Bridge Stormwater Runoff Analysis and Treatment Options (2014)

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B-1 Simple and Complex Assessment Methods and Worked Example Overview of Appendix This appendix provides two methods, the simple assess- ment and complex assessment to estimate the impact of a bridge crossing on receiving water quality and illustrates the dilution calculations. This appendix also provides a worked example problem that uses the entire recommended procedure in the Guide (except the simple and complex examples, previously provided), moving through a bridge analysis in a step-wise example to illustrate the use of the guide, the BMP Evaluation Tool, and the Conveyance Cost Tool. Effect of Bridge Deck Runoff on the Receiving Water One of the largest factors likely to reduce the impact of bridge runoff on receiving waters is dilution. Evaluation of impacts of runoff in general, and that from bridge struc- tures in particular, hinges on the estimation of the con- centration of a constituent in the receiving watercourse downstream of the bridge. Methods for carrying this out have been reviewed and summarized in several key refer- ences, notably NCHRP Report 474 (TRB 2004) and URS (2010). In the present context, we present approximations that will serve as “screening estimates”, exploiting the char- acteristics of bridge discharges to simplify the computation in the majority of cases while remaining justifiable and con- sistent with regulation. The strategy is to specify strongly conservative estimates, i.e., those that will entail maximum impacts of the bridge runoff, and compare these estimates to regulatory thresholds. We emphasize two aspects of each such screening calculation: the data necessary to complete the calculation and the assumptions underlying the for- mula, which implicitly delimit conditions for which the calculation is appropriate. Generally, the concentration c in some volume of influence V in the receiving water is given as a statement of conserva- tion of mass: – – (1) d Vc dt c Q c Q cQ EA c KcVa a s s ( ) = + + where Qa = ambient flow in the watercourse ca = ambient concentration in the watercourse Qs = storm runoff flow into the watercourse cs = storm runoff concentration E = evaporation rate in depth of water per unit time A = surface area of the volume of influence K = first-order decay coefficient in inverse time (i.e., per unit time) The outflow from V is given approximately by: – (2)Q Q Q EAa s= + Concentration is usually a mass ratio, i.e., mass of constitu- ent per unit mass of water, but in water quality evaluations of dilute concentrations in natural watercourses, concentra- tion is typically expressed as the ratio of mass of constituent per unit volume of water, e.g., mg/L. In this case, the prod- ucts caQa and csQs are the loads in the watercourse and the storm runoff, respectively. The concentration c is a spatial mean assumed to be averaged in some way over the volume V, but better definition of V will be dependent upon the water- course and the objective of the estimate, as will be seen. The pollutants of concern in evaluating the impact of bridge deck runoff remain behind in the watercourse when water is lost to evaporation, thereby increasing their concentrations. The term EAc represents the effective mass source from this process. The last term –KcV is a first-order decay, which can A P P E N D I X B

B-2 depict the in-stream kinetics to which some constituents, such as nitrogen compounds and coliforms, are subjected. If K, ca, Qa, E, cs, and V are taken constant, and Qs is the averaged storm flow over the time from 0 to t, the solution to (1) is: 1 (3)/ /c t c Q c Q R K V EA e c ea a s s R K EA V t o R K EA V t( )( ) ( )= + + − − +( ) ( )− + − − + − where R ≡ Q/V and co ≡ c(0) for which co = ca is a realistic value for an isolated event. It should be noted that R is the recipro- cal of the residence time t for the volume of influence (see Section 3.1.1). Frequently, a practical approximation is that of a steady-state balance, i.e., d(Vc)/dt → 0 in (1), whereupon (3) reduces to: (4)c t c Q c Q R K V EA a a s s( ) ( )= + + − These equations apply in general to a storm event in which the runoff is from the entire area affected by the storm, of which the bridge deck is one component. (These separate components, of which the bridge is only one, are examined in the next paragraph.) If the pollutant is unique to the bridge deck, that