In order to measure flow rates and calculate the infiltration capacity of the Grove St and Kettlebell SGFs, a variety of sensors were installed throughout the systems. Information
pertaining to the instrumentation at each monitoring site is provided in Table 2. At the Grove St site, Onset HOBO water level loggers were installed in each catch basin and were used to monitor water levels. The HOBO loggers are sealed, non-vented sensors that measure absolute pressure and temperature according to a set logging interval. The sensors were hung from the catch basin grates using nylon string, as displayed in Figure 8, to keep the sensors at a consistent elevation and allow for easy access throughout the monitoring period. A fifth HOBO water level logger was installed near the end of the lower perforated pipe which runs between CB #3 and CB #4 (see Figure 8). The sensor was used to monitor the water level inside the gravel layer when ponding occurred.
In order to convert the HOBOs’ absolute pressure data to water levels, the data was first converted to gauge pressures by means of barometric compensation. An additional HOBO sensor (PTT-BARO) was set up at the UNH Stormwater Center field site, only 5.5 miles from the monitoring sites, to collect barometric pressure data for the correction. According to the HOBO U20 manual, the barometric compensation sensor is within the acceptable usage range of 10 miles. Details regarding PTT-BARO are provided in Table 2. After performing barometric
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compensation, the gauge pressures were then divided by the specific weight of water
(approximately 62.4 lbf/ft3 at 4°C) to obtain values for the depths of water above the sensors. Water surface elevations were calculated by adding the depths to the known relative elevations of the sensors (see Table 2).
An Aqua TROLL data logger was installed at the end of the outlet pipe at the Grove St site to measure the depth of water flowing through the pipe. The Aqua TROLL is a vented sensor that can measure a variety of different parameters including pressure, water level, temperature, and conductivity. For the purposes of the research described in this report, the Aqua TROLL logger was only used to collect pressure and temperature data. Unlike the HOBOs, the pressure data collected by the Aqua TROLL is automatically corrected for barometric pressure. This data, along with the water level data collected for each of the catch basins, was used to determine inflows and outflows based on calibrated rating curves. Development of the rating curves will be described in the Section 3.4.
At the Seacoast Kettlebell site, a HOBO water level logger was installed in the systems inlet catch basin (CB #1) to collect pressure and water temperature data. The absolute pressure data was converted, as described previously, to elevations which were then used to calculate flows. No other sensors were installed at this site as monitoring data showed that the water level within CB #1 never dropped below the invert of the outlet pipe. This indicated that the system’s gravel storage layer remained completely full throughout the majority of the monitoring period. Possible reasons for this observation will be discussed in Section 4.1.
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Table 2: Information for sensors installed at each monitoring site
System Type of Sensor Manufacturer/ Model Raw Pressure Accuracy Relative Elevation (ft) Description of the Location Unique Name
Grove St Water level logger
Onset HOBO / U20-
001-04 ± 0.3% FS (0.063 psi) 95.929
Suspended in CB#1;
Dover, NH GSt-1 Grove St Water level logger Onset HOBO / U20-001-04 ± 0.3% FS (0.063 psi) 95.717 Suspended in CB#2; Dover, NH GSt-2 Grove St Water level logger Onset HOBO / U20-001-04 ± 0.3% FS (0.063 psi) 95.323 Suspended in CB#3; Dover, NH GSt-3 Grove St Water level
logger
Onset HOBO / U20-
001-04 ± 0.3% FS (0.063 psi) 94.775
Suspended in CB#4;
Dover, NH GSt-4 Grove St Water level
logger
Onset HOBO /
U20L-04 ± 0.3% FS (0.063 psi) 96.545
Inside lower pipe connecting CB#3 &
CB#4; Dover, NH
GSt-4_PrP3 Grove St Data logger Win-Situ Aqua TROLL / 200 ± 0.05% FS (0.015 psi) 95.985 outlet pipe; Dover, NH Downstream end of GSt_Outlet Kettlebell Water level
logger
Onset HOBO / U20-
001-04 ± 0.3% FS (0.063 psi) 95.098
Suspended in CB#1;
Dover, NH KB-1
- Water level logger
Onset HOBO / U20-
001-04 ± 0.3% FS (0.063 psi) -
UNH SC field site,
Durham, NH PTT-BARO Horne St
Bioretention Rain gauge
Campbell Scientific, TE525MM
± 1% at 1 in/hr
(Resolution: 0.004 in) - Horne St, Dover, NH D3BA1HB2
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Figure 8: Diagram showing the general location of instrument installation
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Rainfall data was collected using a tipping-bucket rainfall gauge, located at a bioretention system along Horne St in Dover, NH. The data from the rainfall gauge was applied to the two SGF monitoring sites because their locations were close enough (i.e. less than 2,000 ft away) that rainfall could be considered reasonably consistent for both sites. As a backup, rainfall data was also obtained from the weather station maintained by UNH at Kingman Farm in Durham, NH. Gaps in the gauge data from Horne St were filled in with data from the UNH weather station.
For this project, all sensors at the monitoring sites were set to either 1 or 5-minute logging intervals. Due to the limited size of the systems’ watersheds and high percentage of impervious area, the time of concentration for the stormwater runoff is very short (Table 1). Runoff flows can therefore change quickly during rain events. A short logging interval was used in order to capture the rapid changes in the runoff hydrographs. The HOBO logger used for barometric compensation was set to a slightly longer logging interval of 15 minutes because changes in barometric pressure were more gradual than changes in runoff and the longer interval helped preserve device memory and battery life.
During the 1-year monitoring period, each sensor was periodically checked to verify the accuracy of the device. Accuracy checks were performed by analyzing the sensor reading for a known depth of water in a graduated cylinder. The Aqua TROLL data logger was also calibrated periodically for pressure if the accuracy check showed the sensor’s measurements were shifting. The HOBO sensors were all factory calibrated and could not be recalibrated without being sent to Onset for maintenance. None of the HOBO sensors showed any significant reading drifts during the monitoring period that required the sensors to be recalibrated.
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