Tables: Pervious Paving Systems (Small-Scale GI)
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1. Table 1
BMP 2026, Ch 9.6, p. 1
| N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards |
|
|
| 🔄 |
Green Infrastructure |
Yes |
| 🔥 |
Stormwater Runoff Quantity |
Yes |
| 📷 |
Groundwater Recharge |
Yes, for systems designed to infiltrate into the subsoil |
| 🔅 |
Stormwater Runoff Quality |
80% TSS Removal |
| Stormwater Runoff Quality Mechanisms and Corresponding Criteria |
|
|
| --- |
--- |
|
| Filtering |
|
|
| Maximum Area of Additional Inflow |
≤ 3 x the Area of Pervious Paving System |
|
| Maximum Drain Time |
72 hours, Using Slowest Design Permeability Rate |
|
Porous Asphalt,
Pervious Concrete and
Permeable Interlocking Paver Units |
6.4 inches/hour Minimum Infiltration Rate |
|
Source context and notes
Source revision: January 2026.
# 9.6 PERVIOUS PAVING SYSTEMS
[Figure: img-0.jpeg (original PDF, p. 1)](SOURCE.pdf#page=1)
A pervious paving system is a stormwater management facility used to address the impacts of land development. The system consists of a durable, permeable surface course, which allows stormwater runoff to move through it; this surface course is placed over a transition layer and a storage bed of open-graded, i.e., devoid of fine particles, aggregate. There are two types: underdrained systems and systems designed to infiltrate into the subsoil. When designed in accordance with this chapter, the total suspended solid (TSS) removal rate is 80%.
| N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards | | |
| --- | --- | --- |
| 🔄 | Green Infrastructure | Yes |
| 🔥 | Stormwater Runoff Quantity | Yes |
| 📷 | Groundwater Recharge | Yes, for systems designed to infiltrate into the subsoil |
| 🔅 | Stormwater Runoff Quality | 80% TSS Removal |
| Stormwater Runoff Quality Mechanisms and Corresponding Criteria | |
| --- | --- |
| Filtering | |
| Maximum Area of Additional Inflow | ≤ 3 x the Area of Pervious Paving System |
| Maximum Drain Time | 72 hours, Using Slowest Design Permeability Rate |
| Porous Asphalt,
Pervious Concrete and
Permeable Interlocking Paver Units | 6.4 inches/hour Minimum Infiltration Rate |
2. Table 2
BMP 2026, Ch 9.6, p. 18
| Inflow Drainage Area |
Pavement Type |
Acreage |
CN Value |
| Driveway and Aisles |
Conventional Asphalt |
0.50 |
98 |
| Central Parking Stalls |
Permeable Asphalt |
0.25 |
98 |
| Left Parking Stalls |
Permeable Asphalt |
0.125 |
98 |
| Right Parking Stalls |
Permeable Asphalt |
0.125 |
98 |
| Rooftop |
Roof |
0.25 |
98 |
Source context and notes
- Post-construction testing of the permeable interlocking paver unit surface course is required and must conform to the methods of ASTM C1781: Standard Test Method for Surface Infiltration Rate of Permeable Unit Pavement Systems. At least three locations must be used for the test, and they should be spaced evenly across the pervious paving system. Failure to achieve the minimum design infiltration rate of the surface course at one or more location indicates the system cannot be put in service until the system is corrected to yield all passing values. Unlike the test methodology outlined in the ASTM standard, the test results must not be averaged. The maintenance plan must include a log for recording each location and its test result for future reference.
# Designing Pervious Paving Systems
Below are two examples on the same site showing the basics of designing a pervious paving system to treat the runoff produced by the Water Quality Design Storm (WQDS); these examples are only two of many ways to configure these systems and are not intended to limit the design. The first example is for a system designed to infiltrate and the second for an underdrained system. Note that both examples show the use of pervious paving systems only in the parking stalls; however, these systems may be used in higher traffic areas as long as the system is designed to withstand the anticipated traffic loads.
