Ch9.§5-6 · City of Nowthen public records

Projects shall be designed in accordance with the most recent version of the Engineering Manual, the latest edition of the MN Stormwater Manual as published by the MPCA, and the following: A. Rate Control 1\. All projects will be required to control the rate of runoff from the site. Proposed discharge rates shall be equal to or less than the pre- development discharge rates for the 2-year, 10-year, and 100-year return frequency events. ## B. Volume Control 1\. A volume equal to 1 inch of runoff from the new or fully reconstructed impervious area of the site shall be retained on-site through infiltration or other volume reducing BMPs as approved by the City Engineer. 2\. Infiltration systems must be prohibited when the system would be constructed in one or more of the following areas: a. That receive discharges from vehicle fueling and maintenance areas, regardless of the amount of new and fully reconstructed impervious surface; b. Where high levels of contaminants in soil or groundwater may be mobilized by the infiltrating stormwater. To make this determination, the owners and/or operators of construction activity must complete the Agency's site screening assessment checklist, which is available in the Minnesota Stormwater Manual, or conduct their own assessment. The assessment must be retained with the site plans; C. Where soil infiltration rates are more than 8.3 inches per hour unless soils are amended to slow the infiltration rate below 8.3 inches per hour; d. With less than three (3) feet of separation distance from the bottom of the infiltration system to the elevation of the seasonally saturated soils or the top of bedrock; e. Of predominately Hydrologic Soil Group D (clay) soils;   f. In an Emergency Response Area (ERA) within a Drinking Water Supply Management Area (DWSMA) as defined in Minn. R. 4720.5100, Subp. 13, classified as high or very high vulnerability as defined by the Minnesota Department of Health; g. In an ERA within a DWSMA classified as moderate vulnerability unless the permittee performs or approves a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater; h. Outside of an ERA within a DWSMA classified as high or very high vulnerability unless the permittee performs or approves a higher level of engineering review sufficient to provide a functioning treatment system and to prevent adverse impacts to groundwater; i. Within 1,000 feet upgradient or 100 feet down gradient of active karst features; or j. That receive stormwater runoff from these types of entities regulated under NPDES for industrial stormwater: automobile salvage yards; scrap recycling and waste recycling facilities; hazardous waste treatment, storage, or disposal facilities; or air transportation facilities that conduct deicing activities. 3\. Linear projects a. For linear projects, the water quality volume must be calculated as the larger of one (1) inch times the new impervious surface or one- half (0.5) inch times the sum of the new and fully reconstructed impervious surface. b. Where the lack of right-of-way precludes the installation of volume reducing BMPs, an exception to the volume control standard will be allowed provided that Sections 5.a. thru 5.b. below are met. C. Mill and overlay and other resurfacing activities are not considered fully reconstructed and an exception to the volume control standard will be allowed as the project will not alter the hydrologic response of the watershed. b. A reasonable attempt must be made to obtain right-of-way during the project planning process for volume control practices. 4\. Exceptions. A lesser volume control standard on the site of the original construction activity may be applied, at the discretion of the City, under the   following circumstances: a. The owner and/or operator of a construction activity is precluded from infiltrating stormwater due to limitations under sections 9-5- 6.B.2., 3, or 4, above, and b. The owner and/or operator of the construction activity implements to the Maximum Extent Practical (MEP) volume reduction techniques, other than infiltration, on the site of the original construction activity that reduce stormwater discharge volume but may not meet the requirements of post-construction stormwater management. C. The applicant provides water quality calculations showing that there is no net increase in stormwater discharges of Total Suspended Solids (TSS) and Total Phosphorous (TP) on an average annual basis for new development. d. The applicant provides water quality calculations showing that there is a net decrease in stormwater discharges of TSS and TP on an average annual basis for redevelopment. 5\. Mitigation. If the owner and/or operator of a construction activity believe that the requirements for TSS and/or TP cannot be met on the site of the original construction activity, the owner and/or operator must provide appropriate documentation to the City Engineer as support. Stormwater discharges that do not meet the TSS and/or TP standards on the site of the original construction activity may be mitigated off-site at the City's discretion. The proposed mitigation must meet the following criteria: a. Mitigation project areas should be selected in the following order of preference and in consultation and with approval by the City. 1\) Locations that yield benefits to the same receiving water that receives runoff from the original construction activity. 2\) Locations within the same Department of Natural Resource (DNR) catchment area as the original construction activity. 3\) Locations in the next adjacent DNR catchment area up- stream. 4\) Locations within the City. b. Mitigation projects must involve the creation of new structural stormwater BMPs, the retrofit of existing structural stormwater   BMPs, or the use of a properly designed regional structural stormwater BMP. C. Routine maintenance of structural stormwater BMP's required by this section cannot be used to meet mitigation requirements. d. Mitigation projects must be completed within 24 months after the start of the original construction activity. e. If the mitigation project is a private structural stormwater BMP and the City is not responsible for long-term maintenance of the project, the City will require written and recorded documentation of maintenance responsibilities. These requirements are transferrable to any party that becomes the owner/operator of the site. f. A permanent public easement shall be provided to the City for access for inspection and/or maintenance purposes. g. If site configurations or structural stormwater BMPs change, causing decreased BMP effectiveness, new or improved structural stormwater BMPs must be implemented to meet the requirements of this section. C. Flood Protection 1\. The High Water Level (HWL) for all waterbodies on the site shall be determined by the applicant utilizing the methods outlined in the Engineering Manual, consistent with local and regional watershed plans and acceptable to the City Engineer. 2\. The lowest opening for all structures shall be set 2 feet above the HWL of adjacent water bodies and stormwater BMPs. D. Discharges to Impaired Waters with a United States Environmental Protection Agency ((USEPA) approved Total Maximum Daily Load (TMDL). 1\. Additional BMPs will be required for projects that discharge to impaired waters with a SEPA TMDL. All BMPs deemed necessary to comply with the Waste Load Allocation or the local or regional watershed plans must be implemented. (Ordinance 2015-04, adopted September 8, 2015) 9-5-7: INSPECTION: The stormwater management practices shall be subject to the inspection procedures outlined in Section 9-4-6 of this Chapter and as specified herein:   A. All applicants are required to submit actual "As-Built" plans for any stormwater management practices located on-site after final construction is completed. 1\. The Grading, Drainage, and Erosion Control Plan must have survey elevations shown to adequately show the final grading of all pond, basins, ditches, swales, and berms are built per the stormwater management design. (Ordinance #16, adopted April 13, 2010)

Ch.9 § 5-6

Amendment history