Tables: Small-Scale Infiltration Basins
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Table fragments retain their source context. Refer to the original page for layout, figures, merged headings and notes.
1. Table 1
BMP 2026, Ch 9.8, p. 1
| N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards |
|
|
|
Green Infrastructure |
Yes |
|
Water Quantity |
Yes, when designed as an on-line system |
|
Groundwater Recharge |
Yes |
| % |
Water Quality |
80% TSS Removal |
| Water Quality Mechanisms and Corresponding Criteria |
|
|
| --- |
--- |
|
| Settling |
|
|
| Storage Volume |
Entire Water Quality Design Storm Volume |
|
| Infiltration |
|
|
| Maximum Contributory Drainage Area |
2.5 acres |
|
| Maximum Design Storm Drain Time |
72 hours, Using Slowest Design Permeability Rate |
|
| Permeability Rate Factor of Safety |
2 |
|
| Minimum Subsoil Design Permeability Rate |
0.5 inches/hour |
|
| Maximum Design Permeability Rate |
10 inches/hour |
|
| Soil Testing Consistent with Chapter 12: Soil Testing Criteria |
Required |
|
| Minimum Distance between Basin Bottom and Seasonal High Water Table |
2 feet |
|
| Biological and Chemical Activity |
|
|
| Minimum Sand Layer Thickness |
6 inches |
|
| Minimum Sand Layer Permeability Rate |
20 inches/hour, tested per Chapter 12: Soil Testing Criteria |
|
| Maximum % Fines in Sand Layer |
15% |
|
Source context and notes
Source revision: January 2026.
# 9.8 SMALL-SCALE INFILTRATION BASINS
[Figure: img-0.jpeg (original PDF, p. 1)](SOURCE.pdf#page=1)
Small-scale infiltration basins are stormwater management systems constructed with highly permeable components designed to both maximize the removal of pollutants from stormwater and to promote groundwater recharge. Pollutants are treated through settling, filtration of the runoff through and biological and chemical activity within, the components. The total suspended solids (TSS) removal rate is 80%.
| N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards | | |
| --- | --- | --- |
| | Green Infrastructure | Yes |
| | Water Quantity | Yes, when designed as an on-line system |
| | Groundwater Recharge | Yes |
| % | Water Quality | 80% TSS Removal |
| Water Quality Mechanisms and Corresponding Criteria | |
| --- | --- |
| Settling | |
| Storage Volume | Entire Water Quality Design Storm Volume |
| Infiltration | |
| Maximum Contributory Drainage Area | 2.5 acres |
| Maximum Design Storm Drain Time | 72 hours, Using Slowest Design Permeability Rate |
| Permeability Rate Factor of Safety | 2 |
| Minimum Subsoil Design Permeability Rate | 0.5 inches/hour |
| Maximum Design Permeability Rate | 10 inches/hour |
| Soil Testing Consistent with Chapter 12: Soil Testing Criteria | Required |
| Minimum Distance between Basin Bottom and Seasonal High Water Table | 2 feet |
| Biological and Chemical Activity | |
| Minimum Sand Layer Thickness | 6 inches |
| Minimum Sand Layer Permeability Rate | 20 inches/hour, tested per Chapter 12: Soil Testing Criteria |
| Maximum % Fines in Sand Layer | 15% |
2. Table 2
BMP 2026, Ch 9.8, p. 14
| Inflow Drainage Area = |
1 ac (100% impervious) |
| Pavement NRCS Curve Number (CN) = |
98 |
| Tested Sand Permeability Rate = |
40 in/hr |
| Tested Subsoil Permeability Rate = |
8 in/hr |
| Maximum Depth of Runoff to be Infiltrated = |
2 ft |
| Sand Layer Depth = |
6 in |
Source context and notes
Source revision: January 2026.
# Designing a Small-Scale Infiltration Basin
The following examples show how to design various small-scale infiltration basins to treat the runoff generated by the WQDS. The examples below are two of many possible ways to configure these basins and are not intended to limit the design.
Example 1: For one acre of regulated motor vehicle surface, design a small-scale infiltration basin to infiltrate the runoff generated by the WQDS. Runoff will receive pretreatment by a forebay designed in accordance with the criteria established in this chapter. Runoff volumes in excess of the WQDS generated volume will discharge via an emergency spillway. The following parameters apply:
| Inflow Drainage Area = | 1 ac (100% impervious) |
| --- | --- |
| Pavement NRCS Curve Number (CN) = | 98 |
| Tested Sand Permeability Rate = | 40 in/hr |
| Tested Subsoil Permeability Rate = | 8 in/hr |
| Maximum Depth of Runoff to be Infiltrated = | 2 ft |
| Sand Layer Depth = | 6 in |
## Step 1: Runoff Calculations
Using the NRCS method described in the National Engineering Handbook, Part 630 (NEH) discussed in Chapter 5, the runoff volume for the WQDS was calculated to be 3,755 cf.
