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24 Stormwater and Other Source Control Practices to Consider for All Bridges The objective of this chapter is to present BMPs that should be considered for all bridges, as appropriate, depending on the setting and the type of bridge construction. These source control, operation, and maintenance practices will avoid or reduce loading of pollutants to the receiving water. The prac- titioner must determine which measures discussed herein apply to each bridge crossing. A number of factors must be considered when selecting source control and operation and maintenance BMPs, including the estimated cost of the mea- sure as compared to the estimated benefit. The practitioner must consider the receiving water conditions, pollutants of concern, and sources of those pollutants, and balance these assessments against the effectiveness of the measure, poten- tial operational hazards and liabilities, and long-term cost as compared to other practices that may be as effective but have a lower whole life cost. 4.1 Collection and Conveyance of Deck Runoff Collection and conveyance of runoff along the bridge is important not only from the perspective of maintaining dry lane criteria, but also from the perspective of managing scour at the discharge point. In some instances, runoff is collected and conveyed in pipe systems and directed towards the abut- ments. Such pipe systems can result in issues associated with leaking, additional capital cost, and are generally more dif- ficult to maintain. However, in most instances runoff from a bridge is collected in a comparatively simpler deck drain or scupper system. For the purposes of this guide, a deck drain is considered any drain with a grate opening that is installed flush into the deck of a bridge. A scupper is considered a circu- lar or rectangular slot opening within the bridge railing wall. Care must be taken in assessing slopes and approach areas to bridges, to ensure that runoff is conveyed to the receiving water without the potential to create scour and introduce TSS and turbidity into runoff. 4.1.1 Scour Protection at Collection System Discharge Points A direct discharge of bridge runoff from a deck drain or scupper to the bank areas should not be used without suf- ficient scour protection at the point of impact of the flow. Free fall drainage from the bridge superstructure can have substantial kinetic energy that can loosen soil particles and cause erosion, particularly entraining colloidal particles that contribute to turbidity in runoff. The point of impact from free fall drainage can also be difficult to predict since it is sub- ject to the influence of wind and dispersion. For this reason, riprap pads placed at the anticipated point of impact can be of limited effectiveness in controlling erosion from free fall drainage if they are not properly sized. A minimum size of 3ft. by 3ft. is recommended, with the caveat that the pad be inspected after storm events and enlarged if impact scour is observed adjacent to the pad. Runoff collected from a deck drain or scupper located in stream bank areas can also be conveyed in a pipe either along the rail or under the deck, and then down the abutment, col- umn or the piers (down drain). A 90 degree elbow (or similar) should be used to direct discharge horizontally into a suitably stabilized area (such as with riprap) to create a condition of sheet flow at the discharge point. The use and experience with piping of deck runoff varies by DOT and maintenance per- sonnel should be consulted to ensure the selected system can be maintained. Scour at the outlet of a bridge down drain is a function of the discharge rate, duration of flow, the outlet shape and size, and soil type (Thompson and Kilgore 2006). A drop between the outlet and ground surface should be avoided, but if necessary, considered in the determination of scour. The practitioner is encouraged to review FHWA Publica- tion No. FHWA-NHI-06-086 (HEC 14) for specific methods to predict scour-hole geometry. Predictive methods should be used in combination with estimates of erosion at similar C H A P T E R 4
25 locations. Since most bridge down drains are relatively small in diameter, a riprap apron consisting of suitably sized rock on top of a filter blanket would be an effective approach when placed at the outlet, and computing scour hole dimensions as detailed in HEC 14 would not be necessary. Flow expansion should be computed using a 4:1 ratio to determine the length of the pad. The depth of flow at the edge of the pad should result in a velocity that will not scour native material. Over their service life, riprap aprons may experience a wide variety of flow and tail water conditions. For this reason, maintenance personnel should inspect them after major flood events. If repeated damage occurs, extending the apron or replacing it with another more robust type of energy dissipater (such as a riprap stilling basin) should be considered. Equally important is to assess runoff flow paths around the bridge structure. Abutment walls may create preferential flow paths where they meet the fill slope, with the potential for rills or gullies during runoff events. Care should be taken to ensure that local drainage from the deck, approach roadway, and abutment fill slopes does not cause scour, which can result in increased turbidity in the receiving water. Riprap and concrete ditches can be used to convey runoff where velocities would scour native soils. The need for engineered energy dissipation should always be assessed at discharge points. Concentrated runoff should not be allowed to flow uncontrolled over slopes; grading should include benches and/or terrace drains when slope lengths exceed local requirements. 4.2 Bird Roosting 4.2.1 Background The configuration of the substructure of the bridge may provide habitat for wildlife that can act as a continuing source of indicator bacteria. Wildlife that inhabits the underside of bridges includes cliff swallows, pigeons, and bats. Geese have been observed to rest on bridge piers in Portland, Oregon. This section will provide design recommendations to discourage bird roosting and nesting by cliff swallows. Cliff swallows build mud nests under bridges at any loca- tion where a vertical surface meets a horizontal overhang at a right angle. While such habitat was typically naturally limited to cliff formations, the birdsâ range has expanded substan- tially due to the adequate nesting sites provided by bridges, culverts, and buildings (Figure 4-1). When building their mud nests, the swallows prefer a rougher surface texture, and con- sequently are more prone to roost on concrete structures than on wood and steel. A highly social bird, the swallows form nesting colonies that contain as many as 3,500 individuals. A study in the Austin, Texas, area demonstrated that nesting colonies of cliff swallows on bridges are a significant source of Escherichia coli and fecal coliform for the underlying surface water during the nesting period (Sejkora 2011). The con- centrations of these two indicator organisms downstream of the bridge were significantly higher than the concentrations upstream of the bridge during dry weather. The elevated E. coli concentrations downstream of the bridge nesting site were fairly constant through the day and night and persisted at least three quarters of a mile downstream. In this case, the most likely cause of this pollution is the direct deposition of swallow feces from nests over the water body. The data and visual observation of the swallowsâ behavior indicate that the peak loading of E. coli and fecal coliform corresponded with the approximately 20-day period between the hatching and fledging of the nestlings. Of course, the extent of the impact will depend on the size of the water body and the number of cliff swallows. Sejkora (2011) reported that the average E. coli loading per over-water nest for the nesting periods was about 5.0 Ã 108 MPN/d/nest. This value can be used in conjunction with the number of nests and the flow rate of the water body to calculate the expected impact and determine whether BMPs should be implemented. The feral rock pigeon is a nonnative, non-migratory bird that can be found on ledges under bridges throughout the United States. As a social bird, pigeons often form flocks for feeding, roosting, and breeding purposes. More often, pigeons can be found in greater numbers in more urban set- tings due to the greater availability of food sources and shelter Figure 4-1. Distribution of cliff swallows in North America (Brown and Brown 1995).
