250 R.I. Code R. 250-RICR-150-10-8.33

Current through November 21, 2024

Section 250-RICR-150-10-8.33 - Quantity Control - Stormwater Basins

A. Wet basins may be located in the groundwater table; dry basins do not need a permanent pool and may be designed such that the groundwater table is at or below the bottom of the basin.

B. Feasibility

1. Wet Extended Detention Basins shall have a minimum contributing drainage area of 25 acres, unless groundwater is intercepted.

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

3. The use of basins 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 the CPv through an underdrained gravel trench outlet. Additional storage for Qp may be discharged through traditional basin outlet structures.

4. Basins receiving runoff from LUHPPLs must be lined and shall not intercept groundwater.

5. Basins that do intercept groundwater (allowed as long as not receiving runoff from LUHPPLs) shall not include the volume of the permanent pool in storage calculations.

C. Conveyance

1. The channel immediately below a basin 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.

2. 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/second).

3. Outfalls, where needed, shall be constructed such that they do not increase erosion or have undue influence on the downstream geomorphology of any natural watercourse by discharging at or near the stream water surface elevation or into an energy dissipating step-pool arrangement.

4. All basins shall have an emergency outlet to accommodate the storm flow in excess of the 100-year storm event maintaining at least one foot of freeboard between the peak storage elevation and the top of the embankment crest, and to safely convey the 100-year storm without overtopping the embankment.

D. Treatment: Stormwater basins shall not be used for meeting the water quality treatment standard.

E. Using Basins for Additional Pollutant Loading Reduction: In order to use the pollutant removal rates for dry extended detention basins and wet extended detention basins as listed in § 8.38(E) of this Part, the following design criteria must be met.

1. Each basin shall have a sediment forebay or equivalent upstream pretreatment. The forebay shall be sized to contain 10% of the water quality volume (WQv) sized per § 8.29 of this Part. The forebay storage volume counts toward the total WQv requirement.

2. The minimum detention time for the WQv shall be 24 hours.

3. Storage for the channel protection volume (CPv) and the WQv shall be computed and routed separately. The WQv cannot be met simply by providing CPv storage for the one-year storm.

4. Provide water quality treatment storage to capture the computed WQv from the contributing drainage area through a combination of permanent pool and extended detention, as outlined in the table in § 8.33(F) of this Part.

5. The minimum length to width ratio for a basin shall be 1.5:1 (length relative to width).

6. Provide a minimum Drainage Area: Surface Area Ratio of 75:1.

7. Incorporate an aquatic bench that 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.

F. Minimum Required Storage Volumes for Basins Used for Enhanced Pollutant Removal

Design Variation	%WQv
Design Variation	Permanent Pool	Extended Detention
Dry Extended Detention Basin	20% min.	80% max.
Wet Extended Detention Basin	50% min.	50% max.

G. Vegetation

1. The perimeter of all deep pool areas (four feet or greater in depth) shall be surrounded by a safety bench. Except when basin side slopes are 4:1 (h:v) or flatter, provide a safety bench that generally extends 15 feet outward (a 10' 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 basin side slope. The maximum slope of the safety bench shall be 6%.

2. A planting plan for a stormwater basin and its setback shall be prepared to indicate how the basin perimeter will be stabilized and established with vegetation.

H. Basin Setbacks

1. A basin setback from structures, roads, and parking lots shall be provided that extends 25 feet outward from the maximum water surface elevation of the basin.

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.

I. Maintenance

1. Maintenance responsibility for a basin 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. The principal spillway shall be equipped with a removable trash rack, and generally accessible from dry land.

3. A maintenance right-of-way or easement shall extend to a basin from a public or private road.

4. Sediment shall be removed from stormwater basins when the sediment volume exceeds 10% of the total basin volume.

5. 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 every six months. Inspection shall consist of verifying that the wet basin 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.

6. Annual mowing of the basin setback is only required along maintenance rights-of-way and the embankment. The remaining setback can be managed as rangeland (mowing every other year) or forest.

J. Non-clogging Low-flow Orifice: When CPv is required, a low-flow orifice shall be provided, with the design of the orifice sufficient to ensure that no clogging shall occur.

K. Outlet Control Structure

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

2. The outlet control structure shall be sized and designed for CPv and Qp, as required.

3. For discharges beyond 200 feet from jurisdictional waters 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 basin;

b. Shall be four feet wide, located at least 2 feet from the 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 gravel trench and adjacent soil; and

e. Shall utilize clean poorly-graded (uniform size material) gravel.

L. Basin Drain

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

2. 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.

M. Safety Features

1. Side slopes to the basin shall not exceed 3:1 (h:v) and, for wet basins, shall terminate on a 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.

250 R.I. Code R. 250-RICR-150-10-8.33

Amended effective 11/13/2018