Example 1 – Permeable Asphalt Parking Lot Designed to Infiltrate: A proposed commercial site includes a 1-acre parking lot and 0.25-acre roof. The parking stalls will be permeable asphalt and will manage the runoff generated by the WQDS that falls on both them and the adjacent aisles and driveway. Additionally, roof runoff, pretreated in accordance with the inflow and pretreatment requirements on Pages 3 and 4, will be piped directly to the system's storage bed for volume reduction credit for the WQDS. Runoff from all events larger than the WQDS will be temporarily stored in the system and discharged into a downstream collection system. The following parameters apply:
| Inflow Drainage Area | Pavement Type | Acreage | CN Value |
| --- | --- | --- | --- |
| Driveway and Aisles | Conventional Asphalt | 0.50 | 98 |
| Central Parking Stalls | Permeable Asphalt | 0.25 | 98 |
| Left Parking Stalls | Permeable Asphalt | 0.125 | 98 |
| Right Parking Stalls | Permeable Asphalt | 0.125 | 98 |
| Rooftop | Roof | 0.25 | 98 |
Tested Subsoil Permeability Rate = 1.00 inches/hour
WQDS: = 1.25 inches in 2 hours
Note that even though a pervious paving system does not produce stormwater runoff, the permeable asphalt surface course must be assigned a CN value of 98 in order to calculate the volume of precipitation collected by the system.
3. Table 3
BMP 2026, Ch 9.6, p. 20
| Description of Area Producing Runoff |
Runoff Volume (cf) |
| Driveway and Aisles to Left Pervious Parking Stalls |
469.4 |
| Driveway and Aisles to Right Pervious Parking Stalls |
469.4 |
| Driveway and Aisles to Central Pervious Parking Stalls |
938.9 |
| Left Parking Stalls |
469.4 |
| Right Parking Stalls |
469.4 |
| Central Parking Stalls |
938.9 |
| Roof to Left Pervious Parking Stalls |
469.4 |
| Roof to Right Pervious Parking Stalls |
469.4 |
| Total Runoff Volume |
4,694.2 |
Source context and notes
# Pervious Paving Systems (Small-Scale GI)
Source revision: January 2026.
| Description of Area Producing Runoff | Runoff Volume (cf) |
| --- | --- |
| Driveway and Aisles to Left Pervious Parking Stalls | 469.4 |
| Driveway and Aisles to Right Pervious Parking Stalls | 469.4 |
| Driveway and Aisles to Central Pervious Parking Stalls | 938.9 |
| Left Parking Stalls | 469.4 |
| Right Parking Stalls | 469.4 |
| Central Parking Stalls | 938.9 |
| Roof to Left Pervious Parking Stalls | 469.4 |
| Roof to Right Pervious Parking Stalls | 469.4 |
| Total Runoff Volume | 4,694.2 |
## Step 2: Storage Volume and Depth of Bed Sizing
As shown in the illustration above, there are three separate permeable asphalt paving areas, which will manage the precipitation falling on them and runoff from the adjacent aisles and from the driveway. For the purposes of this example, stormwater runoff is assumed to be evenly distributed across the three areas. Clean roof runoff from the buildings adjacent to both the right and left parking stalls will be piped directly to the storage beds via leaders connected to the downspouts from the right and left sides of the building.
Because the pervious paving system will receive runoff from areas not occupied by the pervious asphalt, a check of the maximum contributrary drainage area limitation are as follows:
4. Table 4
BMP 2026, Ch 9.6, p. 20
| Pervious Asphalt Paving Area (A) |
Additional Flow Area (B) |
Ratio |
| Central Parking Stalls (0.25 acres) |
Driveway and Aisles (0.25 acres) |
1 |
| Left Parking Stalls (0.125 acres) |
Driveway and Aisles (0.125 acres) and Roof (0.125 acres) |
2 |
| Right Parking Stalls (0.125 acres) |
Driveway and Aisles (0.125 acres) and Roof (0.125 acres) |
2 |
Source context and notes
| Total Runoff Volume | 4,694.2 |
## Step 2: Storage Volume and Depth of Bed Sizing
As shown in the illustration above, there are three separate permeable asphalt paving areas, which will manage the precipitation falling on them and runoff from the adjacent aisles and from the driveway. For the purposes of this example, stormwater runoff is assumed to be evenly distributed across the three areas. Clean roof runoff from the buildings adjacent to both the right and left parking stalls will be piped directly to the storage beds via leaders connected to the downspouts from the right and left sides of the building.
