Development Standards for Stormwater
Development Requirements for Stormwater Management in the City of Madison
Stormwater management can be broken up into two main categories water quantity control and water quality control. New development, which we will define as going from farm field to a developed urban condition, has many requirements in both areas. Additionally, at least partially as a result of the intense storms experienced in 2018, Madison is currently in the process of rewriting its’ stormwater management code to address deficiencies identified during the response to those events.
Water quantity control:
- New development is required to match the peak stormwater runoff rate leaving the site to peak predevelopment runoff for varying sized storm events including the 1, 2, 5, 10, 100 and 200 year. This means that a model is used to estimate the rate of discharge from the existing (farmland) site for the above storm events. Then a parallel model is built to show post development conditions and their detention facilities (buildings, parking lots, and ponds that are designed to hold stormwater to reduce the post development runoff rate). The post development model needs to match peak rates to the existing model for the same storm event. Generally, ponds are used to meet this requirement as they act like large bathtubs that fill up and then release the stormwater slowly to make it manageable for downstream facilities.
Water quality control:
- Sediment control: Remove 80% of total suspended solids (sediment), leaving the developed site. This means a model is run to estimate how much sediment will leave the site without treatment, and then devices are designed to trap the sediment and reduce it to 80% of the initial modeling.
- Oil and grease control: Required if the site has a drive through, has over 40 parking spaces, or is a “hot” spot such as a car sales or repair lot.
- Infiltration: Must infiltrate 90 percent of the water that infiltrates during existing conditions. This means you calculate the existing amount of infiltration occurring during an average annual year on the site as farmland and the amount of infiltration that will occur post development with no controls. There is always a drop off from existing to proposed conditions and the amount of infiltration must be 90 percent of the existing amount. This amount is capped at 90 percent by State Statute.
- Thermal control: certain areas of the city (those draining to the Sugar River) are required to complete thermal control to reduce the temperature of the water being discharged off site in an effort to limit temperature increases to a cold-water resource.
Stormwater treatment standards for Re-development were revised in 2020 and for the first time include both peak flow requirements and peak volume requirements for redevelopment sites. In an effort to limit requirements to larger redevelopments, stormwater requirements are only required if the disturbed area exceeds 10,000 square feet.
Peak Flow requirements
If the redevelopment exceeds 10,000 SF and the proposed site impervious area is more than 80% of the existing impervious area then the site must reduce the peak discharge from the site by 15% in the 10-year design storm compared to the existing site.
Peak Volume requirements
If the redevelopment exceeds 10,000 SF and the proposed site impervious area is more than 80% of the existing impervious area then the site must reduce the peak volume being discharged from the site by 5% in the 10-year design storm compared to the existing site.
Total Suspend Solids (TSS) requirements
Redevelopments are required to reduce the TSS from the site by 80% compared to the existing site or by 60%, compared to no controls, off of parking areas. No treatment for TSS is required for rooftop areas and for the purposes of comparison to existing conditions, rooftops are considered to be “clean.”
Oil and Grease requirements
Redevelopments are required to treat oil and grease off of parking areas that exceed 40 parking stalls, have a drive through or are considered to be a “hot spot” for these types of pollutants (gas stations, auto repair sites,…).