250 R.I. Code R. 250-RICR-150-10-8.35

Current through November 21, 2024

Section 250-RICR-150-10-8.35 - Stormwater Infiltration Practices for Recharge/Storage Only

A. On sites where the soil infiltration rate is high (greater than 8.3 inches/hour), infiltration practices cannot be used to treat the water quality volume. However, they may be used to provide recharge (Rev), channel protection (CPv) and/or overbank flood control (Qp) storage (applicants must provide treatment of 100% of the WQv prior to direct infiltration).

B. Feasibility

1. The bottom of the infiltration facility shall be separated by at least 2 feet vertically from the SHGT or bedrock layer, as documented by on-site soil testing, unless a mounding analysis shows that the system will accept the stormwater without causing breakout or backup into the system with less than 2 feet vertical separation. 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.

2. 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 SHGT of less than 4 feet shall provide a groundwater mounding analysis. 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.

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

4. Infiltration facilities must meet the minimum horizontal setbacks 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	10	25
Structures2
Up-gradient from Building Structures2	10	50
Onsite Wastewater Treatment Systems	15	25
1 Refer to DEM Rules Establishing Minimum Standards Relating t Location, Design, Construction and Maintenance of Onsite Wastewater Treatment Systems, Figures 14-16 for maps of the drinking water impoundments. 2 Setbacks from building structures applies only where basement of 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.

D. Design

1. Infiltration practices shall be designed to exfiltrate the design volume through the floor of each practice (sides are not considered in sizing), except where the depth is greater than the square root of the bottom surface area.

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

3. Design infiltration rates shall be determined in accordance with § 8.21(E) (4) of this Part.

E. 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.

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

1. 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 only constructing the infiltration practice last, connecting upstream areas only after construction is complete, and the contributing area 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.

2. 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.

250 R.I. Code R. 250-RICR-150-10-8.35

Amended effective 11/13/2018