Bridge Stormwater Runoff Analysis and Treatment Options (2014) / Chapter Skim
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From page 64... ...
A-1 Literature Review Project Overview NCHRP Project 25-42 provides guidance for assessing potential water quality impacts and selecting BMPs for stormwater runoff from bridge decks and vehicle approaches. The study focuses on bridge structures that cross a waterway and discharge directly to the receiving water.
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From page 65... ...
A-2 DOT Survey Methodology The research team contacted a range of DOTs, including those known to be active in BMP and highway stormwater investigations to gain insight into current issues and practices relating to the management of stormwater discharge from bridges. Several state DOTs have stormwater research divisions that are engaged in original highway runoff assessments and were able to suggest additional studies that were utilized in the literature research effort.
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From page 66... ...
A-3 statistical data comparing bridge deck runoff event mean concentrations (EMCs) to the approach highway revealed only limited instances when parameters were significantly different from each other.
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From page 67... ...
A-4 Cyanide Anti-cake compound used to keep deicing salt granular Sodium, Calcium, Chloride Deicing salts Sulphate Roadway materials, fuel, deicing salts Petroleum Spills, leaks or blow-by of motor lubricants, antifreeze and hydraulic fluids, asphalt surface leachate The Impact of Curbed vs. Uncurbed Sections on Bridge Decks Some research has pointed to the accumulation of pollutants and sediments along curbed sections of bridge decks.
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From page 68... ...
A-5 Literature Research over the Last Decade The primary recent US studies assessing the effect of bridge deck runoff on receiving water beneficial uses (NCDOT/ USGS/URS, 2010, NCHRP, 2006, and Malina et al.
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A-6 • Phosphate concentrations in the creek were on average much higher than concentrations from bridge deck runoff. • Total nitrogen and total Kjeldahl nitrogen (TKN)
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A-7 associated with the characteristic opposite to other total recoverable and dissolved metals and TSS. Because a major source of arsenic in stormwater runoff is air deposition from point sources (e.g., coal-fired power plants)
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A-8 to occur between the pipe outlet (typically located on the bridge embankment or near a bent) and the receiving stream.
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A-9 off from bridge decks were minimal compared to the mass loads of constituents carried by the respective receiving stream." NCDOT pointed out that loads should be evaluated for meeting specific stormwater management goals, such as goals associated with waste load allocations, pollutant trading, stormwater banking programs, or off-site mitigation, as required by a particular program or regulation. Linking Bridge Deck Runoff to Receiving Streams Linking bridge deck runoff to conditions in receiving streams is more difficult than measuring constituents in runoff.
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A-10 abutment, on-deck spread takes valuable surface area and may require construction of a larger bridge deck, with the physical and carbon footprint that entails. Bridge girders, visibility, and space constraints complicate piping.
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A-11 and the limited headroom available within the bridge, pipe slopes are essentially restricted to the bridge slope. Piping located near the top of vertical curves may have very slight slopes and small flow capacities.
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A-12 • Limitation in Right-of-Way. There is no flexibility regarding the size of the foot print.
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A-13 tion Development (LADOTD) treats runoff "in accordance with permit."51.
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A-14 floodplain may be wide and wetlands extensive compared to the bridge project, in addition to the difficulties with draining water on long, flat bridges. Such cases are drivers in considering off-site mitigation.
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A-15 Nearly all DOTs contacted said that treatment for new construction projects is determined on a project-by-project basis with resource and regulatory agencies as part of the project planning phase. Where states consider retrofit measures, those may be selected and designed through the DOT's Highway Stormwater Retrofit Program to meet site-specific water quality goals.73 NCDOT avoids direct discharge off bridge decks whenever possible; they try to discharge to the overbank and collect and convey the stormwater to the stream in a manner that doesn't cause erosion.
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A-16 Some DOTs confine source control to DOT operations only. For example, during construction and maintenance projects, LADOTD limits materials placed on bridges to only that necessary, with special attention to cleaning materials, solvents and/ or fuels.
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A-17 roadway November–April (when rain might freeze) , so sweeping during those seasons is counterproductive.
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A-18 magnesium acetate (CMA) reduces the corrosion of metal bridge supports that can occur when salt is used.
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A-19 used abrasives can be lost to the environment if appropriate containment practices are not used.106 Paint overspray and solvents also may be toxic to aquatic life if they reach the receiving water (Kramme 1985)
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A-20 railings were a source of zinc contamination in runoff. Coatings for exposed galvanized metals have the potential to reduce such discharges.
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A-21 • Description. A brief overview of the BMP, including a description of the basic design concept and functionality.
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A-22 control practices must be executed throughout the sweeping process. Air quality impacts should be minimized by utilizing sweepers that include dust control mechanisms.
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A-23 Conveyance channels may be provided to collect runoff from bridge scuppers. They are typically lined with riprap since immediate stabilization is required.
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A-24 Environmentally sensitive design (ESD) utilizes natural topography downgrade of a bridge deck to receive stormwater from a bridge deck collection system.
