Tables: Dry Wells (Small-Scale GI)¶
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1. Table 1¶
| N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards | ||
|---|---|---|
| Green Infrastructure | Yes | |
| Stormwater Runoff Quantity | Not Allowed | |
| Groundwater Recharge | Yes | |
| % | Stormwater Runoff Quality | Not Allowed |
| Groundwater Recharge Mechanism and Corresponding Criteria | ||
| --- | --- | |
| Infiltration | ||
| Maximum Contributory Drainage Area | 1 acre | |
| Maximum Design Volume | Water Quality Design Storm Volume | |
| Maximum Drain Time | 72 hours, Using Slowest Design Permeability Rate | |
| Permeability Rate Factor of Safety | 2 | |
| Minimum Design Permeability Rate of the Subsoil | 0.5 inches/hour | |
| Maximum Design Permeability Rate | 10 inches/hour | |
| Soil Testing Consistent with Chapter 12: Soil Testing Criteria | Required | |
| Minimum Distance between Dry Well Bottom and Seasonal High Water Table | 2 feet |
Source context and notes
Source revision: January 2026. # 9.2 DRY WELLS [Figure: img-0.jpeg (original PDF, p. 1)](SOURCE.pdf#page=1) Dry wells are subsurface stormwater facilities that are used to collect and temporarily store stormwater runoff generated by clean rooftops; runoff is discharged through infiltration into the subsoil. Dry wells may be used to comply with the groundwater recharge design and performance standard of the Stormwater Management rules. Additionally, they may also be used to reduce the volume of stormwater runoff generated by a clean roof from the Water Quality Design Storm. Dry wells are assigned a 0% total suspended solids (TSS) removal rate. | N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards | | | | --- | --- | --- | | | Green Infrastructure | Yes | | | Stormwater Runoff Quantity | Not Allowed | | | Groundwater Recharge | Yes | | % | Stormwater Runoff Quality | Not Allowed | | Groundwater Recharge Mechanism and Corresponding Criteria | | | --- | --- | | Infiltration | | | Maximum Contributory Drainage Area | 1 acre | | Maximum Design Volume | Water Quality Design Storm Volume | | Maximum Drain Time | 72 hours, Using Slowest Design Permeability Rate | | Permeability Rate Factor of Safety | 2 | | Minimum Design Permeability Rate of the Subsoil | 0.5 inches/hour | | Maximum Design Permeability Rate | 10 inches/hour | | Soil Testing Consistent with Chapter 12: Soil Testing Criteria | Required | | Minimum Distance between Dry Well Bottom and Seasonal High Water Table | 2 feet |2. Table 2¶
| Depth to the SHWT | 10 ft |
|---|---|
| Tested Permeability of Subsoil = | 4 in/hr |
Source context and notes
## Designing a Dry Well The following examples illustrate how to use a dry well to reduce the volume generated by the Water Quality Design Storm. Example 1 features a dry well capable of collecting all of the rooftop runoff generated, whereas Example 2 depicts a dry well that captures only a portion. The examples show possible configurations and flow paths and are not intended to limit the design. Example 1: An undeveloped, 1-acre site is zoned for commercial use. On this site, the proposed development includes the construction of a 5,000 sf building. The stormwater runoff from the parking lot will be directed to the bioretention system shown via a vegetative filter strip and their design is beyond the scope of this example. The following parameters apply: | Depth to the SHWT | 10 ft | | --- | --- | | Tested Permeability of Subsoil = | 4 in/hr | Design a structural chamber dry well, containing stone fill, that will meet the criteria for the stormwater runoff quality volume reduction if all of the rooftop runoff is directed to it, as shown in the illustration below.3. Table 3¶
| Inflow Area = 5,000 sf,100.00% Impervious, Inflow Depth = 1.03" | |||
|---|---|---|---|
| Inflow = 0.33 cfs @ 1.09 hrs, Volume= 431 cf | |||
| Outflow = 0.01 cfs @ 2.05 hrs, Volume= 431 cf, Atten= 98%, Lag= 57.3 min | |||
