Comparative Rain Garden Study

Executive Summary

Stormwater control measures (SCMs) play a large role in mitigating runoff in urban watersheds. Rain gardens are a SCM comprised of soil that supports vegetation and promotes infiltration and evapotranspiration (ET), which is the combined effect of evaporation and transpiration that removes water from the soil and vegetation. Rain gardens are becoming more common in urban and suburban areas to mitigate non-point source pollution in stormwater runoff from impervious surfaces.

Rain gardens have many possible configurations, and can promote treatment, infiltration and ET, as well as peak flow reduction. Some systems are underdrained, which changes the dynamics of the water movement and treatment. Water movement and treatment is dependent on the configuration of the underdrain, as well as the presence of a site liner and the rain garden soil’s clay content. Clay soils and site liners can inhibit infiltration into the underlying soils. Typical underdrained systems are called bioretention rain gardens. Sites without underdrains are called bio-infiltration rain gardens, which promote infiltration into the underlying soil and ET from plants. The difference is fundamentally the speed at which the water moves through the system, and its effect on treatment.

Rain garden soils can be configured to optimize hydrologic performance. However, if native soils have acceptable drainage characteristics, it may be more cost-effective to use native soil or a mix with lower sand content. To further promote infiltration, the soils and the interface with the existing soil is not compacted. All of these components play a major role in determining the optimum rain garden configuration in an urban and impervious area.

At Villanova University, two nearly identical rain gardens were constructed at Fedigan Hall, which was renovated in 2009 to achieve Silver LEED (Leadership in Energy Efficient Design) certification. These renovations included the aforementioned rain gardens, sensor lighting, geothermal heat, and low-flow toilets. The rain garden situated directly to the left of the front entrance is a lined bioretention rain garden, and the rain garden to the right of the front entrance is a bio-infiltration rain garden. The rain gardens are each comprised of an engineered soil mix, an inlet box with a V-notch weir, and an overflow riser with a volumetric weir. Figure 1 shows the rain gardens at Fedigan Hall, with the bioretention rain garden circled in red and the bio-infiltration rain garden circled in blue. An ongoing study will allow a comparison of the bioretention rain garden and bio-infiltration rain garden performance. Additionally, the original Bio-infiltration Rain Garden (formerly the Bio-infiltration Traffic Island) and an additional rain garden will be added to the study. The Bio-infiltration Rain Garden has been studied since its construction in 2001.

Figure 1. The bioretention rain garden and bio-infiltration rain garden at Fedigan Hall (Bing Maps 2013).
Figure 1. The bioretention rain garden and bio-infiltration rain garden at Fedigan Hall (Bing Maps 2013).

Construction

The Fedigan Rain Gardens were constructed in the summer of 2009, during the LEED renovations at Fedigan Hall. Since Fedigan Hall does not have a basement, the rain gardens were situated against the building. Figure 1 and Figure 2 show the construction of the bio-infiltration rain garden at Fedigan Hall, and Figure 3 shows the 12 inch discharge pipe beneath the bio-infiltration rain garden. Figure 4 and Figure 5 show the completed bio-infiltration rain garden.

Figure 1. Construction of the bio-infiltration rain garden at Fedigan Hall.
Figure 1. Construction of the bio-infiltration rain garden at Fedigan Hall.
Figure 2. Construction of the bio-infiltration rain garden at Fedigan Hall.
Figure 2. Construction of the bio-infiltration rain garden at Fedigan Hall.
Figure 3. The 12 inch discharge pipe during construction of the bio-infiltration rain garden.
Figure 3. The 12 inch discharge pipe during construction of the bio-infiltration rain garden.
Figure 4. The completed bio-infiltration rain garden.
Figure 4. The completed bio-infiltration rain garden.
Figure 5. The completed bio-infiltration rain garden, with the bioretention rain garden in the background.
Figure 5. The completed bio-infiltration rain garden, with the bioretention rain garden in the background.