26 (Sacchi et al. 2002). Sacchi et al. (2002) cited several densities of pigeons in urban environments. For example, the densities of pigeons per square kilometer for urban and rural London were 200 to 400 vs. 12 to 14, respectively. Many of the concerns regarding urban pigeon populations stem from aesthetic and health concerns posed by droppings. A single pigeon produces 12 kilograms of excrement per year (Haag-Wackernagel and Geigenfeind 2008). Whereas some birds temporarily vacate their nesting and roosting sites to defecate, pigeons do not discriminate where their feces are deposited (Johnston 1992). Parents do not remove the squabsâ fecal matter from the nest, causing extensive buildup of hard- ened material. Pigeon feces are a known vector of some waterborne patho- gens; their excrement has been shown to harbor water borne pathogens such as Clostridium perfringens, Salmonella enteric, and E. coli (VTEC) (Haag-Wackernagel and Moch 2004). Given this risk, several studies have investigated bacteria spe- cies present in pigeon droppings. Oshiro and Fujioka (1995) found each gram of pigeon excrement contains approximately 1.7 Ã 108 E. coli. As with cliff swallows, this value can be used along with the receiving water characteristics to determine whether BMPs to exclude pigeons from the underside of bridges would be effective in protecting water quality. 4.2.2 BMPs to Discourage Roosting under Bridges The control of migratory birds, such as cliff swallows, is stipulated by the U.S. Dept. of Fish and Wildlife Service (2003). Migratory birds are protected under the Migratory Bird Act of 1918. As such, it is unlawful to harm, kill, or remove swallows or their eggs and nests. In general, per- mits for the removal of swallow nests under bridges must be obtained during the nesting season. A permit may only be obtained for compelling reasons such as safety concerns per- taining to airports or food manufacturing facilities. If such a permit is obtained from the Department of Fish and Wildlife, the nests may be knocked down in the manner specified in the permit (USFW 2003). Most often, the nests are washed off with a hose or knocked down with a pole. Subsequent to the nests being removed, it is imperative that humane exclusion methods are employed; otherwise, swallows might return and recolonize the site. During non-nesting seasons, nests may be removed if it can be ascertained that they are uninhabited. Again, exclusion methods should then be employed to dis- courage re-colonization. Pigeons and other birds that are non-native to the United States are subject to far greater control measures, such as trapping and poisoning. However, such methods are not encouraged as they are usually ineffective and are regarded as inhumane. One of the most common exclusion techniques to dissuade bird colonization of structures is bird nets. Bird nets can be successfully implemented by either pulling the net taut across roosting areas or simply allowing the net to hang loosely sev- eral centimeters from the roosting and nesting area (Gorenzel and Salmon 1982). Palmer (1982) found bird nets to be 95% effective at deterring pigeons. Literature also suggests that bird netting can be used to seal off crevices underneath bridges that might be prone to bat habitation (Kern 1995). However, bird nets can have maintenance issues; if they become torn, they must be mended or replaced (Gorenzel and Salmon 1982). Additionally, debris could potentially get caught within the netting, which makes netting subject to frequent cleaning. The expected life for bird netting cited by Gorenzel and Salmon (1982) is 3 to 5 years. Another common BMP to discourage the roosting birds are wire spikes installed on ledges underneath the bridge. These metal or plastic spikes can be laid in strips on roost- ing sites, making the area unappealing to birds. While these spikes are generally effective at excluding birds, care must be taken at selecting the size and spacing of the spikes in order to make them most appropriate for the bird species in question. Spikes also must be properly maintained, as debris buildup on the spikes can reduce their efficiency (Bishop 2003). When new bridges are being built, architectural consider- ations can also be implemented to deter swallows and other birds from nesting beneath the bridge. Gorenzel and Salmon (1982) note that swallows prefer nesting sites where over- hanging eaves meet the wall of a structure at acute or right angles. Concave or obtuse angle interfaces are rarely used as nest sites. Right angle interfaces on existing structures can be retrofitted with plastic, fiberglass, or metal fittings to make the interface less appealing to swallows. Bird spikes and nets strategically placed at right angle interfaces also might be successful at deterring swallows from nesting. Salmon and Gorenzel (2005) illustrated the four main methods for exclu- sion of birds (Figure 4-2). (a) (b) (c) (d) Figure 4-2. Four methods that may deter bird nesting (a) netting attached from the outer edge of the overhang down to the side of the bridge; (b) a curtain of netting; (c) metal projections along the junction of the wall; (d) fiberglass panel mounted.