is, the bridge deck is the only source of the pollut- ant in the storm runoff, then cs and Qs in (3) and (4) apply solely to the load from the bridge. The solutions (3) and (4) can be exploited for order-of-magnitude estimates. A version of (4) is presented in Method 2 of NCHRP Report 474 (TRB 2004) for salts, in which K and E do not appear. We can estimate the magnitude of the dilution process by separating the storm runoff in the watercourse downstream of the bridge into areal components, viz. that precipitation Qp falling directly on the surface of the watercourse upstream from the bridge structure (which, technically, is not runoff), runoff from the adjacent drainage area Qd likewise upstream from the bridge, and that running off from the bridge struc- ture Qb itself, (5)Q Q Q Qs p d b= + + For a constant rate of precipitation, it is immediate that the relative magnitudes of these flows are proportional to the effective area of each component. The constant of propor- tionality (i.e., runoff coefficient) is a fraction generally less than 1. A bridge deck is assumed impermeable with no pond- ing areas, so a value of 1 is adopted. Clearly, for precipitation directly on the water surface, this coefficient equals 1. For an urbanized drainage area the coefficient ranges from 0.7–1.0, so a value of 0.8 is recommended. For a rural drainage area, the coefficient can range from 0.0 to nearly one, depending upon the condition of the landscape and soils. Absent region- specific information, a value of 0.3 is suggested. “Effective area” means the intersection of the area of pre- cipitation and the surface area of each areal component. The area of precipitation is the “footprint” of the convective storm system giving rise to the precipitation. For deep-convecting systems, precipitation is concentrated in single thunderstorm cells or clusters of cells. The precipitation area is a combina- tion of the time history of precipitation from the convective cells and the trajectory of movement. At the low end of the size spectrum, these cells are air-mass thunderstorms, typical of summer in most of the United States, but in some regions of the country the primary source for rainfall. On average, these single cells are about 30 km2 in area, and rarely smaller than 20 km2 (e.g., Morin 2006). At the other end of the spectrum, “supercell” thunderstorms have precipitation areas ranging up to the order of 104 km2 (e.g., Smith 2001, Bluestein, 2009). Mesoscale convective complexes (MCCs) are clusters of cells exceeding specified thresholds of size and intensity that are longer lived than single-cell storms. These MCCs have areas of significant precipitation about 106 km2 (Kane 1987). At the largest scale are the precipitation pat- terns associated with synoptic systems, such as cyclonic storms, frontal passages and squall lines, whose lifetime trajectories can extend over several states to much of North America, and whose precipitation areas can range up to 108 km2 or more. For order-of-magnitude estimates of Equation (5), con- sider a watercourse of width 100 m and length upstream from the bridge crossing of 100 km. The order of bridge deck dimensions would be 100 m length (the same order as the watercourse width) and 10 m width, so its area is 10-3 km2. Precipitation areas for all meteorological systems described above exceed the bridge deck area, so for the bridge the effec- tive area is the bridge deck itself, 10-3 km2. The upstream surface area of the watercourse is its width times its length, 10 km2. While this is exceeded by the precipitation areas of all of the above meteorological systems, the smallest, the air- mass thunderstorm, is of the same order, but of a circular rather than rectilinear geometry. The radius of the cell is one- tenth the dimension of the upstream watercourse length, so the effective area is about 10 km length times 100 m width, or 1 km2. For the larger meteorological systems, the effec- tive area is the upstream surface area of the watercourse, 10 km2. Finally, the nominal area of the upstream watershed is the square of upstream watercourse length, or 104 km2. For single-cell storms, ranging from air-mass to super cell, the watershed area is on the same order or larger than the precipi- tation area, so the effective area is that of the storm, ranging 10–104 km2. The larger meteorological systems are orders of magnitude larger than the watershed, so the effective area