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A-25 taken during construction activities to protect the infiltration basin area from construction traffic, material laydown, and other activities that can compact soils and reduce infiltration capacity. Construction cost varies by basin size, which is based on drainage area and percent imperviousness, and hydraulic capacity of in-situ soils.
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From page 89... ...
A-26 Stormwater wetlands can be implemented downgrade from the outlet of a bridge deck runoff collection system. These engineered wetlands with dense vegetation remove pollutants primarily through biological processes, evapotranspiration and infiltration.
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A-27 swales are an appropriate means of TSS reduction from highway runoff.137 More information about swales is available in NCDOT's Bioretention and Swale study.138 Emerging BMPs for Potential Use on Bridges Interviewed DOTs staff expressed interest in emerging BMPs and alternative treatment approaches given the difficulties inherent in treating on-site with known methods. Most DOTs did not have emerging treatment mechanisms to share with others, though some said they were "working on it." Emerging BMPs identified in the interview process and literature review included the following: Planning Mechanisms • Louisiana is performing a statewide assessment of water quality at existing and future bridge crossings to assist with the development of runoff management strategies.
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From page 91... ...
A-28 ability to handle larger flows, but FDOT notes that "bridge inlets don't intercept large flow rates. FDOT might use bioactivated media on a bridge deck if the agency is pressed to perform retrofits."143 Researchers anticipate being able to raise the flow rate by a factor of four in the next year, for bridge applications.144 FDOT has already supported research using BAM in retention areas such as swales and pipe-in-pipe wet detention pond harvesting applications.145 The technology could conceivably be used for ultra-urban environments or instead of a bridge collection system.
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From page 92... ...
A-29 The final report, published as NCHRP Report 474, advocates that, "Nonstructural mitigation techniques should always be considered before structural measures because they are cost-effective and sometimes more efficient pollutant removers."150 The practitioner's guide provides a simplified evaluation process to lead the practitioner through the BMP selection process.151 Further information is available in the NCHRP Report 474, Volumes 1 and 2 (see also Figure A-1)
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Figure A-1. Process drivers and treatment identification: NCHRP Report 474, bridge deck runoff practitioner's guide.
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A-31 model results.155 BASINS has an open-source MapWindow GIS interface, a Data Download Tool, project builder, watershed delineation routines, and data analysis and model output visualization tools as well as plug-in interfaces for well-known watershed and water quality models SWMM5, WASP7, and SWAT 2005. It includes a data extractor, projector, project builder, GIS interface, various GIS-based tools, a series of models, and custom databases; a web data extractor provides a tool for dynamic downloading of GIS data and databases from the BASINS web site and a variety of other sources.156 The user specifies a geographic area of interest and the software downloads appropriate data from EPA, USGS and other locations on the Internet.
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A-32 acquire "clear, legally binding written agreements assigning maintenance responsibilities and committing adequate funds for maintenance." Sustainability of performance over time is a design consideration, whether the amount of supplemental irrigation required for the chosen vegetation or clogging of infiltration BMPs when there is upstream development. Design of BMPs must be informed by operation and maintenance performance in the field.
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A-33 • Fracturing geology present • Minimum depth to groundwater • Approximate percent (%) removal efficiencies for select parameters • Safety reference(s)
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A-34 quality goals is also not feasible without extensive studies, the (team) relied upon existing water quality regulations established by NCDENR to define sensitive receiving waters.
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A-35 • BMPs are not typically selected based on identified POCs (see first bullet) and are still widely evaluated based on pollutant removal efficiency and not the irreducible concentration, as is recommended in the International Stormwater BMP Database (Wright Water Engineers and Geosyntec Consultants, 2007)
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A-36 – The bridge project is located as part of a roadway with anticipated average daily traffic greater than 30,000 vehicles per day. – The bridge project is a new location bridge and located in a water quality sensitive area.
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A-37 Methods to Identify an Appropriate Whole Life Cost-Benefit Strategy for Bridge Deck Runoff Mitigation As of December 2012 there were 607,380 bridges in the United States as defined within 23 CFR 650 Subpart C.174 Of those, 504,563 are listed as crossing some type of waterway.175 With this number of bridges to maintain and state and federal budgets as they are, funds must be directed to where they will produce a tangible and worthwhile benefit. Management of bridge deck runoff water quality requires practical solutions, which are easy to retrofit to existing infrastructure, maintainable by DOTs using existing personnel, equipment and techniques, and which will have the lowest possible whole-life cost.
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A-38 while other construction is underway so if the bridge is in STIP, then BMP options should be considered to handle deck runoff prior to the completion of the project. • What is State ACWA (Combined water body sensitivity)
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A-39 DOT's project did not find any bridges where that was the case -- there are a wide variety of BMPs that might be applied, ranging from low cost items such as public education and review of de-icing practices, to more costly items such a street sweeping or drainage modifications.182 When asked about their methods to identify an appropriate cost/benefit strategy for bridge deck runoff mitigation, NCDOT emphasized that "it comes back to a case-by-case situation: what is practical; what can be installed. It's a qualitative assessment.
