250 R.I. Code R. 250-RICR-150-10-8.20

Current through November 21, 2024

Section 250-RICR-150-10-8.20 - Wet Vegetated Treatment Systems (WVTS)

A. Feasibility

1. WVTS designs shall not be located within jurisdictional waters, including wetlands; except that on already developed sites, WTVS designs may be allowed in jurisdictional upland buffers in areas already altered under existing conditions, if acceptable to the approving agency.

2. WVTS designs shall not be located within stream channels in order to prevent habitat degradation caused by these structures.

3. Assess the hazard classification of the structure and consider alternative placement and/or design refinements to reduce or eliminate the potential for the structure being subject to the RI DEM Rules and Regulations for Dam Safety, Part 130-05-1 of this Title.

4. The use of WVTS designs in watersheds draining to cold-water fisheries is restricted to prohibit discharges within 200 feet of streams and any contiguous natural or vegetated wetlands. Discharges beyond 200 feet shall be designed to discharge up to and including the CPv through an underdrained gravel trench outlet. Additional storage for Qp may be discharged through traditional outlet structures.

5. WVTS designs specified to manage LUHPPL runoff require a 3-foot separation to groundwater. All other land uses do not require groundwater separation.

6. The volume below the surface elevation of the permanent pool shall not be included in storage calculations for peak flow management (CPv/Qp).

7. Setbacks for WVTS designs from OWTSs shall be consistent with the setbacks in DEM's Rules Establishing Minimum Standards Relating to Location, Design, Construction and Maintenance of Onsite Wastewater Treatment Systems.

B. Conveyance

1. Flow paths from the inflow points to the outflow points of WVTS shall be maximized through the use of BMP geometry and features such as berms and islands.

2. The channel immediately below a WVTS outfall shall be modified to prevent erosion and conform to natural dimensions in the shortest possible distance, typically by use of appropriately sized riprap placed over filter cloth.

3. A stilling basin or outlet protection shall be used to reduce flow velocities from the principal spillway to non-erosive velocities (3.5 to 5.0 feet per second).

4. A subsurface water level must be maintained in the gravel WVTS through the design of the outlet elevation (invert just below the surface). The outlet invert location must be open or vented to prevent a siphon that would drain the WVTS.

5. For discharges beyond 200 feet from streams (and any contiguous natural or vegetated wetlands) in cold-water fisheries, the underdrained gravel trench shall be designed to meet the following requirements:

a. Shall be sized to release the CPv over at least 12 hours and not more than 24 hours to provide adequate cooling of stormwater runoff discharging from the WVTS;

b. Shall be 4 feet wide, located at least 2 feet from the WVTS permanent pool, and located at the furthest location opposite from the principal inflow location to the facility;

c. The trench shall have a length of 3 feet per 1,000 cubic feet of CPv storage volume, have a depth of at least 3 feet, and maintain 2 feet of gravel cover over a 6-inch diameter perforated pipe outlet (Rigid Schedule 40 PVC or SDR35);

d. Shall utilize geotextile fabric placed between the gravel trench and adjacent soil; and e. Shall utilize clean poorly-graded gravel (i.e., uniform stone size).

C. WVTS Liners: When a WVTS is located in medium to coarse sands and above the average groundwater table, a liner shall be used to sustain a permanent pool of water. If geotechnical tests confirm the need for a liner (soils with an infiltration rate of 0.05 inches/hour or greater), acceptable options include:

1. 6 to 12 inches of clay soil (minimum 15% passing the #200 sieve and a minimum permeability of 1 x 10-5 cm/sec);

2. A 30 mil poly-liner;

3. Bentonite; or

4. Use of chemical additives.

D. Pretreatment - Sediment Forebay

1. Each WVTS shall have a sediment forebay or equivalent upstream pretreatment. The forebay shall consist of a separate cell.

2. The forebay shall be sized to contain a minimum of 10% of the WQv, and shall be at least 3 feet deep. The forebay storage volume counts toward the total WQv requirement.

3. A forebay shall be provided at each inlet, unless the inlet provides less than 10% of the total design storm inflow to the WVTS.

4. The forebay shall be designed with non-erosive outlet conditions.

5. Direct access for appropriate maintenance equipment shall be provided to the forebay.

E. Minimum Water Quality Volume (WQv)

1. The surface area of a shallow WVTS shall be at least 1.5% of the contributing drainage area; the gravel WVTS surface area shall be at least 0.35% of contributing drainage area.

2. For a shallow WVTS: A minimum of 35% of the total surface area shall have a depth of 6 inches or less, and at least 65% of the total surface area shall be shallower than 18 inches. At least 10% of the WQv shall be provided in a sediment forebay or other pretreatment practice, and at least 25% of the WQv shall be provided in "deep water zones" with a depth equal to or greater than 4 feet. The remaining 65% of the WQv shall be provided in some combination of shallow permanent pool (depth less than 4 feet) and the ED storage volume above the permanent pool, as applicable. ED storage volume shall not exceed 50% of the WQv and shall drain over 24 hours.

