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 following criteria:
(A) Pre- versus post-hydrological response of site. An applicant shall design, install or construct, on or for the proposed land-disturbing activity, development activity or redevelopment 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. Post construction storm water management BMPs shall be installed to the maximum extent practical (MEP) and shall show a net per annual basis reduction for volume, total suspended solids (TSS) and total phosphorus (TP) for new or redeveloped sites from pre-project conditions. 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. Design criteria must meet the most current state approved NPDES/SDS and NPDES/MS4 general permit requirements.
(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 section and the city code:
(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, except as provided in storm water management “limitations” noted in the state’s NPDES general permit.
(3) For non-linear projects, where the water quality volume cannot cost effectively be treated on the site of the original construction activity, the applicant is required to identify locations where off-site treatment projects can be completed. If the entire water quality volume is not addressed on the site of the original construction activity, the remaining water quality volume must be addressed through off-site treatment.
(a) Off-site treatment project areas shall be selected in the following order of preference: locations that yield benefits to the same receiving water that receives runoff from the original construction activity; locations within the same Department of Natural Resource (DNR) catchment area as the original construction activity; locations in the next adjacent DNR catchment area up-stream; or locations anywhere within the regulated party’s jurisdiction.
(b) Off-site treatment projects must involve the creation of new structural stormwater BMPs or the retrofit of existing structural stormwater BMPs, or the use of a properly designed regional structural stormwater BMP. Routine maintenance of structural stormwater BMPs already required by the general permit cannot be used to meet this requirement.
(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. In addition, projected velocities in downstream natural or human-made 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.
(2002 Code, § 3.94) (Ord. 109, Sixth Series, effective 6-25-2009; Ord. 24, Seventh Series, effective 5-23-2015; Ord. 31, Eighth Series, effective 12-11-2023)