250 R.I. Code R. 250-RICR-150-10-8.25

Current through November 21, 2024

Section 250-RICR-150-10-8.25 - Open Channel Systems

A. Feasibility

1. Open channels shall have a maximum drainage area of 5 acres draining to any 1 inlet. No maximum drainage area if flow enters via sheet flow along a linear feature, such as a road.

2. Open channels shall have a maximum longitudinal slope of 4%, without check dams.

3. Wet Swales are constructed in groundwater. The bottom of a Dry Swale shall be located at or above the seasonal high groundwater table; the top of a Dry Swale shall be located at least 3 feet above the seasonal high groundwater table.

4. Wet swales shall be placed a minimum 50 feet downgradient of any OWTS drainfield.

B. Conveyance

1. The maximum allowable temporary ponding time within a channel shall be less than 48 hours. An underdrain system shall be used in the dry swale to ensure this ponding time, unless designed as an exfilter in which case an underdrain might not be necessary.

2. The peak velocity for the 1-year storm must be non-erosive ( 3.5-5.0 feet/second).

3. Open channels shall be designed to safely convey the 10-year storm.

4. Channels shall be designed with moderate side slopes (flatter than 3:1) for most conditions. Designers may utilize a 2:1 maximum side slope, where 3:1 slopes are not feasible.

5. If the site slope is greater than 4%, additional measures such as check dams shall be utilized to retain the water quality volume within the swale system.

C. Pretreatment: Provide 10% of the WQv in pretreatment.

D. Treatment

1. Wet swale length, width, depth, and slope shall be designed to temporarily accommodate the WQv through surface ponding.

2. Dry swales shall consist of the following treatment components: A 30 inch deep bioretention soil bed, a surface mulch layer, and no more than a 12 inch deep average surface ponding depth. Soil media shall meet the specifications outlined for bioretention areas.

3. The minimum filter area for dry swales shall be sized based on the principles of Darcy's Law. A coefficient of permeability (k) shall be used as follows:1.0 feet/day for sandy-loam soils. The minimum required filter area is computed using the following equation:

Af = (WQv) (df) / [(k) (hf + df) (tf)]

Where:

Af = Surface area of filter bed (square feet)

Df = Filter bed depth (feet)

K = Coefficient of permeability of filter media (feet/day)

hf = Average height of water above dry swale surface (feet)

tf = Design filter bed drain time (days)

(2 days is maximum tf for dry swales)

4. Swales shall be designed with a bottom width no greater than 8 feet to avoid potential gullying and channel braiding, but no less than 2 feet.

5. The bioretention soil depth of dry swales may be reduced to 12 inches on a case-by-case basis as demonstrated by the designer that 30 inches is not feasible, such as sites with high groundwater or shallow depth to bedrock or clay soils, or in retrofit situations where pre-existing site constraints exist. In these cases, the designer should add 20% (by volume) of well-aged ( 6-12 months), well-aerated, leaf compost (or approved equivalent) to the bioretention soil mixture and will need to provide a calculation to demonstrate that an equal WQv is provided as with a 30 inch deep soil bed.

E. Maintenance

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

2. Open channel practices shall be inspected annually and after storms of greater than or equal to the 1-year, 24-hour Type III precipitation event.

3. Sediment build-up within the bottom of the channel or filter strip shall be removed when 25% of the original WQv volume has been exceeded.

4. Eroded side slopes and channel bottoms shall be stabilized as necessary.

5. In the absence of evidence of contamination, removed debris may be taken to a landfill or other permitted facility.

6. Sediment testing may be required prior to sediment disposal when a LUHPPL is present.

7. Vegetation in dry swales shall be mowed as required to maintain grass heights in the 4-6 -inch range, with mandatory mowing once grass heights exceed 10 inches.

8. Woody vegetation in wet swales shall be pruned where dead or dying branches are observed, and reinforcement plantings shall be planted if less than 50% of the original vegetation establishes after two years.

9. If the surface of the dry swale becomes clogged to the point that standing water is observed on the surface 48 hours after precipitation events, the bottom shall be roto-tilled or cultivated to break up any hard-packed sediment, and then reseeded.

250 R.I. Code R. 250-RICR-150-10-8.25

Amended effective 11/13/2018