Stormwater Management

Uncontrolled stormwater runoff from land development activities have significant adverse impacts upon regional water resources and the health, safety, property and general welfare of the community, and diminish the public enjoyment and use of natural resources.

Madison General Ordinances Chapter 37 stipulates that land development activities within the City of Madison shall be subject to stormwater management requirements to control stormwater runoff and improve stormwater quality. The following activities are subject to stormwater management requirements:

• Any development that results in the cumulative addition of 20,000 square feet of impervious area to the site.
• Any subdivision of land that requires plat approval or any certified survey for property intended for commercial or industrial use.
• Redevelopment projects that disturb 10,000 square feet or more of land area.
• Other land development activities that the Administrative Authority determines may exceed the safe capacity of the existing drainage facilities and/or receiving body, significantly increase downstream runoff volumes, flooding, soil erosion, water pollution or property damage, or significantly impact a lake, stream, wetland or other sensitive area.
• Any land disturbance equal to or greater than one (1) acre regardless of the impervious area that is part of the project after construction.
• New street construction, exceeding 20,000 square feet of new impervious area that is not directly associated with a new plat or CSM development.

New Development Projects and Stormwater Management Requirements

New Development projects that add 20,000 square feet or more impervious area must meet the following stormwater management criteria.

• Sediment Control: 80% Total Suspended Solids (TSS) removal from stormwater runoff leaving the site. Existing parking lots and improvements on a site are exempt provided they are not removed and rebuilt.
• Oil and Grease Control: The first 0.5 inches of runoff shall be treated using the best oil and grease removal technology available. Site must provide Oil and Grease control when they reach 40 or more parking stalls, have high traffic areas (such as drive thru windows/lanes), or commercial/business activities will take place that have the potential for oil and grease pollution (i.e. gas/service stations).
• Runoff Rate Control: Runoff Rate Control or Detention is required upon a cumulative increase in impervious area equal to 20,000 square feet or more. In general sites must maintain predevelopment peak runoff rates for the 1, 2, 5, 10, 100, and 200-year, 24-hour storm events and safely pass the 500-year, 24-hour storm event. Depending on watershed and local drainage conditions additional runoff rate control requirements may also be required (contact Engineering Division staff to determine if there are additional requirements for your site).
• Infiltration: All lots, plats, or Certified Survey Maps that have a total 20,000 square feet or more of impervious area are required to provide infiltration per State, County and City criteria.
• Thermal Control: A site that adds 20,000 square feet or more of impervious area and is located within the watershed of a river or stream identified by the Wisconsin Department of Natural Resources as a Cold Water Community or Class I, Class II, and Class III Trout Stream must provide temperature reduction practices for stormwater runoff leaving the site.

Redevelopment Projects and Stormwater Management Requirements

Redevelopment projects that disturb 10,000 square feet or more land area by removing or demolishing existing site improvements must meet the following stormwater management criteria.

• Sediment Control: 40% Total Suspended Solids (TSS) removal from stormwater runoff leaving the site for projects in the Sugar River Watershed. 80% TSS removal as compared to existing conditions prior to the proposed redevelopment OR 60% TSS removal from redeveloped parking areas for projects in the Rock River Watershed. Existing parking lots and improvements are exempt provided they are not removed and rebuilt. If the site is adding improvements to an existing vegetated area/non-developed portion of the site in excess of 20,000 square feet it is actually considered New Development and will need to meet Sediment Control requirements as if it were New Development.
• Oil and Grease Control: The first 0.5 inches of runoff shall be treated using the best oil and grease removal technology available. Site must provide Oil and Grease control when they reach 40 or more parking stalls, have high traffic areas (such as drive thru windows/lanes), or commercial/business activities will take place that have the potential for oil and grease pollution (i.e. gas/service stations). Only the new surfaces added during redevelopment must be treated, existing parking lots and improvements are exempt provided they are not removed and rebuilt.
• Runoff Rate Control and Infiltration: Runoff Rate Control or Detention is required if the proposed impervious area is > 80% of the existing impervious area. In general sites must reduce the 10-year storm event peak discharge rate by 15% compared to existing conditions. Sites must also reduce the 10-year storm event site volume by 5% compared to existing conditions.  Reductions shall be completed using green infrastructure that captures at least the first 0.5 inches of rain over the total site impervious area.  Additional green or non-green best management practices (BMPs) may be used for volumes beyond the first 0.5 inches of rainfall if needed to comply with other ordinance requirements.

New Development vs. Redevelopment

Normally New Development occurs on a new lot or site that previously has not been developed. In some cases it is possible to have New Development and Redevelopment taking place during the same project. If 20,000 square feet or more impervious area is being added to a portion of a larger that is currently vegetated and has never been developed on it would be considered New Development. The same project could also be removing existing improvements such as a parking lot or building to make room for the proposed improvements making that portion of the work Redevelopment. If you are working on a project which may fall into this situation it is recommended that you contact Engineering Division staff as early as possible to determine the development status of the project or parts thereof.