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Chapter 3
Designing for Environmental Stewardship in Construction & Maintenance
3.7. Design Guidance for Stormwater and Erosion & Sedimentation Control

For "point source" discharges (construction sites, maintenance yards, rest areas) the Clean Water Act requires pollutant removal, depending on the type of pollutant, that represents best conventional pollutant control technology (BCT) or best available technology economically achievable (BAT). In addition, these other point sources are required to comply with local water quality standards if they happen to be higher, even if this means controls beyond BCT/BAT, such as "all known available and reasonable" technology or methods of pollution prevention (AKART). BCT and BAT are numerically defined for most industries but not for construction site runoff. This has meant that construction site requirements, as well as those for municipal runoff, have been something of a moving target.

All projects should incorporate certain minimum design elements with respect to water quality concerns. Such design goals include the following: [N]

  • Minimize Impervious Surfaces: The intent of this goal is to reduce the volume of runoff.
  • Prevent Downstream Erosion: Stormwater drainage systems should be designed to avoid causing or contributing to downstream erosion.
  • Stabilize Disturbed Soil Areas: Disturbed soil areas should be appropriately stabilized.
  • Maximize Vegetated Surfaces Consistent with Existing Policies: Vegetated surfaces prevent erosion, promote infiltration (which reduces runoff), and remove pollutants from stormwater. See the following section on design for sustainable, low maintenance roadsides.

NYSDOT's Operations & Maintenance Manual for Stormwater Facilities contains the following general pre-construction stormwater facility design considerations: [N]

  • Discuss proposed facilities with the Maintenance Environmental Coordinator and Residency personnel.
  • Make facilities visible. Visible structures get more attention.
  • Select low-growing suitable grasses to reduce mowing needs. Add nitrogen fixing plants such as clover to reduce fertilizing needs.
  • Incorporate reference points into basins and other features that require clean-out in regard to an absolute elevation. How would someone know how much sediment has accumulated? Percentage of capacity reduction is difficult to estimate.
  • Consider mosquito control such as introducing natural predators into a permanent pool (for example mosquito eating fish) or placing commercial mosquito traps. Non-native species should not have a means of escape.
  • Features to be maintained must be accessible. Consider access roads, ramps to basin bottoms, sturdy slopes.
  • Trash racks should be accessible at normal pool elevations.
  • Do not plant trees and shrubs on embankments, side slopes or dam areas.
  • Slopes that should be mowed should be 1 on 3 or flatter. If slope is higher than 5 feet, slope should be 1on 4 or flatter. If steeper, explore other treatment options.
  • For non-vegetated covers, loose stone or rip rap, which encourages the growth of weeds, should be discouraged. Can consider using gabion lining.
  • Consider effects of sediment removal from vegetated surfaces. Can vegetative cover recuperate on its own?
  • Aesthetic features of the stormwater management facilities requested by municipalities should be maintained by the requester. Commitments by municipalities must be made by signed resolutions..

Low Impact Design

Low Impact Development (LID) is a stormwater management strategy concerned with maintaining or restoring the natural hydrologic functions of a site to achieve natural resource protection objectives and fulfill environmental regulatory requirements. LID employs a variety of natural and built features that reduce the rate of runoff, filter out its pollutants, and facilitate the infiltration of water into the ground. By reducing water pollution and increasing groundwater recharge, LID helps to improve the quality of receiving surface waters and stabilize the flow rates of nearby streams.

LID incorporates a set of overall site design strategies as well as highly localized, small-scale, decentralized source control techniques known as Integrated Management Practices (IMPs). IMPs may be integrated into buildings, infrastructure, or landscape design. Rather than collecting runoff in piped or channelized networks and controlling the flow downstream in a large stormwater management facility, LID takes a decentralized approach that disperses flows and manages runoff closer to where it originates. The LID effort in Prince George's County was one of the first. It began with the development and use of bioretention cells, which are created by replacing existing soil with a highly porous soil mixture, grading the area to form a shallow depression, and replanting the area with specially selected vegetation. Such vegetation must be able to tolerate temporarily saturated soil conditions as well as the pollutants contained in the local runoff. When it rains, bioretention areas collect the runoff and then filter out the pollutants as the water passes down through the soil. After initial experimentation, the County initiated a full-scale effort to incorporate the LID approach; the Prince George's County manual was expanded into a nationally distributed LID manual published in 2000. LID site design attempts to minimize runoff volume and preserve existing flow paths, in order to minimize infrastructure needs, in contrast to conventional site design, which may generate increases in runoff volume and energy, resulting in concentrated flows that require larger and more extensive stormwater infrastructure.

