Stormwater Treatment with Vegetated Buffer

Focus Area

Water Quality/Wetlands


Natural Resources




Under $99k


1-2 years

Research Idea Scope

TERI Administrator Note (January 2009): Funded under NCHRP 25-25 (53).

Background: The New Hampshire Department of Transportation has recently been issued a number of Water Quality Certifications by the NH Department of Environmental Services (DES) that require a “no net increase” for pollutants in stormwater leaving a proposed facility.  The requirement has its


origins in the anti-degradation language in the Clean Water Act.  DES has approached this requirement in two ways.  The first is to require the applicant to provide evidence that the receiving water body can assimilate the pollutants generated by the proposed action and not degrade the water quality to the point where fish, shellfish, and wildlife and recreation in and on the water become impaired.  The second and more applicable method is to provide evidence that the proposed action will not have a “net increase” in pollutant loads to the receiving water body.

To aid the applicant in meeting this requirement, DES has issued “Interim BMP Guidance to Satisfy Loading Analyses” while the regulators develop rules that will be more broadly applied to terrain altering activities that are greater than 1-acre. The Department has had the opportunity to apply the guidance to number of projects since it was issued on May 21, 2007.  The use of the guidance is required to stay compliant with our Water Quality Certification, but it has generated a number of questions that we think need further investigation.

Our major concern is the applicability of the guidance to rural sections of highway where there is no proposed closed drainage system.  The guidance has a section, which allow the use of vegetated buffers as a stormwater treatment structure.  However, the requirement does not allow the use of engineered slopes that are steeper that 15% (6.7:1).  We can take a small credit for slopes flatter than 3:1, but it does not account for even a small incremental widening of the shoulder.   The Department also maintains vegetated slopes as steep as 2:1, with little or no overland flow. The guidance also requires that a buffer be at least 50 to 80 feet long to receive full removal credits. 

We believe these requirements are too stringent because we rarely see overland flow from roadways travel more than a few feet from the edge of pavement.  A quick scan of stormwater management guidance from around the country revealed very different criteria being applied to the same situation, and much of the guidance is based on “Big Box” development that is inappropriately applied to rural highway situations. 


The objective of this research proposal is to review the literature and other states’ guidance to come to a common understanding of how highway stormwater treatment is modeled throughout the country, share best practices and develop guidance for effective stormwater treatment on rural highways.


The tasks shall include, but not be limited to the documentation of the physical characteristics of vegetated buffers, filter strips and swales utilized by state transportation agencies around the country, and documentation of the modeled removal efficiencies associated with the physical characteristics of each stormwater treatment structure.  Specifically:

  1. Identify, compile, analyze, and document the current reports, publications, tools, guidance, training programs, and successful practices that are available.
  1. Submit an interim report, within 5 months, to document the results for review by the NCHRP.
  1. Based on the results, develop guidance on effective best management practices to facilitate the regulatory process
  1. Submit a final report that documents the entire research effort.  In addition, provide a PowerPoint presentation summarizing the background, objectives, research method, results, and benefits.

Suggested By

Mr. Mark Hemmerlein, Water Quality Program Manager NH Department of Transportation

[email protected]