27 Bats are often found roosting under bridges and their pres- ence may need to be controlled. In some locations, bats may only be managed by licensed pest control services. However, great care must be taken in mitigating bats due to their impor- tant ecological role. It is estimated that the bats from the Con- gress Avenue Bridge in Austin, Texas, consume upwards of 10 to 15 tons of insects per night (Keeley and Tuttle 1999). Bats are noted to most often use parallel box beams with small crevices between them as a dwelling site. Expansion joints and other crevices also should be designed with care; it is cited that bats prefer crevices with a width of 0.75 to 1 inch (Keeley and Tuttle 1999). The humane exclusion of bats is described thoroughly by Bat Conservation International (Keeley and Tuttle 1999). All crevices present in the bridge superstructure greater than 0.25 inches must be sealed to prevent bats from entering or reentering. This can be accomplished with wood, backer rod, expanding foam, or caulk. The primary exit points used by the bats are then fitted with one-way valves such as PVC pipes, which allow any bats remaining in the structure to exit without allowing any more to enter. An alternative to deter bats and birds from inhabiting bridges overlying water bodies is to provide them with a pref- erential roosting site nearby (Keeley and Tuttle 1999). In this case, the nuisance animals might vacate the site of concern and inhabit the preferential site at a less environmentally sen- sitive location. 4.3 Bridge Construction Materials Previous research by NCHRP has shown that portland cement concrete (PCC) and asphaltic concrete (AC) and con- stituents used in their production represent the largest volume of construction and repair material for highways. Addition- ally, it is known that many agencies are routinely using indus- trial by-products in construction materials. AC is the most widely used road surfacing material in the world, constituting more than 90% of the surfaced roads in the United States. Asphalt products are also used in surface treatments and base courses of roads and as repair materials. The wide application of asphalt has also spawned a large number of additives. A list of common additives includes liquid and fibrous poly- mers; rejuvenating agents (light-molecular weight petroleum products); carbonblack; sulfur; and crumb rubber (ground scrap-tires). PCC is also associated with transportation infra- structure construction. In addition to its use in pavements, PCC is a particularly relevant material in the discussion of bridge construction and stream stabilization. As with AC, the wide range of uses has led to a prolifera- tion of admixtures for PCC. These admixtures are used to improve the concrete properties with respect to workability, durability, and strength. A list of common additives includes air entraining agents (e.g., organic salts, organic acids, fatty acids, detergents); water reducers (e.g., lignosulfates, ligno- sulfonic acids, sulfonated melamine, sulfonated naphthalene, zinc salts); strength accelerating agents (e.g., calcium chlo- ride, calcium acetate, carbonates, aluminates, nitrates, cal- cium butyrate, oxalic acids, lactic acids, formaldehyde); and other, less common admixtures (e.g., coloring agents [iron oxides and titanium dioxide]; corrosion inhibitors [sodium benzoate]; fillers[fly ash, bottom ash, furnace slags]; and pumping acids [acrylic polymers, polyethylene oxides, poly- vinyl alcohol]) (Nelson et al. 2001). Although AC and PCC constitute the majority of the con- struction and repair materials commonly used, other materials are also routinely included in bridge and other transportation projects. These materials include treated timber, reinforcing steel, reinforcement fibers, epoxy-based materials, cathodic protective coatings, pipes, and bridge deck sealers. Use of such materials brings additional chemicals (e.g., creosote, ammoniacal-copper-zinc-arsenate or ACZA, and copper- chromated-zinc, or CCA) (Nelson et al., 2001). The impact of the most common construction and repair materials and their mobile constituents on surface and ground waters were studied within NCHRP Report 448. This report found that in their âpureâ form, that is, prior to incor- poration into an âassemblageâ such as AC mix or PCC, many highway construction and repair materials exhibit high toxic- ity. However, in most of the construction and repair materials, toxicity is considerably reduced after incorporation into the final assemblage (e.g., pavement or fill). Further investiga- tion of leaching rates also showed that toxicities to aquatic organisms are generally much lower under field conditions because of reduced mass transfer and soil sorption (Nelson et al. 2001). One material not specifically addressed in NCHRP Report 448 that has been historically significant in bridge construction is galvanized steel. Bridge components have been hot-dip galvanized for many years. This process places the entire steel component into a vessel of molten zinc. The zinc coats the steel with the heat of the process causing the formation of several metallurgical transition layers between the steel and zinc. This process results in a corrosion-resistant, adherent coating on the steel (Kogler 2012). Galvanized steel compo- nents exposed to rainfall can result in high concentrations of dissolved zinc in runoff. Barrett (2010) reports that samples of rainfall dripping from galvanized bridge rail were collected at a site in Texas, and concentrations of zinc in the sample ranged from 3,260 mg/l to 9,480 mg/l. For this reason, DOTs should consider painting bridge components that are hot-dip galvanized with a non-lead based paint to reduce the poten- tial for zinc transport to receiving waters. One potential alternative to the use of galvanized steel for corrosion protection is the use of weathering steel. The pri- mary benefit of weathering steel is the promise of long-term
28 corrosion protection without the need for either initial or main- tenance painting (Kogler 2012). Bridge painting is discussed further in Section 4.4.1, and can be significant as a potential source of lead and other pollutants in receiving waters during maintenance. This is because lead-containing alkyd paint was used to protect steel bridges for several decades. However, the receiving water risk associated with weathered steel as a poten- tial source of iron and other metals that make up the alloy is unknown. The practitioner should weigh the risk associated with potential metal transport against the benefits of elimi- nating paint maintenance over the long term to determine the viability of weathered steel as an alternative bridge construc- tion material. 