The bioretention rain garden at Fedigan Hall was constructed similarly to the bio-infiltration rain garden, with the addition of the liner and underdrain. Construction of the bioretention rain garden is shown in Figure 6. The completed bioretention rain garden is shown in Figure 7. The estimated construction cost of both rain gardens was around $30,000.

Figure 6. Construction of the bioretention rain garden at Fedigan Hall.
Figure 6. Construction of the bioretention rain garden at Fedigan Hall.
Figure 7. The completed bioretention rain garden at Fedigan Hall.
Figure 7. The completed bioretention rain garden at Fedigan Hall.

Design

The initial design of the rain gardens called for a 5:1 drainage area-to-practice area ratio. All of the impervious runoff entering the rain gardens comes from the roof of Fedigan Hall, which has an area of 1725 ft2. Both rain gardens have an engineered soil that contains 85% sand, 10% fines (silt or clay), and 5% organics. Both rain gardens also have a ponding depth of 6 inches, and both are designed to empty within 72 hours to mitigate the possibility of mosquito growth at Fedigan Hall. Initial infiltration tests show that the infiltration rate ranges from 0.625 to 2.25 inches per hour. Both rain gardens have an 18 inch PVC riser with an 8 inch volumetric weir to measure outflow, and a concrete inlet box with a 60° sharp crested V-notch weir to measure inflow. Figure 1 shows one of the inlet boxes, and Figure 2 shows one of the overflow risers.

Figure 1. A concrete inlet box with a V-notch weir at Fedigan Hall.
Figure 1. A concrete inlet box with a V-notch weir at Fedigan Hall.
Figure 2. An overflow PVC riser without the metal casing.
Figure 2. An overflow PVC riser without the metal casing.

While the bioretention rain garden still requires surveying, it is believed that it has a drainage area-to-practice area ratio of 2:1. The liner is a 45 mil EDPM liner, and the overflow riser connects to a 4 inch perforated PVC underdrain. The engineered soil depth at the bioretention rain garden is 2.5 feet. Figure 3 shows a design drawing of the bioretention rain garden at Fedigan Hall.

Figure 3. The bioretention rain garden at Fedigan Hall.
Figure 3. The bioretention rain garden at Fedigan Hall.

Similarly to the bioretention rain garden, the bio-infiltration rain garden requires surveying. However, it is believed that it also has a drainage area-to-practice area ratio of 2:1. The bio-infiltration rain garden has an engineered soil depth of 1.5 feet. There is no liner or underdrain at this rain garden, so water infiltrates directly into the native soil where it recharges the groundwater table. Figure 4 shows a schematic drawing of the bio-infiltration rain garden at Fedigan Hall.

Figure 4. The bio-infiltration rain garden at Fedigan Hall.
Figure 4. The bio-infiltration rain garden at Fedigan Hall.

Current Instrumentation

Currently, the there are three pressure transducers at each rain garden. At each rain garden, one pressure transducer is situated horizontally behind the V-notch weir in the inlet box, another is in the center of the garden bowl, and another is in the overflow pipe before a volumetric weir. Figure 1 shows the location of the pressure transducer at one of the inlets.

Figure 1. A pressure transducer behind a V-notch weir at one of the inlet boxes.
Figure 1. A pressure transducer behind a V-notch weir at one of the inlet boxes.

Both sites have a datalogger box connected to the pressure transducers and are connected to the internet to collect data. The rain gauge on the roof of the Center for Engineering Education and Research (CEER) building is used to determine rainfall at the site. The estimated instrumentation cost for the site is $10,000. Currently, the site does not contain any instrumentation to collect samples for water quality testing.

Calibration and Maintenance

The pressure transducers at the Fedigan Rain Gardens are calibrated in the field at the site every three months, or when data shows that a calibration is required. There is no required pressure transducer setup for rain storm events. The rain gauge on the CEER roof is calibrated by the graduate student who is monitoring the Green Roof. Calibration is performed by removing the pressure transducers from their locations and placing them in containers which are gradually filled with water. The water is filled to five or six points and measured manually. Figure 1 and Figure 2 shows calibration of the pond pressure transducers. Figure 3 shows calibration of an inlet box pressure transducer.