B-3 becomes that of the watershed, 104 km2. In summary, the orders of magnitude of the terms in Equation (5) are given in Table B-1. Clearly, the contribution of bridge runoff to the total storm flow given by Equation (1) is at least three orders of magnitude smaller than the other terms. That the bridge run- off is negligible compared to the other terms is unaffected by considering a bridge deck of greater width, and to assum- ing smaller values of the runoff coefficient for the upstream watershed, and is equally applicable to the stream, arm of a lake, or arm of an estuary. For larger watercourse systems, the watershed area scales with the upstream length, while bridge dimensions scale with watercourse width, so similar orders of magnitude result. Even though concentrations of various constituents in bridge deck runoff can be elevated compared to those measured in the receiving water, the actual impact would be expected to be small because of the relative volumes of runoff illustrated in Table B-1. This is an expected result based on mass balance of constit- uent and hydrologic inputs because the bridge is such a small fraction of the watershed. In addition to the water balance, a comparison of the associated pollutant loads can be made, if estimates of the constituent concentrations are available. For short residence times (i.e., large R), the reactive rates EA/V and K may be neglected in comparison to R. With concentra- tions cp, cd, and cb assigned to each of Qp, Qd, and Qb, resp., the flow-weighted concentration upstream from the bridge: c c Q c Q c Q Q Q Qu a a p p d d a p d( ) ( )= + + + + and the dilution of the bridge runoff defined to be D ≡ Q/Qb, (4) becomes: 1 1 1 (6)c c D c D u b( )= − + or, in perhaps a more transparent form: –c c c c D u b u( ) = − Equation (6) is a suitable screening test. For storm runoff from even a small single-cell storm, the comparative mag- nitudes of Table B-1 indicate that D is about 104. The ratio of cb to cu would therefore have to approach this order to be problematic for acute concentrations. In such a situation, the more accurate Equations (3) or (4) should be used with better estimates of stream flow and runoff magnitudes. If we assume that the concentration in bridge runoff is as much as an order of magnitude greater than that in the precipitation and storm runoff loads, then the bridge runoff load cbQb is at least two orders of magnitude smaller than either of the load to the stream in rainfall directly on the water surface cpQp or the load from upstream runoff cdQd. The general conclusion must be that the impact of runoff from any individual bridge in a rural area on the receiving water is de minimis. This qualitative conclusion is supported by a scenario from a bridge crossing in Texas. Malina et al. (2005) evaluated the load increase associated with the Loop 360 bridge discharge to Barton Creek in Austin, Texas. This site was selected as a worst-case scenario, because it was a six-lane bridge discharg- ing to an ephemeral stream (big bridge/stream with small flow). Average annual loads were calculated based on typical flow rates and average precipitation. The watershed tributary to this crossing is about 120 square miles, with a 30,000 ft2 bridge deck. The watershed however, is largely undeveloped, with wet season flows averaging about 25 cfs at the crossing. Table B-2 summarizes the findings from this Austin, Texas study. For no constituent was the increase downstream of the bridge even as large as 0.1% and most were smaller by at least a factor of 10. These data indicate that even if all discharge from the bridge were eliminated, the change in receiving water quality downstream of the bridge would be undetect- able. This suggests that under most conditions, bridge run- off has a de minimis impact on water quality that does not require the installation of BMPs. As mentioned previously, it is likely that any impairment in an urban watershed is probably associated with impervious cover and it is the cumulative impact of many small contribu- tors that creates the problem. Consequently, the de minimis argument does not apply and bridges should generally have Source of runoff Meteorological Precipitation System Area (km 2) Bridge Deck (Qb) All 10-3 Upstream Surface of Watercourse (Qp) Airmass Cell 1 Others 10 Upstream Watershed (Qd) Single-Cell Storms 10-104 Others 104 Table B-1. Comparative example orders of magnitude for flow components of Equation (5).