4.4 Bridge Maintenance 4.4.1 Painting Materials and Methods 4.4.1.1 Pre-Maintenance Assessment of Bridge Paint As noted in NCHRP Report 474, bridge repainting is prob- ably the most common bridge maintenance practice and the one with potentially the greatest adverse effect on the receiv- ing water. Blasting abrasives and paint chips from preparation activities may fall into the receiving waters below the bridge during surface preparation. Surveys (CTC 2009) have indi- cated that up to 80% of existing bridges that require repaint- ing have paint containing lead. In addition, lead bridge paint can also contain other constituents including asbestos, ar- senic, chromium, and cadmium. The surveys also indicated that substantial amounts of used abrasives could be lost to the environment if appropriate containment practices are not followed. Prior to initiation of a maintenance plan involving bridge painting, an assessment should be performed to iden- tify the presence of lead, asbestos, arsenic, chromium, and cad- mium, as the presence of any or all of these constituents will directly impact the ability to recycle residual paint, or trans- port and dispose of it. Contractor noncompliance with con- tract specifications for handling and disposal of solid waste/ hazardous waste are among the greatest challenges to the suc- cessful completion of bridge repainting. The assessment of paint materials should follow the EPA âToxicity Characteristic Leaching Procedureâ (TCLP) and EPA SWA-846. For more information, the practitioner should refer to http://www.epa. gov/osw/hazard/testmethods/sw846/index.htm and http:// www.epa.gov/osw/hazard/testmethods/faq/faq_tclp.htm 4.4.1.2 Paint Selection, Storage and Handling The following information was consolidated from the AASHTO Center for Environmental Excellence web site (envi- ronment.transportation.org). Proper paint selection, storage, and handling will reduce potential impacts to receiving water quality. Paint with a long service life should be used to reduce the frequency of removal and reapplication. Before starting work, verify that the paint has not exceeded its shelf life or pot life. Pot life refers to the length of time paint is useful after its original package has been opened or, for two-component systems, the length of time after it has been mixed. Pot life is temperature dependent. The pot life on the product data sheet is generally for 21°C (70°F). Contact the manufacturer for additional pot life information if the paint has been stored in temperatures outside of this general range. Exceeding the pot life can result in sagging of the fresh paint along with poor performance attributable to film porosity and/or poor paint adhesion. Two-component paints tend to become unwork- able at or beyond their pot life. Paint should be kept in a secure location to avoid vandalism and accidental spills. It should also be stored in an area that will not be subject to temperatures beyond the recommended limits. Going beyond the acceptable temperature range can cause changes in viscosity and shelf life. Water-based paint will spoil when stored below freezing. Solvent-based paint, on the other hand, may gel or become flammable or explosive when stored at high temperatures. When transporting paint to and from the job site, use containers with secure lids, and ensure that containers are tied down to the transport vehicle. Do not transfer or load paint near storm drain inlets or watercourses. When mixing paint or using thinner, the instructions on the product data sheet should be strictly followed. Paints have dif- ferent mixing requirements. The product data sheet will indi- cate the specific type and maximum amount of thinner to be used. Check drying and curing times on the product data sheet to determine when the next coat of paint can be applied. Recoat- ing before enough time has passed can seriously affect the cur- ing and integrity of the layer being over coated. Some paints, particularly two-component paints, have a maximum time to re-coat as well. Exceeding this could jeopardize the adherence of the top coat. Recycle paint when possible and dispose of unused paint at an appropriate hazardous waste facility. All clean-up water should be captured and disposed of properly. Collect runoff from sand blasting and high-pressure wash- ing. Filter runoff through an appropriate filtering device (e.g., filter fabric) to keep sand, particles, and debris out of storm drains if the wash water (without cleaning agents) will be discharged to land. If wash water containing a cleaning com- pound (such as high-pressure washing with a cleaning com- pound) is generated, plug nearby storm drains and vacuum/ pump wash water to the sanitary sewer. 4.4.1.3 BMPs During Painting A variety of BMPs can be implemented during painting to limit pollutant discharge. When possible, schedule painting
29 activities for dry weather, and test and inspect spray equip- ment prior to starting to paint. Tighten all hoses and con- nections and do not overfill the paint container. Plug nearby storm drains (using sandbags or fabric and stone) prior to starting painting or sandblasting where there is significant risk of a spill reaching storm drains. Perform work on a maintenance platform, or use sus- pended netting or tarps to capture paint, rust, paint-removing agents, or other materials, to prevent discharge of materials to surface waters if the bridge crosses a watercourse. A floating silt mat can be used to protect receiving water systems from debris generated during routine paint blasting and other maintenance operations. A silt mat is similar to a turbidity curtain but its primary purpose is to collect debris and waste that might otherwise fall directly in the water. Floating silt mats have been shown to perform effectively in high current areas where conventional turbidity curtains might fail. Accu- mulated material and debris collected within the floating silt mat will usually require handling, transport, and disposal as solid waste or hazardous waste. Sand blasting can be performed as an âopenâ or âclosedâ operation. An open operation requires full containment. A project-specific containment plan should be developed including drawings, equipment specifications, and calcula- tions (wind load, air flow, and ventilation when negative pressure is specified) prior to the start of work. The plan should also include copies of the manufacturerâs specifica- tions for the containment materials and equipment that will be used to accomplish containment and ventilation. Closed abrasive blasting or vacuum blasting allows dust, abrasive, and paint debris to be vacuumed simultaneously with the blasting operation. Debris is separated for disposal and the abrasive is returned for reuse. Closed vacuum blasting equip- ment is expensive; however, both worker exposure to dust and environmental emissions can be minimized if opera- tions are conducted properly. Closed blasting is limited by its reduced production rate and operational problems cleaning edges and irregular surfaces. To be completely effective, the whole nozzle assembly must be sealed against a surface to maintain proper suction for the vacuum operation. Clean up afterwards by sweeping or vacuuming thoroughly, and/or by using absorbent and properly disposing of the absor- bent. Disposal of lead-based paint must follow the applicable procedures specified within the Resource Conservation and Recovery Act (RCRA). As a contingency measure, the contrac- tor should keep clean-up materials readily available and in a known location so that spills can be cleaned up immediately. 