Rhode Island Code of Regulations › Title 250 - Department of Environmental Management › Chapter 150 - Water Resources › Subchapter 10 - Wastewater And Storm Water › Part 8 - Stormwater Management, Design and Installation Rules › Section 250-RICR-150-10-8.10 - Minimum Standard 4: Conveyance and Natural Channel Protection

Current through June 14, 2024
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)
Click
					 here to view
(3)
				  Approximate Detention Basin Routing For Rainfall Types I, IA, II, and III.
				  (TR-55, incorporated above at §
													8.4(A) of
				  this Part)
Click
					 here to view
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)