northeastern illinois planning commission


Reducing the Impacts of Urban Runoff
with Alternative Site Design Approaches
NIPC Bulletin - May 1997


How to Get the Full Report:

This topic is covered in much greater detail in a report titled Reducing the Impacts of Urban Runoff: The Advantages of Alternative Site Design Approaches. This report, which includes extensive references, is available from NIPC's Publications Department (312-454-0400 extension 210).


The Problem

Conventional urban development dramatically increases the amount of stormwater runoff generated by the landscape. The principal causes of this effect are impervious surfaces -- streets, parking lots, and buildings -- and compaction of the soil due to construction activities. Instead of soaking into the ground, rainfall is converted quickly to runoff and is then eliminated from the site via sewers and man made channels.

Some common site development standards may actually worsen stormwater runoff problems. For example, modern standards which require wide streets, expansive parking lots, and artificial drainage systems produce even more runoff than similar developments of 40 to 50 years ago.

In recognition of the effect that increased runoff has had on flooding, new development often incorporates stormwater detention to slow the release to downstream rivers. Unfortunately, this still leaves several runoff-related problems inadequately addressed.

  • Stormwater runoff is contaminated with various water pollutants which are byproducts of urban activities such as automobile use, lawn care, and industrial fallout. If unchecked, these pollutants will damage the aquatic life, including fish, in downstream lakes, streams, and wetlands
  • Water which runs off of urban landscapes can no longer recharge groundwater supplies. For communities which depend on locally recharged aquifers, resultant water shortages could limit future development and necessitate sprinkling bans and other restrictions.
  • Urban runoff causes instability in the drainage system by 1) increasing the high flows, which can cause streams to rapidly erode and 2) decreasing the low flows (or baseflows), which literally causes small streams and lakes to dry up and concentrates pollutants to damaging levels.
  • While stormwater detention can effectively reduce runoff rates, thereby controlling localized flooding, it does little to control the increased volume of runoff caused by urbanization. As a consequence, flooding continues to worsen on larger drainage systems, such as the Des Plaines and Fox rivers.


A Solution -- Alternative Site Designs

Fortunately, there are development options involving alternative stormwater drainage and site design approaches which can substantially reduce the identified impacts. These alternative development techniques, commonly called best management practices, or BMPs, involve measures which accomplish two basic objectives:

  • reduce the amount of impervious surface area, thereby reducing runoff and
  • utilize the landscape to naturally filter and infiltrate runoff before it leaves the development site.

Interestingly, the recommended alternatives reflect both old and new design philosophies. Some mirror a design philosophy which existed prior to the 1950s-1960s when "modern" subdivisions began to spread across the landscape. Older developments, for example, often utilized natural drainage approaches and narrower street widths. In other instances the alternative approaches, such as landscaping with native vegetation, emulate conditions prior to the arrival of European settlers. Also recommended are innovative planning approaches, such as cluster developments, which have not yet been widely implemented in this region.


Recommended Site Design Alternatives

  • Natural drainage measures: Use of drainage swales, vegetated filter strips, and other natural drainage approaches -- in contrast to storm sewers, lined channels, and curbs and gutters -- will reduce runoff volumes and greatly enhance the removal of pollutants from runoff water.
  • Natural detention basin designs: Natural detention designs incorporate features of natural wetland and lake systems, such as gradual shoreline slopes, a border of wetland vegetation, and areas of open water -- in contrast to conventional designs which feature dry bottoms or riprap-edged wet basins. Natural designs are much more effective in removing stormwater pollutants than conventional wet and dry bottom basins.
  • Infiltration practices: Where soils are sufficiently permeable, infiltration trenches and basins dramatically reduce surface runoff volumes and naturally recharge groundwater.
  • Permeable paving: The use of permeable paving blocks is a recommended alternative for low traffic parking areas, emergency access roads, and driveways to reduce runoff volumes and pollutant loads.
  • Natural landscaping: Natural landscaping approaches utilize native plants, particularly wildflowers, prairie grasses, and wetland species, as an alternative to conventional turf grass and ornamental plants, to reduce stormwater runoff and to reduce the maintenance needs of conventional turf grass landscaping.
  • Reduced imperviousness via alternative residential streetscapes: The area of impervious surfaces in a residential development can be reduced in several ways: utilizing narrower streets; reducing setbacks between streets and homes, thereby reducing the length of driveways; and by reducing sidewalk widths.
  • Reduced imperviousness via alternative parking lot designs: Impervious surfaces also can be reduced in parking lots by downsizing individual parking stalls, sharing parking between adjacent users, adjusting peak demand assumptions, and/or banking parking until it is needed.
  • Cluster development/PUDs: Cluster development increases densities on portions of a development site to preserve natural land amenities and common open space, resulting in substantially less overall impervious area. Planned unit developments (PUDs) provide for greater flexibility in the site planning process, allowing the inclusion of many of the site design alternatives described above.


Summary of Benefits

When used in combination on a development site, these techniques can remarkably reduce both stormwater-related impacts and construction costs. Based on assessments of case studies in northeastern Illinois and other parts of the country, it is estimated that alternative site design approaches can:

  • reduce stormwater runoff volumes by 20 to 70 percent (in comparison to conventional development);
  • reduce runoff pollutant loads by 60 to 90 percent;
  • reduce site development costs by $1,000 to over $4,000 per lot for residential developments and by $4,000 to $10,000 per acre for commercial/industrial developments.

Other documented benefits of these approaches include reduced infrastructure maintenance and replacement costs, improved protection of sensitive natural areas, enhanced site aesthetics, improved property values, and greater flexibility of site design.


Tradeoffs

Clearly, not all of the recommended site design approaches are applicable on all development sites. While the recommended alternatives have obvious documented benefits, they also may have some disadvantages. From a developer's perspective, some of these approaches may entail a more difficult and time consuming local government approval process. From a local government perspective, acceptance of some of these approaches will require education of local residents and still might result in complaints from some residents about "standing water" or "weedy conditions." Some local planners or engineers may be hesitant because there is relatively little experience in northeastern Illinois with certain alternative design practices.

Ultimately, it is hoped that local officials and developers will thoroughly consider the tradeoffs between conventional and alternative site design approaches. In this consideration they should weigh all the relevant factors, including construction costs, maintenance needs, public safety, aesthetics, marketing considerations, as well as the obvious environmental benefits

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Conclusion

The floods of 1996 and 1997 which caused damage in much of the region, are recent reminders of the need to better control the effects of new development. Experts, both regional and national, are coming to the realization that while engineered solutions are important tools in flood prevention, over-reliance on artificial drainage approaches has serious negative consequences. This truth actually has been known for quite some time as evidenced by the following observation contained in a joint publication of the Urban Land Institute, American Society of Civil Engineers, and National Association of Homebuilders from 1975:

"Past philosophy sought maximum convenience at an individual site by the most rapid possible elimination of excess surface water after a rainfall and the containment and disposal of that water as quickly as possible through a closed system. The cumulative effects of such approaches have been a major cause of increased frequency of downstream flooding, often accompanied by diminishing groundwater supplies." -- from Residential Storm Water Management


This document was prepared using U.S. Environmental Protection Agency funds under Section 604(b) of the Clean Water Act distributed through the Illinois Environmental Protection Agency. The findings and recommendations contained herein are not necessarily those of the funding agencies