B-4 to comply with local NPDES requirements regarding storm- water treatment of runoff to the MEP. In this case, installing structural BMPs on bridges is probably the correct decision for urban watersheds, although feasibility will be affected by cost and limited area to work with: The DOT may find that equivalent or better pollutant removal to the receiving water can be achieved for a lower cost by treating runoff from the adjacent roadway where construction cost and maintenance cost are much more favorable. Simple Assessment Procedure Accessing Upstream Water Quality and Flow Data The best source of river flow and quality data is a USGS website, which can be accessed at: http://maps.waterdata. usgs.gov/mapper/index.html. Zoom in to area of bridge proj- ect and locate a monitoring station on the river of interest. If none exists, use another site in the general area. Click on the site to bring up “Site Information.” Figure B-1 shows a screen shot of the map with the monitoring site located on the North Bosque River. Click on “Access Data” on the “Site Information” tab to move to the data selection page, which is presented in Figure B-2. Flow Data To access average flow data click on “Annual Statistics”, which brings up the page presented in Figure B-3. Click the box next to “Discharge” and select “Table of Annual Means” under output format. Then click on “Submit”. This brings up the page presented in Figure B-4. These values are easily copied and pasted into Excel to calculate average annual dis- charge, which is 237 cfs. If the monitoring site is not precisely at the bridge location, then normalize the flow by drainage area, which is 1146 mi2 in this case resulting in 0.207 cfs/mi2. Determine the upstream area at your location of interest and multiply by normalized flow to determine expected discharge at another bridge location. Water Quality Data To access water quality data from the site, go back to the page shown in Figure B-2 and select “Field/lab water-quality samples,” which brings up the page shown in Figure B-5. Select “Table of Data” with Default attributes and click on “Submit.” This brings up the page shown in Figure B-6, which can be easily copied and pasted into Excel to calculate average values for the constituents of interest. Based on the data pro- vided and using the detection limit for all censored values, the average nitrate concentration is 0.58 mg/L as N and dissolved P is 0.015 mg/L as P. Calculation of Constituents of Concern Load Increase The load increase is calculated as: Load Increase Bridge Load Bridge Load Upstream Load 100 ( )= + × Constituent Annual Load Barton Creek Upstream of Loop 360 Bridge (kg/yr) Annual Load Contributed by Loop 360 Bridge Runoff (kg/yr) % Increase Copper, Total 214 0.04 0.018 Lead, Total 135 0.023 0.017 Zinc, Total 680 0.38 0.056 Nitrate, as N 10,625 0.79 0.007 Total N 47,610 3.03 0.006 COD 9.44 x 105 77 0.008 Phosphorus, Total 6,165 0.26 0.004 Phosphorus, Dissolved 2,148 0.19 0.008 TSS 7.0 x 106 258 0.004 VSS 2.64 x 105 52 0.02 Fecal Coliform (cfu/yr) 1.13 x 1016 1.16 x 1011 0.001 (Modified from Malina et al., 2005) Table B-2. Comparison of average storm flow loads in Barton Creek at the Loop 360 Bridge, Austin, TX.

B-5 where the Load Increase is the percentage of the load down- stream of the bridge contributed by the bridge itself, Bridge Load is the load conveyed by the bridge runoff, and Upstream Load is the load in the receiving water upstream of the bridge. Bridge Load Rainfall Runoff Coefficient Area of the Bridge Deck Concentration = × × × Where the Rainfall is the average annual rainfall for the spe- cific location, the runoff coefficient is typically about 0.9, and the Concentration is the average concentration of the constitu- ent of concern (see Table B-2). This is a low traffic site, so con- centrations typical of AADT of 0-25,000 are appropriate. There are many sources of rainfall data, including the performance and cost tool developed as part of this project. Average rainfall in North Central Texas is 29 inches/yr and the area of the bridge is 20,000 ft2. Consequently, the Bridge Load for nitrate is: Nitrate Bridge Load 29 in/yr/12 in/ft 1.0 20,000 ft 0.2 mg/L 28.3 L/ft 2 3 = × × × × Nitrate Bridge Load 273,567 mg/yr 0.274 kg/yr= = Similarly, the dissolve P load can be calculated as: DP Bridge Load 29 in/yr/12 in/ft 1.0 20,000 ft 0.072 mg/L 28.3 L/ft 2 3 = × × × × DP Bridge Load 98,433 mg/yr 0.098 kg/yr= = The Nitrate Upstream Load can be calculated as: Nitrate Upstream Load Annual discharge of the receiving water Average Stream Concentration = × Nitrate Upstream Load 237 ft /s 86,400 s/d 365 d/yr 0.58 mg/L 28.3 L/ft 3 3 = × × × × Nitrate Upstream Load 122,678,761, 248 mg/yr 122,679 kg/yr = = Figure B-1. Screen shot of the USGS mapper.

