1. Minimum Requirement. To minimize downstream bank erosion and the failure of downstream conveyance systems, the peak stormwater discharge rate shall not exceed the rates as described below. Modeling requirements for this provision are further described Section II.F.

Note: If the conveyance system downstream of a BMP is undersized based on the proposed peak discharge rates in accordance with Sec. 17.09-3(f), P&D may require a more stringent release rate than required in Sec 17.09-3(a).

2. Des Plaines River Watershed Release Rate Applicability. P&D shall establish a maximum allowable release rate on a per acre basis that would supercede the requirements of Section 17.09-3(a)1. for new developments located in the Des Plaines River watershed that meet any of the following conditions:

3. Des Plaines River Watershed Per Acre Release Rates. Peak rates of runoff within the Des Plaines River watershed from new development shall be controlled as follows:

4. Per Acre Release Rate Exemptions. Certain sites may be exempt from the per acre discharge requirements of this subsection as follows:

1. By design, each stormwater management plan shall to the maximum extent practicable meet the following post-development total suspended solids reduction targets, based on average annual rainfalls, as compared to no runoff management controls:

1. Low imperviousness. For development up to 40 percent connected imperviousness, such as parks, cemeteries, and low density residential development, infiltrate sufficient runoff volume so that the post−development infiltration volume shall be at least 90 percent of the pre−development infiltration volume, based on an average annual rainfall. However, when designing appropriate infiltration systems to meet this requirement, no more than one percent of the post−construction site is required as an effective infiltration area.

2. Moderate imperviousness. For development with more than 40 percent and up to 80 percent connected imperviousness, such as medium and high density residential, multi−family development, industrial and institutional development, and office parks, infiltrate sufficient runoff volume so that the post−development infiltration volume shall be at least 75 percent of the pre−development infiltration volume, based on an average annual rainfall. However, when designing appropriate infiltration systems to meet this requirement, no more than 2 percent of the post−construction site is required as an effective infiltration area.

3. High imperviousness. For development with more than 80 percent connected imperviousness, such as commercial strip malls, shopping centers, and commercial downtowns, infiltrate sufficient runoff volume so that the post−development infiltration volume shall be at least 60 percent of the pre−development infiltration volume, based on an average annual rainfall. However, when designing appropriate infiltration systems to meet this requirement, no more than 2 percent of the post−construction site is required as an effective infiltration area.

4. Modeling. Refer to Section II.F. for details on calculating runoff volumes and pre-development conditions.

5. Pretreatment. Pretreatment shall be required before infiltrating parking lot and road runoff from commercial, industrial and institutional areas. The pretreatment shall be designed to protect the infiltration system from clogging prior to scheduled maintenance and to protect groundwater quality in accordance with Section 17.09-3(c)9. Pretreatment options may include, but are not limited to, oil/grease separators, sedimentation or bioretention basins, infiltration swales or filter strips. All designs shall comply with the technical standards in Section II.F.

Note: To achieve the infiltration requirement for the parking lots or roads, “maximum extent practicable” should not be interpreted to require significant topography changes that create an excessive financial burden. To minimize potential groundwater impacts, it is desirable to infiltrate the cleanest runoff. To achieve this, a design may propose greater infiltration of runoff from low pollutant sources such as roofs, and less from higher pollutant source areas such as parking lots.

6. Infiltration Prohibitions. Due to potential for groundwater contamination, runoff shall not be infiltrated and will not be credited toward meeting the requirements of this subsection for the following:

7. Separation Distances. Infiltration BMPs shall be located so the characteristics of the soil and the separation distance between the bottom of the infiltration BMP and the elevation of the highest groundwater table or the top of bedrock are in accordance with Table 1.

8. Infiltration Exemptions. The infiltration requirements of this subsection do not apply to sites that meet the exemption requirements described in DNR Technical Standard 1002 – Site Evaluation for Stormwater Infiltration and P&D determines it would be impracticable to modify existing soil conditions. Other sites may be exempted in accordance with Section 17.09-4.

9. Groundwater protection.

1. For outstanding resource waters and exceptional resource waters, 75 feet.

2. For perennial and intermittent streams deemed navigable as identified on the Kenosha County Zoning Maps or determined by the Department of Natural Resources, 50 feet.

3. For lakes, 50 feet.

4. For highly susceptible wetlands, as determined by the Department of Natural Resources or the Southeastern Regional Planning Commission, 75 feet. Highly susceptible wetlands include the following types: calcareous fens, sedge meadows, bogs, low prairies, conifer swamps, lowland hardwood swamps, and ephemeral ponds.

5. For wetlands not subject to (4.), 50 feet.

6. Wetland boundary delineations shall be made in accordance with Chapter NR 103 Wis. Admin. Code. This paragraph does not apply to wetlands that have been completely filled in accordance with all applicable state and federal regulations. The protective area for wetlands that have been partially filled in accordance with all applicable state and federal regulations shall be measured from the wetland boundary delineation after fill has been placed.

