250 R.I. Code R. 250-RICR-150-10-8.21

Current through November 21, 2024

Section 250-RICR-150-10-8.21 - Stormwater Infiltration Practices

A. Stormwater infiltration practices in this section capture and temporarily store the WQv before allowing it to infiltrate into the soil over a maximum period of 48 hours. The requirements for stormwater infiltration practices in this section apply to infiltration basins, infiltration trenches, subsurface chambers and dry wells. This section does not apply to those practices that infiltrate stormwater into the soil addressed in § 8.22 of this Part (Permeable Pavement), § 8.23 of this Part (Filtering Systems), and § 8.25 of this Part (Open Channel Systems).

B. Feasibility

1. Roof runoff from non-LUHPPL sites can be infiltrated directly, without pretreatment, and counted toward both Rev and WQv requirements.

2. In order to meet the water quality standard, the bottom of infiltration practices must be located in the soil profile. Where a TMDL or CRMC goal requires maximum treatment of runoff, the bottom of infiltration practices shall be within the uppermost soil horizons (A or B) or another BMP is required.

3. To be suitable for infiltration, underlying soils shall have an in-situ infiltration rate of at least 0.5 inches per hour, as initially determined from US Natural Resources Conservation Service soil textural classification, and subsequently confirmed by field geotechnical tests in accordance with § 8.21(E)(4)(b) of this Part. The minimum geotechnical testing at the site of a proposed infiltration practice is 1 test hole per 5,000 square feet, with a minimum of 1 boring or test pit (taken within the proposed limits of the facility). However, for residential rooftop runoff, testing requirements are reduced to 1 infiltration test and 1 test pit per 5 lots assuming consistent terrain and within the same US Natural Resources Conservation Service soil series. If terrain and soil series are not consistent, then requirements increase to 1 infiltration test and 1 test hole per 1 lot.

4. Soils shall also have a clay content of less than 20% and a silt content of less than 60%.

5. The bottom of infiltration practices cannot be located in fill with the exception for strictly residential land uses, for which the bottom of practices may be located in up to 2 feet of fill consisting of material suitable for long-term infiltration after placement. Practices for non-residential sites that cannot be placed in natural soil may be designed as filtering systems. Such cases shall meet the media requirements of sand filters as described in § 8.23 of this Part.

6. To protect groundwater from possible contamination, runoff from designated LUHPPLs or activities shall not be directed to an infiltration facility.

7. The bottom of the infiltration facility shall be separated by at least 3 feet vertically from the SHGT and the bedrock layer (when treating WQv), as documented by on-site soil testing. The SHGT elevation in the area of each infiltration facility must be verified by a DEM-licensed Class IV Soil Evaluator or RI-registered Professional Engineer. The distance may be reduced to 2 feet for strictly residential land uses, excluding roadways.

8. Infiltration practices that are designed for the 10-year storm event or greater and have a separation from the bottom of the system to the seasonal high groundwater of less than four feet shall provide a groundwater mounding analysis.

a. The groundwater mounding analysis must show that the groundwater mound that forms under the infiltration system will not break out above the land or jurisdictional water.

b. Infiltration practices designed for residential rooftops < 1,000 ft² are exempt from this requirement.

9. Infiltration practices cannot be placed in locations that cause water problems (such as seepage which may cause slope failure) to downgrade properties.

10. Infiltration facilities must meet the minimum horizontal setbacks in the table below:

	Minimum Horizontal Setbacks
	From small-scale facilities serving residential properties (feet)	From all other infiltration facilities (feet)
Public Drinking Water Supply Well - Drilled (rock), Driven, or Dug	200	200
Public Drinking Water Supply Well - Gravel Packed, Gravel Developed	400	400
Private Drinking Water Wells	50	100
Surface Water Drinking Water Supply Impoundment with Supply Intake1	100	200
Tributaries that Discharge to the Surface Drinking Water Supply Impoundment1	50	100
Coastal Features	50	50
All Other Surface Waters	50	50
Up-gradient from Natural slopes > %15	25	50
Down-gradient from Building Structures2	10	25
Up-gradient from Building Structures2	10	50
Onsite Wastewater Treatment Systems	15	25

1 Refer to DEM Rules Establishing Minimum Standards Relating to Location, Design, Construction and Maintenance of Onsite Wastewater Treatment Systems, Figures 14-16 for maps of the surface water drinking water impoundments.

2 Setbacks from building structures applies only where basement or slab is below the ponding elevation of the infiltration facility.

C. Conveyance

1. Adequate stormwater outfalls shall be provided for the overflow associated with the 1-year design storm event (non-erosive velocities on the down-slope).

2. The overland flow path of surface runoff exceeding the capacity of the infiltration system shall be evaluated to preclude erosive concentrated flow during the overbank events. If computed flow velocities exiting the system overbank exceed erosive velocities (3.5 to 5.0 feet/second) for the 1-year storm event, an overflow channel and/or level spreader shall be provided.

3. All infiltration systems shall be designed to fully de-water the entire WQv within 48 hours after the storm event.

4. If runoff is delivered by a storm drain pipe or along the main conveyance system, the infiltration practice must be designed as an off-line practice, except when used exclusively to manage CPv and Qp.