## Step 2: Forebay Sizing
3. Table 3
BMP 2026, Ch 9.8, p. 15
| Inflow Area = |
43,560 sf,100.00% Impervious, Inflow Depth = 1.03" |
|
|
| Inflow = |
2.66 cfs @ |
1.13 hrs, Volume= |
3,755 cf |
| Outflow = |
0.14 cfs @ |
0.75 hrs, Volume= |
3,755 cf, Atten= 95%, Lag= 0.0 min |
| Discarded = |
0.14 cfs @ |
0.75 hrs, Volume= |
3,755 cf |
| Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs |
|
|
|
| Peak Elev= 2.00' @ 1.94 hrs Surf.Area= 1,500 sf Storage= 3,005 cf |
|
|
|
| Plug-Flow detention time= 187.9 min calculated for 3,752 cf (100% of inflow) |
|
|
|
| Center-of-Mass det. time= 188.1 min (261.2 - 73.1) |
|
|
|
| Volume |
Invert |
Avail.Storage |
Storage Description |
| #1 |
0.00' |
4,500 cf |
Custom Stage Data (Prismatic) Listed below (Recalc) |
| Elevation (feet) |
Surf.Area (sq-ft) |
Inc.Store (cubic-feet) |
Cum.Store (cubic-feet) |
| 0.00 |
1,500 |
0 |
0 |
| 1.00 |
1,500 |
1,500 |
1,500 |
| 2.00 |
1,500 |
1,500 |
3,000 |
| 3.00 |
1,500 |
1,500 |
4,500 |
| Device |
Routing |
Invert |
Outlet Devices |
| #1 |
Discarded |
0.00' |
4.00 in/hr Exfiltration over Surface area |
| Discarded OutFlow Max=0.14 cfs @ 0.75 hrs HW=0.04' (Free Discharge) |
|
|
|
| †-1=Exfiltration (Exfiltration Controls 0.14 cfs) |
|
|
|
Source context and notes
# Small-Scale Infiltration Basins
Source revision: January 2026.
An initial routing calculation for the basin is performed. It is assumed the basin shape is a rectangular prism with vertical sides.
## Summary Report:
| Inflow Area = | 43,560 sf,100.00% Impervious, Inflow Depth = 1.03" | | |
| --- | --- | --- | --- |
| Inflow = | 2.66 cfs @ | 1.13 hrs, Volume= | 3,755 cf |
| Outflow = | 0.14 cfs @ | 0.75 hrs, Volume= | 3,755 cf, Atten= 95%, Lag= 0.0 min |
| Discarded = | 0.14 cfs @ | 0.75 hrs, Volume= | 3,755 cf |
| Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs | | | |
| Peak Elev= 2.00' @ 1.94 hrs Surf.Area= 1,500 sf Storage= 3,005 cf | | | |
| Plug-Flow detention time= 187.9 min calculated for 3,752 cf (100% of inflow) | | | |
| Center-of-Mass det. time= 188.1 min (261.2 - 73.1) | | | |
| Volume | Invert | Avail.Storage | Storage Description |
| #1 | 0.00' | 4,500 cf | Custom Stage Data (Prismatic) Listed below (Recalc) |
| Elevation (feet) | Surf.Area (sq-ft) | Inc.Store (cubic-feet) | Cum.Store (cubic-feet) |
| 0.00 | 1,500 | 0 | 0 |
| 1.00 | 1,500 | 1,500 | 1,500 |
| 2.00 | 1,500 | 1,500 | 3,000 |
| 3.00 | 1,500 | 1,500 | 4,500 |
| Device | Routing | Invert | Outlet Devices |
| #1 | Discarded | 0.00' | 4.00 in/hr Exfiltration over Surface area |
| Discarded OutFlow Max=0.14 cfs @ 0.75 hrs HW=0.04' (Free Discharge) | | | |
| †-1=Exfiltration (Exfiltration Controls 0.14 cfs) | | | |
Source: HydroCAD® Summary Report; HydroCAD is a register trademark of HydroCAD Software Solutions LLC. Used with permission
## Routing Report Excerpt:
| Time (hours) | Elevation (feet) | Discarded (cfs) |
| --- | --- | --- |
| 1.00 | 0.25 | 0.14 |
| 1.05 | 0.42 | 0.14 |
| 1.10 | 0.67 | 0.14 |
| 1.15 | 0.96 | 0.14 |
| 1.20 | 1.23 | 0.14 |
| 1.25 | 1.44 | 0.14 |
| 1.30 | 1.58 | 0.14 |
| 1.35 | 1.68 | 0.14 |
| 1.40 | 1.75 | 0.14 |
| 1.45 | 1.79 | 0.14 |
| 1.50 | 1.83 | 0.14 |
| 1.55 | 1.86 | 0.14 |
| 1.60 | 1.89 | 0.14 |
| 1.65 | 1.92 | 0.14 |
| 1.70 | 1.94 | 0.14 |
| 1.75 | 1.96 | 0.14 |
| 1.80 | 1.98 | 0.14 |
| 1.85 | 1.99 | 0.14 |
| 1.90 | 2.00 | 0.14 |
| 1.95 | 2.00 | 0.14 |
| 2.00 | 2.00 | 0.14 |
| 2.05 | 2.00 | 0.14 |
| 2.10 | 1.99 | 0.14 |
| 2.15 | 1.98 | 0.14 |
| 2.20 | 1.97 | 0.14 |
4. Table 4
BMP 2026, Ch 9.8, p. 15
| Time (hours) |
Elevation (feet) |
Discarded (cfs) |
| 1.00 |
0.25 |
0.14 |
| 1.05 |
0.42 |
0.14 |
| 1.10 |
0.67 |
0.14 |
| 1.15 |
0.96 |
0.14 |
| 1.20 |
1.23 |
0.14 |
| 1.25 |
1.44 |
0.14 |
| 1.30 |
1.58 |
0.14 |
| 1.35 |
1.68 |
0.14 |
| 1.40 |
1.75 |
0.14 |
| 1.45 |
1.79 |
0.14 |
| 1.50 |
1.83 |
0.14 |
| 1.55 |
1.86 |
0.14 |
| 1.60 |
1.89 |
0.14 |
| 1.65 |
1.92 |
0.14 |
| 1.70 |
1.94 |
0.14 |
| 1.75 |
1.96 |
0.14 |
| 1.80 |
1.98 |
0.14 |
| 1.85 |
1.99 |
0.14 |
| 1.90 |
2.00 |
0.14 |
| 1.95 |
2.00 |
0.14 |
| 2.00 |
2.00 |
0.14 |
| 2.05 |
2.00 |
0.14 |
| 2.10 |
1.99 |
0.14 |
| 2.15 |
1.98 |
0.14 |
| 2.20 |
1.97 |
0.14 |
Source context and notes
| #1 | Discarded | 0.00' | 4.00 in/hr Exfiltration over Surface area |
