Skip to main content
padlock icon - secure page this page is secure


The full text article is not available for purchase.

The publisher only permits individual articles to be downloaded by subscribers.

In 2002 the Milwaukee Metropolitan Sewerage District (MMSD) began to examine the potential for Low-Impact Development (LID) techniques to help developments meet regulatory peak runoff targets and to increase the number of options suitable for compliance with the District's storm water management regulations. Various stakeholders were known to have misgivings about the technical effectiveness of LID components, and it was apparent that a major disincentive to the use of LID was the complexity of modeling the flow and storage of runoff among interconnected LID components (such as green roofs, bioretention cells, permeable pavement areas, infiltration trenches, etc.). Consequently, MMSD sought an approach to LID hydrologic analysis that was technically reasonable, not difficult to execute by development engineers, and relatively easy to check by permitting personnel.

An approach that came to the forefront for serious consideration consisted of a straightforward adaptation of the TR-20 unit hydrograph method. The adaptation takes the sum of all the LID retention volumes and treats it as a depth of retention storage across the site. The depth of runoff is calculated using conventional TR-20 formulas, but only after the runoff depth exceeds the retention depth is a component of the runoff hydrograph calculated for each subsequent time step. In other words, rather than calculating hydrograph components on the basis of excess rainfall, the components are calculated on the basis of excess runoff, i.e., the runoff that exceeds the available retention capacity.

A technical review of this approach has led to favorable comparisons against other alternatives for accounting for retention storage. These alternatives consisted of a curve number adjustment, truncating the runoff hydrograph, treating the retention volume as an initial abstraction, and taking the runoff hydrograph representing the developed conditions and multiplying each ordinate by the ratio of runoff volumes with retention and without it.

However, because the most desirable approach could not be readily incorporated into existing software, the need to develop a customized computational tool became apparent. Ultimately, a spreadsheet was created with a simple interface that allows the user to input the amount of retention provided by each of several LID components, and observe immediately the predicted impact of that retention on the runoff hydrograph and the amount of detention storage that might otherwise be required at the drainage area outlet. Stakeholder involvement before and during the development and testing of the tool has been a key aspect of this project. Stakeholder groups have included representatives from the development community, design engineers, and various government agencies. Their input helped identify the initial needs to be met, and their response to a draft version of the spreadsheet has spurred the effort on to testing the method on actual design projects.
No Reference information available - sign in for access.
No Citation information available - sign in for access.
No Supplementary Data.
No Article Media
No Metrics

Document Type: Research Article

Publication date: 01 January 2004

More about this publication?
  • Proceedings of the Water Environment Federation is an archive of papers published in the proceedings of the annual Water Environment Federation® Technical Exhibition and Conference (WEFTEC® ) and specialty conferences held since the year 2000. These proceedings are not peer reviewed.

    A subscription to the Proceedings of the Water Environment Federation includes access to most papers presented at the annual WEF Technical Exhibition and Conference (WEFTEC) and other conferences held since 2000. Subscription access begins 12 months after the event and is valid for 12 months from month of purchase. A subscription to the Proceedings of the Water Environment Federation is included in Water Environment Federation (WEF) membership.

    WEF Members: Sign in (right panel) with your IngentaConnect user name and password to receive complimentary access. Access begins 12 months after the conference or event
  • Subscribe to this Title
  • Membership Information
  • About WEF Proceedings
  • WEFTEC Conference Information
  • Learn about the many other WEF member benefits and join today
  • Ingenta Connect is not responsible for the content or availability of external websites
  • Access Key
  • Free content
  • Partial Free content
  • New content
  • Open access content
  • Partial Open access content
  • Subscribed content
  • Partial Subscribed content
  • Free trial content
Cookie Policy
Cookie Policy
Ingenta Connect website makes use of cookies so as to keep track of data that you have filled in. I am Happy with this Find out more