B-6 Similarly, the dissolved P upstream load can be calculated as: DP Upstream Load Annual discharge of the receiving water Average Stream Concentration = × DP Upstream Load 237 ft /s 86,400 s/d 365 d/yr 0.015 mg/L 28.3 L/ft 3 3 = × × × × DP Upstream Load 3,172,726,584 mg/yr = 3,173 kg/yr= The nitrate load increase is given by: Nitrate Load Increase 0.274 kg/yr/ 0.274 kg/yr 122,679 kg/yr 100( )= + × Nitrate Load Increase 0.0002%= Finally, the dissolved phosphorus load increase is given by: ( )= + × DP Load Increase 0.098 kg/yr/ 0.098 kg/yr 3,173 kg/yr 100 DP Load Increase 0.003%= Since the load increase for the constituents of concern, nitrate, and dissolved phosphorus are 0.0002% and 0.003% respectively, which is substantially less than 1%, we can con- clude that the impact is de minimus. Complex Assessment Procedure For any of the reasons listed in the main text (Section 3.1.3), the designer may be faced with assessing the impact on the receiving water either by comparison of concentrations with and without the bridge structure, or relative to stream standards Figure B-2. Screenshot of the data selection page.

B-7 (or some other quantitative criterion for a specific water- quality parameter). The first step is to determine the applicable stream flow. For water-quality standards, this will be the critical low- flow, typically defined for streams to be the 7Q10, or in some states the 7Q2 or 3Q10. (For reservoirs, the 30Q10 may be the appropriate choice. Unfortunately, these statistics are not routinely provided by the USGS. They have been computed for several states and are available on the Internet (search for “7Q10” + the name of the state). They may be computed from daily data at a stream gauge using Windows-based programs, either DFLOW downloadable from the EPA web- site, or SWSTAT from the USGS website. There are also a script file available for MATLAB application and a macro for EXCEL, both available on the Internet. The first place to check, however, is the water quality regulatory office of the state. Many states publish the computed 7Q10s or other regulatory streamflows as a part of their water quality stan- dards. Even though the period of record for which these cal- culations were made may be long out of date or unknown, without instruction to the contrary from the cognizant state agency, these low-flow statistics should be regarded as jurisdictional. For the example site of the FM 56 crossing of the North Bosque, this bridge is found to lie in Water Quality Segment 1226, delineated in the State of Texas Surface Water Quality Standards. In an Appendix to the Standards, the 7Q2 (the statistic used for the low-flow limit for standards applica- tion in Texas) throughout this segment is 10.1 cubic feet per second. The next step is to estimate the rainfall from the event giv- ing rise to storm runoff from the bridge. The worst-case sce- nario is a small single-cell thunderstorm, say, with area 20 km2 and rainfall 0.2 inches delivered in an hour or less (because this small a storm will afford minimum dilution). Convective events with rainfalls not exceeding 0.2 inches make up about 75% of storm events in Central Texas and the plateau regions of Texas, e.g., Owens and Lyons (2004). Figure B-3. Screenshot of the annual statistics page.

Figure B-4. Output of annual mean discharges. Figure B-5. Screenshot of the water quality data selection page.