| Discarded OutFlow Max=0.14 cfs @ 0.75 hrs HW=0.04' (Free Discharge) | | | |
| †-1=Exfiltration (Exfiltration Controls 0.14 cfs) | | | |
Source: HydroCAD® Summary Report; HydroCAD is a register trademark of HydroCAD Software Solutions LLC. Used with permission
## Routing Report Excerpt:
| Time (hours) | Elevation (feet) | Discarded (cfs) |
| --- | --- | --- |
| 1.00 | 0.25 | 0.14 |
| 1.05 | 0.42 | 0.14 |
| 1.10 | 0.67 | 0.14 |
| 1.15 | 0.96 | 0.14 |
| 1.20 | 1.23 | 0.14 |
| 1.25 | 1.44 | 0.14 |
| 1.30 | 1.58 | 0.14 |
| 1.35 | 1.68 | 0.14 |
| 1.40 | 1.75 | 0.14 |
| 1.45 | 1.79 | 0.14 |
| 1.50 | 1.83 | 0.14 |
| 1.55 | 1.86 | 0.14 |
| 1.60 | 1.89 | 0.14 |
| 1.65 | 1.92 | 0.14 |
| 1.70 | 1.94 | 0.14 |
| 1.75 | 1.96 | 0.14 |
| 1.80 | 1.98 | 0.14 |
| 1.85 | 1.99 | 0.14 |
| 1.90 | 2.00 | 0.14 |
| 1.95 | 2.00 | 0.14 |
| 2.00 | 2.00 | 0.14 |
| 2.05 | 2.00 | 0.14 |
| 2.10 | 1.99 | 0.14 |
| 2.15 | 1.98 | 0.14 |
| 2.20 | 1.97 | 0.14 |
Source: HydroCAD® Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission.
The routing report excerpt depicted at left shows that the maximum depth of stormwater runoff generated by the WQDS will be 2 ft when the basin bottom area, or "footprint" is 1,500 sf.
Because the maximum slope of the earthen embankments may not be steeper than 3:1, the small-scale infiltration basin shape cannot be a simple rectangular prism. Therefore, the shape of the basin will initially be set as a trapezoidal prism, as shown below. The infiltration area is shown in blue, the side slopes in pink and the end slopes in yellow. The dimensions of this assumed shape can be calculated by computer programs based on the depth and side slopes.
5. Table 5
BMP 2026, Ch 9.8, p. 16
| Inflow Area = 43,560 sf,100.00% Impervious, Inflow Depth = 1.03" |
|
|
|
| Inflow = 2.66 cfs @ 1.13 hrs, Volume= 3,755 cf |
|
|
|
| Outflow = 0.19 cfs @ 1.89 hrs, Volume= 3,755 cf, Atten= 93%, Lag= 45.3 min |
|
|
|
| Discarded = 0.19 cfs @ 1.89 hrs, Volume= 3,755 cf |
|
|
|
| Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs |
|
|
|
| Peak Elev= 1.80' @ 1.89 hrs Surf.Area= 2,045 sf Storage= 2,911 cf |
|
|
|
| Plug-Flow detention time= 155.5 min calculated for 3,752 cf (100% of inflow) |
|
|
|
| Center-of-Mass det. time= 155.7 min (228.8 - 73.1) |
|
|
|
| Volume Invert Avail.Storage Storage Description |
|
|
|
| #1 0.00' 5,790 cf Custom Stage Data (Prismatic) Listed below (Recalc) |
|
|
|
| Elevation (feet) Surf.Area (sq-ft) Inc.Store (cubic-feet) Cum.Store (cubic-feet) |
|
|
|
| 0.00 |
1,200 |
0 |
0 |
| 1.00 |
1,656 |
1,428 |
1,428 |
| 2.00 |
2,142 |
1,899 |
3,327 |
| 3.00 |
2,784 |
2,463 |
5,790 |
| Device Routing Invert Outlet Devices |
|
|
|
| #1 Discarded 0.00' 4.00 in/hr Exfiltration over Surface area |
|
|
|
| Discarded OutFlow Max=0.19 cfs @ 1.89 hrs HW=1.80' (Free Discharge) |
|
|
|
| +1=Exfiltration (Exfiltration Controls 0.19 cfs) |
|
|
|
Source context and notes
Source revision: January 2026.
Example 1 - Geometry
[Figure: img-9.jpeg (original PDF, p. 16)](SOURCE.pdf#page=16)
Summary Report:
| Inflow Area = 43,560 sf,100.00% Impervious, Inflow Depth = 1.03" | | | |
| --- | --- | --- | --- |
| Inflow = 2.66 cfs @ 1.13 hrs, Volume= 3,755 cf | | | |
| Outflow = 0.19 cfs @ 1.89 hrs, Volume= 3,755 cf, Atten= 93%, Lag= 45.3 min | | | |
| Discarded = 0.19 cfs @ 1.89 hrs, Volume= 3,755 cf | | | |
| Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs | | | |
| Peak Elev= 1.80' @ 1.89 hrs Surf.Area= 2,045 sf Storage= 2,911 cf | | | |
| Plug-Flow detention time= 155.5 min calculated for 3,752 cf (100% of inflow) | | | |
| Center-of-Mass det. time= 155.7 min (228.8 - 73.1) | | | |
| Volume Invert Avail.Storage Storage Description | | | |
| #1 0.00' 5,790 cf Custom Stage Data (Prismatic) Listed below (Recalc) | | | |
| Elevation (feet) Surf.Area (sq-ft) Inc.Store (cubic-feet) Cum.Store (cubic-feet) | | | |
| 0.00 | 1,200 | 0 | 0 |
| 1.00 | 1,656 | 1,428 | 1,428 |
| 2.00 | 2,142 | 1,899 | 3,327 |
| 3.00 | 2,784 | 2,463 | 5,790 |
| Device Routing Invert Outlet Devices | | | |
| #1 Discarded 0.00' 4.00 in/hr Exfiltration over Surface area | | | |
| Discarded OutFlow Max=0.19 cfs @ 1.89 hrs HW=1.80' (Free Discharge) | | | |
| +1=Exfiltration (Exfiltration Controls 0.19 cfs) | | | |
Source: HydroCAD® Summary Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission.