Because the pervious paving system will receive runoff from areas not occupied by the pervious asphalt, a check of the maximum contributrary drainage area limitation are as follows:
| Pervious Asphalt Paving Area (A) | Additional Flow Area (B) | Ratio |
| --- | --- | --- |
| Central Parking Stalls (0.25 acres) | Driveway and Aisles (0.25 acres) | 1 |
| Left Parking Stalls (0.125 acres) | Driveway and Aisles (0.125 acres) and Roof (0.125 acres) | 2 |
| Right Parking Stalls (0.125 acres) | Driveway and Aisles (0.125 acres) and Roof (0.125 acres) | 2 |
The area of additional inflow from conventional pavement is not greater than three times the area occupied by the pervious asphalt paving area.
The storage bed underneath the parking stalls is filled with AASHTO No. 2 coarse aggregate assumed to have 40% voids. The design permeability rate, 0.5 in/hr, is used as the exfiltration rate in the routing calculation. The footprints of the storage beds are the same as the surface areas of the pervious
5. Table 5
BMP 2026, Ch 9.6, p. 21
| Inflow Area = 16,335 sf,100.00% Impervious, Inflow Depth = 1.03" for Custom event |
|
|
|
|
| Inflow = 1.09 cfs @ 1.09 hrs, Volume= 1,408.3 cf |
|
|
|
|
| Outflow = 0.06 cfs @ 0.70 hrs, Volume= 1,408.3 cf, Atten= 94%, Lag= 0.0 min |
|
|
|
|
| Discarded = 0.06 cfs @ 0.70 hrs, Volume= 1,408.3 cf |
|
|
|
|
| Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs |
|
|
|
|
| Peak Elev= 0.49' @ 1.86 hrs Surf.Area= 5,445 sf Storage= 1,076.7 cf |
|
|
|
|
| Plug-Flow detention time= 148.9 min calculated for 1,408.3 cf (100% of inflow) |
|
|
|
|
| Center-of-Mass det. time= 148.9 min (219.2 - 70.3) |
|
|
|
|
| Volume |
Invert |
Avail.Storage |
Storage Description |
|
| #1 |
0.00' |
1,089.0 cf |
Custom Stage Data (Prismatic) Listed below (Recalc) |
|
|
|
|
2,722.5 cf Overall x 40.0% Voids |
|
| Elevation (feet) |
Surf.Area (sq-ft) |
Inc.Store (cubic-feet) |
Cum.Store (cubic-feet) |
|
| 0.00 |
5,445 |
0.0 |
0.0 |
|
| 0.50 |
5,445 |
2,722.5 |
2,722.5 |
|
| Device |
Routing |
Invert |
Outlet Devices |
|
| #1 |
Discarded |
0.00' |
0.50 in/hr Exfiltration over Surface area |
|
| Discarded OutFlow Max=0.06 cfs @ 0.70 hrs HW=0.01' (Free Discharge) |
|
|
|
|
| 1=Exfiltration (Exfiltration Controls 0.06 cfs) |
|
|
|
|
Source context and notes
Source revision: January 2026.
parking areas. Therefore, to manage the entire volume produced by the WQDS for the inflow contributory drainage area, the depth of stormwater runoff in the storage bed is calculated by software while performing the routing calculation, as depicted in the exhibits on the following pages for each of the areas depicted in the previous illustration:
Left Pervious Parking Area (Note that the same report is generated for the right pervious parking area.)
