Guidelines for Evaluating and Selecting Modifications to Existing Roadway Drainage Infrastructure to Improve Water Quality in Ultra-Urban Areas (2012) / Chapter Skim
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Pages 109-123
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109 This section describes general retrofitting approaches and strategies. A general process for planning and implementing ultra-urban highway stormwater control retrofits is presented.
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110 NCDOT for retrofit prioritization and is used in the CWP retrofit guidance manual (Schueler et al., 2007) for watershed restoration.
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111 broad and early coordination with regulators, local officials, and personnel familiar with the site can benefit and reduce likelihood of encountering problems and increasing costs in the later project stages. Recommended project planning and coordination practices include the following: • Proactively engage regulators and local officials.
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112 costs can potentially increase significantly due to redesign and/or removal of the contamination or infrastructure during construction. • Strive for vision and creativity in developing BMP alternatives.
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113 and lists potential data sources. The data sources may include DOT policy documents, DOT monitoring data, and regulatory and stakeholder reports.
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114 In setting the objectives, it is important to keep in mind the potential limitations of retrofitting in ultra-urban settings. The objectives may include alternatives to retrofitting and/or reduced performance metrics as allowable or pollutant trading amongst project areas (i.e., over-treating some areas and under-treating others)
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115 BMP-driven approach where the project team first identifies appropriate BMPs, followed by site scoping, conceptualization, and evaluation. The process is iterative and includes the following tasks: 1.
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116 – Bioretention – Infiltration BMPs (basins, trenches) • Underground and non-traditional approaches for spacelimited sections: – Various proprietary underground detention vaults for water quality treatment.
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117 in ROW strips are embankment cuts along downslope sections and at down-gradient outfalls. Such areas are potentially suitable for aboveground filtration systems, sand filters, and GSRDs.
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118 – Using guard rails and concrete barriers if clear zone setbacks or slopes are not adequate, and – Modifying BMP sizing and design to fit within space and contours, or to meet design constraints. • Drainage and connectivity: Look for sufficient hydraulic gradient to potential connection points with existing conveyances, taking into account the BMP head requirements.
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119 Task 2b: Consider Jurisdictional Partnerships. If aboveground BMPs within the ROW are not feasible or excessively costly or where a partnership would be more cost effective overall, planners should consider and investigate potential cross-jurisdictional partnerships with local municipalities to develop off-site aboveground retrofit solutions.
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120 which to choose. Considerations when evaluating underground BMPs include the following: • Existing infrastructure.
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121 – Review of independent performance evaluations, and/or – Modeling of expected performance by the DOT. Consider retrofit pilot tests for promising BMP approaches with incomplete performance information.
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122 Section 6 provide a means of assessing sizing tradeoffs with alternative operation strategies, such as batch mode operation of detention facilities versus extended detention. For proprietary systems, manufacturer sizing criteria should be used as minimum sizing guidance.
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From page 123... ...
123 at the design capacity, and for obtaining feedback about BMP design and ongoing performance. • Ongoing BMP inspection and maintenance: As discussed in various sections previously, an initial BMP O&M plan should be coordinated and developed early in the retrofit planning process.
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