: Snowmelt at Traffic Island. Water remains within basin.
Figure 6: Snowmelt at Traffic Island. Water remains within basin.

Figures 7-9 illustrate the performance of the Bio-Infiltration Traffic Island during warmer seasons.

Graphical representation of storm events in July 2005.

    Figure 7: Lush vegetation within and surrounding the basin.
Figure 7: Lush vegetation within and surrounding the basin.
Full Basin
Figure 8: A full basin during a storm event. Plants are capable of surviving in water for long periods.
Weir Flow
Figure 9: Weir flow during a storm event.

Special thanks to all involved in the development of the Bio-Infiltration Traffic Island and their continuous hard work towards its maintenance, improvement and research advancements.

PADEP Growing Greener Program
EPA Nonpoint Source National Monitoring Program
Villanova Urban Stormwater Partnership
Villanova Facilities Management

Graduate Students:
Matthew Prokop
Tyler Ladd
Jordan Ermilio
Erika Tokarz
Keisha Isaac-Ricketts

Research Fellows:
William Heasom, PhD, PE
Clay Emerson 

Q: What is the drainage area for the VUSP bio-infiltration Traffic Island?
A: The drainage area for this site is approximately 50,000 sq.ft or 1.15 acres

Q: How much rain can the VUSP-TI retain in a given even?
A: This site is designed to hold approximately 1.0 inches of rain over the watershed. When the water level in the basin reaches the level of the overflow weir (at 1.72 ft.), this site begins to overflow and any rain into the system is bypassed to a stormwater basin off site. Click here to see how this site performed during an event of July 12, 2004 where approximately 4.00 inches of rain fell over a 16 hour period.

Q: How does the site overflow?
A: This site has a unique design in that it was retrofit from an existing storm drain. The original drain was bypassed so that it enters the BMP prior to continuing downgrade to a stormwater detention basin. Thus, when this site overflows, the water in the system backs up into the original drain and continues to the original stormwater detention basin.

Q: What percentage of annual rain does the site retain?
A: We currently do not have figures for the exact percentage of annual rainfall retained by this system due to difficulties in measuring precipitation during the winter months because of snow and icing at the site. It is interesting however to note that during the month of July 2004 a total of 10.48 inches of rain fell at the site. During this particularly rainy month, a total of 3.39 inches of rain overflowed from the BMP which reflects a performance of 67.7%. Click here to see an interesting summary of our research for the month of July 2004.

Q: What are the water quality parameters studied at the VUSP bio-infiltration BMP?
A: At the moment VUSP is monitoring a number of water quality parameters at this site to determine the effectiveness of bio-infiltration in reducing pollutants. Our water resources laboratory at Villanova University uses a variety of methods to analyze pollutants. Using HPLC we analyze stormwater runoff samples for Chlorides, Nitrite, Nitrate and Phosphate. Using spectrophotometery we analyze for total Nitrogen and Phosphorous. We analyze for a variety of metals including copper, lead, zinc, chromium and cadmium using a graphite furnace and, we also analyze for suspended and dissolved solids as well as conductivity, pH and temperature using standard laboratory techniques. Click here to see a summary or water quality parameters analyzed on July 12, 2004.

Q: What other water quality issues should be considered when using bio-infiltration BMP?
A: (1) Organic Carbon is an important parameter because as organics decompose, they can deplete dissolved oxygen in lakes and rivers which adversely impacts aquatic life.

(2) Hydrocarbons can be be an issue due to oil and grease which leaks from vehicles onto roadway surfaces and is carried to streams by precipitation runoff. Low concentrations of some hydrocarbons can be toxic to aquatic life.

(3) Pesticides, insecticides and herbicides used in agricultural lands as well as in suburban areas have been detected in stormwater runoff.