7. For less susceptible wetlands, 10 percent of the average wetland width, but no less than 10 feet nor more than 30 feet, unless otherwise required by another applicable regulation. Less susceptible wetlands include degraded wetlands dominated by invasive species such as reed canary grass.

8. In Sections 17.09-3(d)4., 5., and 6., determinations of the extent of the protective area adjacent to wetlands shall be made on the basis of the sensitivity and runoff susceptibility of the wetland in accordance with the standards and criteria in Chapter NR 103 Wis. Admin. Code.

9. For concentrated flow channels with drainage areas greater than 130 acres, 10 feet.

10. Requirements. The following requirements shall be met for all land development activity located within a protective area:

11. Protective Area Exemptions. The protective area requirements of this subsection may be exempted in accordance with Section 17.09-4 and do not apply to the following:

1. Minimum Design Standards.

2. Drainage easement. Perpetual drainage easements or other deed restrictions shall be recorded on the property to preserve major stormwater flow paths and permanent stormwater BMP locations. Covenants in these areas shall not allow buildings or other structures and shall prevent any grading, filling or other activities that interrupt or obstruct flows in any way. Covenants shall also specify maintenance responsibilities and authorities in accordance Section II.E.

3. Site grading. Site grading shall ensure positive flows away from all buildings, roads, driveways and septic systems, be coordinated with the general stormwater drainage patterns for the area, and minimize adverse impacts on adjacent properties. All drainage facilities shall have an overland flow path which does not impact any structures for storms larger than the design event.

4. Street drainage. All street drainage shall be designed to prevent concentrated flows from crossing the traffic lanes to the maximum extent practicable. Design flow depths at the road centerline for on-street drainage, shall not exceed six (6) inches during the peak flows generated by the 100-year, 24-hour design storm, using planned land use conditions for the entire contributing watershed area.

5. Bridges and cross-culverts over or in navigable waterways. A hydrologic and hydraulic analysis shall be required for all projects impacting a navigable waterway. This analysis must demonstrate no adverse impacts, onsite or offsite. All new or modified bridges and cross-culverts shall comply with applicable design standards and regulations, facilitate fish passage and prevent increased flooding or channel erosion upstream or downstream from the structure. For bridges or culverts associated with collector streets, the peak water surface elevation from a 10-year recurrence interval storm event shall have at least two feet of freeboard to the roadway elevation. For arterial streets and highways, the peak water surface elevation from a 50-year recurrence interval storm event shall have at least two feet of freeboard to the roadway elevation. All predevelopment runoff storage areas within the regulatory floodplain or, in the absence of a regulatory floodplain, the area inundated during a 100-year storm event, upstream of bridges and cross-culverts shall be preserved or shall be designated as drainage easements, unless compensatory storage is provided and accounted for in modeling. As-built documentation shall be submitted in accordance with Section 17.09-8 for all new or modified structures that are located within a mapped floodplain as determined by P&D.

6. Subsurface drainage. Basement floor surfaces shall be built a minimum of one (1) foot above the high ground water elevation, as documented in the submitted soil evaluations, and shall avoid hydric soils as much as possible. The high ground water elevations can be extrapolated based on the site borings conducted as part of the soil evaluation in order to determine the high ground water elevation at each basement location. P&D shall be notified of any drain tiles that are uncovered during construction, which P&D may require to be restored or connected to other drainage systems.

7. Open channels. All open channel drainage systems shall at a minimum be designed to carry the peak flows from a 10-year, 24-hour design storm using planned land use for the entire contributing watershed area. Side slopes shall be no steeper than 3h:1v unless otherwise approved by P&D for unique site conditions. Water surface elevations for the 100-year, 24-hour design storm shall be calculated for all existing and proposed open channels. All channels shall be designed with a minimum 1.0% gradient and be centered on lot lines wherever possible.

8. Storm sewers. All storm sewers shall be designed in accordance with applicable community technical standards and specifications.

9. Changes to stormwater discharges. For sites where P&D determines the post-development stormwater discharge flow paths will be significantly different than pre-development conditions, or where proposed storm water discharges may otherwise have a significant negative impact on downstream property owner(s), the P&D may require the applicant to submit written authorization, record a drainage easement, or complete other legal arrangements with the affected property owner(s) prior to permit issuance.

10. Structure protection and safety. Flows generated by the 100-year, 24-hour design storm under planned land use conditions may exceed the design capacity of conveyance systems, but shall not come in contact with any buildings. For buildings designed for human occupation on a regular basis the lowest elevation of the structure that is exposed to the ground surface shall be a minimum of two (2) feet above the maximum water surface elevation produced by the 100-year, 24-hour design storm, including flows through any stormwater BMP that may temporarily or permanently store water at a depth of greater than one (1) foot.

1. A cold water stream, an outstanding water resource, or an exceptional water resource;

2. Environmentally sensitive area;

3. A downstream property;

4. Public health or safety. [Code § 17.09-3.]