| Discarded = 0.01 cfs @ 0.60 hrs, Volume= 431 cf | |||
| Primary = 0.00 cfs @ 2.05 hrs, Volume= 0 cf | |||
| Routing by Stor-Ind method, Time Span= 0.00-58.00 hrs, dt= 0.05 hrs | |||
| Peak Elev= 6.33' @ 2.07 hrs Surf.Area= 154 sf Storage= 390 cf | |||
| Plug-Flow detention time= 464.3 min calculated for 431 cf (100% of inflow) | |||
| Center-of-Mass det. time= 464.7 min (535.0 - 70.3) | |||
| Volume | Invert | Avail.Storage | Storage Description |
| #1 | 0.00' | 431 cf | 14.00'D x 7.00'H Vertical Cone/Cylinder 1,078 cf Overall x 40.0% Voids |
| Device | Routing | Invert | Outlet Devices |
| #1 | Discarded | 0.00' | 2.00 in/hr Exfiltration over Surface area |
| #2 | Primary | 6.33' | 1.0" Vert. Orifice/Grate X 12.00 columns X 4 rows with 1.0" cc spacing C= 0.600 |
| Discarded OutFlow Max=0.01 cfs @ 0.60 hrs HW=0.07" (Free Discharge) ↑-1=Exfiltration (Exfiltration Controls 0.01 cfs) |
|||
| Primary OutFlow Max=0.00 cfs @ 2.05 hrs HW=6.33" (Free Discharge) ↑-2=Orifice/Grate (Orifice Controls 0.00 cfs @ 0.02 fps) |
Source context and notes
# Dry Wells (Small-Scale GI) Source revision: January 2026. WQDS Routing Summary Report | Inflow Area = 5,000 sf,100.00% Impervious, Inflow Depth = 1.03" | | | | | --- | --- | --- | --- | | Inflow = 0.33 cfs @ 1.09 hrs, Volume= 431 cf | | | | | Outflow = 0.01 cfs @ 2.05 hrs, Volume= 431 cf, Atten= 98%, Lag= 57.3 min | | | | | Discarded = 0.01 cfs @ 0.60 hrs, Volume= 431 cf | | | | | Primary = 0.00 cfs @ 2.05 hrs, Volume= 0 cf | | | | | Routing by Stor-Ind method, Time Span= 0.00-58.00 hrs, dt= 0.05 hrs | | | | | Peak Elev= 6.33' @ 2.07 hrs Surf.Area= 154 sf Storage= 390 cf | | | | | Plug-Flow detention time= 464.3 min calculated for 431 cf (100% of inflow) | | | | | Center-of-Mass det. time= 464.7 min (535.0 - 70.3) | | | | | Volume | Invert | Avail.Storage | Storage Description | | #1 | 0.00' | 431 cf | 14.00'D x 7.00'H Vertical Cone/Cylinder1,078 cf Overall x 40.0% Voids | | Device | Routing | Invert | Outlet Devices | | #1 | Discarded | 0.00' | 2.00 in/hr Exfiltration over Surface area | | #2 | Primary | 6.33' | 1.0" Vert. Orifice/Grate X 12.00 columns
X 4 rows with 1.0" cc spacing C= 0.600 | | Discarded OutFlow Max=0.01 cfs @ 0.60 hrs HW=0.07" (Free Discharge)
↑-1=Exfiltration (Exfiltration Controls 0.01 cfs) | | | | | Primary OutFlow Max=0.00 cfs @ 2.05 hrs HW=6.33" (Free Discharge)
↑-2=Orifice/Grate (Orifice Controls 0.00 cfs @ 0.02 fps) | | | | Source: HydroCAD® Summary Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission. WQDS Water Depth and Flow Rate Routing Report
4. Table 4¶
| Time (hours) | Elevation (feet) | Discarded (cfs) | Primary (cfs) |
|---|---|---|---|
| 0.00 | 0.00 | 0.00 | 0.00 |
| 1.00 | 1.24 | 0.01 | 0.00 |
| 2.00 | 6.32 | 0.01 | 0.00 |
| 3.00 | 5.97 | 0.01 | 0.00 |
| 4.00 | 5.55 | 0.01 | 0.00 |
| 5.00 | 5.13 | 0.01 | 0.00 |
| 6.00 | 4.72 | 0.01 | 0.00 |
| 7.00 | 4.30 | 0.01 | 0.00 |
| 8.00 | 3.88 | 0.01 | 0.00 |
Source context and notes
| Primary OutFlow Max=0.00 cfs @ 2.05 hrs HW=6.33" (Free Discharge)↑-2=Orifice/Grate (Orifice Controls 0.00 cfs @ 0.02 fps) | | | | Source: HydroCAD® Summary Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission. WQDS Water Depth and Flow Rate Routing Report | Time (hours) | Elevation (feet) | Discarded (cfs) | Primary (cfs) | | --- | --- | --- | --- | | 0.00 | 0.00 | 0.00 | 0.00 | | 1.00 | 1.24 | 0.01 | 0.00 | | 2.00 | 6.32 | 0.01 | 0.00 | | 3.00 | 5.97 | 0.01 | 0.00 | | 4.00 | 5.55 | 0.01 | 0.00 | | 5.00 | 5.13 | 0.01 | 0.00 | | 6.00 | 4.72 | 0.01 | 0.00 | | 7.00 | 4.30 | 0.01 | 0.00 | | 8.00 | 3.88 | 0.01 | 0.00 | Source: HydroCAD® Routing Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission. A structural chamber 7 feet in depth and 14 feet in diameter will provide sufficient volume to contain the WQDS volume below the invert of the overflow. ## Step 4: Estimated Drain Time Calculations
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