Summary Report:
| Inflow Area = 16,335 sf,100.00% Impervious, Inflow Depth = 1.03" for Custom event | | | | |
| --- | --- | --- | --- | --- |
| Inflow = 1.09 cfs @ 1.09 hrs, Volume= 1,408.3 cf | | | | |
| Outflow = 0.06 cfs @ 0.70 hrs, Volume= 1,408.3 cf, Atten= 94%, Lag= 0.0 min | | | | |
| Discarded = 0.06 cfs @ 0.70 hrs, Volume= 1,408.3 cf | | | | |
| Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs | | | | |
| Peak Elev= 0.49' @ 1.86 hrs Surf.Area= 5,445 sf Storage= 1,076.7 cf | | | | |
| Plug-Flow detention time= 148.9 min calculated for 1,408.3 cf (100% of inflow) | | | | |
| Center-of-Mass det. time= 148.9 min (219.2 - 70.3) | | | | |
| Volume | Invert | Avail.Storage | Storage Description | |
| #1 | 0.00' | 1,089.0 cf | Custom Stage Data (Prismatic) Listed below (Recalc) | |
| | | | 2,722.5 cf Overall x 40.0% Voids | |
| Elevation (feet) | Surf.Area (sq-ft) | Inc.Store (cubic-feet) | Cum.Store (cubic-feet) | |
| 0.00 | 5,445 | 0.0 | 0.0 | |
| 0.50 | 5,445 | 2,722.5 | 2,722.5 | |
| Device | Routing | Invert | Outlet Devices | |
| #1 | Discarded | 0.00' | 0.50 in/hr Exfiltration over Surface area | |
| Discarded OutFlow Max=0.06 cfs @ 0.70 hrs HW=0.01' (Free Discharge) | | | | |
| 1=Exfiltration (Exfiltration Controls 0.06 cfs) | | | | |
Source: HydroCAD® Summary Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission
Tabular Hydrograph Report Excerpt:
| Time (hours) | Inflow (cfs) | Storage (cubic-feet) | Elevation (feet) | Discarded (cfs) |
| --- | --- | --- | --- | --- |
| 0.00 | 0.00 | 0.0 | 0.00 | 0.00 |
| 1.00 | 0.69 | 187.8 | 0.09 | 0.06 |
| 2.00 | 0.04 | 1,066.9 | 0.49 | 0.06 |
| 3.00 | 0.00 | 852.4 | 0.39 | 0.06 |
| 4.00 | 0.00 | 625.6 | 0.29 | 0.06 |
| 5.00 | 0.00 | 398.7 | 0.18 | 0.06 |
| 6.00 | 0.00 | 171.8 | 0.08 | 0.06 |
| 7.00 | 0.00 | 0.0 | 0.00 | 0.00 |
6. Table 6
BMP 2026, Ch 9.6, p. 21
| Time (hours) |
Inflow (cfs) |
Storage (cubic-feet) |
Elevation (feet) |
Discarded (cfs) |
| 0.00 |
0.00 |
0.0 |
0.00 |
0.00 |
| 1.00 |
0.69 |
187.8 |
0.09 |
0.06 |
| 2.00 |
0.04 |
1,066.9 |
0.49 |
0.06 |
| 3.00 |
0.00 |
852.4 |
0.39 |
0.06 |
| 4.00 |
0.00 |
625.6 |
0.29 |
0.06 |
| 5.00 |
0.00 |
398.7 |
0.18 |
0.06 |
| 6.00 |
0.00 |
171.8 |
0.08 |
0.06 |
| 7.00 |
0.00 |
0.0 |
0.00 |
0.00 |
Source context and notes
| #1 | Discarded | 0.00' | 0.50 in/hr Exfiltration over Surface area | |
| Discarded OutFlow Max=0.06 cfs @ 0.70 hrs HW=0.01' (Free Discharge) | | | | |
| 1=Exfiltration (Exfiltration Controls 0.06 cfs) | | | | |
Source: HydroCAD® Summary Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission
Tabular Hydrograph Report Excerpt:
| Time (hours) | Inflow (cfs) | Storage (cubic-feet) | Elevation (feet) | Discarded (cfs) |
| --- | --- | --- | --- | --- |
| 0.00 | 0.00 | 0.0 | 0.00 | 0.00 |
| 1.00 | 0.69 | 187.8 | 0.09 | 0.06 |
| 2.00 | 0.04 | 1,066.9 | 0.49 | 0.06 |
| 3.00 | 0.00 | 852.4 | 0.39 | 0.06 |
| 4.00 | 0.00 | 625.6 | 0.29 | 0.06 |
| 5.00 | 0.00 | 398.7 | 0.18 | 0.06 |
| 6.00 | 0.00 | 171.8 | 0.08 | 0.06 |
| 7.00 | 0.00 | 0.0 | 0.00 | 0.00 |
Source: HydroCAD® Output; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission.
The greatest depth of stormwater runoff in the storage bed of the left porous pavement area is 0.49 ft. The same result is calculated for the right porous pavement area.
