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Protecting Our Waters -- Stormwater

Runoff Quantity Control Requirements | Infiltration Devices | Rain Gardens | Other Structural BMPs | Stormwater Regulations

Image of a storm drain at a curbEach municipality should develop a stormwater management plan if it has not already done so. It is recommended that all municipalities adopt a stormwater control ordinance to address both water quality and quantity control.

Ordinances adhere to the requirements of the Natural Resources (NR) codes as well as the MMSD requirements. Municipalities not currently under a WPDES Municipal Stormwater Discharge Permit should begin development of an ordinance in advance of the permit issuance. The NR codes outline the permit requirements as well as the performance standards for stormwater management practices.

Peak stormwater flows after development can be two to five times higher than pre-development conditions.Increases in stormwater runoff rates and volume have been shown to have a detrimental effect on stream quality and habitat. While many measures exist to reduce peak flow rates, there are not many practical ways to reduce runoff volumes unless soil conditions permit. With the predominantly clay soils present throughout the Milwaukee River Basin, there is not much chance to reduce runoff volume through infiltration unless it is in small areas like rain gardens. Volume controls such as collecting rooftop runoff in rain barrels or rain gardens help reduce runoff from residential areas but such techniques are often not feasible in commercial or industrial areas where the percentage of impervious surface is high.

Many municipalities in the basin have adopted stormwater ordinances that address the issue of peak flow reduction. The Milwaukee Metropolitan Sewerage District (MMSD) has also implemented peak flow reduction requirements (Chapter 13 Surface Water and Storm Water guidelines) that further limit the allowable peak flow rate from a development within their service area. More than 30 municipalities and 500 industries in the Milwaukee River Basin are covered by a stormwater discharge permit.In addition, many municipalities have been issued a WPDES stormwater discharge permit from the WDNR that requires treatment of stormwater runoff prior to discharge into receiving waters.

The WDNR has created several administrative rules that apply to stormwater management. Administrative Code Chapter NR216 deals with stormwater discharge permits for municipalities as well as from private industry and construction sites. NR216 and the proposed Chapter NR151 of the Wisconsin Administrative Code establishes runoff pollution performance standards for new development, existing urban areas, transportation facilities and agricultural operations.

Runoff Quantity Control Requirements -- Back to top
As previously mentioned, many municipalities have stormwater ordinances that require peak flow rates from new development be reduced to a specified level. Typical storm frequencies evaluated include the two-, five-, 10-, 25-, 50-, and 100-year storms. A few of the methods for determining the required reduction are described below.

--- Requiring peak flow rates to be reduced to existing conditions. Under this scenario, peak flow rates from proposed development are reduced to rates under existing conditions for the identified storm frequency. This method is best suited for areas where little development has already occurred, however existing conditions can include agricultural land uses.

--- Requiring peak flow rates to be reduced to pre-existing conditions. Under this scenario, peak flow rates from proposed development are reduced to rates that would have occurred before any development. New developments, however, are not the only ones affected. If a property owner wants to redevelop a site, this type of rule will require comparing the site to its “natural” condition. This method can be economically and practically prohibitive for redevelopment because it can result in a larger detention area than is physically feasible for the site.

--- Requiring peak flow rates to be reduced to rates from smaller storm events. In this case, peak flow rates from proposed development are reduced to rates that would have occurred before any development for a smaller storm. For example, it may be required to reduce the 10-year developed conditions peak flow rate to that of the two-year existing conditions. The previous worked best for areas where little development has already occured, however, this method can be used in areas where a significant amount of development has occurred before establishment of the ordinance.

Outlet with no water quality benefit.
In addition to DNR rules and local ordinances, MMSD has developed additional peak flow reduction rules for municipalities within their service area that may be more restrictive that the local ordinances. These rules state that individual developments cannot discharge more than 0.5 cubic feet per second (cfs) per acre for the 100-year storm nor more than 0.15 cfs per acre for the two-year storm. Stormwater management plans for the development must be submitted to the municipality and the MMSD.

Because infiltration is typically limited in the basin, detention basins are widely used to achieve required levels of peak flow reduction. To learn more about the infiltration and detention basins follow the link here. In situations where the required reduction is small, it may be possible to oversize the site storm sewer or use a grassed swale system with check dams to temporarily store runoff.

