Green Infrastructure and Storm Depth Retention Criteria explained by Andy Reese


The Shift to Storm Depth Criteria

In the July-August 2010 issue of Stormwater Magazine, Andy Reese et al write about the dawning awareness of a ‘need for change’ that is currently being expressed through changes in regulatory criteria throughout the United States.

The authors state that the purpose of their article is to explore the proposed single-criterion 95% storm capture requirement as a suitable volume-based hydrology criterion; and to explore and propose different options. To accomplish this objective, they performed a continuous simulation analysis of hourly rainfall and runoff across four soil types for both pre- and post-development conditions.

95% Storm Capture Requirement

The 95% storm is calculated by taking standard daily rainfall values, removing all records less than one-tenth of an inch, arranging smallest to largest, and finding the value for which 95% of all the values are less.

The ‘95% rule’ requires that drainage system designers “prevent the off-site discharge of the precipitation from all rainfall events less than or equal to the 95th percentile rainfall event”.

Unintended Consequences

Andy Reese and his co-authors are concerned about the set of unintended consequences that result from a simplified abstraction of reality.

In the article, they state that: “In our experience, volume-based criteria can only be rightly developed by understanding the long-term flux of water volume; and the only way to accurately do this is through well-constructed continuous simulation modeling that accounts for drying processes between storms. And the need for long-term data may not be as critical as suggested.”


“Continuous simulation models may be an effective answer to volume-based criteria and design generation because the driving data are long-term rainfall and evapotranspiration patterns. Some of the inherent power in continuous simulation can be harnessed through the development of generic design curves based on the data output,” write Andy Reese et al.

Andy reese (120p) - amec“There is a need for a simple, hardwired, and intuitive software tool for the design of volume-based sites that contains sufficient flexibility to model differing soils and groundwater configurations, has local data hardwired into it, and has an effective pre- and post-processor (or takes advantage of existing software and export capabilities) for visual input and output.”

Related Articles on Water Bucket by Andy Reese

Volume-Based Hydrology explained by Andy Reese – Examining the shift in focus from peak flows and pollution treatment to mimicking predevelopment volumes, August 2009

Voodoo Hydrology explained by Andy Reese – Magical practice considered to be a form of black magic but also considered a religion to some, August 2006

Do you know where you really are in the shifting paradigms of stormwater management? –  Article by Andy Reese examines how our ideas about stormwater have changed over time, August 2008 (note: source story published in 2001)


Stormwater magazine - andy reese story - cover (150p)Before STORMWATER, The Journal for Surface Water Quality Professionals, there was no single publication written specifically for  the professional involved with surface water quality issues, protection, projects, and programs. To download a copy of the Andy Reese article, click on this link to Green Infrastructure and Storm Depth Retention Criteria.

About Andy Reese

Andrew (Andy) J. Reese has over 30 years experience in a wide variety of stormwater management, water resources, hydraulic and hydrologic engineering, and management roles. He is known for this mantra: Stormwater – Back to the Future.

Municipal stormwater management - cover (120p) - by andy reese & tom deboAndy Reese has been a popular speaker at over 200 conferences, short courses and meetings including the keynote for the first annual STORMCON conference. He has published over fifty articles and has co-authored a best-selling 1400 page textbook on Municipal Stormwater Management, now in second edition.

He has worked in all fifty states in a wide variety of assignments from highly technical modeling and criteria development to stakeholder group facilitation and stormwater utility implementation. He is known as a leading expert in municipal stormwater program development, LID and green infrastructure design and planning, and stormwater program and funding implementation.