D. Pretreatment

1. For infiltration basins, chambers, and trenches, a minimum pretreatment volume of at least 25% of the WQv must be provided to protect the long-term integrity of the infiltration rate. This must be achieved by using one of the following options (see §§ 8.26 through 8.31 of this Part):

a. Grass channel;

b. Filter strip;

c. Sediment forebay; or

d. Deep sump catch basin and one of the following:

(1) Upper sand layer (6 inch minimum with filter fabric at the sand/gravel interface);

(2) Washed pea gravel (1/8 inch to 3/8 inch); or

(3) Proprietary device.

2. Exit velocities from pretreatment chambers flowing over vegetated channels shall be non-erosive (3.5 to 5.0 feet/second) during the 1-year design storm.

E. Treatment

1. If the in-situ infiltration rate for the underlying soils is greater than 8.3 inches per hour, 100% of the WQv shall be treated by an acceptable water quality practice prior to entry into an infiltration facility.

2. Infiltration practices shall be designed to exfiltrate the entire WQv through the floor of each practice, unless the depth is greater than 1/2 the square root of the bottom surface area.

3. The construction sequence and specifications for each infiltration practice shall be precisely followed.

4. Design infiltration rates shall be determined by using either §§ 8.21(E)(4) (a) or (b) of this Part:

a. Design Infiltration Rates for Different Soil Textures (from Rawls, W. I., D. L. Brakensiek, and K. E. Saxton. 1982. Soil water characteristics. Trans. ASAE, 25(5): 13l6-1328.)

US Department of Agriculture Soil Texture	Design Infiltration Rate (inches/hour)	Design Infiltration Rate (feet/minute)
Sand	8.27	0.0115
Loamy Sand	2.41	0.0033
Sandy Loam	1.02	0.0014
Loam	0.52	0.0007
Silt Loam	0.27	0.0004

b. In-situ rates established by one of the approved methods listed below in §§ 8.21(E)(4)(b) ((1)) through ((4)) of this Part. Rates derived from standard percolation tests are not acceptable. Field test methods to assess saturated hydraulic conductivity must simulate the "field-saturated" condition and must be conducted at the depth of the bottom of the proposed infiltrating practice. Design infiltration rates shall be determined by using a factor of safety of 2 from the field-derived value. The saturated hydraulic conductivity analysis must be conducted by a DEM-licensed Class IV Soil Evaluator or RI-registered Professional Engineer.

(1) Guelph permeameter - ASTM D5126 - 16e1 Standard Guide for Comparison of Field Methods for Determining Hydraulic Conductivity in Vadose Zone, incorporated above at § 8.4(C) of this Part.

(2) Falling head permeameter - ASTM D5126 - 16e1 Standard Guide for Comparison of Field Methods for Determining Hydraulic Conductivity in Vadose Zone, incorporated above at § 8.4(C) of this Part.

(3) Double ring permeameter or infiltrometer - ASTM D3385 - 18 Standard Test Method for Infiltration Rate of Soils in Field Using Double-Ring Infiltrometer, incorporated above at § 8.4(D) of this Part; ASTM D5093 - 15e1 Standard Test Method for Field Measurement of Infiltration Rate Using a Double-Ring Infiltrometer with a Sealed-Inner Ring, incorporated above at § 8.4(E) of this Part; ASTM D5126 -16e1 Standard Guide for Comparison of Field Methods for Determining Hydraulic Conductivity in Vadose Zone, incorporated above at § 8.4(C) of this Part.

(4) Amoozemeter or Amoozegar permeameter (Amoozegar, A. 1992. Compact constant head permeameter: a convenient device for measuring hydraulic conductivity. In G.C. Topp et al., Eds. Advances in Measurement of Soil Physical Properties: Bringing Theory into Practice. Soil Science Society of America Special Publication, 30. Soil Science Society of America, Madison, WI, pp. 31-42)

F. Vegetation: Upstream construction shall be completed and stabilized before connection to a downstream infiltration facility. A dense and vigorous vegetative cover shall be established over the contributing pervious drainage areas before runoff can be accepted into the facility.

G. Maintenance

1. A legally binding and enforceable maintenance agreement shall be executed between the facility owner and the responsible authority to ensure the following:

2. Infiltration practices shall never serve as a sediment control device during site construction phase. Great care must be taken to prevent the infiltration area from compaction by marking off the location before the start of construction at the site and constructing the infiltration practice last, connecting upstream drainage areas only after construction is complete, and the contributing area is stabilized. In addition, the SESC plan for the site shall clearly indicate how sediment will be prevented from entering the site of an infiltration facility.

3. An observation well shall be installed in every infiltration trench or chamber system, consisting of an anchored 4- to 6-inch diameter perforated PVC pipe with a lockable cap installed flush with the ground surface. The approving agency may require multiple observation wells for large underground chamber systems.

4. Infiltration practices shall be inspected annually and after storms equal to or greater than the 1-year, 24-hour Type III storm event.

5. If sediment or organic debris build-up has limited the infiltration capabilities (infiltration basins) to below the design rate, the top 6 inches shall be removed and the surface roto-tilled to a depth of 12 inches.

250 R.I. Code R. 250-RICR-150-10-8.21

Amended effective 11/13/2018