Storm Water Regulations

Storm Water Ordinance

Current storm water management is monitored under City of Oshkosh Municipal Code: Chapter 14.

Chapter 14 Post-Construction Stormwater Management Requirement Quick Facts

City of Oshkosh Stormwater Reference Guide

The information contained here is only a summary of some of the technical requirements of the City of Oshkosh Municipal Ordinance and the Stormwater Reference Guide. Questions regarding the Reference Guide or the Post-Construction Stormwater Management requirements should be directed to the City of Oshkosh Engineering Division at 920-236-5065.

Table 1: Post-Construction Performance Standard Requirements

(See Chapter 14 and Reference Guide for specific requirements, exemptions, and prohibitions)

Site	Sediment (TSS)	Total Phosphorus (TP)	Peak Discharge	Infiltration (Developed Lands Imperviousness Categories)
				Low (< 40% connected)	Moderate (40% - 80% connected)	High (> 80% connected)
< 20,000 ft2 Impervious Surface (Cumulative since adoption date of Chapter 14 Ordinance – 7/1/2012)
	No Numeric Standard	No Numeric Standard	No Numeric Standard	No Numeric Standard	No Numeric Standard	No Numeric Standard
> 20,000 ft2 Impervious Surface
New Development and Infill > 5 acres	80%	54%	Reduce 1-, 2-, & 10-yr Storms to Pre-Development Conditions; Reduce 100-yr Storm to 10-yr Pre-Development	90% of pre- development infiltration volume Max Infiltration Area = 1%	75% of pre- development infiltration volume Max Infiltration Area = 2%	60% of pre- development infiltration volume Max Infiltration Area = 2%
Infill < 5 acres	80%	54%	Reduce 10- & 100- yr Storms to 10-yr Pre- Development	90% of pre-development infiltration volume	75% of pre- development infiltration volume	60% of pre- development infiltration volume
Redevelopment	58.4%	30%	No Requirements	Potentially Exempt	Potentially Exempt	Potentially Exempt
Routine Maintenance (Mill and Overlay)	None, unless discharging into a BMP	None, unless discharging into a BMP	None, unless discharging into a BMP	Exempt	Exempt	Exempt

The impervious surface areas after the adoption date of the Ordinance are cumulative. For example, if a site first adds 18,000 square feet of parking lot and then adds a 2,001 square foot building the following year, the site is held to the >20,000 square foot requirements at that time. These requirements are applicable to the entire 20,000 square feet of impervious area.

Properties that install post-construction best management practices (BMP's) will require (Sections 14-22 & 24):

• Operation and Maintenance Agreement recorded with the City.
• A final site walk-through of the storm water BMP's with the Contractor, Site Owner Representative, and a City of Oshkosh Civil Engineer.
• An As-Built Survey and Drawing.
• A BMP Certification for the storm water facilities provided by design engineer once construction is complete.

If peak flow reductions or conveyance systems for the site are required, the modeling shall use the rainfall depths found in Table 2 and assume a Type II rainfall distribution Section 14-21 (D) (2).

Table 2: SCS Type II 24-Hour Design Rainfall Depths

Commonly Referenced Definitions:

• Routine maintenance means that portion of a post-construction site where pre-development impervious surfaces are being maintained to preserve the original line and grade, hydraulic capacity, drainage pattern, configuration, or purpose of the facility. Remodeling of buildings and resurfacing or milling and overlaying (or some combination of these two methods) of parking lots, streets, driveways, and sidewalks are examples of routine maintenance, provided the impervious surface's granular base is not exposed. The disturbance shall be classified as redevelopment if the granular base associated with the pre-development impervious surface is exposed or if the soil located beneath the impervious surface is exposed. For purposes of this chapter, a post-construction site is classified as new development, redevelopment, routine maintenance, or some combination of these three classifications as appropriate.
• Land disturbing construction activity (or disturbance) means any man-made alteration of the land surface resulting in a change in the topography or existing vegetative or non-vegetative soil cover, that may result in runoff and lead to an increase in soil erosion and movement of sediment into waters of the state. Land disturbing construction activities include, but are not limited to: clearing and grubbing, demolition, excavating, pit trench dewatering, filling and grading activities, and soil stockpiling.
• Existing development means development in existence on October 1, 2004, or development for which a Notice of Intent to apply for a storm water permit in accordance with subch. Ill of ch. NR 216 was received by the DNR or the Department of Commerce on or before October 1, 2004.
• Pre-development means the extent and distribution of land cover types present before the initiation of land disturbing construction activity, assuming that all land uses prior to development activity are managed in an environmentally sound manner.
  • The Pre-development Curve Number (CN) shall be calculated from the maximum Pre-development Curve Numbers in Table 3 and as outlined in City of Oshkosh Municipal Code Chapter 14 Section 14-21(D)(2)(a). For example: A 1.0 acre site with a Kewaunee Silty Loam (Type D) soil contains 0.50 acres of woodland (CN = 77) and 0.50 acres of cropland (CN=83). The Predevelopment Curve Number for this site would be 80.

Table 3: Maximum Pre-Development Runoff Curve Numbers

Other Potential Requirements:

• Sites with more than 10,000 square feet of disturbed area shall collect and convey storm water in accordance with City of Oshkosh Municipal Code Chapter 14 Section 14-21(D)(2)(b). Any pipes installed within the right of way shall convey storm water up to and including the 10-year storm event.
• Protective Area means an area of land that commences at the top of the channel of lakes, streams and rivers, or at the delineated boundary of wetlands. Impervious areas shall be kept out of the protective area to the maximum extent practicable. If protective areas are disturbed, they shall be restored with self-sustaining vegetative cover of 70% or greater. Protective area setback distances are outlined in City of Oshkosh Municipal Code Chapter 14 Section 14-21(D)(4).
• Fuel and Vehicle Maintenance Areas applies to modifications to existing and new fueling and vehicle maintenance areas. Fuel and vehicle maintenance areas shall have BMP’s designed, installed and maintained in order to reduce the possibility of petroleum in runoff per Section 14-21(D)(5) of City of Oshkosh Municipal Code Chapter 14.