The summary above shows that with a reduced footprint of 1,200 sf, the greatest depth of stormwater runoff temporarily stored above the basin bottom is 1.8 ft, which is less than the 2 ft maximum depth of stormwater runoff requirement. An excerpt from the tabular hydrograph is provided on the following page.
6. Table 6
BMP 2026, Ch 9.8, p. 17
| Time (hours) |
Inflow (cfs) |
Storage (cubic-feet) |
Elevation (feet) |
Discarded (cfs) |
| 1.00 |
1.20 |
397 |
0.31 |
0.12 |
| 1.05 |
1.95 |
658 |
0.50 |
0.13 |
| 1.10 |
2.52 |
1,036 |
0.75 |
0.14 |
| 1.15 |
2.63 |
1,472 |
1.03 |
0.15 |
| 1.20 |
2.15 |
1,874 |
1.26 |
0.17 |
| 1.25 |
1.55 |
2,177 |
1.43 |
0.17 |
| 1.30 |
1.10 |
2,385 |
1.54 |
0.18 |
| 1.35 |
0.79 |
2,523 |
1.61 |
0.18 |
| 1.40 |
0.59 |
2,615 |
1.65 |
0.18 |
| 1.45 |
0.48 |
2,677 |
1.69 |
0.18 |
| 1.50 |
0.43 |
2,725 |
1.71 |
0.19 |
| 1.55 |
0.40 |
2,766 |
1.73 |
0.19 |
| 1.60 |
0.37 |
2,802 |
1.75 |
0.19 |
| 1.65 |
0.33 |
2,831 |
1.76 |
0.19 |
| 1.70 |
0.31 |
2,855 |
1.77 |
0.19 |
| 1.75 |
0.30 |
2,876 |
1.78 |
0.19 |
| 1.80 |
0.29 |
2,895 |
1.79 |
0.19 |
| 1.85 |
0.24 |
2,908 |
1.80 |
0.19 |
| 1.90 |
0.17 |
2,911 |
1.80 |
0.19 |
| 1.95 |
0.13 |
2,904 |
1.80 |
0.19 |
| 2.00 |
0.11 |
2,892 |
1.79 |
0.19 |
Source context and notes
# Small-Scale Infiltration Basins
Source revision: January 2026.
Tabular Hydrograph Report Excerpt:
| Time (hours) | Inflow (cfs) | Storage (cubic-feet) | Elevation (feet) | Discarded (cfs) |
| --- | --- | --- | --- | --- |
| 1.00 | 1.20 | 397 | 0.31 | 0.12 |
| 1.05 | 1.95 | 658 | 0.50 | 0.13 |
| 1.10 | 2.52 | 1,036 | 0.75 | 0.14 |
| 1.15 | 2.63 | 1,472 | 1.03 | 0.15 |
| 1.20 | 2.15 | 1,874 | 1.26 | 0.17 |
| 1.25 | 1.55 | 2,177 | 1.43 | 0.17 |
| 1.30 | 1.10 | 2,385 | 1.54 | 0.18 |
| 1.35 | 0.79 | 2,523 | 1.61 | 0.18 |
| 1.40 | 0.59 | 2,615 | 1.65 | 0.18 |
| 1.45 | 0.48 | 2,677 | 1.69 | 0.18 |
| 1.50 | 0.43 | 2,725 | 1.71 | 0.19 |
| 1.55 | 0.40 | 2,766 | 1.73 | 0.19 |
| 1.60 | 0.37 | 2,802 | 1.75 | 0.19 |
| 1.65 | 0.33 | 2,831 | 1.76 | 0.19 |
| 1.70 | 0.31 | 2,855 | 1.77 | 0.19 |
| 1.75 | 0.30 | 2,876 | 1.78 | 0.19 |
| 1.80 | 0.29 | 2,895 | 1.79 | 0.19 |
| 1.85 | 0.24 | 2,908 | 1.80 | 0.19 |
| 1.90 | 0.17 | 2,911 | 1.80 | 0.19 |
| 1.95 | 0.13 | 2,904 | 1.80 | 0.19 |
| 2.00 | 0.11 | 2,892 | 1.79 | 0.19 |
Source: HydroCAD® Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission.
## Step 4: Estimated Drain Time Calculation
As previously stated, the drain time of the basin is determined by the design permeability rate of the subsoil. Note that only the infiltration area, i.e. the footprint, can be credited for infiltration, meaning infiltration may not be applied to the side slopes. The drain time calculation is based on the area of the footprint, which is 1,200 sf.