7. Table 7
BMP 2026, Ch 9.6, p. 22
| Inflow Area = |
21,780 sf,100.00% Impervious, Inflow Depth = 1.03" for Custom event |
|
|
| Inflow = |
1.45 cfs @ |
1.09 hrs, Volume= |
1,877.7 cf |
| Outflow = |
0.13 cfs @ |
0.80 hrs, Volume= |
1,877.7 cf, Atten= 91%, Lag= 0.0 min |
| Discarded = |
0.13 cfs @ |
0.80 hrs, Volume= |
1,877.7 cf |
| Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs |
|
|
|
| Peak Elev= 0.29' @ 1.80 hrs Surf.Area= 0.250 ac Storage= 0.029 af |
|
|
|
| Plug-Flow detention time= 90.5 min calculated for 1,877.7 cf (100% of inflow) |
|
|
|
| Center-of-Mass det. time= 90.5 min (160.7 - 70.3) |
|
|
|
| Volume |
Invert |
Avail.Storage |
Storage Description |
| #1 |
0.00' |
0.050 af |
Custom Stage Data (Prismatic) Listed below (Recalc)
0.125 af Overall x 40.0% Voids |
| Elevation (feet) |
Surf.Area (acres) |
Inc.Store (acre-feet) |
Cum.Store (acre-feet) |
| 0.00 |
0.250 |
0.000 |
0.000 |
| 0.50 |
0.250 |
0.125 |
0.125 |
| Device |
Routing |
Invert |
Outlet Devices |
| #1 |
Discarded |
0.00' |
0.50 in/hr Exfiltration over Surface area |
Discarded OutFlow Max=0.13 cfs @ 0.80 hrs HW=0.01' (Free Discharge)
1=Exfiltration (Exfiltration Controls 0.13 cfs) |
|
|
|
Source context and notes
# Pervious Paving Systems (Small-Scale GI)
Source revision: January 2026.
## Summary Report:
| Inflow Area = | 21,780 sf,100.00% Impervious, Inflow Depth = 1.03" for Custom event | | |
| --- | --- | --- | --- |
| Inflow = | 1.45 cfs @ | 1.09 hrs, Volume= | 1,877.7 cf |
| Outflow = | 0.13 cfs @ | 0.80 hrs, Volume= | 1,877.7 cf, Atten= 91%, Lag= 0.0 min |
| Discarded = | 0.13 cfs @ | 0.80 hrs, Volume= | 1,877.7 cf |
| Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.05 hrs | | | |
| Peak Elev= 0.29' @ 1.80 hrs Surf.Area= 0.250 ac Storage= 0.029 af | | | |
| Plug-Flow detention time= 90.5 min calculated for 1,877.7 cf (100% of inflow) | | | |
| Center-of-Mass det. time= 90.5 min (160.7 - 70.3) | | | |
| Volume | Invert | Avail.Storage | Storage Description |
| #1 | 0.00' | 0.050 af | Custom Stage Data (Prismatic) Listed below (Recalc)
0.125 af Overall x 40.0% Voids |
| Elevation (feet) | Surf.Area (acres) | Inc.Store (acre-feet) | Cum.Store (acre-feet) |
| 0.00 | 0.250 | 0.000 | 0.000 |
| 0.50 | 0.250 | 0.125 | 0.125 |
| Device | Routing | Invert | Outlet Devices |
| #1 | Discarded | 0.00' | 0.50 in/hr Exfiltration over Surface area |
| Discarded OutFlow Max=0.13 cfs @ 0.80 hrs HW=0.01' (Free Discharge)
1=Exfiltration (Exfiltration Controls 0.13 cfs) | | | |
Source: HydroCAD® Summary Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission.
## Tabular Hydrograph Report Excerpt:
| Time (hours) | Inflow (cfs) | Storage (acre-feet) | Elevation (feet) | Discarded (cfs) |
| --- | --- | --- | --- | --- |
| 0.00 | 0.00 | 0.000 | 0.00 | 0.00 |
| 1.00 | 0.92 | 0.005 | 0.05 | 0.13 |
| 2.00 | 0.05 | 0.028 | 0.28 | 0.13 |
| 3.00 | 0.00 | 0.018 | 0.18 | 0.13 |
| 4.00 | 0.00 | 0.008 | 0.08 | 0.13 |
| 5.00 | 0.00 | 0.000 | 0.00 | 0.00 |
8. Table 8
BMP 2026, Ch 9.6, p. 22
| Time (hours) |
Inflow (cfs) |
Storage (acre-feet) |
Elevation (feet) |
Discarded (cfs) |
| 0.00 |
0.00 |
0.000 |
0.00 |
0.00 |
| 1.00 |
0.92 |
0.005 |
0.05 |
0.13 |
| 2.00 |
0.05 |
0.028 |
0.28 |
0.13 |
| 3.00 |
0.00 |
0.018 |
0.18 |
0.13 |
| 4.00 |
0.00 |
0.008 |
0.08 |
0.13 |
| 5.00 |
0.00 |
0.000 |
0.00 |
0.00 |
Source context and notes
| Device | Routing | Invert | Outlet Devices |
| #1 | Discarded | 0.00' | 0.50 in/hr Exfiltration over Surface area |
| Discarded OutFlow Max=0.13 cfs @ 0.80 hrs HW=0.01' (Free Discharge)
1=Exfiltration (Exfiltration Controls 0.13 cfs) | | | |
Source: HydroCAD® Summary Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission.