B-9 The simplest, and most conservative, calculation is the relative dilution given by Equation (5), in which E = 0 and K = 0. Equation (4) reduces to: (7)c t c Q c Q Q a a s s( ) = + A runoff coefficient of 1 is used (because of the intense rainfall during a short period of time on the essentially imper- vious bridge deck). The storm runoff from the bridge deck Qb is given by: Bridge deck runoff = Rainfall coefficient rainfall rate bridge deck area =1 0.2 ins/hr 1/12 ft/in 1/3600 hrs/sec 20,000 ft2 ( ) × × × × × × Bridge deck runoff 0.0926 cfs= from which the bridge load is calculated by Bridge Load Bridge deck runoff Concentration= × which is exactly the same equation as in Section 6.5.2, but the meanings and some of the values of the terms are different. As an example, nitrate load is computed. Using the same value of runoff concentration of 0.2 mg/L NO3-N from Table B-2: Bridge nitrate load 0.926 cfs 0.2 mg/L 28.3 L/ft3= × × Bridge nitrate load 5.2 mg/s= The ambient flow is 10.1 cfs. For ambient concentration ca, one option is the long-term mean concentration of 0.58 mg/L, see above. With these values, Upstream ambient nitrate load 10.1 cfs 0.58 mg/L 28.3 L/ft3 ( ) = × × Upstream nitrate load 165.8 mg/s= Figure B-6. Screenshot of “Parameter Group Period of Record Table”.

B-10 If we assume that the only runoff from the storm originates from the bridge, i.e., we neglect Qp and Qd in (5), then (4) reduces to Downstream concentration Upstream load bridge load downstream flow( ) ( )= + Downstream concentration 165.8 5.2 mg/s / 10.1 0.093 cfs 0.0353 ft /L 0.59 mg/L 3( ) ( )= + + × = There is no stream standard for nitrate in Texas. The mini- mum detection limit for nitrate-nitrogen ranges 0.01–0.10 mg/L depending upon the methodology. Even with the conservative assumptions made, the effect of the bridge is at or below detec- tion limits. The above calculation makes several assumptions that magnify the effects of the bridge, which can be improved to arrive at a more accurate estimate. Foremost is the absurd assumption that rainfall only occurs on the bridge deck. In order to include these effects, it is necessary to estimate the rainfall rate on the stream itself Qp and that falling on the adjacent drainage area Qd, with the corresponding runoff concentrations. Assuming a circular storm of area 20 km2 = 215.3 × 106 ft2, its diameter is 5.05 km = 16560 ft, which, if centered on the stream, is the length of stream covered by the storm. The nominal stream width is about 50 ft = 0.0153 km, giving a surface area of 0.0773 km2 = 828,000 ft2. Thus Drainage area affected by storm 215.2782 10 – 0.8278 10 – 0.0200 106 6 6( )= × × × = ×Drainage area affected by storm 214.4304 10 ft6 2 With a runoff coefficient for the rural drainage area of 0.3, the flow equivalents of rainfall become: Runoff from drainage area runoff coefficient rainfall rate drainage area = × × Runoff from drainage area 0.3 0.2 ins/hr 1/12 ft/in. 1/3600 hrs/sec 214.4304 10 ft6 2 ( )= × ×× × × Runoff from drainage area 297.820 cfs= Rainfall on stream surface runoff coefficient rainfall rate stream surface area = × × ( )( )= × ×× × Rainfall on stream surface 1.0 0.2 ins/hr 1/12 ft/in. 1/3600 hrs/sec 82779 ft2 Rainfall on stream surface 0.3832 cfs= and from above Bridge deck runoff 0.0926 cfs= The dilution factor D then becomes Dilution Ambient flow runoff from drainage area rainfall on stream runoff from bridge runoff from bridge( )= + + +       Dilution 10.1 297.82 0.383 0.0926 cfs 0.0926 cfs 3330 ( ) ( )= + + + = With much more work, reasonable estimates of the nitrate concentration in rainfall cp and in runoff from the adjacent drainage cd could be computed and used along with that of ambient ca to estimate the resulting downstream concentra- tion. A simpler, equally effective calculation is the increase in concentration due to the bridge runoff given by: =     increased concentration bridge runoff concentration – downstream concentration that would occur without bridge dilution The worst-case assumption is that the difference in the numerator is exactly the concentration