4.4.2 Bridge Washing Washing of bridges is known to produce the potential for migration of pollutants associated within winter mainte- nance, such as excess deicing agents, sand, and chlorides. In the case of steel bridges, wash effluent can produce a solution containing high levels of copper, zinc, and lead. A range of best management practices should be followed to limit the potential for transport of these and other pollut- ants. These best management practices should be used prior to, and during washing. Prior to washing the bridge surface, sweep sand, debris, and sediment from the bridge. Sand can be transported to a main- tenance yard for storage and subsequent recycling. Material accumulated on the bridge deck should never be swept into open deck drains or over the edge of the bridge. All scuppers and other drains should be blocked with unbroken sand bags or as discussed in Section 4.4.5 to prevent accidental discharge of wash water to the surface waters under the bridge. Sweep- ing, instead of washing, may be preferred near sensitive water areas or where there is direct discharge to waters of the United States; however, washing of the bridge bearings, joints, and sub-structure may be necessary for structure maintenance. When washing the bridge surface and superstructure, aim water hose nozzles to minimize overspray into surface waters or the roads below the bridge. Whenever possible, water should be aimed in a manner to force any remaining sediment or debris towards a flat vegetated area. Water washed over a veg- etated area must not cause scour or contribute to sedimenta- tion of the waterway. Limit water pressure when washing steel bridge components so as to avoid the accidental dislodging of paint, which might end up in the water body beneath the bridge. If paint is observed being displaced, cease washing operations. Pressure washing shall also be limited to prevent undercut of grout or harm to the masonry plates beneath the bearings. To the extent practicable, washing of bridges should be scheduled on structures during a time that coincides with high-flow periods or periods following storm events. If dis- placement of bats or nesting birds is observed, cease washing operations. Promptly report and document any accidental discharges to water bodies and the corrective measures taken to cease the discharge and prevent additional discharges. Safety is a large concern during washing operations and must be planned at off-peak times and appropriate traffic con- trol must be provided. 4.4.3 Winter Maintenance Winter maintenance activities such as salt and sand and other product applications are a potential threat to receiv- ing water quality. These activities substantially increase the sediment and/or chloride (salt) load in bridge runoff. Chlo- ride is extremely mobile and soluble, and once it has been introduced to the environment it is practically impossible to remove without advanced treatment. The only practical option is to minimize the use of salt. Sand on bridges can
30 be removed through an aggressive street sweeping program. Street sweeping issues will be discussed in Section 4.4.4. A balanced approach should be used during application of salt for snow and ice control (The Salt Institute 2007). This will result in providing the necessary level of safety for traf- fic, while minimizing the potential for transport of pollutant constituents that results from over application. Determining a properly calibrated application rate in conjunction with the use of automated spreader control systems can keep the amount of salt needed for adequate traffic protection to a minimum. Proper application rates will consider variations in road surface temperature, type of precipitation, and the tendency for accumulation. The practitioner should keep in mind that there is no direct correlation between yearly snow- fall and the total quantity of salt required for effective traf- fic protection. The type of storm dictates the frequency of application and total amount of salt necessary. For example, a short-term freezing rain or ice storm may require large amounts of salt, perhaps even more than a prolonged snow- storm. Table 4-1 illustrates the relationship between road temperature, meteorological condition, ideal salt application rate, and resulting coverage per two-lane mile of bridge. It is important to note that most typically available tem- perature information from traditional meteorological sources is measured at 30 feet above the ground (The Salt Institute 2007). Determining the optimal salt application rate for a bridge should be based upon the actual deck or roadway tem- perature, as opposed to air temperature. Gaining this type of information requires road sensing systems or having access to a Road Weather Information System (RWIS) (The Salt Insti- tute, 2007). Calibration of spreaders is critical in ensuring that the planned application rate achieves the actual application rate. Calibration involves calculating the pounds per mile actu- ally discharged at various spreader control settings and truck speeds. It is carried out by first counting the number of auger or conveyor shaft revolutions per minute, measuring the salt discharged in one revolution, then multiplying the two and finally multiplying the discharge rate by the minutes it takes to travel one mile. An example of a calibration chart in spread- sheet format can be found on the Salt Institute website at: http://www.saltinstitute.org/images/calibrationchart.xls. 4.4.3.1 Salt Application Techniques for Pollutant Minimization Several techniques should be observed when applying salt on or near a bridge deck. These techniques include pre- wetting, determining the proper spread width, consider- ation of wind effects, consideration of plow timing relative Condition Suggested Application Rate Coverage Per Cubic Yard Salt Per Two Lane Mile Temperature near 30°F Precipitation snow, sleet, or freezing rain Bridge surface wet If snow or sleet, apply salt at 500 lbs. per two-lane mile. If snow or sleet continues and accumulates, plow and salt simultaneously. If freezing rain, apply salt at 200 lbs. per two-lane mile. If rain continues to freeze, re-apply salt at 200 lbs. per two-lane mile. Consider anti-icing procedures. Snow/Sleet â 4 Freezing Rain â 10 Temperature Below 30°F or falling Precipitation snow, sleet or freezing rain Bridge surface wet or sticky Apply salt at 300-800 lbs. per two-lane mile, depending on accumulation rate. As snowfall continues and accumulates, plow and repeat salt application. If freezing rain, apply salt at 200-400 lbs. per two-lane mile. Consider anti-icing and deicing procedures as warranted. Snow â 6 to 2 ½ Freezing Rain â 10 to 5 Temperature below 20°F and falling Precipitation dry snow Bridge surface dry Plow as soon as possible. Do not apply salt. Continue to plow and patrol to check for wet, packed or icy spots; treat only those areas with salt applications. N/A Temperature below 20°F Precipitation snow, sleet, or freezing rain Bridge surface wet Apply salt at 600-800 lbs. per two-lane mile, as required. If snow or sleet continues and accumulates, plow and salt simultaneously. If temperature starts to rise, apply salt at 500-600 lbs. per two-lane mile, wait for salt to react before plowing. Continue until safe pavement is obtained. Snow or Sleet â 4 to 2 ½ Temperature below 10°F Precipitation snow or freezing rain Bridge surface accumulation of packed snow or ice Apply salt at rate of 800 lbs. per two-lane mile or salt- treated abrasives at rate of 1,500 to 2,000 lbs. per two- lane mile. When snow or ice becomes mealy or slushy, plow. Repeat application and plowing as necessary. Snow or Freezing Rain â 2 ½ Source: The Salt Institute, 2007 Table 4-1. Salt application guidelines for bridges.
