Tables: Green Roofs (Small-Scale GI)¶
Back to source | Source PDF | Structured table records
Table fragments retain their source context. Refer to the original page for layout, figures, merged headings and notes.
1. Table 1¶
| N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards | ||
|---|---|---|
| Green Infrastructure | Yes | |
| Stormwater Runoff Quantity | Yes | |
| GR | Groundwater Recharge | Not Allowed |
| % | Stormwater Runoff Quality | Not Allowed |
| Stormwater Runoff Quantity Control Mechanisms and Corresponding Criteria | ||
| --- | --- | |
| Volume Reduction* | ||
| Reduced Curve Number | Based on the available water capacity of the growing medium | |
| Growing Medium | Must meet the design criteria specifications. See Pages 6 and 7, plus either Page 9 for Extensive Green Roofs or Page 10 for Intensive Green roofs, as appropriate | |
| Maximum Roof Slope | 20% | |
| Evapo-transpiration | ||
| Appropriate Species Selection | See Page 6 | |
| Minimum Density of Vegetation | 85% |
Source context and notes
Source revision: March 2021. # 9.4 GREEN ROOFS [Figure: img-0.jpeg (original PDF, p. 1)](SOURCE.pdf#page=1) A green roof, also known as a vegetated roof, is a roof that has been covered with a growing medium and vegetation. Green roofs are effective for reducing the amount of stormwater runoff leaving a site. A green roof consists of vegetation planted in growing media on top of a drainage layer that intercepts stormwater runoff and reduces the total volume of runoff through evapotranspiration. | N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards | | | | --- | --- | --- | | | Green Infrastructure | Yes | | | Stormwater Runoff Quantity | Yes | | GR | Groundwater Recharge | Not Allowed | | % | Stormwater Runoff Quality | Not Allowed | | Stormwater Runoff Quantity Control Mechanisms and Corresponding Criteria | | | --- | --- | | Volume Reduction* | | | Reduced Curve Number | Based on the available water capacity of the growing medium | | Growing Medium | Must meet the design criteria specifications. See Pages 6 and 7, plus either Page 9 for Extensive Green Roofs or Page 10 for Intensive Green roofs, as appropriate | | Maximum Roof Slope | 20% | | Evapo-transpiration | | | Appropriate Species Selection | See Page 6 | | Minimum Density of Vegetation | 85% |2. Table 2¶
| Growing Medium and Composition (% by Volume) | *Effective Porosity (cf/cf) | *Field Capacity (cf/cf) | *Wilting Point (cf/cf) | Maximum Water Capacity, Using FLL Method, (% by Volume) | Maximum Available Water Capacity For Runoff Retention (cf/cf) |
|---|---|---|---|---|---|
| • 20% 1-7 mm pumice • 60% 4-10 mm pumice • 20% composted pine bark fines |
0.30 | 0.349 | 0.118 | 49.6 | 0.231 |
| • 50% 4-10 mm pumice • 30% 1-8 mm zeolite • 20% composted pine bark fines |
0.30 | 0.295 | 0.053 | 46.6 | 0.242 |
| • 70% 4-10 mm pumice • 10% zeolite <3 mm • 15% pine bark fines + mushroom compost • 5% sphagnum peat |
0.35 | 0.358 | 0.069 | 63 | 0.289 |
Source context and notes