from Table B-2, i.e., that there is no liability assigned (or no credit taken) for what- ever nitrate concentrations are already present in the water- course due to the storm, whereupon increased concentration 0.2 mg/L 3330 0.00006 mg/L( )= = which is far below the minimum detection limits for this parameter. A similar procedure can be followed for total copper. The same flow and dilution values apply. Bridge total copper load 0.926 cfs 9.3 / L 28.3 L/ft3g= × µ × Bridge total copper load 243.7 g/s= µ The ambient concentration at this station according to the above USGS website is low, frequently below detection limits, and averaging about 2 mg/L when detectable. Then ( ) = × µ × Upstream ambient total copper load 10.1 cfs 2 g/L 28.3 L/ft3 Upstream total copper load 573 g/s= µ

B-11 If we assume that the only runoff from the storm originates from the bridge, i.e., we neglect Qp and Qd in (5), then (4) reduces to Downstream concentration Upstream load bridge load downstream flow( ) ( )= + Downstream concentration 573 244 g/s 10.1 0.093 cfs 0.0353 ft /L 2.83 g/L 3)( )(= + µ + × = µ In this case, there is a state stream standard for total copper, which for average hardness 160 mg/L as CaCO3, is 22 mg/L. The calculated concentration of 2.8 mg/L is well below this concentration. As noted above, this assumes that rainfall from the storm falls only on the bridge deck. If the other storm inflows to the stream are included, then, using (6) with zero concentrations in rainfall and land-surface runoff Upstream concentration Ambient concentration ambient flow Ambient flow runoff from drainage area rainfall on stream ( )= × + +       Upstream concentration 2 g/L 10.1 cfs 10.1 cfs 297.820 cfs 0.3832 cfs 0.065 g/L ( ) ( )= µ × + + = µ Downstream concentration upstream concentration 1 – 1 dilution bridge runoff concentration dilution ( )= × + Downstream concentration 0.065 1 – 1 3330 9.3 3330 0.068 g/L, well below the stream standard. ( )= + = µ Worked Example Problem The bridge selected for the worked example is the new bridge on FM 56 over the North Bosque River, just west of Waco, Texas. A picture of the bridge is provided in Fig- ure B-7. Segments of the North Bosque River are suffer- ing from eutrophication resulting from high nutrient concentrations; consequently, nitrate and dissolved phos- phorus have been selected as the constituents of interest. The primary sources are agriculture and dairy cattle. For a conservative assessment, the entire length of the bridge over the floodplain will be considered contributing directly to the river, rather than just the very small section over water. The length of this portion of the bridge is about 500 feet and the width is 40 feet. The example follows the steps described in Chapter 1 and summarized in Figure 1-1. Although most of the emphasis in this example falls within Step 5, the steps are: • Step 1. Development of project environmental documen- tation • Step 2. Consideration of source controls and O&M BMPs • Step 3. Determine if bridge is subject to NPDES permit • Step 4. Determine NPDES permit treatment requirements • Step 5. Determine if bridge is subject to 404 permit and 401 certification Step 1: Development of Project Environmental Documentation This particular segment of the North Bosque is not listed for any pollutants, but other segments are listed for concerns related to organic matter. Therefore, this analysis will focus on the nutrients nitrate and dissolved phosphorus. During the development of the project environmental documentation it was determined that a 404 permit and 401 certification are required for the replacement project. In addition, the construc- tion contractor is required to take measures to prevent harm to active swallow nests on the existing bridge. The Bosque River eventually drains into Lake Waco. Step 2: Consideration of Source Controls and O&M BMPs Several source controls and operations and maintenance BMPs for consideration for all bridges are described in Chap- ter 4 of this guide. These can be low cost and effective means Figure B-7. Crossing of North Bosque River.

