A Watershed Approach to Mitigating Stormwater Impacts (2017) / Chapter Skim
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From page 15... ...
15 Defining Overall Stormwater Management Objectives Determining the regulatory requirements and identifying available BMP planning and design options is an essential step in developing criteria for selecting mitigation options. Each state or even watershed may have different requirements depending on the regulatory needs, capacity, or resources.
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16 A Watershed Approach to Mitigating Stormwater Impacts Category Typical Objectives of DOT Stormwater Management Projects Hydraulics Manage flow characteristics within and downstream of stormwater system Hydrology Mitigate floods; groundwater recharge; improve runoff characteristics (peak shaving and/or volume reduction) ; improve stream channel protection Water quality Reduce downstream pollutant loads and concentrations Avoid/minimize downstream temperature increases Achieve a target reduction of a not-to-exceed concentration or load in outflow Toxicity Reduce acute or chronic toxicity of runoff Regulatory Comply with NPDES permit, 401 certification, ESA, CERCLA, RCRA, UIC Program Meet local, state, or federal water quality, flow, and flood control criteria Mitigate development in other areas Create banking credits or advance mitigation for scheduled projects Address/avoid third party CWA and other lawsuits Implementation Function within management and oversight structure Cost Minimize capital, operation, and maintenance (life-cycle)
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Methods to Develop Mitigation Options 17 groundwater recharge, flood reduction and stream protection, and reduced stormwater infrastructure capacity requirements. Potential runoff pollutant loads and pollutant concentrations, rates, or discharge volumes can be related to highway attributes such as the volume of traffic, roadway classification, or impervious surface area.
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18 A Watershed Approach to Mitigating Stormwater Impacts • BMP selection, sizing, and performance – The appropriate type and size of BMP to mitigate the pollutants of concern will be based on the scale of implementation, site-specific conditions, recognized treatment processes, and calculation results that are acceptable to the regulatory agencies. This can include determination of the effect of the BMP systems(s)
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Methods to Develop Mitigation Options 19 One innovative technique DOTs have employed is to seek off-site partners and/or mitigation banks to care for off-site mitigation over the long-term. DOTs have also sought opportunities to sell "excess parcels" of/in the ROW that may be maintained for natural resource or other non-transportation corridor needs (Venner Consulting and ICF International 2012)
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20 A Watershed Approach to Mitigating Stormwater Impacts Step 1: Identify Appropriate and Acceptable BMPs Regulatory requirements and guidance often dictate what design standard should be met (Caltrans 2010)
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Methods to Develop Mitigation Options 21 Off-Site Treatment Options. Off-site options to treat stormwater that originates outside the project footprint may be necessary to meet mitigation objectives where on-site BMP options are limited or where on-site designs achieve only part of the treatment goals.
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22 A Watershed Approach to Mitigating Stormwater Impacts using an offline configuration may not be an option and/or might require additional considerations for energy dissipation and overflow. • Flow duration sizing.
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Methods to Develop Mitigation Options 23 where V _ i is the total runoff volume and Ai is the total area for land use i. P is the average annual rainfall depth.
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24 A Watershed Approach to Mitigating Stormwater Impacts Category Constraint Ve ge ta te d s w al es R oc k sw al es Fi lte r s tri ps D ry d et en tio n ba si ns R et en tio n ba si n s In fil tra tio n ba si n /tr en ch Co m po st a m en de d sl op e W et la nd sw al e/ ch an ne l/b as in O il/ w at er /g rit s ep ar at or v au lt B io re te nt io n / ra in ga rd en Physical Limited available ROW Obstructions (existing structures) Utility conflicts On or near top of steep slopes Poor soils/rocks High groundwater High hydraulic loads High pollutant loads Lack of vertical relief Deficient storm drain infrastructure Limited access Drainage patterns (adverse slopes)
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Methods to Develop Mitigation Options 25 It is recognized that there are additional local factors that may be applied and one factor may outweigh all of the others, such as ROW acquisition or the presence of other protected resources in the potential BMP location. Each DOT must customize this part of the toolbox.
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26 A Watershed Approach to Mitigating Stormwater Impacts the credit-generating BMP from the project site. For example, the Connecticut Nitrogen Credit Exchange (CNCE)
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Methods to Develop Mitigation Options 27 a BMP, the needed percent capture can be computed. This percent capture can be used to estimate the design volume through the use of a sizing nomograph, such as the example shown in Figure 3, for an extended dry detention basin with a 72-hour drain time (figure shows various precipitation patterns represented by differing U.S.
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28 A Watershed Approach to Mitigating Stormwater Impacts selection, cost, life-cycle, and sizing of BMPs, whether on-site or off-site, can be accomplished with a series of recognizable and readily understood procedures and formulas that can be adapted for use by any DOT. The issues of permit processing times and ROW acquisition for BMPs are more complex as are the considerations of institutional issues and stewardship.
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Methods to Develop Mitigation Options 29 1999a)
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30 A Watershed Approach to Mitigating Stormwater Impacts Pre-construction Costs. Before construction can begin, remediation sites have to be prepared.
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Methods to Develop Mitigation Options 31 in out-of-kind watershed-based approaches all center on the use and application of mitigation and restoration approaches that have been developed for other environmental impacts. Ecosystem Services The concept of ecosystem services has been around for many years, but the use of eco system services in designing environmental mitigation approaches is still evolving.
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32 A Watershed Approach to Mitigating Stormwater Impacts patterns of aggradation and sedimentation (supporting services)
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Methods to Develop Mitigation Options 33 • The Ecosystem Based Management (EBM) Tool Network is an organization coordinated by NatureServe that provides information on ecosystem measurement tools.
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34 A Watershed Approach to Mitigating Stormwater Impacts status of these benefits, the ecosystems that provide and protect them, and related health and economic impacts. The EnviroAtlas is available at www.epa.gov/enviroatlas.
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From page 35... ...
Methods to Develop Mitigation Options 35 • Delaware DOT. DelDOT participates in stakeholders groups called Tributary Action Teams that have been convened by the state Department of Natural Resources and Environmental Control (DNREC)
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