Examples of LID site design strategies and practices include:

  • Grade to encourage sheet flow and lengthen flow paths.
  • Maintain natural drainage divides to keep flow paths dispersed.
  • Disconnect impervious areas such as pavement and roofs from the storm drain network, allowing runoff to be conveyed over pervious areas instead.
  • Preserve the naturally vegetated areas and soil types that slow runoff, filter out pollutants, and facilitate infiltration.
  • Direct runoff into or across vegetated areas to help filter runoff and encourage recharge.
  • Provide small-scale distributed features and devices that help meet regulatory and resource objectives.
  • Treat pollutant loads where they are generated, or prevent their generation

Bioretention, dry wells, filter strips, grassed swales, infiltration trenches, and inlet pollution traps/removal devices, and permeable pavers and pavement are some of the common LID tools. Greater use of soil amendments increase the capacity of soil to absorb moisture and sustain vegetation, curbside or in swales, which in turns removes water through transpiration.

Resources for LID design include:

EPA maintains a low impact development methods website among its on-line non-point source pollution resources. [N] The Department of Defense has also created a Low Impact Development manual, which reviews LID principles, the cost, maintenance, and other considerations of various LID BMPs, in Chapter 8. [N]

Recent DOT research has shown that some of the initial assumptions guiding LID have not been valid and that the concept can be more widely applied, in less space than previously thought. One particularly important study found that: [N]

  • Over the range of slopes and system specific conditions studied, runoff did not correlate positively to slope (i.e., higher runoff was not observed at higher slopes). Most importantly, the current design guideline requiring LID slopes to be not steeper than 7:1 is not justified and thus eliminates many areas with acceptable levels of LID infiltration or increases the LID length well beyond what is necessary at many roadside locations.
  • Saturated conductivity (Ks) was found to be the most critical design parameter. Required minimum values of Ks (102 mm/hr) found in existing natural dispersion guidance were found to be not physically based, again eliminating areas that are well suited for LID application or resulting in LID lengths greater than needed in some situations. As a result of this research, a simplified, user friendly design equation (LID Design Equation) was developed and shown to be an accurate means of determining LID length by comparison to field data and calibrated model simulations using a finite difference solution to the hydrodynamic wave equation dynamically coupled with the Green-Ampt infiltration equation. [N]

Federally Sponsored Stormwater BMP Manuals

Detailed selection guidance and information on BMP effectiveness will be available from NCHRP 25-20(01) in late 2006. Existing federally sponsored stormwater BMP manual include the following.

Guidance on Low Volume Road Design

Erosion and sedimentation control is a primary consideration in the design and maintenance of low volume roads. Some of the best guidance for transportation agencies has been developed by the USDA Forest Service and the PennDOT supported State Conservation Commission Dirt & Gravel Road Program. The Forest Service and U.S. Agency for International Development Guide covers best environmental practices in the following areas: [N]

State Sponsored Stormwater BMP Manuals

Almost every state DOT has a guide to development of such plans and design of stormwater BMPs. The U.S. EPA Region 10: The Pacific Northwest provides web links to Stormwater BMP manuals from various State agencies: According to a 2003 survey by the author, 54 percent of all the states have developed a Highway Runoff Manual; Caltrans, FDOT, Illinois DOT, MoDOT, Ohio DOT, and TxDOT completed revisions in the last two years. Almost 30 percent of state DOTs have developed manuals for stormwater management at non-highway facilities (AR, CA, FL, GA, IH, MO, MT, NH, NV, WA) and stormwater manuals for construction (AR, CA, FL, GA, IH, IA, IN, LA, MI, MO, MT, NM, OH, WA.) [N] Following is list of manuals available on-line:

