Bridge Stormwater Runoff Analysis and Treatment Options (2014)

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C-1 Quick Start Guide The purpose of this Quick Start Guide is to assist the expe- rienced practitioner in developing information for regulatory agencies to document the likely impact of bridge runoff on a specific receiving water (Impact Assessment) and to deter- mine the most cost effective BMP using the BMP Evaluation Tool if runoff treatment is required. Each of these elements is described below. Runoff Impact Assessment Strategy Two general cases are presented for determining if treat- ment BMPs for bridge deck runoff are appropriate. The cases are differentiated according to the surrounding general land use, either rural or urbanized, which is consistent with the approach taken by USEPA for implementation of the NPDES permit program. Rural Location For the rural case, treatment of bridge deck runoff is gen- erally not recommended since the impacts to the receiving stream are usually de minimis. This is an expected result since, as noted by NCHRP Report 474, “Highways typically constitute a very small fraction of a watershed’s total drain- age area, and bridges often constitute a small portion of the highway drainage area. Thus, highways often, but not always, contribute a small fraction of the overall pollutant load to a given receiving water body, and bridges contribute even less.” In addition, studies evaluating water and sediment quality, as well as biological systems have failed to document environ- mental impact associated with bridge runoff. Therefore, the default DOT position for rural bridges (defined as those outside of an NPDES Permitted area) is that no treatment BMPs are needed. It is only when a numeric analysis of impacts is required by the environmental docu- ment, or treatment BMPs are required pursuant to a resource agency permit, that an assessment of the impact of bridge deck runoff on the quality of the receiving stream should be performed. The practitioner can provide evidence of the de minimis nature of the impact by performing either the Simple or Complex Assessment. The Simple Assessment calculates the percentage change in load in the water body resulting from dis- charge of bridge runoff. This assessment uses average annual values to make this determination. The Complex Assessment calculates the change in concentration for any constituent of concern based on a single, worst case event (historical low flow in the water body, resulting in the least possible dilution). These two procedures are described in detail in Chapter 3 and worked examples are provided in Appendix B. Before performing either assessment, the DOT should con- sult with the regulatory agency to discuss the analysis approach. There may be areas of the country or specific receiving water requirements where treatment of bridge runoff will be required, regardless of the de minimis nature of the impact. In that case, the DOT should skip the assessment step and proceed directly to selecting the most cost effective BMPs for the constituents of concern from Chapter 4 (source control) and Chapter 5 (treatment control) using the BMP Evaluation Tool. Urban Location The primary factor affecting receiving water health in urbanized areas is the volume and quality of runoff from impervious surfaces in the watershed. The bridge itself is one of many small impervious parcels contributing runoff and, consequently, it is logical that it be subject to the same regulations as other impervious area. That is, the de minimis assessment does not apply because of the cumulative impact of many small impervious parcels. Since DOTs are subject to stormwater permit requirements in urban areas, the level of mitigation for bridge runoff should be guided by the DOT MS4 permit. If implementation of treatment BMPs for run- off from new impervious cover is considered necessary to comply with the maximum extent practical (MEP) reduction A P P E N D I X C