7. Table 7
BMP 2026, Ch 9.8, p. 20
| New Jersey Groundwater Recharge Research Institute (13 December 2011) |
Annual Groundwater Recharge Analysis (based on GSR-32) |
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Project Name: |
Sample Project |
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Select Township ↓ |
Antique Annual P (in) |
Climatic Factor |
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Description: |
This is a test application |
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MORCER CO., HAMELTON TWP |
44.9 |
1.43 |
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Analysis Date: |
09/01/03 |
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| Pre-Developed Conditions |
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Post-Development Conditions |
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| Land Segment |
Area (acres) |
TR-65 Land Cover |
Soil |
Annual Recharge (in) |
Annual Recharge (cu.ft) |
Land Segment |
Area (acres) |
TR-65 Land Cover |
Soil |
Annual Recharge (in) |
| 1 |
0.25 |
Impervious areas |
Woodstown |
0.6 |
|
1 |
0.75 |
Impervious areas |
|
0.6 |
| 2 |
0.75 |
Open space |
Woodstown |
11.3 |
30,748 |
2 |
0.25 |
Open space |
Woodstown |
11.3 |
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3 |
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| 4 |
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5 |
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6 |
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8 |
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11 |
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| 12 |
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12 |
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13 |
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14 |
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15 |
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| Total = |
1.0 |
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Total Annual Recharge (in) |
Total Annual Recharge (cu.ft) |
Total = |
1.0 |
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Total Annual Recharge (in) |
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0.5 |
30,748 |
Annual Recharge Requirements Calculation ↓ |
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2.8 |
| Procedure to fill the Pre-Development and Post-Development Conditions Tables |
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% of Pre-Developed Annual Recharge to Preserve = |
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100% |
Department (area cu.ft) |
32,670 |
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| For each land segment, first enter the area, then select TR-65 Land Cover, then select Soil. Start from the top on the land and proceed downward. Don't leave blank rows (with A=0) in between your segment entries. Rows with A=0 will not be displayed or used in calculations. For impervious areas outside of standard lots select "impervious Areas" as the Land Cover. Soil type for impervious areas are only required if an infiltration facility will be built within these areas. |
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Post-Development Annual Recharge Deficit= |
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20,499 |
Grades Test) |
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Recharge Efficiency Parameters Calculations (area averages) |
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RWC= 2.99 |
(st) |
DRWC= 0.81 |
(st) |
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ERWC = 0.83 |
(st) |
EDRWC= 0.22 |
(st) |
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Source context and notes
# Small-Scale Infiltration Basins
Source revision: January 2026.
Example 2 – Post-Development Annual Recharge Deficit Calculation
| New Jersey Groundwater Recharge Research Institute (13 December 2011) | Annual Groundwater Recharge Analysis (based on GSR-32) | | | | | | Project Name: | Sample Project | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| | Select Township ↓ | Antique Annual P (in) | Climatic Factor | | | Description: | This is a test application | | | |
| | MORCER CO., HAMELTON TWP | 44.9 | 1.43 | | | Analysis Date: | 09/01/03 | | | |
| Pre-Developed Conditions | | | | | Post-Development Conditions | | | | | |
| Land Segment | Area (acres) | TR-65 Land Cover | Soil | Annual Recharge (in) | Annual Recharge (cu.ft) | Land Segment | Area (acres) | TR-65 Land Cover | Soil | Annual Recharge (in) |
| 1 | 0.25 | Impervious areas | Woodstown | 0.6 | | 1 | 0.75 | Impervious areas | | 0.6 |
| 2 | 0.75 | Open space | Woodstown | 11.3 | 30,748 | 2 | 0.25 | Open space | Woodstown | 11.3 |
| 3 | | | | | | 3 | | | | |
| 4 | | | | | | 4 | | | | |
| 5 | | | | | | 5 | | | | |
| 6 | | | | | | 6 | | | | |
| 7 | | | | | | 7 | | | | |
| 8 | | | | | | 8 | | | | |
| 9 | | | | | | 9 | | | | |
| 10 | | | | | | 10 | | | | |
| 11 | | | | | | 11 | | | | |
| 12 | | | | | | 12 | | | | |
| 13 | | | | | | 13 | | | | |
| 14 | | | | | | 14 | | | | |
| 15 | | | | | | 15 | | | | |
| Total = | 1.0 | | | Total Annual Recharge (in) | Total Annual Recharge (cu.ft) | Total = | 1.0 | | | Total Annual Recharge (in) |
| | | | | | 0.5 | 30,748 | Annual Recharge Requirements Calculation ↓ | | | 2.8 |
| Procedure to fill the Pre-Development and Post-Development Conditions Tables | | | | | % of Pre-Developed Annual Recharge to Preserve = | | 100% | Department (area cu.ft) | 32,670 | |
| For each land segment, first enter the area, then select TR-65 Land Cover, then select Soil. Start from the top on the land and proceed downward. Don't leave blank rows (with A=0) in between your segment entries. Rows with A=0 will not be displayed or used in calculations. For impervious areas outside of standard lots select "impervious Areas" as the Land Cover. Soil type for impervious areas are only required if an infiltration facility will be built within these areas. | | | | | Post-Development Annual Recharge Deficit= | | 20,499 | Grades Test) | | |
| | | | | | Recharge Efficiency Parameters Calculations (area averages) | | | | | |
| | | | | | RWC= 2.99 | (st) | DRWC= 0.81 | (st) | | |
| | | | | | ERWC = 0.83 | (st) | EDRWC= 0.22 | (st) | | |
The design volume for a basin treating the WQDS is 2,817 cf. Setting the exfiltration rate to 2 in/hr, results from stormwater modeling software show that an infiltration basin with a footprint of 1,250 sf will provide the required storage volume for the stormwater runoff generated by the WQDS and will yield a maximum depth of runoff of 1.99 ft, which is less than the 2 ft maximum allowable depth. For design purposes, 1 ft of freeboard is included in the software model. The summary report and routing table excerpt are provided on the following page.