## Tabular Hydrograph Report Excerpt:
| Time (hours) | Inflow (cfs) | Storage (acre-feet) | Elevation (feet) | Discarded (cfs) |
| --- | --- | --- | --- | --- |
| 0.00 | 0.00 | 0.000 | 0.00 | 0.00 |
| 1.00 | 0.92 | 0.005 | 0.05 | 0.13 |
| 2.00 | 0.05 | 0.028 | 0.28 | 0.13 |
| 3.00 | 0.00 | 0.018 | 0.18 | 0.13 |
| 4.00 | 0.00 | 0.008 | 0.08 | 0.13 |
| 5.00 | 0.00 | 0.000 | 0.00 | 0.00 |
Source: HydroCAD® Output; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission.
The greatest depth of stormwater runoff in the storage bed of the central pervious pavement area is 0.28 ft.
## Step 3: Drain Time Calculation
9. Table 9
BMP 2026, Ch 9.6, p. 24
| Storm Event |
Rainfall (in) |
Direct Runoff Volume without Pervious Paving (cf) |
Retention Volume (cf) |
Runoff Volume Leaving Pervious Paving (cf) |
Reduced Direct Runoff (in) |
Reduced CN (from Graph) |
| WQ |
1.25 |
4,695 |
4,694 |
0 |
0.00 |
65 |
| 2-year |
3.31 |
13,946 |
4,694 |
9,252 |
2.04 |
88 |
| 10-year |
5.01 |
21,633 |
4,694 |
16,939 |
3.73 |
88 |
| 100-year |
8.33 |
36,666 |
4,694 |
31,972 |
7.05 |
89.5 |
Source context and notes
# Pervious Paving Systems (Small-Scale GI)
Source revision: January 2026.
# Step 5: Reduced CN
The volume retained in the pervious paving system reduces the direct runoff leaving the site. Therefore, a reduced CN can be calculated by using the reduced direct runoff, rainfall depth and Fig 2-1 in NEH, Part 630, Chapter 2, as shown in the graph following the table below. The contributory drainage area is 1.25 acre including the pervious driveway, aisles, permeable asphalt and the roof area.
| Storm Event | Rainfall (in) | Direct Runoff Volume without Pervious Paving (cf) | Retention Volume (cf) | Runoff Volume Leaving Pervious Paving (cf) | Reduced Direct Runoff (in) | Reduced CN (from Graph) |
| --- | --- | --- | --- | --- | --- | --- |
| WQ | 1.25 | 4,695 | 4,694 | 0 | 0.00 | 65 |
| 2-year | 3.31 | 13,946 | 4,694 | 9,252 | 2.04 | 88 |
| 10-year | 5.01 | 21,633 | 4,694 | 16,939 | 3.73 | 88 |
| 100-year | 8.33 | 36,666 | 4,694 | 31,972 | 7.05 | 89.5 |
[Figure: img-13.jpeg (original PDF, p. 24)](SOURCE.pdf#page=24)
The Reduced CN can be used to calculate the runoff volume. However, when calculating peak flow rate, a routing calculation must be performed on the pervious paving system.
10. Table 10
BMP 2026, Ch 9.6, p. 25
| Inflow Drainage Area |
Pavement Type |
Acreage |
CN Value |
| Driveway and Aisles |
Conventional Asphalt |
0.50 |
98 |
| Central Parking Stalls |
Permeable Asphalt |
0.25 |
98 |
| Left Parking Stalls |
Permeable Asphalt |
0.125 |
98 |
| Right Parking Stalls |
Permeable Asphalt |
0.125 |
98 |
| Rooftop |
Roof |
0.25 |
98 |
Source context and notes
# Pervious Paving Systems (Small-Scale GI)
Source revision: January 2026.