3. For a gravel WVTS: At least 10% of the WQv shall be provided in a sediment forebay or other pretreatment practice. The remaining 90% of the WQv shall be provided in some combination of one or more basins or chambers filled with a minimum 24-inch gravel layer and the open, ED storage volume above the gravel, as applicable. ED storage volume shall not exceed 50% of the WQv and shall drain over 24 hours.

F. Minimum WVTS Geometry

1. Flow paths from the inflow points to the outflow points of WVTS shall be maximized through the use of BMP geometry and features such as berms and islands. The minimum length to width ratio for a shallow WVTS is 2:1.

2. For a gravel WVTS: length to width ratio of 1:1 or greater is needed for each treatment cell with a minimum flow path within the gravel substrate of 15 feet.

G. Shallow WVTS Benches: The perimeter of all deep pool areas (four feet or greater in depth) shall be surrounded by two benches as follows:

1. Except when side slopes are 4:1 (h:v) or flatter, provide a safety bench that generally extends 15 feet outward (a 10 foot minimum bench is allowable on sites with extreme space limitations at the discretion of the approving agency) from the normal water edge to the toe of the WVTS side slope. The maximum slope of the safety bench shall be 6%; and

2. Incorporate an aquatic bench that generally extends up to 15 feet inward from the normal edge of water, has an irregular configuration, and a maximum depth of 18 inches below the normal pool water surface elevation.

H. Planting Plan

1. A planting plan for a WVTS and its setback shall be prepared to indicate how aquatic and terrestrial areas will be stabilized and established with vegetation. Minimum elements of a plan include: delineation of pondscaping zones, selection of corresponding plant species, plant locations, sequence for preparing WVTS bed (including soil amendments, if needed), and sources of plant material. Donor plant material must not be from natural wetlands.

2. Donor soils for WVTS mulch shall not be removed from natural wetlands.

I. WVTS Setbacks

1. A WVTS setback shall be provided that extends 25 feet outward from the maximum design water surface elevation of the WVTS.

2. Woody vegetation shall not be planted or allowed to grow on a dam, or within 15 feet of a dam or toe of the embankment, or within 25 feet of a principal spillway outlet.

J. Maintenance

1. Maintenance responsibility for a WVTS and its setback shall be vested with a responsible authority by means of a legally binding and enforceable maintenance agreement that is executed as a condition of plan approval.

2. General inspections shall be conducted on an annual basis and after storm events greater than or equal to the 1-year, 24-hour Type III precipitation event.

3. The principal spillway shall be equipped with a removable trash rack, and generally accessible from dry land.

4. A maintenance and operation plan must specify that sediment removal in the forebay shall occur every 5 years or after 50% of total forebay capacity has been lost, whichever occurs first.

5. An operation and maintenance plan shall specify that if a minimum vegetative coverage of 50% is not achieved in the planted areas after the second growing season, a reinforcement planting is required.

6. Sediment and organic build-up shall be removed from a gravel WVTS every 2 years, as needed.

7. In a gravel WVTS, vertical cleanouts must be constructed that are connected to the distribution and collection subdrains at each end.

8. For discharges beyond 200 feet from streams (and any contiguous natural or vegetated wetlands) in cold-water fisheries, the gravel trench outlet shall be inspected after every storm in the first 3 months of operation to ensure proper function. Thereafter, the trench shall be inspected at least once annually. Inspection shall consist of verifying that the WVTS is draining to the permanent pool elevation within the 24-hour design requirement and that potentially clogging material, such as accumulation of decaying leaves or debris, does not prevent the discharge through the gravel. When clogging occurs, at least the top 8 inches of gravel shall be replaced over with new material. Sediments shall be disposed of in an acceptable manner.

9. A maintenance right of way or easement shall extend to a WVTS from a public or private road.

10. A low-flow orifice or weir shall be provided when a WVTS is sized for the CPv. The low-flow orifice or weir shall be designed to ensure that no clogging shall occur.

11. The outlet control structure shall be located within the embankment for maintenance access, safety and aesthetics.

12. Except where local slopes prohibit this design, each WVTS shall have a drain pipe that can completely or partially drain the practice. The drain pipe shall have an elbow or protected intake within the WVTS to prevent sediment deposition, and a diameter capable of draining the permanent pool within 24 hours.

13. Access to the drain pipe shall be secured by a lockable structure to prevent vandalism and/or accidental draining of the pond, which could pose a safety hazard due to high drainage velocities.

K. Safety Features

1. Proposed graded side slopes to the WVTS shall not exceed 3:1 (h:v), and shall terminate on the safety bench.

2. The principal spillway opening shall not permit access by small children, and endwalls above pipe outfalls greater than 48 inches in diameter shall be fenced to prevent a hazard.

3. Token or emergency spillways (those placed above the water elevation of the largest managed storm) are required if not already provided as part of the conveyance of the 100-year storm event and must be a minimum 8 feet wide, 1 foot deep, with 2:1 channel side slopes.

250 R.I. Code R. 250-RICR-150-10-8.20

Amended effective 11/13/2018