8. Table 8
BMP 2026, Ch 9.8, p. 21
| Inflow Area = 32,670 sf,100.00% Impervious, Inflow Depth = 1.03" |
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| Inflow = 2.17 cfs @ 1.09 hrs, Volume= 2,817 cf |
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| Outflow = 0.06 cfs @ 0.65 hrs, Volume= 2,817 cf, Atten= 97%, Lag= 0.0 min |
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| Discarded = 0.06 cfs @ 0.65 hrs, Volume= 2,817 cf |
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| Primary = 0.00 cfs @ 0.00 hrs, Volume= 0 cf |
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| Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs |
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| Peak Elev= 1.99' @ 2.05 hrs Surf.Area= 1,250 sf Storage= 2,490 cf |
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| Plug-Flow detention time= 367.4 min calculated for 2,814 cf (100% of inflow) |
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| Center-of-Mass det. time= 367.7 min (438.0 - 70.3) |
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| Volume Invert Avail.Storage Storage Description |
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| #1 0.00' 3,750 cf Custom Stage Data (Prismatic) Listed below (Recalc) |
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| Elevation Surf.Area Inc.Store Cum.Store |
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| (feet) (subc-feet) (cubic-feet) |
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| 0.00 1,250 0 0 |
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| 3.00 1,250 3,750 3,750 |
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| Device Routing Invert Outlet Devices |
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| #1 Discarded 0.00' 2.00 in/hr Exfiltration over Surface area |
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| #2 Primary 2.00' 2.5" Vert. Orifice/Grate C= 0.600 |
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| Discarded OutFlow Max=0.06 cfs @ 0.65 hrs HW=0.03' (Free Discharge) |
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| +1=Exfiltration (Exfiltration Controls 0.06 cfs) |
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| Primary OutFlow Max=0.00 cfs @ 0.00 hrs HW=0.00' (Free Discharge) |
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| +2=Orifice/Grate (Controls 0.00 cfs) |
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Source context and notes
# Small-Scale Infiltration Basins
Source revision: January 2026.
WQDS Summary Report
| Inflow Area = 32,670 sf,100.00% Impervious, Inflow Depth = 1.03" | | | |
| --- | --- | --- | --- |
| Inflow = 2.17 cfs @ 1.09 hrs, Volume= 2,817 cf | | | |
| Outflow = 0.06 cfs @ 0.65 hrs, Volume= 2,817 cf, Atten= 97%, Lag= 0.0 min | | | |
| Discarded = 0.06 cfs @ 0.65 hrs, Volume= 2,817 cf | | | |
| Primary = 0.00 cfs @ 0.00 hrs, Volume= 0 cf | | | |
| Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs | | | |
| Peak Elev= 1.99' @ 2.05 hrs Surf.Area= 1,250 sf Storage= 2,490 cf | | | |
| Plug-Flow detention time= 367.4 min calculated for 2,814 cf (100% of inflow) | | | |
| Center-of-Mass det. time= 367.7 min (438.0 - 70.3) | | | |
| Volume Invert Avail.Storage Storage Description | | | |
| #1 0.00' 3,750 cf Custom Stage Data (Prismatic) Listed below (Recalc) | | | |
| Elevation Surf.Area Inc.Store Cum.Store | | | |
| (feet) (subc-feet) (cubic-feet) | | | |
| 0.00 1,250 0 0 | | | |
| 3.00 1,250 3,750 3,750 | | | |
| Device Routing Invert Outlet Devices | | | |
| #1 Discarded 0.00' 2.00 in/hr Exfiltration over Surface area | | | |
| #2 Primary 2.00' 2.5" Vert. Orifice/Grate C= 0.600 | | | |
| Discarded OutFlow Max=0.06 cfs @ 0.65 hrs HW=0.03' (Free Discharge) | | | |
| +1=Exfiltration (Exfiltration Controls 0.06 cfs) | | | |
| Primary OutFlow Max=0.00 cfs @ 0.00 hrs HW=0.00' (Free Discharge) | | | |
| +2=Orifice/Grate (Controls 0.00 cfs) | | | |
Source: HydroCAD® Summary Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission.
Routing Table Excerpt
| Time (hours) | Elevation (feet) | Outflow (cfs) | Discarded (cfs) |
| --- | --- | --- | --- |
| 0.00 | 0.00 | 0.00 | 0.00 |
| 0.50 | 0.01 | 0.02 | 0.02 |
| 1.00 | 0.39 | 0.06 | 0.06 |
| 1.50 | 1.80 | 0.06 | 0.06 |
| 2.00 | 1.99 | 0.06 | 0.06 |
| 2.50 | 1.93 | 0.06 | 0.06 |
| 3.00 | 1.84 | 0.06 | 0.06 |
| 3.50 | 1.76 | 0.06 | 0.06 |
| 4.00 | 1.68 | 0.06 | 0.06 |
| 4.50 | 1.59 | 0.06 | 0.06 |
9. Table 9
BMP 2026, Ch 9.8, p. 21
| Time (hours) |
Elevation (feet) |
Outflow (cfs) |
Discarded (cfs) |
| 0.00 |
0.00 |
0.00 |
0.00 |
| 0.50 |
0.01 |
0.02 |
0.02 |
| 1.00 |
0.39 |
0.06 |
0.06 |
| 1.50 |
1.80 |
0.06 |
0.06 |
| 2.00 |
1.99 |
0.06 |
0.06 |
| 2.50 |
1.93 |
0.06 |
0.06 |
| 3.00 |
1.84 |
0.06 |
0.06 |
| 3.50 |
1.76 |
0.06 |
0.06 |
| 4.00 |
1.68 |
0.06 |
0.06 |
| 4.50 |
1.59 |
0.06 |
0.06 |
Source context and notes
| +1=Exfiltration (Exfiltration Controls 0.06 cfs) | | | |
| Primary OutFlow Max=0.00 cfs @ 0.00 hrs HW=0.00' (Free Discharge) | | | |
| +2=Orifice/Grate (Controls 0.00 cfs) | | | |
Source: HydroCAD® Summary Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission.
