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Page 102
Suggested Citation:"Glossary." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Page 102
Page 103
Suggested Citation:"Glossary." National Academies of Sciences, Engineering, and Medicine. 2017. A Watershed Approach to Mitigating Stormwater Impacts. Washington, DC: The National Academies Press. doi: 10.17226/24753.
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Page 103

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102 Banking systems – A centralized system that establishes and enables the trading of various credits that are related to watershed health such as water quality, stormwater, or wetlands. Baseline watershed assessment – An accounting of the existing conditions in a watershed. Best management practices (BMP) – Control measures taken to mitigate stormwater impacts caused through land use changes. Stormwater BMPs may be classified as structural or non-structural. Conservation mitigation – Preservation of natural lands through easement purchases or other legal instruments. Hydrologic unit (HU) – A hierarchical system of nesting, successively smaller watersheds that are delineated and georeferenced to USGS 1:24,000 scale topographic base maps according to com- pilation criteria monitored by the national Subcommittee on Spatial Water Data. Watersheds are represented as hydrologic units, which are classified into six levels. Each is identified by a unique Hydrologic Unit Code (HUC) consisting of two to 12 digits based on the six levels of classifications. In-kind mitigation – This approach uses accepted BMPs that are designed to address a specific stormwater pollutant associated with the project. The BMPs will directly reduce the same impair- ment that is caused by the project, such as reducing total suspended solids (TSS) in runoff. In lieu fee – A payment to a fund maintained by a mitigation sponsor to create one or a number of sites that will meet the payee mitigation requirements. The mitigation sponsor will create sites based on agreements with the necessary regulatory bodies upon receiving the required funds from one or multiple payees (permittees). Mitigation offset – Any program that is established to mitigate project impacts that cannot be met at the project-site. Examples include trading systems, banking facilities, in lieu fee programs, off-site mitigation, and out-of-kind mitigation. Non-structural BMPs – Procedures focused on source control of pollutants by incorporating landscape features or practices such as street sweeping to mitigate stormwater impacts. Off-site – Any area outside of the immediate project limit. On-site – The area of the ROW within the project’s limits that is available for stormwater mitigation. This is essentially the area owned and maintained by the DOT that is within the project’s boundary. Off-site treatment – Stormwater mitigation measures located outside the project’s limits that treat runoff not associated with the project. Off-site mitigation options include those that are within the existing ROW but outside of the immediate project limits, and those that are outside of the existing ROW and the project limit. Glossary

Glossary 103 On-site treatment – Stormwater mitigation measures that are performed within the on-site area which treat runoff generated from the on-site areas (roadway and non-roadway ROW areas). It may also include treatment for off-site runoff from nearby adjacent areas. Out-of-kind mitigation – This approach uses mechanisms (e.g., offset programs, conservation, in lieu fee) other than conventional BMPs to provide an equivalent or greater benefit to the watershed than mitigating the direct project impact. The benefit to the watershed may be directly connected to the impact or it may otherwise be indirectly connected to the impact by improving a different, but related, watershed function. Restoration mitigation – Restoration of impaired natural lands to a prior state that provides an improved watershed benefit. Structural BMPs – Engineered physical structures that are installed or constructed on a site to mitigate stormwater impacts such as a sand filter. Trading opportunities – A non-centralized system that provides opportunities for purchasing (paying into) projects or parts of projects that improve watershed health to meet project impact mitigation requirements that cannot be met at the project-site. The agency pays the required fees that are determined by the regulatory agency to fund part or all of the treatment facility which will offset the impacts from the project to the watershed. Watershed – An area of land from which all of the water drains to the same place. Watershed-based approaches – These refer to on- or off-site stormwater mitigation measures that take place within the same watershed in order to address the direct impacts of the project. Such mitigation measures may also serve to address other watershed issues and priorities. The watershed scale or size of the watershed may vary depending on the scope of the project or regu- latory requirements.

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A Watershed Approach to Mitigating Stormwater Impacts Get This Book
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TRB's National Cooperative Highway Research Program (NCHRP) Research Report 840: A Watershed Approach to Mitigating Stormwater Impacts provides a practical decision-making framework that will enable state departments of transportation (DOTs) to identify and implement offsite cost-effective and environmentally beneficial water quality solutions for stormwater impacts when onsite treatment and/or mitigation is not possible within the right-of-way.

The report is accompanied by the Watershed-Based Stormwater Mitigation Toolbox, a Microsoft Excel-based program to facilitate the characterization of the project watershed and the identification of mitigation options at the planning level.

Disclaimer - This tool is offered as is, without warranty or promise of support of any kind either expressed or implied. Under no circumstance will the National Academy of Sciences, Engineering, and Medicine or the Transportation Research Board (collectively "TRB") be liable for any loss or damage caused by the installation or operation of this product. TRB makes no representation or warranty of any kind, expressed or implied, in fact or in law, including without limitation, the warranty of merchantability or the warranty of fitness for a particular purpose, and shall not in any case be liable for any consequential or special damages.

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