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A-40 NCDOT developed cost estimates for each of 50 pilot study sites to characterize costs for the particular SCMs at each site and to provide an additional means of identifying costs for bridge SCMs; when actual construction costs or preliminary construction estimates were unavailable, some known data, typically impervious drainage area, were used to estimate construction costs.191 NCDOT described how they calculated operating costs:192 Operating costs represent the costs necessary to inspect, operate, and maintain an SCM. Typical operating cost estimates were derived from the following sources (in order of preference)
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A-41 Table A-4. Arizona DOT annual maintenance BMP costs.
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A-42 runoff to the abutment for treatment requires a structural BMP as well as an outlet structure to the receiving water, which requires environmental permitting and potentially an engineered energy dissipater. The whole-life cost of the traditional approach is high compared to passive methods on an at-grade highway cross section, such as engineered vegetative filter strips.
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A-43 • Infiltration trenches may require little routine maintenance outside of litter and debris removal. The whole life cost driver is the frequency with which the trench must be rehabilitated.
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A-44 DOTs can preserve functioning and extend the life cycle of BMPs if they prevent sedimentation of permanent BMPs during construction on the project or upstream, as "the majority of sediment problems" in permanent controls are caused by inadequate erosion and sedimentation control from construction upstream of the structure. In a stable urban watershed, WEF estimates that normal annual accumulation of sediment would be less than 1 cm per year.201 A UK survey identified that no upstream pretreatment was provided in 85% of the stormwater controls where sediment was a problem, a particular issue in the more expensive maintenance involved in wet basins.202 Heavier solids, leaves, trash, and debris frequently outweigh the load based on total suspended solids.203 To facilitate comparison of costs among BMP types, Barrett et al.
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A-45 is necessary to get an improved range for the various device types, but even then the data are limited (only 17 studies with construction cost information; only eight with maintenance cost information)
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A-46 BMP ranking procedure. The objective of using a BMP is often to protect aquatic life or prevent spills from entering a receiving water.
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A-47 209 NCHRP 25-40 Interim Report (Literature Review Results) , June 2012 209 Strecker, E
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A-48 North-West Florida Water Management District Compensatory Treatment Guidelines Compensating Stormwater Treatment Occasionally, applicants find that it is impractical to construct a stormwater management system to capture the runoff from a portion of the project site due to on-site conditions such as extreme physical limitations, availability of right-of-way, or maintenance access. Two methods have been developed to compensate for the lack of treatment for a portion of a project.
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A-49 A T T A C H M E N T A - 2 DOT BMP Selection Matrices Table A-8. MassDOT BMP selection matrix.
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A-50 Table A-9. Georgia DOT, general application BMPs.
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A-51 Methodologies for Discerning Appropriate Treatment of Bridge Deck Runoff Summarized from NCHRP Report 474 Vol. 2, Practitioner's Guide: METHOD 1: CALCULATION OF IN-STREAM POLLUTANT CONCENTRATION AT THE ZONE OF INITIAL DILUTION.
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A-52 adopt sediment criteria and implementing procedures are published and widely adopted into state water quality regulations, it can reasonably be argued that practitioners should not be expected to evaluate sediment impacts.
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A-53 METHOD 9: FATE AND TRANSPORT MODELS. If a more rigorous analysis of fate and transport of pollutants is warranted (i.e., for long-term pollutant loading effects and sediment accumulation)
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A-54 include USGS, watershed councils, or university extension offices. Such watershed-specific information will be superior to nonspecific literature values for particular land use types that have often been used in the past (Dupuis et al., 1985b)
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A-55 A T T A C H M E N T A - 4 BMP Inspection and Reporting Tables (WERF 2005) Table A-10.
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A-56 Table A-11. Vegetation management with trash and minor debris removal for swales and strips (WERF 2005)
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A-57 Table A-12. Intermittent maintenance for swales and strips (WERF 2005)
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A-58 Table A-13. Summary method to estimate effort for inspection, reporting, and information management for wet ponds (WERF 2005)
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A-59 Table A-14. Summary of vegetation management and trash and minor debris removal for wet ponds (WERF 2005)
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A-60 Table A-15. Vegetation management with trash and minor debris removal for dry extended detention ponds (WERF 2005)
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A-61 Table A-16. Summary method to estimate costs for inspection, reporting, and information management for media filters (WERF 2005)
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Table A-17. Filter maintenance for media filters (WERF 2005)
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A-63 Table A-18. Trash and minor sediment and debris removal for infiltration trenches (WERF 2005)
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Table A-19. Sediment removal for infiltration trenches (WERF 2005)
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A-65 Table A-21. Intermittent facility maintenance: structural repairs for pervious pavement (WERF 2005)
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A-66 Catch Basin Cleaning Many municipalities, especially those with combined sewer systems, have catch basins that maintain a permanent pool of water. These inlets retain sediment and floatables, which must be periodically removed.
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