Section 250-RICR-150-10-8.9 - Minimum Standard 3: Water QualityA. Stormwater runoff must be treated before discharge. The amount that must be treated from each rainfall event is known as the required water quality volume (WQv). The required WQv is calculated as described in §§ 8.9(E) through (J) of this Part and excludes LID credits allowed under § 8.18 of this Part.B. The WQv must be treated by at least one of the structural BMPs listed in §§ 8.19 through 8.25 of this Part at each location where a discharge of stormwater will occur. Structural BMPs are generally required to achieve the following minimum average pollutant removal efficiencies: 85% removal of total suspended solids, 60% removal of pathogens, 30% removal of total phosphorus for discharges to freshwater systems, and 30% removal of total nitrogen for discharges to saltwater or tidal systems. Based upon results published in the scientific literature, the structural BMPs listed in §§ 8.19 through 8.25 of this Part will meet these standards when properly designed, constructed, and maintained. Pretreatment is required for water quality treatment practices where specified in §§ 8.19 through 8.25 of this Part.C. BMPs targeted to remove other pollutant(s) of concern and/or to achieve higher pollutant removal efficiencies may be required for impaired receiving waters, drinking water reservoirs, bathing beaches, shellfishing grounds, Outstanding National Resource Waters, Special Resource Protection Waters, tributaries thereto, and for those areas where watershed plans, including Special Area Management Plans or Total Maximum Daily Loads, have been completed. In some cases, the permitting agencies may require that an applicant prepare and submit a pollutant loading analysis developed in accordance with the provisions of §§ 8.36 through 8.38 of this Part.D. Applicants or other interested parties may petition DEM and CRMC to add one or more BMPs to the list of acceptable structural stormwater controls described in §§ 8.19 through 8.25 of this Part by submitting monitoring results and supporting information developed in accordance with the provisions of the Technology Assessment Protocol in §§ 8.39 and 8.40 of this Part.E. The required WQv, which results in the capture and treatment of the entire runoff volume for 90% of the average annual storm events, is equivalent to the runoff associated with the first 1.2 inches of rainfall over the impervious surface (i.e., 1 inch of runoff). The water quality volume requirement may be waived or reduced by applying the LID Stormwater Credit outlined in § 8.18 of this Part. The WQv is calculated using the following equation: WQv = (1") (I) / 12
Where:
WQv = water quality volume (in acre-feet)
I = impervious area (acres)
F. A minimum WQv value of 0.2 watershed inches (0.2 inches over the entire disturbed area) is required, which requires the calculation of the total site disturbance. This minimum treatment volume is necessary to fully treat the runoff from pervious surfaces on sites with low impervious cover, i.e., less than 20% of the disturbed area. However, this requirement does not imply that every pervious subarea of disturbance must be treated with a structural water quality BMP.G. For facility sizing criteria, the basis for hydrologic and hydraulic evaluation of development sites should be as follows:1. Impervious cover is measured from the site plan and includes all impermeable surfaces; and2. Off-site areas shall be assessed based on their "pre-development condition" for computing the water quality volume (i.e., treatment of only on-site areas is required). However, if an off-site area drains to a proposed BMP, flow from that area must be accounted for in the sizing of a specific practice.H. Acceptable water quality treatment BMPs are described in detail in §§ 8.19 through 8.25 of this Part. Other practices may be used to meet other criteria, such as recharge or flood control, but only the practices §§ 8.19 through 8.25 of this Part may be used to meet the water quality criterion. In addition, disconnection of impervious areas (see § 8.18 of this Part) may be used to meet some or all of the WQv, including the minimum WQv.I. Although most of the stormwater treatment practices in this Rule are sized based on WQv, flow diversion structures for off-line stormwater treatment practices must be designed to bypass flows greater than the WQf. The WQf shall be calculated using the WQv described above and a modified curve number (CN) for small storm events. 1. The following equation shall be used to calculate a modified CN. This modified CN can then be used in a traditional TR-55 model, incorporated above at § 8.4(A) of this Part, or spreadsheet in order to estimate peak discharges for small storm events. Using the water quality volume, a corresponding CN is computed utilizing the following equation: CN = 1000 / [10 + 5P +10Q - 10(Q² + 1.25 QP)1/2]
Where:
P = rainfall, in inches (use 1.2 inches for the Water Quality Storm that produces 1 inch of runoff)
Q = runoff volume, in watershed inches (equal to WQv ÷ total drainage area)
2. When using a hydraulic/hydrologic model for facility sizing and WQf determination, designers must use this adjusted CN for the drainage area to generate runoff equal to the WQv for the 1.2-inch precipitation event.3. Designers can also use a TR-55 model, incorporated above at § 8.4(A) of this Part, spreadsheet to find the WQf. Using the computed CN from the equation above, the time of concentration (tc), and drainage area (A); the WQf for the water quality storm event can be computed with the following steps: a. Read initial abstraction (Ia) from TR-55, incorporated above at § 8.4(A) of this Part, Table 4.1 or calculate using Ia = 200/CN - 2b. Compute Ia/P (P = 1.2 inches)c. Approximate the unit peak discharge (qu) from TR-55, incorporated above at § 8.4(A) of this Part, Exhibit 4-III using tc and Ia/Pd. Compute the peak discharge (WQf) using the following equation: WQf = qu * A * Q
Where:
WQf = the peak discharge for water quality event, in cubic feet per second
qu = the unit peak discharge, in cubic feet per second/square mile/inch
A = drainage area, in square miles
Q = runoff volume, in watershed inches (equal to WQv ÷ A)
250 R.I. Code R. 250-RICR-150-10-8.9
Amended effective 11/13/2018