Storm water control facilities included as part of the final design for a permanent development shall be addressed in the storm water management plan and shall meet the requirements of the State NPDES general permit to discharge stormwater associated with construction activity and the following criteria.
(A) Pre- versus post-hydrological response of site. An applicant shall install or construct, on or for the proposed land disturbing activity or development activity, all storm water management facilities necessary to manage runoff such that increases in flow under the design conditions will not occur that could exceed the capacity of the outlet, or the storm water management system, into which the site discharges or that would cause the storm water management system to be overloaded or accelerate channel erosion as a result of the proposed land disturbing activity or development activity. Under no circumstances shall the two-, ten- or 100-year developed peak flow exceed the two-, ten- or 100-year existing peak flow without prior written approval by the City Engineer. For regional detention or storm water management system, the City Engineer shall recommend a proposed system charge or assessment to be approved by the City Council based upon an approved watershed master plan and an analysis of required drainage systems, projected costs and flood protection benefits provided to those properties directly or indirectly impacted by the regional detention or storm water management system.
(B) Natural features of the site. The applicant shall give consideration to reducing the need for storm water management system facilities by incorporating the use of natural topography and land cover such as wetlands, ponds, natural swales and depressions as they exist before development to the degree that they can accommodate the additional water flow without compromising the integrity or quality of these natural features.
(C) Storm water management strategies.
(1) The following storm water management practices shall be investigated when developing a storm water management plan in accordance with this chapter and with § 53.139 of this chapter:
(a) Natural infiltration of precipitation and runoff on-site, if suitable soil profiles can be created during site grading. The purpose of this strategy is to encourage the development of a storm water management plan that encourages natural infiltration. This includes, providing as much natural or vegetated area on the site as possible, minimizing impervious surfaces and directing runoff to vegetated areas rather than onto adjoining streets, storm sewers and ditches;
(b) Flow attenuation by use of open vegetated swales and natural depressions;
(c) Storm water detention facilities;
(d) Storm water retention facilities (on a case by case basis); and
(e) Other facilities requested by the City Engineer.
(2) A combination of successive practices may be used to achieve the applicable minimum control requirements specified. Justification shall be provided by the applicant for the method selected. For sites of a redevelopment construction activity where, prior to the start of construction, the areas to be disturbed have 15% or more of impervious surface, then the practices must be designed to meet the following conditions:
(a) A new reduction from pre-project conditions (on an annual average basis) of:
1. Stormwater discharge volume, unless precluded by the stormwater management limitations in § 54.038(F) of this code;
2. Stormwater discharges of TSS; or
3. Stormwater discharges of TP.
(D) Adequacy of outlets. The adequacy of any outlet used as a discharge point for proposed storm water management system must be assessed and documented to the satisfaction of the City Engineer. To the extent practicable, hydraulic capacities of downstream natural channels, storm sewer systems or streets shall be evaluated to determine if they have sufficient conveyance capacity to receive and accommodate post-development runoff discharges and volumes without causing increased property damages or any increase in the established base flood elevation. If a flood plain or floodway has not been established by the Federal Emergency Management Agency, the applicant shall provide a documented analysis and estimate of the base flood elevation as certified by a professional engineer registered in the state. In addition, projected velocities in downstream natural or manmade channels shall not exceed that which is reasonably anticipated to cause erosion unless protective measures acceptable to the City Engineer are approved and installed as part of the storm water management plan. The assessment of outlet adequacy shall be included in the storm water management plan.
(E) Storm water detention/retention facilities. Storm water detention or retention facilities proposed to be constructed in the storm water management plan shall be designed according to the most current practices as reflected in the Nationwide Urban Runoff Program study.
(F) Infiltration. Infiltration facilities are prohibited, unless approved by the City Engineer, when receiving discharges from, or are to be constructed in areas:
(1) Where industrial facilities are not authorized to infiltrate industrial stormwater under an NPDES/SDS industrial stormwater permit issued by the Minnesota Pollution Control Agency;
(2) Where vehicle fueling and maintenance occur;
(3) With less than three feet of separation distance from the bottom of the infiltration system to the elevation of the seasonally saturated soils or the top of bedrock;
(4) Where high levels of contaminants in soil or groundwater will be mobilized by the infiltrating stormwater;
(5) With predominately Hydrologic Soil Group D (clay) soils; or
(6) Within a Drinking Water Supply Management Area (DWSMA) as defined in Minn. Rules 4720.5100, subpart 13.
(G) Mitigation provisions. There may be circumstances where the owner of a construction activity cannot cost effectively meet the conditions of the State NPDES general permit to discharge stormwater associated with construction activity on the site of the original construction activity. For this purpose the owner of a construction activity may identify locations where mitigation projects can be completed. Mitigation must be accomplished using the following:
(1) Mitigation project areas are selected in the following order of preference:
(a) Locations that yield benefits to the same receiving water that receives runoff from the original construction activity;
(b) Locations within the same Department of Natural Resource (DNR) catchment area as the original construction activity;
(c) Locations in the next adjacent DNR catchment area up-stream; and
(d) Locations anywhere within city jurisdiction.
(2) Mitigation projects must involve the creation of new structural storm water BMPs or the retrofit of existing structural storm water BMPs, or the use of a properly designated regional structural storm water BMP.
(3) Routine maintenance of existing structural storm water BMPs cannot be used to meet mitigation requirements.
(4) Mitigation projects shall be completed within 24 months after the start of the original construction activity.
(2004 Code, § 54.23) (Ord. 07-0462, passed 4-24-2007; Ord. 08-0497, passed 3-25-2008; Ord. 15- 0737, passed 3-24-2015; Ord. 16-0757, passed 4-26-2016; Ord. 17-773, passed 6-27-2017)