(A) Applicability and exemptions.
(1) The stormwater application must include post-construction stormwater quality measures. These measures are incorporated as a permanent feature into the site plan and are left in place following completion of construction activities to continuously treat stormwater runoff from the stabilized site. Any project located within the Town that includes clearing, grading, excavation, and other land disturbing activities, resulting in the disturbance of one (1) acre or more of total land area, is subject to the requirements of this section. This includes land disturbances of less than one (1) acre of land that are part of a larger common plan of development or sale if the larger common plan will ultimately disturb one (1) or more acres of land.
(2) The requirements under this section do not apply to the following activities:
(a) Agricultural land disturbing activities; or
(b) Forest harvesting activities; or
(c) Construction activities associated with a single-family residential dwelling disturbing less than five (5) acres, when the dwelling is not part of a larger common plan of development or sale; or
(d) Single-family residential developments consisting of four (4) or fewer lots; or
(e) A single-family residential strip development where the developer offers for sale or lease without land improvements and the project is not part of a larger common plan of development of sale; or
(f) Individual building lots within a larger permitted project; or
(g) Underground utility projects with no change to the surface conditions; or
(h) Land disturbances with no improvements or impervious surfaces
(3) The requirements under this section do not apply to the following activities, provided other applicable state permits contain provisions requiring immediate implementation of soil erosion control and pollution prevention measures:
(a) Landfills that have been issued a certification of closure under 329 IAC 10.
(b) Coal mining activities permitted under I.C. 14-34.
(c) Municipal solid waste landfills that are accepting waste pursuant to a permit issued by IDEM under 329 IAC 10 that contains equivalent stormwater requirements, including the expansion of landfill boundaries and construction of new cells either within or outside the original solid waste permit boundary.
(4) It will be the responsibility of the project site owner to ensure proper construction and installation of all stormwater BMPs in compliance with this chapter and with the approved Stormwater Management Plan, and to notify the Town with a sufficient NOT letter upon completion of the project and stabilization of the site. However, all eventual property owners of stormwater quality facilities meeting the applicability requirements must comply with the requirements of this chapter and this section.
(B) Policy on stormwater quality management.
(1) BMPs must be implemented for the management of total suspended solids (TSS) including floatables. BMP systems or treatment trains shall have at least one component of the system providing floatable control.
(2) The project site owner must submit to the Town a SWPPP that would show placement of appropriate BMPs designed, constructed, and maintained.
(3) For ease of maintenance and to reduce the frequency of pond dredging, all detention/retention facilities (including underground detention) shall have an upstream BMP installed for each inlet pipe, to collect sediments, debris, floatables, and other material prior to entering the detention/retention facility. Forebays are not acceptable. Position BMPs outside right-of-way and not more than fourteen (14) feet from a drivable surface for vac-truck and maintenance vehicle access.
(4) Gasoline outlets and refueling areas must install appropriate practices to reduce lead, copper, zinc, and polyaromatic hydrocarbons in stormwater runoff. These requirements will apply to all new facilities and existing facilities that replace their fuel tanks.
(5) Infiltration practices will not be allowed in wellhead protection areas. Discharges will not be allowed directly into sinkholes or fractured bedrock, without treatment that results in the discharge meeting Indiana groundwater quality standards as referenced in 327 IAC 2-11.
(6) Any stormwater practice that is a Class V injection well must ensure that the discharge from such practices meets Indiana groundwater quality standards as referenced in 327 IAC 2-11.
(C) Calculations and design standards and specifications.
(1) Calculation of land disturbance should follow the guidelines contained in § 151.22(C).
(2) The target TSS removal rate is eighty percent (80%). For the purposes of this requirement, TSS is defined as particles smaller than one hundred twenty-five (125) microns in diameter.
(3) There are two methods for calculating the required size of a BMP. The first method calculates the water quality volume to be treated, which applies to detention-based BMPs. The second method calculates the water quality peak flow rate to be treated, which applies to filtration processes and mechanical-type BMPs such as hydrodynamic devices.
(4) The appropriate stormwater quality volume (WQv) and/or stormwater quality flow rate (Qwq) generated from a qualifying site shall be adequately treated before discharge. Traditional structural BMPs are presumed to comply with the eighty percent (80%) TSS removal rate if BMPs are:
(a) Sized to capture the prescribed water quality volume or flow rate, as applicable;
(b) Designed according to the specific performance criteria outlined in Appendix D of the Hendricks County Stormwater Technical Standard Manual or the Indiana Stormwater Quality Manual;
(c) Constructed properly; and
(d) Maintained regularly.