31 to salt application, and how plowing influences the need for re-application. Pre-wetting salt with brine speeds the reaction time of salt and keeps salt from bouncing off the road so more of it is available to melt the ice and snow. This effect also helps minimize salt transport potential. However, brine use should include careful consideration of how varying concentration and temperature influence the effectiveness in deicing and snowmelt. Brine will only be effective on bridge decks with a temperature between â6°F and 30°F, and in concentrations of 5 to 23% salt, by weight (The Salt Institute 2007). Various material alternatives to brine exist for use in pre-wetting. A tool has been developed by the National Academy of Sciences to evaluate relative tradeoffs between cost, performance, and environmental impact of brine application that can be used by DOT practitioners. Refer to the following web site for a copy of the tool and additional information regarding the selection of environmentally sensitive pre-wetting materials: www.saltinstitute.org/snowfighting/index.html. Salt spreading on bridges is typically done by applying a windrow of salt in a 4 â8 foot strip along the centerline. This technique is effective on two-lane pavements with a low to medium traffic count (The Salt Institute 2007). Less salt is required with this pattern and quickly gives vehicles clear pavement under the wheel areas. Traffic will soon move some salt off the centerline and the salt brine will move toward both shoulders for added melting across the entire road width. It is important in this scenario to remove remaining snow from the shoulder area as quick as possible, since when snowmelt occurs, it will potentially re-freeze and necessitate re-application. As snow melts within the shoulder area, avoid or minimize the use of salting directly onto deck drains and scupper areas. Consciousness of wind conditions is also an important aspect when spreading salt. A strong wind blowing across a bridge can cause salt to drift as it comes out of the spreader, pushing it onto the shoulder area where deck drains are located. This is particularly true in rural areas where there are few windbreaks. How the wind affects spreading depends on both velocity and pavement condition. The operator or application crew should avoid areas where high wind has the potential to blow salt over the side rails or into the deck drains or scuppers. It is important also to know when to plow and re-apply salt. Salt use can be minimized by giving it appropriate time to work. Plowing operations should be timed to allow maxi- mum melting by salt. The need for another salt application can be determined by watching melting snow kicked out behind the vehicle tires. If the slush is soft and fans out like water, the salt is still effective. Salt should only be re-applied once the slush begins to stiffen and is thrown directly to the rear of vehicle tires (The Salt Institute 2007). 4.4.4 Sweeping Street sweeping is a practice that DOTs use to remove accu- mulated trash, debris and sediment along roadways. The tech- nology of street sweeping continues to improve, and sweepers have become much more effective at removing finer sedi- ment. There have been a variety of studies to evaluate whether removal of sediment and associated pollutants would improve stormwater runoff quality. Sweeping is an effective BMP for use on bridge decks, and is a practical alternative to washing the roadway. WSDOT (Nguyen 2013) is using sweeping as a primary BMP for the SR 520 Floating Bridge Project, set to finish con- struction by 2016. The SR 520 floating bridge is an example of a design with very little longitudinal grade, restricting the use of a bridge deck conveyance system. WSDOT success- fully negotiated a defined sweeping program as a BMP for the bridge partially in lieu of other deck runoff treatment options. The main factors effecting the removal of solids from the street for sweeping are: ⢠The type of equipment used and speed ⢠The frequency of sweeping ⢠Other important variables include the time of day, and the time to the next rain event. Pollutants that can be reduced through sweeping are: ⢠Sediment ⢠Organic debris ⢠Trash/litter Secondary pollutants associated with sediment and likely to also be reduced include: ⢠Bacteria ⢠Heavy Metals ⢠Phosphorus There are three principal types of street sweepers currently available: mechanical, vacuum, and high-efficiency regenera- tive air. Mechanical sweepers are equipped with water tanks and sprayers used to loosen particles and reduce dust. Mechan- ical brooms gather debris under the sweeper and the vacuum system pumps debris into the hopper. The majority of debris, especially the heavy debris, is collected within 36 inches of the curb line: mechanical sweepers are designed to do an effec- tive job of cleaning within this area. Even though this type of sweeper typically uses water-based dust suppression systems, they exhaust a high level of particulates into the atmosphere on a continual basis. Vacuum-assisted street sweepers use a high-powered vac- uum to suction debris directly from the road surface and
32 transfer the debris into the hopper. Research has shown that these machines are significantly more effective at removing sediment, nutrients, and metals than standard mechanical sweepers (Weston Solutions 2010). Regenerative air systems are more environmentally friendly than mechanical sweepers (Southerland 2011). Regenerative air sweepers employ a closed-loop âcyclonic effectâ to clean the air before reusing it again to clean the street surface. They are similar to vacuum sweepers in that there is a vacuum inlet located on one side of the sweeping head. Unlike vac- uum machines, however, regenerative air sweepers constantly recirculate (regenerate) their air supply internally. Regen- erative air technology has become widely seen as having a number of advantages: cleaning a wider path, removing small particles more effectively, and limiting the amount of dust- laden air that is exhausted back into the atmosphere. Since these machines âair-blastâ the pavement across the entire width of the sweeping head, regenerative air sweepers tend to do a more effective job of cleaning over the entire pavement surface covered. The optimum frequency of sweeping is discussed exten- sively