# Green Roofs (Small-Scale GI) Source revision: March 2021. # Volume Reduction The growing medium on a green roof will absorb and retain a portion of the rainfall that lands on it, thus reducing the volume of runoff that will reach downstream stormwater facilities. For the calculation of runoff retention, the available water capacity for runoff reduction is the difference in the water content between the field capacity and the wilting point, both of which are defined below. Therefore, the values for field capacity and the wilting point of the selected growing medium must be obtained either from a published research article or tested in accordance with either the ASTM D-6836 method or the Soil Survey Investigations Report No. 42, Kellogg Soil Survey Laboratory Methods Manual, published by NRCS. The field capacity is tested at -10 KPa, for sandy material, or -0.33 KPa, for other materials. The wilting point is tested at 1,500 KPa under the ASTM D-6836 method or Kellogg Soil Survey Laboratory Methods Manual by NRCS. Information regarding some examples of growing media mixes are provided below. | Growing Medium and Composition (% by Volume) | *Effective Porosity (cf/cf) | *Field Capacity (cf/cf) | *Wilting Point (cf/cf) | Maximum Water Capacity, Using FLL Method, (% by Volume) | Maximum Available Water Capacity For Runoff Retention (cf/cf) | | --- | --- | --- | --- | --- | --- | | • 20% 1-7 mm pumice• 60% 4-10 mm pumice
• 20% composted pine bark fines | 0.30 | 0.349 | 0.118 | 49.6 | 0.231 | | • 50% 4-10 mm pumice
• 30% 1-8 mm zeolite
• 20% composted pine bark fines | 0.30 | 0.295 | 0.053 | 46.6 | 0.242 | | • 70% 4-10 mm pumice
• 10% zeolite <3 mm
• 15% pine bark fines + mushroom compost
• 5% sphagnum peat | 0.35 | 0.358 | 0.069 | 63 | 0.289 | *Note: Effective porosity is the porosity available to fluid flow and is typically less than the total porosity. Field capacity is the water content of the soil after free drainage has ceased. Wilting point is the minimum soil moisture required by a plant to not wilt. Note the assumption that the maximum available water capacity for runoff retention assumes the growing medium has dried to the wilting point of the plants between each storm event. In many storm events, especially in the case of back-to-back storms, the green roof may not provide the full calculated volume reduction, as stormwater runoff from the first storm may still be retained within the growing medium
3. Table 3¶
| Roof Area: | 1 | acre |
|---|---|---|
| WQDS rainfall, $P_{WQ}$: | 1.25 | inches |
| 2-year storm rainfall, $P_{2}$: | 3.31 | inches |
| 10-year storm rainfall, $P_{10}$: | 5.06 | inches |
| 100-year storm rainfall, $P_{100}$: | 8.52 | inches |
| Drainage layer depth: | 6 | inches |
| Drainage layer field capacity: | 0.02% | by volume |
| Slope | 2.5% | |
| Vegetation | dense native grass |
Source context and notes
Green roofs may be incorporated with other BMPs to retain the entire roof runoff from the WQDS or to reduce roof runoff produced by larger storms. For example, dry wells may be designed to receive and infiltrate some discharge from the downspouts of a green roof in order to achieve retention of the entire WQDS volume. Cisterns, rain gardens and pervious paving systems may also be incorporated with green roofs. Note that if the discharge from a green roof is mixed with runoff that must be treated to reduce the total suspended solids (TSS), the entire mixed runoff volume is subject to the 80% removal rate for TSS. # Designing a Green Roof The following example illustrates how to use a green roof to reduce the volume of runoff generated by the WQDS, plus the 2-, 10- and 100-year storms. The example shows a possible configuration and flow path and is not intended to limit the design. Example: A 1 acre roof of a building in Camden County is to be designed as a green roof based on the following parameters, assuming the entire roof is vegetated: | Roof Area: | 1 | acre | | --- | --- | --- | | WQDS rainfall, $P_{WQ}$: | 1.25 | inches | | 2-year storm rainfall, $P_{2}$: | 3.31 | inches | | 10-year storm rainfall, $P_{10}$: | 5.06 | inches | | 100-year storm rainfall, $P_{100}$: | 8.52 | inches | | Drainage layer depth: | 6 | inches | | Drainage layer field capacity: | 0.02% | by volume | | Slope | 2.5% | | | Vegetation | dense native grass | | Growing Medium: The growing medium has a composition of 20% 1 - 7 mm pumice by volume, 60% 4 - 10 mm pumice by volume, and 20% composted pine bark fines by volume. The characteristics of the growing medium are as the follows: Growing medium depth: 6 inches Field capacity: 0.349% by volume Wilting point: 0.118% by volume Available Water Capacity for Stormwater Runoff Retention 0.231% by volume4. Table 4¶
| Storm Event | (A) Runoff Volume (cf) (Impervious Surface) | (B) Storage Volume (cf) | (A)-(B) Discharged Runoff Volume (cf) (Green Roof) |
|---|---|---|---|
| WQDS | 3,755 | 5,467 | 0 |
| 2-year | 11,170 | 5,467 | 5,703 |
| 10-year | 17,508 | 5,467 | 12,041 |
| 100-year | 30,056 | 5,467 | 24,589 |
Source context and notes
$$ S _ {v} = \frac {4 3 , 5 6 0 x [ (6 x 0 . 2 3 1) + (6 x 0 . 0 2) ]}{1 2} = 5, 4 6 7 \mathrm {c f} $$ ## Step 2: Calculate the discharged runoff volume Using the storage volume calculated above, the discharged runoff volume for each storm is calculated by subtracting the storage volume calculated in Step 1 from the stormwater runoff volume from a traditional roof of the same area, as shown in the following table: | Storm Event | (A) Runoff Volume (cf) (Impervious Surface) | (B) Storage Volume (cf) | (A)-(B) Discharged Runoff Volume (cf) (Green Roof) | | --- | --- | --- | --- | | WQDS | 3,755 | 5,467 | 0 | | 2-year | 11,170 | 5,467 | 5,703 | | 10-year | 17,508 | 5,467 | 12,041 | | 100-year | 30,056 | 5,467 | 24,589 | ## Step 3: Calculate the direct runoff depth Next, the direct runoff depth, $Q$, is calculated by dividing the discharged runoff volume by the roof area, yielding the following results: | Storm Event | Discharged Runoff Volume (cf) (Green Roof) | Green Roof Area (sf) | Direct Runoff Depth (in) | | --- | --- | --- | --- | | WQDS | 0 | 43,560 | 0 | | 2-year | 5,703 | 43,560 | 1.57 | | 10-year | 12,041 | 43,560 | 3.32 | | 100-year | 24,589 | 43,560 | 6.77 |5. Table 5¶
| Storm Event | Discharged Runoff Volume (cf) (Green Roof) | Green Roof Area (sf) | Direct Runoff Depth (in) |
|---|---|---|---|
| WQDS | 0 | 43,560 | 0 |
| 2-year | 5,703 | 43,560 | 1.57 |
| 10-year | 12,041 | 43,560 | 3.32 |
| 100-year | 24,589 | 43,560 | 6.77 |
Source context and notes
| 2-year | 11,170 | 5,467 | 5,703 | | 10-year | 17,508 | 5,467 | 12,041 | | 100-year | 30,056 | 5,467 | 24,589 | ## Step 3: Calculate the direct runoff depth Next, the direct runoff depth, $Q$, is calculated by dividing the discharged runoff volume by the roof area, yielding the following results: | Storm Event | Discharged Runoff Volume (cf) (Green Roof) | Green Roof Area (sf) | Direct Runoff Depth (in) | | --- | --- | --- | --- | | WQDS | 0 | 43,560 | 0 | | 2-year | 5,703 | 43,560 | 1.57 | | 10-year | 12,041 | 43,560 | 3.32 | | 100-year | 24,589 | 43,560 | 6.77 |6. Table 6¶
| Storm Event | Rainfall Depth (in) | Direct Runoff Depth (in) | Adjusted Curve Number |
|---|---|---|---|
| WQDS | 1.25 | 0 | 61 |
| 2-year | 3.31 | 1.57 | 81 |
| 10-year | 5.06 | 3.32 | 84 |
| 100-year | 8.52 | 6.77 | 85 |
Source context and notes
