CNU's Rainwater-in-Context Initiative Subcommittee Reviews LEED-ND Rainwater Management Credit

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December 17, 2010

With LEED for Neighborhood Development up for another round of public comment, the CNU Rainwater-in-Context initiative took another look at both the Stormwater Management credit and the 2009 position statement that was drafted by this same committee. Following a point-by-point review, we have determined that the LEED-ND Stormwater Credit (GIBc8) continues to lack methodological rigor and may not adequately support the development patterns that are best for watershed health. Please note that there is a name change proposed to the credit, from Stormwater Management to Rainwater Management.

The philosophy of LEED-ND is that compact development works by orchestrating the design elements of the public and private realms to use and conserve land more effectively and efficiently. This concept extends to rainwater management, and can help reform the current requirements for site-by-site BMPs. Lot-level standards alone deliver lower overall performance than holistically-planned shared systems at the community or watershed scale.

Today’s most advanced stormwater policies for BMPs in constrained environments seek to obtain as much onsite management as is feasible, with shared solutions covering the rest. The beauty of LEED-ND is that shared solutions (like shared parking, landscaping, parks, etc.) may be planned ahead of time. Unfortunately, the draft stormwater credit does not address this potential.

We should note that many permitting authorities actually express disfavor for shared solutions, so it is essential that USGBC signal that the ultimate client is the watershed. If shared solutions ultimately deliver better performance, the watershed, not ease of permit administration, should be the driver.

The critiques and recommendations of the task force are as follows:

1) Shared Practices should be recognized throughout LEED rating systems, not just LEED for Retail. LEED for Retail allows the credit requirement to be met by using a centralized approach in projects that are part of a multi-tenant complex. It is recognizing that a shop built within a mall or retail complex may not be able to install onsite Low Impact Design measures. There is no reason this allowance should be limited to retail. Any land use may have similar constraints if the site is compactly and densely developed.

Recommendation: LEED rating systems should credit offsite rainwater management if it is part of a district, small-area, or master plan.

2) Intent fails to recognize the watershed scale. In 2009, we noted that the intent was divorced from reality because restoration of urban watersheds to pre-development hydrological conditions would entail removing all water-related infrastructure from urban areas. Additionally, the intent fails to recognize the watershed-scale stormwater benefits of good urbanism, and therefore encourages suburban sprawl. The proposed change to the intent does nothing to address these concerns.

Current Language: To reduce pollution and hydrologic instability from stormwater, reduce flooding, promote aquifer recharge, and improve water quality by emulating natural hydrologic conditions.

Proposed Language: To restore or maintain the natural hydrology and water balance of the site based on historical conditions and undeveloped ecosystems in the region.

Recommendation: Revise the intent to the following: To improve watershed catchment areas by using natural or historical hydrologic conditions as benchmarks.

This reinforces the focus on the watershed and allows the applicant to prioritize the biggest problems and utilize a variety of mitigation options. 

3) Hydrology science should not be discarded. The credit uses metrics – percentile rainfall events and storage on a site of a prescribed, arbitrary volume – that have very little direct relationship to the impacts of urbanization. The hydrological impacts of urbanization have been scientifically researched, published, tested, and proven for decades through national hydrological standards. See, for example, USDA Technical Release 55, which presents the theoretical science and simplified procedures to calculate rainfall runoff volume, peak rate of discharge, time of concentration, and estimated detention or retention volumes required to mitigate the impacts of urbanization. These procedures are applicable to varying scales from small sites to small watersheds, and for rainfall events from low intensity to high. The application of the science results in hydrographs that can range from pre-Columbian, to pre-project, to post-project – all of which can be compared in order to directly measure the impacts of urbanization.

Recommendation: Base credit points on the actual impacts and mitigation practices as measured by the science of hydrology.  For example, “x” points for a post-project hydrograph that resembles the pre-Columbian hydrograph, “y” points for a post-project hydrograph that brings the pre-project hydrograph 50% closer to the Pre-Columbian hydrograph, “z” points for a post-project hydrograph that resembles the pre-project hydrograph.

4) The requirements are not context-sensitive. The same storage requirements apply to every site no matter if it is downtown or in the country. While lower-density, single-family areas would only need depressions in the yards, smaller, higher-density projects will find it harder to meet the standard. Downtown lots will require large storage facilities in most cases, incurring high costs and significant functional disruption. The misapplication of low impact designs hinders good placemaking. Rainwater management should use designs that are tailored to the broad range of urban to rural conditions. The current language encourages lower-density development and/or disruptive designs to fulfill this credit – working against many of the other goals within LEED-ND.

Recommendation: Allow requirements to vary based on the context of the project, provided that performance at the sub-watershed or watershed scale is maintained or improved. Controls would be looser for downtowns and town centers than for large-lot, single-family areas. The credit would correlate to a spectrum of contexts, such as the rural-to-urban transect, or pervious surfaces, or dwelling density / FAR.

5) Additional points for location are ineffective. The points for previously developed, brownfield and transit-oriented sites do not act as an adequate incentive because a site must achieve “storage of the 85th percentile storm volume” in order to qualify. That standard is unrealistic for most downtown or dense infill sites. Infill sites of any significant density, such as 70% or more lot coverage, would require an expensive and/or functionally disruptive and unattractive rainwater storage facility to qualify.

Recommendation: Making the credit context-sensitive, and basing credit points on the degree of watershed hydrograph modification as described above, will encourage better urban patterns.

CNU Rainwater-in-Context Initiative Subcommittee

Paul Crabtree, PE, CNU, Crabtree Group, Inc.

Laurence Aurbach, LEED Location and Planning Technical Advisory Group

Sandy Sorlien, CNU-A, Technical Advisor, Center for Applied Transect Studies

Tom Low, Duany Plater-Zyberk & Company

John Jacob, Texas Sea Grant, Texas A&M University


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