in the literature, although there does not appear to be full agreement on the issue (EOA and Geosyntec 2011). Most sources conducted sweeping tests with a bi-weekly or a weekly schedule, although one study examined a frequency of three times per week (Pitt 1985) and for another, a frequency of five times per week (Pitt and Shawley 1981). A study by the City of San Diego (Weston Solutions 2010) found that increasing sweeping from once to twice per week with a vacuum sweeper did not increase the amount of material collected; however, this was not the case for mechanical sweepers, which showed a lower volume of collected material with the increased frequency. The ideal goal is to sweep prior to a forecasted storm with as little lag time as possible, but this is difficult given logistical and resource constraints. Some references suggest that the frequency of sweeping should be set to conduct, on average, one or two sweepings between storms. In semi- arid climates, some references recommended more inten- sive sweeping prior to the onset of the wet season. Given the potential for street dirt to blow off the bridge deck and directly into the receiving water, more frequent sweeping is likely beneficial. Southerland (2013) recommends a site-specific investi- gation using a calibrated model to determine an optimum sweeping schedule, but notes that it will likely be in a range from about 17 sweepings per year to 52 sweepings per year. In general, for maximum particle removal, sweeping frequency should be increased until there is a decrease in the mass of material removed per curb mile. Pavement conditions are known to significantly affect the pickup performance of street cleaners (Sartor and Boyd 1972). Street sweepers have considerable difficulty effectively picking up particulate material from streets whose pavements are classified as poor, because this usually indicates the pres- ence of significant surface cracks and deep depressions where dirt can accumulate. The uneven surfaces that accompany poor pavement conditions make it difficult for the sweep- ers to operate effectively, especially the newer regenerative air machines. The forward speed of a street cleaner while sweeping will significantly affect its ability to pick up particulate material. Other factors being equal, the pickup effectiveness increases as the forward speed decreases (Sartor and Boyd 1972); how- ever, the URS study (2011) did not find that speed (within a defined range) had a significant influence on material pickup for mechanical sweepers. The optimum average forward sweeping speed is believed to be approximately 5 miles per hour. This is good balance for the tradeoff between pickup performance effectiveness and the need to sweep a reasonable length of streets in a given day. Southerland (2013) reports that sweeping in the range of 8â10 mph reduces particulate pickup performance by 10â15% compared to the optimum average speed. There are two main areas of research regarding street sweep- ing effectiveness. The first of these is the amount of material removed from the street and the factors that influence sweep- ing effectiveness. The second area of research focuses on whether removal of street dirt and associated pollutants has any impact on runoff quality. EOA and Geosyntec (2011) reviewed a number of street sweeping studies and developed Figure 4-3 to compare the observed removal efficiencies. Removal efficiencies of the material accumulated on the street varied from about 20% to 70% depending on the type of sweeper evaluated and the pavement condition. Other factors being equal, the regenera- tive air sweepers and vacuum-assisted sweepers were shown to be more effective. These results were confirmed by the results of the City of San Diego study (Weston Solutions 2010). The removal of sediment is important, but other pol- lutants of concern are metals and potentially bacteria. No studies were identified that examined street sweeping as a practice to remove bacteria from paved surfaces. In the envi- ronment, bacteria are generally associated with the smallest size fraction of particles, which are removed least effec- tively by street sweeping programs. Consequently, bacteria removal efficiency may be only 10â50% of that observed for sediment. Several studies were identified that evaluated removal of other pollutants through street sweeping. Kurahashi and Associates (1997) reported 45â65% removal of total sus- pended solids, 30â55% of total phosphorus, 35â60% of total lead, 25â50% of total zinc, and 30â55% of total cop- per. Montgomery County Department of Environmental
33 Protection (2002) provided removal effectiveness data from studies performed by the Center for Watershed Protection. Total suspended solids reduction ranged from 5% (major road) and 30% (residential street) for mechanical sweep- ers to 22 and 64%, respectively, for regenerative air and 79 to 78%, respectively, for high-efficiency vacuum sweepers. Law et al. (2008) also developed estimates for percent total solids and nutrient removal (Table 4-2). Southerland (2013) reports that street sweeping opera- tions have the ability to remove bioavailable or soluble metals before they are wetted by rainfall and dissolve. The street dirt with a size less than about 2,000 microns accounts for over 80% of the total particle mass. 4.4.5 Scupper Plugs One relatively simple alternative management technique available for use on urban bridges that span impaired water bodies is scupper plugs. This technique is in use by the Oregon Department of Transportation; more information can be found at: http://www.fhwa.dot.gov/environment/wildlife_ protection/index.cfm?fuseaction=home.viewArticle&article ID=62 Scupper plugs are formed by maintenance crews from fast- setting grout or spray foam and used to close off drainage openings on existing bridges. The plugs prevent solids during dry weather from discharging into the receiving water. Their Frequency Technology TS TP TN Monthly Mechanical 9 3 3 Regenerative Air/Vacuum 22 4 4 Weekly Mechanical 13 5 6 Regenerative Air/Vacuum 31 8 7 Table 4-2. Estimated total solids and nutrient removal (percent). Figure 4-3. Comparison of pre- and post-sweeping solids data. From the various street-sweeping studies reviewed, in the form of lbs of solids per curb-mile (EOA and Geosyntec, 2011).