From Figure 10-2 in the NEH, the adjusted curve number can be found using the rainfall depths and the stormwater runoff depths calculated in Step 3 for each storm, as depicted below, in blue, green and red for the 2-, 10- and 100-year storms, respectively. [Figure: img-14.jpeg (original PDF, p. 18)](SOURCE.pdf#page=18) Figure 10-2 ES-1001 graphical solution of the equation $Q = \frac{(P - 0.2S)^2}{P + 0.8S}$ From the figure above, the adjusted curve numbers for the 2-, 10- and 100-year storm events are approximately 81, 84 and 85, respectively. For the WQDS, there is no runoff, so the curve number should be 61, which produces no runoff in the WQDS. | Storm Event | Rainfall Depth (in) | Direct Runoff Depth (in) | Adjusted Curve Number | | --- | --- | --- | --- | | WQDS | 1.25 | 0 | 61 | | 2-year | 3.31 | 1.57 | 81 | | 10-year | 5.06 | 3.32 | 84 | | 100-year | 8.52 | 6.77 | 85 |7. Table 7¶
| Storm Event | Adjusted CN | Runoff Volume Discharged from the Green Roof (cf) | Peak Flow (cfs) |
|---|---|---|---|
| WQDS | 61 | 0 | 0 |
| 2-year | 81 | 5,703 | 1.45 |
| 10-year | 84 | 12,041 | 3.11 |
| 100-year | 85 | 24,589 | 6.10 |
Source context and notes
Source revision: March 2021. Step 5: Calculated Peak Flow Rates by Adjusted Curve Number Next, the peak flow rate can be calculated by using the adjusted curve number with the NRCS method. The time of concentration must be calculated based on the vegetation condition, roof slope and drainage path. In order to avoid potential erosion that may result from the concentration of flow on the rooftop, drains are located on the roof so that the maximum flow length is 100 feet. The Manning's roughness for sheet flow is 0.24. Under these conditions, the green roof will have a time of concentration of 12.8 minutes. Note that the time of concentration for a traditional impervious surface under the post-construction condition has a default value of 6 minutes. The peak flow rates for different storms under the adjusted curve numbers and estimated time of concentration were calculated by hydrologic modeling software and the results are as follows: | Storm Event | Adjusted CN | Runoff Volume Discharged from the Green Roof (cf) | Peak Flow (cfs) | | --- | --- | --- | --- | | WQDS | 61 | 0 | 0 | | 2-year | 81 | 5,703 | 1.45 | | 10-year | 84 | 12,041 | 3.11 | | 100-year | 85 | 24,589 | 6.10 | Compare these results with that from an impervious roof: | Storm Event | CN | Runoff Volume Discharged From an Impervious Roof (cf) | Peak Flow (cfs) | | --- | --- | --- | --- | | WQDS | 98 | 3,755 | 2.93 | | 2-year | 98 | 11,170 | 3.22 | | 10-year | 98 | 17,508 | 4.95 | | 100-year | 98 | 30,056 | 8.37 | ## Considerations8. Table 8¶
| Storm Event | CN | Runoff Volume Discharged From an Impervious Roof (cf) | Peak Flow (cfs) |
|---|---|---|---|
| WQDS | 98 | 3,755 | 2.93 |
| 2-year | 98 | 11,170 | 3.22 |
| 10-year | 98 | 17,508 | 4.95 |
| 100-year | 98 | 30,056 | 8.37 |
Source context and notes
| --- | --- | --- | --- | | WQDS | 61 | 0 | 0 | | 2-year | 81 | 5,703 | 1.45 | | 10-year | 84 | 12,041 | 3.11 | | 100-year | 85 | 24,589 | 6.10 | Compare these results with that from an impervious roof: | Storm Event | CN | Runoff Volume Discharged From an Impervious Roof (cf) | Peak Flow (cfs) | | --- | --- | --- | --- | | WQDS | 98 | 3,755 | 2.93 | | 2-year | 98 | 11,170 | 3.22 | | 10-year | 98 | 17,508 | 4.95 | | 100-year | 98 | 30,056 | 8.37 | ## Considerations A number of factors should be considered when using a green roof to manage stormwater runoff. ## Structural StrengthImages on table pages¶
Page associations do not establish that a crop depicts a specific table.