250 R.I. Code R. 250-RICR-150-10-8.18

Current through November 21, 2024

Section 250-RICR-150-10-8.18 - LID Stormwater Credit: Rooftop, Roadway, Driveway or Parking Lot Runoff Directed to Qualifying Pervious Areas (QPAs)

A. The LID Stormwater Credit using QPAs may be used to reduce the required Recharge Volume and Water Quality Volume, provided that any pervious surfaces used to treat and infiltrate stormwater runoff meet the requirements set forth herein. The application of the Credit does not relieve the design engineer or reviewer from meeting the remaining minimum standards described in §§ 8.6 through 8.17 of this Part or the standard of engineering practice associated with safe conveyance of stormwater runoff and good drainage design.

B. Stormwater Credit shall not be applied:

1. At sites where stormwater runoff is directed to non-permeable soils, such as bedrock and soils classified as Hydrologic Soil Group D;

2. At sites with urban fill, soils classified as contaminated, and soils with a seasonal high groundwater elevation within 18 inches of the land surface.

C. QPAs are natural or landscaped vegetated areas fully stabilized, with runoff characteristics at or lower than the US Natural Resources Conservation Service Curve Numbers in the table below in § 8.18(D) of this Part. All QPAs must be shown on site plans, must have a minimum of 4 inches of topsoil or organic material, and must be located outside of regulated wetland areas and regulated buffer to a waterbody or wetland. In order for lawns to be considered as QPAs, they must consist of low-maintenance grasses adapted to the New England region.

D. Maximum US Natural Resources Conservation Service Hydrologic Soil Group (HSG) Runoff Curve Numbers for QPAs

Cover Type	HSG A	HSG B	HSG C
Natural: Woods Good Condition	30	55	70
Natural: Brush Good Condition	30	48	65
Landscaped: Good Condition (grass cover > 75% or equivalent herbaceous plants)	39	61	74

E. A LID Stormwater Credit is available when rooftop, roadway, driveway, or parking lot runoff is directed to a QPA where it can either infiltrate into the soil or flow over it with sufficient time and reduced velocity to allow for adequate filtering. QPAs are generally flat locations, where the discharge is directed via sheet flow and not as a point source discharge. The credit may be obtained by grading the site to induce sheet flow over specially designed, gently sloped vegetated areas that can treat and infiltrate the runoff. This credit is available for impervious cover associated with all land uses, except for runoff from that portion of a LUHPPL that may generate runoff with a higher potential pollutant load.

F. If runoff from impervious areas is adequately directed to a QPA, the area can be deducted from total impervious area, therefore reducing the required Water Quality Volume and the size of the structural BMPs used to meet the removal requirement of § 8.9 of this Part (Standard 3). Redirected runoff can also be used to meet the recharge requirement as a non-structural practice.

G. The LID Stormwater Credit is subject to the following restrictions:

1. To prevent compaction of the soil in the QPA, construction vehicles must not be allowed to drive over the area. If it becomes compacted, the soil must be suitably amended, tilled, and re-vegetated once construction is complete to restore infiltration capacity.

2. The QPA must be designed to not cause basement seepage. To prevent basement seepage, at a minimum, runoff must be directed away from the building foundation and be infiltrated at least 10 feet away from the foundation.

3. The rooftop area contributing runoff to any one downspout and/or the non-rooftop impervious areas draining to any one discharge location cannot exceed 1,000 square feet.

4. The length of the QPA (in feet) shall be equal to or greater than the contributing rooftop area (in square feet) divided by 13.3 and the maximum contributing flow path from non-rooftop impervious areas shall be 75 feet.

5. For non-rooftop runoff, the length of the QPA must be equal to or greater than the length of the contributing impervious area.

6. For roof runoff, the width of the QPA (in feet) shall be equal to or greater than the roof length.

7. For non-roof runoff, the width of the QPA shall be no less than the width of the contributing impervious surface.

8. Although they may abut, there shall be no overlap between QPAs. They shall not be directed to the same area.

9. The lot must be greater than 6,000 square feet.

10. The slope of the QPA shall be less than or equal to 5.0%.

11. Where provided, downspouts must be at least 10 feet away from the nearest impervious surface to prevent reconnection to the stormwater management system.

12. Where provided, downspouts must have appropriate provisions to induce sheet flow.

13. Where a gutter/downspout system is not used, the rooftop runoff must be designed to sheet flow at low velocity away from the structure housing the roof.

14. A DEM-licensed Class IV Soil Evaluator or RI-registered Professional Engineer shall confirm that the depth to the seasonal high groundwater table is 18 inches or greater. The soil evaluation must identify the soil texture, HSG (from US Natural Resources Conservation Service soil maps), and depth to the seasonal high groundwater table.

15. If a QPA is located in less permeable soils (HSG "C"), the water table depth and soil texture shall be evaluated by a DEM-licensed Class IV Soil Evaluator or RI-registered Professional Engineer to determine if a level spreading device is needed to sheet flow stormwater over vegetated surfaces.

16. Runoff from driveways, roadways, and parking lots may be directed over soft shoulders, through curb cuts, or level spreaders to QPAs. Measures must be employed at the discharge point to the QPA to prevent erosion and promote sheet flow.

17. To take credit for rooftop disconnection associated with a LUHPPL, the rooftop runoff must not commingle with runoff from any paved surfaces or activities or areas on the site that may generate higher pollutant loads.

18. The Operation and Maintenance Plan required by § 8.17 of this Part (Minimum Standard 11) must include measures to inspect the QPA at least yearly to remove any deposited sediment.

19. The QPA must be owned or controlled by the property owner.

20. In locations where there is a history of groundwater seepage and/or basement flooding, the credit shall not be utilized.

H. The impervious areas contributing runoff to the QPA can be deducted from the impervious surfaces used to calculate the WQv, and can meet the Rev requirement if enough area is disconnected in accordance with the Percent Area Method, described below.

1. The amount of impervious area that needs to be disconnected to meet the recharge requirement is referred to as the recharge area. It is equivalent to the recharge volume but can be achieved by filtration of sheet flow over a QPA. Recharge area is calculated according to the equation below:

Recharge area = (F) (I)

Where:

Recharge area = Required impervious area to be directed to a QPA (acres)

F = Recharge factor based on HSG (dimensionless) § 8.8(F) of this Part

I = Impervious area (acres)

2. If only a portion of the recharge area can be directed to a QPA due to site constraints, a designer must use a structural BMP to recharge the difference. This amount can be determined by the following approach:

a. Calculate both the Rev and recharge area for the site;

b. The site impervious area draining to a QPA is subtracted from the recharge area calculation from Credit Step 1, above in § 8.18(H)(1) of this Part;

c. The remaining recharge area is divided by the original recharge area to calculate a pro-rated percentage that must be directed to structural infiltration BMPs; and

d. The pro-rated percentage is multiplied by the original Rev to calculate a new Rev that must be met by an approved structural practice(s).

250 R.I. Code R. 250-RICR-150-10-8.18

Amended effective 11/13/2018