(5) Mechanical-type BMPs must be approved by the Town and meet ASTM standard methods for verifying performance and must be certified by a professional engineer. The BMP must meet the 80% TSS removal rate without resuspension of particles at the design water quality flow rate resulting from a one (1)-inch rainfall depth. Testing of the TSS removal rate must be conducted by an independent testing facility rather than by the manufacturer.
(6) Water quality volume (WQv). The WQV is the storage needed to capture and treat the runoff from the first one (1)-inch of rainfall. A twenty-four (24) to forty (40)-hour emptying time must be used for the release of the WQv, with no more than fifty percent (50%) of the water quality volume being released in the first twelve (12) hours. The WQv is equivalent to one (1) inch of rainfall multiplied by the volumetric runoff coefficient (Rv) and the site area. The volume of runoff is directly related to the amount of impervious cover at the site and is calculated using the following equation:
WQv = PRVA
12
where:
WQv = | water quality volume (acre-feet, ac-ft) |
P = | rainfall depth (inches, in) = 1 inch |
Rv = | volumetric runoff coefficient - see the following equation |
A = | site area (acres) |
Rv = 0.05 + 0.009 I
where:
Rv = | volumetric runoff coefficient |
I = | percentage of impervious cover (%) |
(7) Water quality flow rate (Qwq). The Qwq is needed to size BMP devices designed to treat runoff at a peak design flow rate through the system. Conventional NRCS (SCS) methods have been found to underestimate the volume and rate of runoff for rainfall events less than two (2) inches. The following procedure can be used to calculate the Qwq. The method relies on the water quality volume in conjunction with an adjusted curve number (CNwq) and the NRCS TR-55 methodology.
Step 1 - Using the water quality volume, calculate the adjusted CNwq using the following equation:
CNwq = | 1000 |
10+5 P+10 WQvi-10 (WQvi2+1.25 WQviP)½ |
where:
CNwq = | adjusted curve number for water quality flow rate |
WQvi = | water quality volume (inches, in) - see equation below |
P = | rainfall depth (inches, in) = 1 inch |
WQvi = (1 inch) Rv
where:
WQvi = | water quality volume (inches, in) |
Rv = | volumetric runoff coefficient |
Step 2 - Calculate the site time of concentration in minutes (Tc) and area in acres (A).
Step 3 - Use the adjusted CNwq, Tc and A as input for TR-55 calculations in conjunction with the Huff II rainfall distribution, for 1-inch of rainfall depth using the storm duration that produces the highest peak flow rate to calculate the Qwq.
(D) Easement requirements. All stormwater quality management systems, including detention or retention basins, filter strips, pocket wetlands, in-line filters, infiltration systems, conveyance systems, structures and appurtenances are to be located outside of the right-of-way and shall be incorporated into permanent easements as listed in § 151.21(J).
(E) Submittal requirements. A post-construction stormwater pollution prevention plan is required in addition to the submittal requirements listed in § 151.20. This plan must include the following information:
(1) A description of potential pollutant sources from the proposed land use that may reasonably be expected to add a significant amount of pollutants to stormwater discharges.
(2) Location, dimensions, detailed specifications, and construction details of all post-construction stormwater quality measures.
(3) A description of measures that will be installed to control pollutants in stormwater discharges that will occur after construction activities have been completed. Such practices include infiltration of runoff, flow reduction by use of open vegetated swales and natural depressions, buffer strip and riparian zone preservation, filter strip creation, minimization of land disturbance and surface imperviousness, maximization of open space, and stormwater retention and detention ponds.
(4) Calculations demonstrating the required water quality treatment volume or water quality treatment rate for post-construction stormwater quality measures.
(5) A sequence describing when each post-construction stormwater quality measure will be installed.
(6) Stormwater quality measures that will remove or minimize pollutants from stormwater runoff.
(7) Stormwater quality measures that will be implemented to prevent or minimize adverse impacts to stream and riparian habitat.
(8) An O&M Manual shall be provided for each stormwater BMP in accordance with § 151.29.
(Ord. 2017-28, passed 10-12-17)