34 primary use is on bridges where there is sufficient longitudi- nal slope to avoid flooding the travel lane if the scuppers are plugged during runoff. For bridges that require the scuppers to fulfill dry lane criteria, maintenance crews must quickly remove the plugs to allow for drainage if a storm is forecast. This is an approach that, for practical reasons, would be lim- ited to only existing bridges crossing highly sensitive receiv- ing waters. Sweeping should be performed prior to removal of scupper plugs. 4.4.6 Summary The focus of a street sweeping program should be on bridges that have a solid bridge rail. Bridges without a solid railing wall are unlikely to accumulate much material since it will be mobi- lized off of the bridge by traffic induced wind currents. The fre- quency of sweeping can be optimized by recording the mass of material collected in the sweeper, and increasing the sweeping frequency until a decline in the mass is detected per curb mile. This is an indication that the frequency exceeds the time for equilibrium build-up, and that more frequent sweeping would have only marginal benefit. Modern equipment will perform best. Regenerative air sweepers exhaust less particulate material than vacuum sweep- ers, and have about the same performance, so they are pre- ferred. Optimum speed is 4 to 6 mph, but operating at 10 mph is an appropriate trade of time vs. efficiency. The condition of the bridge deck is also important. Rough or uneven surfaces will retain particulates with less sweeper efficiency. Optimum conditions have low humidity and moisture on the pavement. Weston Solutions (2010) reports the effectiveness of sweep- ing in Table 4-3. The âunsweptâ column refers to streets that were swept once every two months prior to the storm event. The other rows indicate the type of sweeping performed once per week for three weeks prior to the sampling event. Ten stormwater runoff samples were collected for each event at each site, and the final row represents the mean of three sampled storm events. 4.5 Bridge Inspection The AASHTO Manual for Condition Evaluation of Bridges is recognized as a national standard for bridge inspections and load rating. The routine structural inspection frequency is 24 months unless FHWA approval is given for a 48-month cycle. Elements of the AASHTO Manual for Condition Evalu- ation for Bridges that directly or indirectly relate to bridge source control and operational best management practices are as follows: ⢠Substructure inspection requirements for abutments and piers ⢠Superstructure inspection of painted components ⢠Drainage systems within the deck and approach areas The AASHTO Manual for Condition Evaluation for Bridges requires inspection for significant scour or undercutting of abutment areas. If the abutments are submerged in water, then probing is also normally performed. The inspector is required to identify instability of slope areas and accumula- Storm Event Type of Sweeping Copper (µg/L) Lead (µg/L) Zinc (µg/L) TSS (µg/L) 12/07/2009 Unswept 143.0 71.8 1,689.4 703.8 Mechanical 50.9 30.7 443.6 112.8 Vacuum 51.2 22.3 362.7 130.2 1/18/2010 Unswept 218.4 234.0 1,210.9 1,719.6 Mechanical 83.1 77.8 610.1 431.6 Vacuum 34.1 23.5 307.6 145.2 2/5/2010 Unswept 73.7 59.2 452.1 357.6 Mechanical 55.4 38.5 353.8 187.1 Vacuum 39.4 15.2 366.1 132.0 Mean of Three Storms Unswept 145.0 121.7 1,117.5 927.0 Mechanical 63.1 49.0 469.2 243.8 Vacuum 41.6 20.3 345.5 135.8 Table 4-3. Constituent concentrations in stormwater runoff for swept and unswept roadways.
35 tion of sediment. Measures similar to abutments are required for the inspection of piers. If riprap has been used as a counter- measure against pier scour, then it should be inspected for stability and adherence to original design specifications (i.e., size and angularity). Steel piers should also be inspected for signs of corrosion. Painted areas are required by AASHTO to be inspected for signs of rust, chalking, pitting, crazing, and staining. AASHTO requires drainage systems within the deck and approach areas to be inspected for adequacy and physical condition. Grating over deck drains must be observed as physically intact. Missing or broken grates must be docu- mented. Clogged deck drains or scuppers are to be identi- fied and documented. Down drains are to be inspected to confirm that they terminate at splash blocks or other suit- able facilities to prevent erosion or structural undermining. Areas of ponding water are also to be identified and docu- mented. Unintended discharge of water through cracks, joints, spall areas, etc. must also be identified. Drainage within the bridge approach must be inspected to verify that runoff does not bypass and cause erosion of embankment fill areas. In addition to protocol established by AASHTO, the prac- titioner should consider the following enhancements as part of a comprehensive âBridge Stormwater Conveyance and Collection System Assessmentâ program for routine and post storm inspections: 1. Identify and document existence of trash and debris. Remove and dispose of this in a suitable manner based upon the material type. 2. Identify and document buildup of sediment or ponding water within riprap pad areas, or evidence of scour at the downstream end. Identify and document any occurrences of riprap material transport. 3. Inspect drainage pipes, inlets, and structural BMPs (as applicable) for presence of stagnant water and mainte- nance needs. 4. Identify and document occurrences of flaking or dis- lodging paint into or near the receiving waters. 5. Verify the functionality of temporary drainage blocks such as scupper plugs, or non-traditional drainage inlets such as offset deck drains and raised scuppers. 6. If bird nets are being used, check to ensure they are not torn or clogged with dirt or debris. If bird spikes are used, check similarly to ensure they have not experienced buildup of debris or dirt. 7. Check bridge decks after snowstorms for excess residual sand or salt deposits. Wash or sweep in accordance with the practices discussed in this chapter.