Figure 1. Pressure transducer calibration in the pond.
Figure 1. Pressure transducer calibration in the pond.
Figure 2. Pressure transducer calibration in the pond.
Figure 2. Pressure transducer calibration in the pond.
Figure 3. Pressure transducer calibration at an inlet box.
Figure 3. Pressure transducer calibration at an inlet box.

The calibration points start from zero and increase until the water level has reached the maximum expected value at the site. The slope in the datalogger box interface is set to 1, and the y-intercept is set to 0. A plot is constructed with the actual depth on the y-axis and the manually recorded water depth on the x-axis. A linear trend line is applied and the new slope and y-intercept are input into the datalogger box, as long as the R2 value is above 0.97.

The datalogger box must be kept dry. A sealed bag of desiccant, which is changed regularly, is used inside the box to keep moisture levels low. Indicator paper is used to monitor ambient moisture within the box, which notes when the desiccant bag must be replaced. A tube of desiccant is attached to each pressure transducer wire and changes color when it indicates saturation. Putty seals, which are used to keep the box dry, are checked periodically. Higher ambient moisture or insects within the box indicate the need for new putty seals.

The rain gardens are inspected quarterly to note bare spaces, invasive species, and debris. These inspections are also used to determine possible deficiencies in plant life, which indicate the need for new plants. The inlet boxes and overflow risers are also inspected quarterly to remove trash, debris, and leaves which may affect flow rates. Figure 4shows a rain garden with an excessive amount of leaves in the pond, and Figure 5 shows leaf removal. The rain gardens are also checked for long periods of ponding, which would indicate soil clogging. However, the runoff is not expected to have a high amount of suspended solids, so clogging is very unlikely.

Figure 4. Leaves in a rain garden at Fedigan Hall.
Figure 4. Leaves in a rain garden at Fedigan Hall.
Figure 5. Leaf removal at the bio-infiltration rain garden at Fedigan Hall
Figure 5. Leaf removal at the bio-infiltration rain garden at Fedigan Hall.

Future Renovations

As the site is prepared for the comparison study, new instrumentation is being installed at the Fedigan Rain Gardens. To assist the inflow measurements, it has been proposed to use Senix TSPC-30S ultrasonic sensors to replace the pressure transducers in the inlet boxes, since the ultrasonic sensors will be able to detect lower levels of water. The Senix ultrasonic sensor is shown in Figure 1. Additionally, screens were recently placed on the inlet boxes to protect from debris entry.

Figure 1. A Senix TSPC-30S ultrasonic sensor (Senix 2014).
Figure 1. A Senix TSPC-30S ultrasonic sensor (Senix 2014).

It has also been proposed to replace the existing pressure transducers with Campbell Scientific CS451 2.9 PSIG pressure transducers, as it has the lowest pressure range and will be more accurate in shallow waters. Figure 2 shows the Campbell Scientific CS451 2.9 PSIG pressure transducer, which is currently being utilized at the Bio-infiltration Rain Garden at Villanova University. Additionally, it has been proposed to use radio to transmit data from the pressure transducers and ultrasonic sensor at the bioretention rain garden. These instruments would run on batteries and transmit data to the datalogger box.

Figure 2. A Campbell Scientific CS451 pressure transducer (Campbell Scientific 2014).
Figure 2. A Campbell Scientific CS451 pressure transducer (Campbell Scientific 2014).

To measure the bioretention underdrain flow, it has been proposed to place a cap at the underdrain with a small orifice. A Campbell Scientific CS451 2.9 PSIG pressure transducer can be utilized to measure the underdrain flow with this configuration. It has also been proposed to install three Decagon 5TE soil moisture sensors at each of the rain gardens. These are shown in Figure 3. Finally, both rain gardens will be surveyed to determine accurate surface areas.

Figure 3. A Decagon 5TE Soil Moisture sensor (Decagon 2014).
Figure 3. A Decagon 5TE Soil Moisture sensor (Decagon 2014).