250 R.I. Code R. 250-RICR-150-10-8.10

Current through November 21, 2024

Section 250-RICR-150-10-8.10 - Minimum Standard 4: Conveyance and Natural Channel Protection

A. Open drainage and pipe conveyance systems must be designed to provide adequate passage for flows leading to, from, and through stormwater management facilities for at least the peak flow from the 10-year, 24-hour Type III design storm event. Protection for natural channels downstream must be supplied by providing 24-hour extended detention of the 1-year, 24-hour Type III design storm event runoff volume.

B. The channel protection volume (CPv) is the 24-hour extended detention of the post-development runoff volume from the 1-year, 24-hour Type III design storm event. If a stormwater discharge is proposed within 200 feet of streams and any contiguous natural or vegetated wetlands in watersheds draining to cold-water fisheries, surface detention practices are prohibited (underground detention or infiltration practices will be required). Consult DEM's Water Quality Regulations to determine if a project is in a watershed directly draining to a cold-water fishery. Discharges beyond 200 feet shall be designed to discharge up to the CPv through an underdrained gravel trench outlet.

C. For facility sizing criteria, the basis for hydrologic and hydraulic evaluation of development sites are as follows:

1. The models TR-55, incorporated above at § 8.4(A) of this Part, or TR-20, incorporated above at § 8.4(B) of this Part, (or approved equivalent) shall be used for determining the CPv.

2. The Rational Method may be used for sizing the conveyance system.

3. Off-site areas draining to proposed facility shall be modeled as "present condition" for the 1-year storm event.

4. The length of sheet flow used in time of concentration (tc) calculations is limited to no more than 100 feet for post-development conditions.

5. The required minimum CPv shall be computed using either §§ 8.10(C)(5) (a) or (b) of this Part below:

a. A modified version of the TR-55, incorporated above at § 8.4(A) of this Part, short-cut sizing approach.

(1) This modification (Harrington, 1987. Design Procedures for Stormwater Management Extended Detention Structures. Maryland Department of Environment, Dundalk, MD) is for applications where the peak discharge is very small compared with the uncontrolled discharge. This often occurs in the 1-year, 24-hour Type III detention sizing. Using TR-55, incorporated above at § 8.4(A) of this Part, the unit peak discharge (qu) can be determined based on the curve number and time of concentration. Knowing qu and T (extended detention time), qo/qI (peak outflow discharge/peak inflow discharge) can be estimated from Figure in § 8.10(C)(5)(a) ((2)) of this Part. Figure in § 8.10(C) (5)(a) ((3)) of this Part can also be used to estimate Vs/Vr. When qo/qI is <0.1 and off the graph, Vs/Vr can also be calculated using the following equation for Type II/III rainfall distributions:

Vs/Vr = 0.682 - 1.43 (qo/qI) + 1.64 (qo/qI)2 - 0.804 (qo/qI)3

Where:

Vs = required storage volume (acre-feet)

Vr = runoff volume (acre-feet)

qo = peak outflow discharge (cubic feet per second)

qI = peak inflow discharge (cubic feet per second)

(2) Detention Time vs. Discharge Ratios. (Maryland Stormwater Design Manual. Maryland Department of the Environment (MDE) 2000)

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(3) Approximate Detention Basin Routing For Rainfall Types I, IA, II, and III. (TR-55, incorporated above at § 8.4(A) of this Part)

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b. By calculating 65% of the direct runoff volume from the post-development 1-year, 24-hour Type III storm based on one of the approved models listed above, using the following equation:

Vs = 0.65 * Vr Where:

Vs = CPv = required channel protection storage volume; and

Vr = runoff volume from 1-year, 24-hour Type III storm.

6. The CPv shall be released at roughly a uniform rate over a 24-hour duration. To determine the average release rate, use the following equation:

Average release rate = Vr / T

Where:

Vr = defined above; and

T = extended detention time (24 hours)

D. The CPv criterion can be waived for sites that:

1. Direct discharge to a large river (4th-order stream or larger), bodies of water > 50 acres in surface area, or tidal waters.

2. Small facilities with impervious cover less than or equal to 1 acre.

3. Projects when the post-development peak discharge from the facility without attenuation is less than 2 cubic feet per second for the 1-year, 24hour Type III design storm event.

250 R.I. Code R. 250-RICR-150-10-8.10

Amended effective 11/13/2018