Routing Table Excerpt
| Time (hours) | Elevation (feet) | Outflow (cfs) | Discarded (cfs) |
| --- | --- | --- | --- |
| 0.00 | 0.00 | 0.00 | 0.00 |
| 0.50 | 0.01 | 0.02 | 0.02 |
| 1.00 | 0.39 | 0.06 | 0.06 |
| 1.50 | 1.80 | 0.06 | 0.06 |
| 2.00 | 1.99 | 0.06 | 0.06 |
| 2.50 | 1.93 | 0.06 | 0.06 |
| 3.00 | 1.84 | 0.06 | 0.06 |
| 3.50 | 1.76 | 0.06 | 0.06 |
| 4.00 | 1.68 | 0.06 | 0.06 |
| 4.50 | 1.59 | 0.06 | 0.06 |
Source: HydroCAD® Routing Table; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission.
However, the calculated area must be checked using the NJGRS to see if it is large enough to meet the annual recharge deficit requirement. To do this, first enter 1,250 for the area of the proposed small-scale infiltration basin, which is represented by the variable ABMP, in Cell C5 on the BMP Calculations Worksheet of the NJGRS and set the BMP Effective Depth (dBMP) to 1.99 ft x 12 in/ft = 23.88 in (Cell C6). The value to be entered in Cell C7 for dBMPu, which is the vertical distance from the vegetated ground surface to the maximum water surface level in the BMP and is positive when the maximum water surface level is below the vegetated ground surface, is 12 in, which is the 1 ft of freeboard mentioned above. The next value entered is that for dEXC, which is the vertical distance
10. Table 10
BMP 2026, Ch 9.8, p. 22
| 1 Project Name Description Analysis Date BMP or LiD Type |
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| 2 Sample Project This is a test application 09/01/03 |
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| 3 Recharge BMP Input Parameters |
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4 Heat Zone Water capacity Calculated Parameters |
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5 Recharge Design Parameters |
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| Parameter |
Symbol |
Value |
Unit |
Parameter |
Symbol |
Value |
Unit |
Parameter |
Symbol |
Value |
Unit |
| BMP Area |
ABMP |
1290.0 |
sq ft |
Empty Portion of RVIC under Prod-D Natural Recharge |
ERVIC |
1.32 |
m |
Inches of Runoff to capture |
Qdesign |
0.96 |
m |
| BMP Effective Depth has is the design variable |
dBMP |
23.9 |
m |
ERVIC Modified to consider dEXC |
EDRVIC |
0.53 |
m |
Inches of Keertel to capture |
Pdesign |
1.17 |
m |
| Upper level of the BMP surface (negative if above ground) |
dBMPs |
12.0 |
m |
Empty Portion of RVIC under Infilt BMP |
RERVIC |
0.41 |
m |
Recharge Provided Avg. over Imp. Area |
|
28.4 |
m |
| Depth of lower surface of BMP, must be= dBMPs |
dEXC |
35.9 |
m |
|
Runoff Captured Avg. over Imp. Area |
|
29.7 |
m |
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| Post-development Land Segment Location of BMP |
SegBMP |
2 |
unitless |
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Annual BMP Recharge Volume Required |
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| Area Decif Locations dedicated to sedimentation |
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| 6 Recharge Replacement from Imp. Area |
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BMP Calculated Size Parameters |
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CALCULATION CHECK MESSAGES |
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ABMP/Amp |
Acetic |
0.04 |
unitless |
Volume Balance = Below Problems to satisfy Annual Recharge |
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BMP Volume |
VBMP |
2,400 |
sq ft |
dBMP Check=0.8 |
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dEXC Check=0.8 |
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Annual BMP Recharge Volume Provided |
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Source context and notes
# Small-Scale Infiltration Basins
Source revision: January 2026.
from the vegetated ground surface to the top of the sand layer in the bottom of the BMP, and is equal to 23.88 in (maximum runoff depth) + 12 in (freeboard) = 35.88 in. Lastly, we must identify the post-developed condition land segment from the Annual Recharge Worksheet in which this BMP is located, which is represented by the variable segBMP, and is 2 in this case. The NJGRS calculates the annual BMP recharge volume provided by the proposed small-scale infiltration basin is 77,328 cf, which is greater than the deficit volume (Vdef) of 20,499 cf, as shown in the image below.
