Site Descriptions

One of the subsurface gravel filters that was evaluated in this study is located under Grove St, near the intersection of Grove and Chesley St in Dover, NH. A picture of the SGF system after construction is presented in Figure 3. The system was statically designed according to NH stormwater guidelines to store and treat the runoff from its estimated drainage area, which consists of 1.44 acres of residential neighborhood with 22% impervious cover (Table 1). The second SGF evaluated for this study was constructed underneath the paved parking lot of the Seacoast Kettlebell workout facility off of Horne St in Dover, NH (Figure 4). The system was designed to treat the runoff from a 2.09-acre residential/commercial watershed of which 72% is impervious cover (Table 1). Due to spatial constraints, the Kettlebell system is undersized and designed to store and treat 30.5% of its WQV, or approximately 1620 ft3 _{of runoff. The Grove St} SGF was fully sized to store its entire WQV of 1320 ft3 _{of runoff. The design procedure and} associated sizing calculations for each system are provided in the Appendices.

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When the systems were originally designed, their watershed areas were roughly estimated based site assessments and the AutoCAD shapefiles provided by the City of Dover Community Services Department (CSD). After construction, the watershed areas for both systems were reassessed using a 1-meter resolution digital elevation map (DEM) from the NH GRANIT website and elevation data collected from site survey. The Grove St SGF’s watershed was found to be over twice the size of the original estimate, with an area of approximately 4.10 acres. The watershed for the Kettlebell SGF was also calculated to be larger than the original estimate, with an area of 2.67 acres. Even though the Grove St system was sized to fully store and treat its WQV, these new drainage area estimates show that both systems are undersized for their actually watersheds. The Grove St and Kettlebell gravel filters both provide storage for just under 30% of their respective WQVs, based on their actual watersheds. The updated watershed characteristics for each system are presented in Table 1.

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Table 1: Original and updated watershed characteristics for the Grove St and Kettlebell SGF systems

Parameters

Grove St SGF Kettlebell SGF Original

Values Updated Values Original Values Updated Values

Drainage Area (acres) _{1.44 4.10 2.09 2.67} Time of Concentration (min) 8.3 13.7 12.2 11.4

Weighted Curve Number (-) 88 83 92 92

Potential Maximum Retention (in) 1.36 2.05 0.87 0.87 Initial Abstraction (in) 0.27 0.41 0.17 0.17

% Impervious Area 22% 31% 72% 61%

WQV (Ac-In) _{0.36 1.35 1.46 1.59}

WQV (ft3_{) 1307 4910 5296 5771}

Constructed Storage Volume (ft3₎

1320 1320 1620 1620

% of WQV _{101% 27% 31% 28%}

The Grove St system design includes four catch basins, referred to as CB #1-4, located at the four corners of the system’s rectangular, gravel filtration layer. Horizontal, perforated and slotted, HDPE pipes are embedded within the gravel layer and run along the perimeter of the system, connecting the catch basins. The whole system is approximately 60 feet long by 25 feet wide with a 2.2-foot deep layer of ¾-inch crushed stone. Figure 5 presents a simplified diagram of the stormwater control measure. Figure 59, Figure 60, and Figure 61 in the Appendix, are the original design diagrams for the system. Various changes to the system design, which occurred during and after construction, will be discussed in Section 3.4 of this report.

During rain events, runoff flows into the system’s four catch basins. The water can then enter the SGF’s stone layer through the five perforated and slotted inlet pipes which run between the catch basins. CB #1 and 2 are connected by a 6-inch diameter, slotted lower pipe and a 12- inch diameter, perforated upper pipe; CB #1 and 4 are connected by a single 6-inch diameter, slotted pipe; and CB #3 and 4 are connected by upper and lower 6-inch diameter, slotted pipes.

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The lower pipes allow water to enter the gravel filter, filling the system from the bottom up, during low flow conditions. In the gravel layer, runoff water filters through the media and can infiltrate into the native soils surrounding the system. If the inflow rate exceeds the infiltration rate, water will pond in the gravel layer, filling the pore spaces of the stone media.

The two lower inlet pipes between CB #1, 3, and 4 also act as underdrains which drain the gravel filter layer to CB #4. Water can then flow out of the system though a 12-inch diameter pipe that discharges to a rip-rap lined swale draining towards Berry Brook. According to the design, outflows are regulated by the tee fitting on the outlet pipe in CB #4. Low flows are controlled by 1-inch orifice hole in the restrictor plate at the bottom of the tee fitting, while high flows are controlled by the open top of the tee. If the system’s gravel layer fills, excess runoff will bypass the filter layer and flow directly the CB #4 through the 6-inch diameter, slotted pipe connecting CB #3 to CB #4. Due to the elevation of the outflow pipe in relation to the system’s other pipes, the system will drain between rain events, even if the permeability of native soils limits infiltration.

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Figure 5: Diagram of the Grove St subsurface gravel filter

The Kettlebell System, presented in Figure 6, has a simpler design with a single 12-inch diameter, slotted, HDPE pipe running through the center of the gravel filtration layer between a catch basin (CB #1) and a manhole (MH). According to the design, surface runoff drains to CB #1 from the surrounding parking lot and an inline network of three other catch basins located throughout the paved portion of the watershed. The runoff then enters the slotted inlet pipe and drains down through the gravel layer, where it may be temporarily stored until it can infiltrate into the surrounding native soils underneath the parking lot. The Kettlebell SGF’s filtration layer is approximately 60 feet long by 30 feet wide and consists of a 2.5-foot deep layer of ¾-inch crushed stone. When the system’s storage fills, water bypasses the gravel filter layer and flows directly to Berry Brook through the 12-inch diameter, corrugated metal, outlet pipe attached to CB #1. Detailed design diagrams for the system are presented in Figure 62 and Figure 63 in the Appendix. CB #2 CB #4 CB #1 CB #3 CB #2 NativeSoil Gravel Filter Layer

AsphaltSubbase Asphalt 6” slotted low flow pipe 6” slotted high flow pipe 6” slotted low

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Figure 6: Diagram of Kettlebell subsurface gravel filter

Figure 7: The siphon in the Kettlebell SGF

Berry Brook

Outlet pipe

Gravel Filter Layer CB #1

MH Asphalt Subbase

Gravel Filter Layer

Native Soil Asphalt

12” slotted inlet pipe 12” drainage network pipe

12” high flow bypass pipe Asphalt subbase Asphalt Siphon CB #1

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Unlike the Grove St SGF, the Kettlebell system can only drain by infiltration because the invert of the system’s outlet is at a higher elevation than the inlet pipe. The elevation of the outlet could not be lowered due to the small elevation difference between the outlet and the water surface elevation (WSE) of Berry Brook. If the outlet were to be lowered, it would be below the WSE of the Brook during high flows and could allow water to back up into system. In order to drain the system between rain events, a 1-inch diameter, PVC pipe was installed along the base of the gravel layer, run up through the outlet pipe to the brook, and extended approximately 40 feet downstream to a location where the water level is significantly lower than the bottom elevation of the gravel layer (see Figure 7). The pipe is designed to act as a siphon for draining the water level down between storms. When the water surface in the system rises above the top of the PVC pipe, water starts to flow through the pipe and out of the system. Theoretically, as the water level drops below the top of the pipe, the system will continue to drain because the

difference in elevation between the bottom of the gravel storage zone and the downstream outlet of the PVC pipe should create a positive suction head to maintain flow through the siphon.