Example 2 – Permeable Asphalt Parking Lot Designed with an Underdrained System: A proposed commercial site includes a 1-acre parking lot and 0.25-acre roof. The parking stalls will be permeable asphalt and will manage the runoff produced by the WQDS that falls on them and the adjacent impervious aisles and driveway. In this example, the permeable asphalt system will be designed solely to meet the stormwater runoff quality design and performance standards; stormwater runoff quantity control requirements will be met by a downgradient subsurface infiltration system. The following parameters apply:
| Inflow Drainage Area | Pavement Type | Acreage | CN Value |
| --- | --- | --- | --- |
| Driveway and Aisles | Conventional Asphalt | 0.50 | 98 |
| Central Parking Stalls | Permeable Asphalt | 0.25 | 98 |
| Left Parking Stalls | Permeable Asphalt | 0.125 | 98 |
| Right Parking Stalls | Permeable Asphalt | 0.125 | 98 |
| Rooftop | Roof | 0.25 | 98 |
Tested Subsoil Permeability Rate = 0.50 inches/hour
WQDS: = 1.25 inches in 2 hours
Note that even though a pervious paving system does not generate runoff, the permeable asphalt surface course must be assigned a CN value of 98 in order to calculate the volume of precipitation collected by the system.
11. Table 11
BMP 2026, Ch 9.6, p. 26
| Pervious Asphalt Paving Area (A) |
Additional Flow Area (B) |
Ratio (B)/(A) |
| Central Parking Stalls (0.25 acres) |
Driveway and Aisles (0.25 acres) |
1 |
| Left Parking Stalls (0.125 acres) |
Driveway and Aisles (0.125 acres) |
1 |
| Right Parking Stalls (0.125 acres) |
Driveway and Aisles (0.125 acres) |
1 |
Source context and notes
# Pervious Paving Systems (Small-Scale GI)
Source revision: January 2026.
The drainage system design is shown in the illustration below. For illustrative purposes, a horizontal slice has been taken through each of the storage beds to show both the layout of the underdrain manifold with the laterals and the relationship of the storage beds to the areas of conventional asphalt pavement.
Because the pervious paving system will receive runoff from areas not occupied by the permeable asphalt, a check of the maximum contributory drainage area limitation is performed, as follows:
| Pervious Asphalt Paving Area (A) | Additional Flow Area (B) | Ratio (B)/(A) |
| --- | --- | --- |
| Central Parking Stalls (0.25 acres) | Driveway and Aisles (0.25 acres) | 1 |
| Left Parking Stalls (0.125 acres) | Driveway and Aisles (0.125 acres) | 1 |
| Right Parking Stalls (0.125 acres) | Driveway and Aisles (0.125 acres) | 1 |
The area of additional inflow from conventional pavement is not greater than three times the area occupied by the pervious asphalt paving area.
[Figure: img-15.jpeg (original PDF, p. 26)](SOURCE.pdf#page=26)
Example 2 – Proposed Site Drainage Layout
## Step 1: Runoff Calculations for the Water Quality Design Storm
12. Table 12
BMP 2026, Ch 9.6, p. 27
| Area Producing Runoff |
Runoff Volume (cf) |
| Driveway and Aisles |
1,878 |
| Parking Stalls |
1,878 |
| Total Runoff Volume |
3,765 |
Source context and notes
# Pervious Paving Systems (Small-Scale GI)
Source revision: January 2026.
| Area Producing Runoff | Runoff Volume (cf) |
| --- | --- |
| Driveway and Aisles | 1,878 |
| Parking Stalls | 1,878 |
| Total Runoff Volume | 3,765 |
## Step 2: Storage Volume and Depth of Bed Sizing
As shown in the preceding illustrations, there are three separate permeable asphalt paving areas, which will manage the precipitation falling on them and runoff from the adjacent aisles and from the driveway. For the purpose of this example, the runoff is assumed to be evenly distributed across the three areas. The storage bed underneath the parking stalls is filled with AASHTO No. 2 coarse aggregate assumed to have 40% voids. The contributory drainage area and surface area of storage bed are provided in the table below for each of the three permeable asphalt paving areas.