C-2 in the discharge of pollutants as implemented in the applica- ble NPDES or other regulatory permit, then treatment BMPs should be implemented either at the bridge crossing itself or offsite (preferred for performance and cost reasons). Since the level of treatment required is specified in the stormwater permit, an assessment of water quality impacts is generally unnecessary in this case as well. Figure C-1 provides an overview of the recommended ana l- ysis process for any bridge project crossing waters of the US. Step 1: Development of the project environmental docu- mentation will provide information related to the condi- tion and status of the receiving water. Items that should be identified include whether the receiving water has site spe- cific requirements such as a 303(d) listing or TMDL, whether it’s included as an Outstanding National Resource Waters (ONRW), a domestic water supply reservoir, or has endan- gered species. These classifications will help determine the need for special consideration where the de minimis assess- ment is not allowable (due to the requirement for a zero increase in constituent load to the receiving water), and iden- tify specific constituents of concern for performing either the Simple of Complex Assessment and selecting a BMP to achieve the desired discharge quality. Step 2: All bridges should consider applicable source control and operation and maintenance BMPs; these are described in Chapter 4 of this guide. Source control BMPs include design and operational provisions to ensure that the bridge struc- ture or traffic operations do not contribute pollutants to the receiving water during dry or wet weather to the extent practicable. Step 3: Determine if the bridge is subject to an NPDES Permit. Bridges not subject to an NPDES Permit skip to Step 5, otherwise move to Step 4 to determine what BMPs are required by the DOT’s MS4 Permit. The BMPs identified in Step 2 may meet NPDES Permit requirements; if this is the case, proceed to Step 5. Step 4: Treatment requirements in the MS4 Permit, if any, should be incorporated into the project. If none are required beyond those already incorporated in Step 2, proceed to Step 5. If treatment is required by the DOT’s NPDES Permit, proceed to Step 4a. The least cost and highest benefit can be achieved by treating a comparable section of terrestrial roadway (with similar AADT, adjacent land use and impervious area) rather than the bridge deck runoff. The tool described in Chapter 6 can be used to document the cost basis for treatment at an off- site location. The off-site treatment location should be within the same watershed or upstream of the bridge crossing. The recommended approach follows the basic tenants of MEP to select the location and BMP with the least cost and highest environmental benefit. Step 5: Determine if a 404 Permit is required to construct or rehabilitate the bridge. Bridges that require a 404 permit will also require the companion 401 water quality certifica- tion. The 401 Certification may contain requirements for treatment of deck runoff. The agency responsible for pro- viding the 401 Certification should be consulted early in the project development process to determine if BMPs beyond those described in Chapter 4, or the DOTs MS4 Permit (for crossings in urban areas) will be included in the 401 Certifi- cation. If the resource agency is requiring BMPs beyond those in Chapter 4 or required as a part of the DOTs MS4 Permit, it is recommended that a simple or complex assessment be performed (Step 5a) to demonstrate that the bridge runoff will not have impacts on the receiving water. Treatment BMP Selection and Assessment This Guide provides a BMP Evaluation Tool to assist the practitioner in documenting the benefit and cost analysis associated with treatment BMP implementation. The tool facilitates the computation of treatment BMP whole life cost and performance information as well as the whole life cost of a bridge deck drain collection system. This information can be quickly compared by the practitioner, for example, to an alternative land-based in-lieu treatment location to deter- mine the treatment strategy with the least cost and highest benefit. The BMP Evaluation Tool is described in detail in Chapter 6 with a full user’s guide presented in Appendix D. The selection of the type of BMP for treatment of runoff either at the off-site in lieu location or at the bridge abutment is largely at the discretion of the designer. Several NCHRP publications can assist the designer in treatment BMP selec- tion. Recent publications include, NCHRP Report 565: Evalua- tion of Best Management Practices for Highway Runoff Control, NCHRP Report 728: Guidelines for Evaluating and Selecting Modifications to Existing Roadway Drainage Infrastructure to Improve Water Quality in Ultra-Urban Areas, and NCHRP Project 25-20(01), “Evaluation of Best Management Practices and Low Impact Development for Highway Runoff Control.” (published as NCHRP Report 565). Selection of the type of BMP will be driven largely by physical site constraints, since the BMPs described in these publications are targeted at con- stituents of concern for highways. The tool accompanying this guide includes four treatment BMPs that are suitable for use at the abutment or on a con- vention highway section, and one that can be used on the bridge deck: At the abutment: • Swales • Dry Detention Basin • Bioretention • Media Filter

C-3 Figure C-1. BMP flowchart.

C-4 On the bridge deck: • PFC These BMPs were selected for their performance, generally broad compatibility with highway physical site constraints, and familiarity to, and common use by DOTs. The practitio- ner is not constrained by these choices, and other BMPs may be a better fit for site conditions. A separate study, in process at the time this guide was prepared, “Long-Term Performance and Life-Cycle Costs of Stormwater Best Management Prac- tices,” under NCHRP Program 25-40. This report provides an expanded list of BMPs for the practitioner to consider, as well as a tool similar to the one provided with this Guide to evaluate performance and whole life cost. If treatment of runoff from the bridge deck is required, a PFC overlay could be considered to avoid the cost of a bridge conveyance system. PFC is a thin lift of gap-graded asphalt, placed over a conventional hot mix asphalt section. The PFC layer has been demonstrated to improve water quality, as well as provide ancillary benefits such as reduced tire noise and improved visibility and stopping distance during rain events. In all cases, treatment of an off-site at grade location is rec- ommended in lieu of treating the actual deck runoff for bridges determined to require treatment. There are a variety of rea- sons why offsite mitigation of the impacts of bridge runoff on receiving water quality is preferred. These include the cost and technical feasibility of retrofitting existing or planned bridges, the fact that the majority of the contribution of pollutants from bridges actually occurs during dry weather, the lack of available space at the bridge landings, and the difficulty of pro- viding routine maintenance for facilities installed on the bridge structure itself.