Example 2 – Calculation of Groundwater Recharge Volume Provided by the BMP
| 1 Project Name Description Analysis Date BMP or LiD Type | | | | | | | | | | | |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 2 Sample Project This is a test application 09/01/03 | | | | | | | | | | | |
| 3 Recharge BMP Input Parameters | | | | 4 Heat Zone Water capacity Calculated Parameters | | | | 5 Recharge Design Parameters | | | |
| Parameter | Symbol | Value | Unit | Parameter | Symbol | Value | Unit | Parameter | Symbol | Value | Unit |
| BMP Area | ABMP | 1290.0 | sq ft | Empty Portion of RVIC under Prod-D Natural Recharge | ERVIC | 1.32 | m | Inches of Runoff to capture | Qdesign | 0.96 | m |
| BMP Effective Depth has is the design variable | dBMP | 23.9 | m | ERVIC Modified to consider dEXC | EDRVIC | 0.53 | m | Inches of Keertel to capture | Pdesign | 1.17 | m |
| Upper level of the BMP surface (negative if above ground) | dBMPs | 12.0 | m | Empty Portion of RVIC under Infilt BMP | RERVIC | 0.41 | m | Recharge Provided Avg. over Imp. Area | | 28.4 | m |
| Depth of lower surface of BMP, must be= dBMPs | dEXC | 35.9 | m | | Runoff Captured Avg. over Imp. Area | | 29.7 | m | | | |
| Post-development Land Segment Location of BMP | SegBMP | 2 | unitless | | | | | Annual BMP Recharge Volume Required | | | |
| Area Decif Locations dedicated to sedimentation | | | | | | | | | | | |
| 6 Recharge Replacement from Imp. Area | | | | BMP Calculated Size Parameters | | | | CALCULATION CHECK MESSAGES | | | |
| | | | | ABMP/Amp | Acetic | 0.04 | unitless | Volume Balance = Below Problems to satisfy Annual Recharge | | | |
| | | | | BMP Volume | VBMP | 2,400 | sq ft | dBMP Check=0.8 | | | |
| | | | | | | | | dEXC Check=0.8 | | | |
| | | | | | | | | Annual BMP Recharge Volume Provided | | | |
| | | | | | | | | | | | |
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## Step 3: Initial Drain Time Calculation
As previously stated, the drain time of the basin is determined by the design permeability rate of the subsoil.
$$
\text{Drain Time} = \frac{\text{WQDS Volume}}{\text{Infiltration Area} \times \text{Design Permeability Rate}} = \frac{2,817 \text{ cf} \times (12 \text{ in/ft})}{(1,250 \text{ sf} \times 2 \text{ in/hr})} = 13.52 \text{ hr}
$$
11. Table 11
BMP 2026, Ch 9.8, p. 23
|
A |
B |
C |
D |
E |
F |
G |
H |
I |
J |
K |
L |
| 6 |
Input Values |
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| 7 |
2.00 |
R |
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Recharge rate (permeability rate) (in/hr) |
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| 8 |
0.150 |
Sy |
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Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted |
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| 9 |
10.00 |
Kh |
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Horizontal hydraulic conductivity (in/hr) |
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| 10 |
25.000 |
x |
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Kh = 5xRecharee Rate (R) in the coastal plain: Kh=R outside the coastal plain |
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| 11 |
12.500 |
y |
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1/2 length of basin (x direction, in feet) |
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| 12 |
13.52 |
t |
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Duration of infiltration period (hours) |
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| 13 |
10.000 |
hi[0] |
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Initial thickness of saturated zone (feet) |
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| 14 |
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| 15 |
14.15 |
h(max) |
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Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) |
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| 16 |
9.15 |
Δh(max) |
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Maximum groundwater mounding (beneath center of basin at end of infiltration period) |
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Source context and notes
# Small-Scale Infiltration Basins
Source revision: January 2026.
surface of the required sand layer at 2.99 ft below ground level. The sand layer is 0.5 ft in thickness. Therefore, the SHWT is 6.51 ft below the bottom of the sand layer, which meets the separation requirement. However, a groundwater mounding analysis is required for any infiltration practice to check whether infiltration of stormwater creates any adverse hydraulic impacts to . For the purpose of this example, assume the basin footprint measures 50 ft by 25 ft and the project area lies within the portion of Hamilton Township that is within the coastal plain. Following the requirements for using the Hantush Spreadsheet as addressed in Chapter 13, the following results were calculated.
Hantush Spreadsheet Input Values and Calculated Height of Groundwater Mounding
| | A | B | C | D | E | F | G | H | I | J | K | L |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 6 | Input Values | | | | | | | | | | | |
| 7 | 2.00 | R | | Recharge rate (permeability rate) (in/hr) | | | | | | | | |
| 8 | 0.150 | Sy | | Specific yield, Sy (dimensionless)default value is 0.15; max value is 0.2 provided that a lab test data is submitted | | | | | | | | |
| 9 | 10.00 | Kh | | Horizontal hydraulic conductivity (in/hr) | | | | | | | | |
| 10 | 25.000 | x | | Kh = 5xRecharee Rate (R) in the coastal plain: Kh=R outside the coastal plain | | | | | | | | |
| 11 | 12.500 | y | | 1/2 length of basin (x direction, in feet) | | | | | | | | |
| 12 | 13.52 | t | | Duration of infiltration period (hours) | | | | | | | | |
| 13 | 10.000 | hi[0] | | Initial thickness of saturated zone (feet) | | | | | | | | |
| 14 | | | | | | | | | | | | |
| 15 | 14.15 | h(max) | | Maximum thickness of saturated zone (beneath center of basin at end of infiltration period) | | | | | | | | |
| 16 | 9.15 | Δh(max) | | Maximum groundwater mounding (beneath center of basin at end of infiltration period) | | | | | | | | |
[Figure: img-12.jpeg (original PDF, p. 23)](SOURCE.pdf#page=23)
The maximum mounding height is 4.19 feet. In the Hantush Spreadsheet, the SHWT forms the x-axis. For this example, ground level is 10 ft above the SHWT, meaning, the top of the groundwater mounding will be 5.81 feet below the surface. As stated above, the surface of the basin sand layer is 2.99 ft below the adjacent ground surface. Therefore, the top of the groundwater mounding is 2.82 ft below the top of the sand layer forming the basin bottom and will not interfere with the infiltration of stormwater runoff, as depicted in the image to the right. However, the design engineer must also check if there are any underground structures within the extents of the groundwater mounding curve plotted by the Hantush Spreadsheet, and the temporary rise in the elevation of the groundwater table, caused by the small-scale infiltration basin, must not adversely impact any underground structure.
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