| Location of the Permeable Paving System | Contributory Drainage Area | Storage Bed Surface Area |
| --- | --- | --- |
| Central Parking Stalls | 0.50 ac (21,780 sf) | 10,890 sf |
| Left Parking Stalls | 0.25 ac (10,890 sf) | 5,445 sf |
| Right Parking Stalls | 0.25 ac (10,890 sf) | 5,445 sf |
13. Table 13
BMP 2026, Ch 9.6, p. 27
| Location of the Permeable Paving System |
Contributory Drainage Area |
Storage Bed Surface Area |
| Central Parking Stalls |
0.50 ac (21,780 sf) |
10,890 sf |
| Left Parking Stalls |
0.25 ac (10,890 sf) |
5,445 sf |
| Right Parking Stalls |
0.25 ac (10,890 sf) |
5,445 sf |
Source context and notes
| Driveway and Aisles | 1,878 |
| Parking Stalls | 1,878 |
| Total Runoff Volume | 3,765 |
## Step 2: Storage Volume and Depth of Bed Sizing
As shown in the preceding illustrations, there are three separate permeable asphalt paving areas, which will manage the precipitation falling on them and runoff from the adjacent aisles and from the driveway. For the purpose of this example, the runoff is assumed to be evenly distributed across the three areas. The storage bed underneath the parking stalls is filled with AASHTO No. 2 coarse aggregate assumed to have 40% voids. The contributory drainage area and surface area of storage bed are provided in the table below for each of the three permeable asphalt paving areas.
| Location of the Permeable Paving System | Contributory Drainage Area | Storage Bed Surface Area |
| --- | --- | --- |
| Central Parking Stalls | 0.50 ac (21,780 sf) | 10,890 sf |
| Left Parking Stalls | 0.25 ac (10,890 sf) | 5,445 sf |
| Right Parking Stalls | 0.25 ac (10,890 sf) | 5,445 sf |
## Step 3: Quantity Control for Large Storm Events and Overflow Measures
In this design example, the permeable asphalt system is not designed to provide stormwater runoff quantity control; therefore, additional storage volume for the larger storm events is not required. However, the outlet control structure design criteria found on Pages 6 through 8 still apply. Solid pipe will be provided to convey stormwater runoff from larger storms from the outlet control structures to the down-gradient subsurface infiltration system.
## Step 4: Drain Time Calculation
14. Table 14
BMP 2026, Ch 9.6, p. 32
| Test Methods Per ASTM C1701 or C1781 |
|
|
| Time to Infiltrate the Specified Amount of Water (seconds) |
Approximate Surface Infiltration Rate (inches per hour) |
|
|
M = 8 lbs |
M = 40 lbs |
| 30 |
235 |
1175 |
| 60 |
118 |
587 |
| 100 |
70.5 |
352 |
| 200 |
35.2 |
176 |
| 350 |
20.1 |
100.7 |
| 360 |
19.6 |
97.9 |
| 380 |
18.5 |
92.7 |
| 900 |
7.8 |
39.2 |
| 1760 |
4.0 |
20.0 |
| 1910 |
3.7 |
18.5 |
| 3600 |
2.0 |
9.8 |
| 5400 |
1.3 |
6.5 |
| 5470 |
1.3 |
6.4 |
| 6000 |
1.2 |
5.9 |
Source context and notes
$$
D = \text{ring diameter} = 12 \text{ inches}
$$
$$
t = \text{time, in seconds}
$$
| Test Methods Per ASTM C1701 or C1781 | | |
| --- | --- | --- |
| Time to Infiltrate the Specified Amount of Water (seconds) | Approximate Surface Infiltration Rate (inches per hour) | |
| | M = 8 lbs | M = 40 lbs |
| 30 | 235 | 1175 |
| 60 | 118 | 587 |
| 100 | 70.5 | 352 |
| 200 | 35.2 | 176 |
| 350 | 20.1 | 100.7 |
| 360 | 19.6 | 97.9 |
| 380 | 18.5 | 92.7 |
| 900 | 7.8 | 39.2 |
| 1760 | 4.0 | 20.0 |
| 1910 | 3.7 | 18.5 |
| 3600 | 2.0 | 9.8 |
| 5400 | 1.3 | 6.5 |
| 5470 | 1.3 | 6.4 |
| 6000 | 1.2 | 5.9 |
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