California Georgia Idaho Illinois
  • Illinois Department of Transportation. Erosion and Sediment Control NPDES for Standard Specifications for Road & Bridge Construction.
Maine Maryland Massachusetts Michigan Minnesota Missouri Montana New Hampshire New Jersey New York North Carolina Ohio Oregon Pennsylvania South Carolina Tennessee Texas Utah Virginia Washington Wisconsin Wyoming

State Stormwater BMP Manual Builder

The Stormwater Manager's Resource Center has developed a "Manual Builder" on-line, a toolbox for developing a stormwater treatment practice (STP) design manual. It includes stormwater treatment plan design and construction criteria and provides schematics and graphics for each practice. It also provides information about maintenance requirements and the typical local review process for treatment plan design and construction. As manuals need to be customized to meet the needs of the state or community where they are being applied, the manual builder does not prescribe one specific set of criteria, but instead presents a series of options for stormwater managers to choose from. A good manual contains specific guidance on how to select, size, design, construct, and maintain practices at each development site.

A typical manual will contain the following elements:

  • Basic Stormwater requirements.
  • Procedures for Reviewing Stormwater Plans. In this section, the manual outlines the process a community should go through to review the stormwater plan.
  • Basic Sizing Criteria. The manual needs to identify minimum sizing criteria for practices to meet groundwater recharge, water quality, channel protection, and flood control requirements.
  • List of Acceptable Practices. The manual should include a list of practices that can meet water quality requirements. An engineer can choose from this menu of practices to treat stormwater runoff from a new development.
  • Performance Criteria. The performance criteria provide required minimum elements and guidance to ensure that practices are designed and maintained to ensure practice longevity and performance.
  • Guidance on STP Selection. This section presents criteria to guide the design engineer to select the best practice for the site, based on characteristics such as soil type, site slope, and the local watershed conditions.
  • Stormwater Credits. Stormwater credits are reductions in stormwater volume requirements given in exchange for incorporating site design techniques that minimize the need for STPs on the site. Many manuals do not incorporate credits, because they can increase the burden of review on local governments substantially.
  • Design Examples. Design examples step the engineer through designs for a representative group of STP design variations. They should illustrate how to select, size, and locate the practice on the site.
  • Construction Specifications. Construction specifications detail specific materials and construction standards that ensure that the practice will function as designed.
  • Checklists for Construction Inspection. These checklists outline what minimum elements are needed for each practice group during construction.

In addition to extensive design guidance available in both manual and on-line formats, a number of BMP selection and evaluation systems are emerging. NCHRP 25-25(01) is designing a BMP effectiveness and evaluation system that will be available in late 2004. MDSHA has developed an evaluation system for all stormwater facilities and criteria for improvements. In the late 1990s WSDOT and FDOT also developed systems for categorizing outfalls and, in the case of WSDOT, assessing which projects provide the best return on investment in terms of environmental effectiveness and pollution reduction. [N] WSDOT's system included a condition indexing methodology and support program that enables users to quickly evaluate and compare projects and generate benefit-cost ratios for projects. [N]

 

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Table of Contents
 
Chapter 3
Designing for Environmental Stewardship in Construction & Maintenance
3.1 Beyond Mitigation: Projects to Achieve Environmental Goals
3.2 Context Sensitive Design/Solutions
3.3 Avoiding Impacts to Historic Sites
3.4 Designing to Accommodate Wildlife, Habitat Connectivity, and Safe Crossings
3.5 Culverts and Fish Passage
3.6 Stream Restoration and Bioengineering
3.7 Design Guidance for Stormwater and Erosion & Sedimentation Control
3.8 Drainage Ditches, Berms, Dikes, and Swales
3.9 Design for Sustainable, Low Maintenance Roadsides
3.10 Designing to Reduce Snow, Ice, and Chemical Accumulation
3.11 Designing to Minimize Air Quality Problems
3.12 Design and Specification for Recycling
3.13 Designing to Minimize Noise
3.14 Lighting Control/Minimization
3.15 Design for Sustainability and Energy Conservation
3.16 Safety Rest Areas, Traveler Services, and Parking Area Design
   
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