Wet Detention Basins
Wet detention basins have a permanent pool of water year-round. The permanent pool allows pollutant particles in stormwater runoff to settle out over an extended period of time, and nutrient uptake also occurs through biological activity. (1) Wet detention basins can be used to treat runoff from a single property or can be incorporated into a regional stormwater management plan where runoff from a large area discharges into a single basin or series of basins. For more information on infiltration or detention basins link here.

An outlet modified for water quality.Infiltration Devices -- Back to top
Proposed Administrative Code Chapter NR151 will require infiltration for all new developments in the Milwaukee River Basin where soil and groundwater conditions permit. Infiltration can be achieved through use of infiltration basins, trenches, grass swales, or rain gardens.

Dry detention basins that are designed to function as infiltration basins have the added benefits of providing some pollutant removal and reducing the volume of stormwater released from the basin.

Pollutants contained in the infiltrated runoff are absorbed by the soil particles and are considered permanently removed from the water. Infiltration basins also provide for some groundwater recharge if the soil conditions permit.

Infiltration trenches and grass swales can also be constructed to infiltrate stormwater. Porous media such as sand and washed gravel are placed in the bottom of the trench or swale. Loosely placed sandy topsoil can also be placed if the trench or swale will become vegetated. The runoff draining to these trenches and swales has the opportunity to infiltrate rather than discharge into waterways. Ditch checks (also referred to as check dams) can also be installed to slow or temporarily store the runoff to allow it to infiltrate.

Rain Gardens -- Back to top
Rain gardens are another infiltration device in which stormwater runoff is the main water supply for the plantings. The garden is planted at the end of a downspout or at a low area where water collects, such as a drainage swale. The plants used in the garden are selected based on site-specific growing conditions such as the amount of sunlight available and the underlying soil conditions. During typical rains, the gardens infiltrate most of the runoff generated from the area and use it to sustain the plantings. As such, much of the pollutant load is removed as well. To learn more about rain gardens follow the link here.

Visit the WDNR rain garden site.

Other Structural BMPs -- Back to top
There are many types of stormwater treatment devices on the market that remove sediments and pollutant in stormwater runoff directly inside the storm sewer system. Oil and grease are also trapped in these devices. These types of systems are often referred to as in-line or in-pipe treatments and are typically used to treat smaller areas than those treated by a wet detention basin.

Some manufacturers claim to also get adequate treatment for large areas through combined use of several large structures installed in series. Treatment is achieved through confining the “first flush” of stormwater to a chamber where suspended solids can settle out of the water. Oil, grease, and other floatables are also trapped in the chamber. Higher flow rates bypass the treatment chamber and are discharged into the downstream storm sewer system as normal. It should be noted that these devices do not provide any reduction in peak flow rates or runoff volume. In-pipe treatment devices require periodic removal of accumulated sediment and floatables by a vacuum truck to maintain specified performance. The frequency of cleaning is dependent on the rate sediment and floatables accumulate.

Sand filters are devices that allow runoff to infiltrate through a deep sand media that filters most pollutants from the runoff. The filters typically have a drain system that conveys the filtered runoff to a storm sewer or drainage swale. In areas where soils have higher infiltration rates, these devices may also reduce runoff volume and further reduce pollutants by allowing runoff to infiltrate beyond the constructed filter. Routine maintenance consists of removing visible sediments and debris accumulated on the filter surface and periodic raking or replacement of the top layer of sand. Long-term use of the sand filter may require complete replacement of the sand media if the accumulated sediment plugs the filter and reduces performance.

Stormwater Regulations
For more information on Wisconsin's Stormwater Regulations, follow the link here.

Source for text in oval:
First Oval: DNR. "The Wisconsin Stormwater Manual."
Second Oval: DNR. Stormwater Discharge Permit Records.

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The Milwaukee River Basin Partnership is a voluntary coalition of businesses, non-profit groups, public agencies, educational institutions, organizations, and individuals committed to restoring and sustaining the ecosystem of the Milwaukee River Basin while ensuring its economic viability. To learn more visit,
Photos by Susan Beaumier, stormwater specialist, Wisconsin DNR and Nancy Froggatt

This page was created on March 26, 2003.
This page was last updated on August 26, 2003.