B-12 of protecting water quality and are briefly considered here for this example. • Collection and conveyance. Preventing the direct discharge of stormwater from the bridge deck to the receiving water by collecting and conveying it in pipes or on the deck to treatment facilities on land may be considered. For this bridge, this option will be evaluated in conjunction with the treatment BMPs in Step 5. • Bird and bat roosting. Roosting birds and bats contribute organic material directly to receiving waters below. Since swallow roosting occurs on the existing bridge, it is rea- sonable to assume swallows will return to the new bridge unless mitigation measures are applied. Forms of netting, projections, and panels, as described in Section 4.2 of this guide should be considered for the new bridge. • Bridge construction materials. Bridge decking and metal finishing materials should be evaluated for potential depo- sition during construction and maintenance activities. • Bridge maintenance. Maintenance activities, e.g., painting materials and methods, bridge washing, winter maintenance, and sweeping, may reduce pollutant introduction to receiv- ing waters. Long-term requirements for painting and bridge washing should consider this potential. Given the climate in this area, winter maintenance requirements will not be sig- nificant. Furthermore, given the rural nature of this bridge, sweeping is unlikely to be cost effective given the significant distance to travel to bring a sweeper to this location. • Bridge inspection. In addition to inspection for structural integrity, regular bridge inspection may provide early iden- tification of maintenance needs to mitigate peeling paint and other maintenance conditions affecting water quality. Step 3: Determine if Bridge is Subject to NPDES Permit This bridge is not subject to an NPDES permit since it is in a rural location. Skip Step 4 (Determine NPDES permit treatment requirements) and proceed to Step 5. Step 5: Determine if Bridge is Subject to 404 Permit and 401 Certification As mentioned previously, this bridge is subject to a 404 permit and 401 certification. Therefore, an assessment of water quality impacts must be completed (Step 5a). Step 5a: Perform Water Quality Assessment Earlier in this appendix, an analysis of the contribution of runoff to water quality was provided for this site. Using the Simple Assessment method, the conclusion is that the contri- bution of runoff from the bridge for nitrate and dissolved phosphorus are de minimis because the bridge loading is substantially less than one percent of the watershed load- ing as noted in the appendix. Using the Complex Assessment method a similar conclusion is made. In that analysis it is concluded that the effect of the bridge on nitrate and dis- solved phosphorus concentrations is below detection limits and the nitrate and dissolved phosphorus concentrations are below the stream standard. However, for the purposes of this example, it is assumed that the 401 certification requires the use of treatment BMPs to reduce annual loading of nitrate and dissolved phosphorus by at least 50%. This reduction is not meant to imply a local or broad standard, but is simply used as an example. Proceed to Step 5b to analyze the treatment options. Step 5b: Analyze Treatment Options This bridge runs approximately north south and is designed with a vertical curve resulting in drainage to both the north and south abutments. Direct discharge to the river is not allowed. Three hundred ninety-one feet of the total 580 ft span drain to the north, while the remaining 189 ft drain to the south. This analysis demonstrates the required techniques applied to the north side. The process would be similar for the south side. For the north side drainage, a BMP may be sited north of the north abutment and on the west side of FM 56. It is antici- pated that the BMP will not fit within the proposed right-of- way requiring the purchase of an additional easement. The contributing area to the BMP includes the entire bridge deck, a portion of the approach roadway and a portion of the approach roadway embankment. For the approach roadway and approach roadway embankment, it is estimated that a 150 ft long section will drain to the BMP. Since only half of the approach roadway and embankment will drain to the west side, the widths for the roadway and embankment are both estimated at 22 ft. The total contributing area to the site is summarized in Table B-3 along with the applicable percent imperviousness. The percent imperviousness of 100 percent Contributing Area Size (ft2) Percent Imperviousness Bridge deck 391 x 44 = 17,204 100 Roadway 150 x 22 = 3,300 100 Embankment 150 x 22 = 3,300 0 Total 23,804 86 Table B-3. Contributing area characteristics.

B-13 is used for the bridge deck and roadway and zero percent is used for the embankment. Tools for five BMP treatment options are provided with this guide and are considered here. However, listing in the guide should not limit the BMPs considered for a particular site. The listed BMPs are: 1. Bioretention 2. Dry Detention 3. Permeable Friction Course 4. Sand Filter 5. Swale All five BMPs require common input data for the respec- tive Tools. The Project Location tab provides for determina- tion of rainfall characteristics. For this example, one clicks on the portion of the map that contains Texas for Step 1 and then selects Texas under the pull-down menu under Step 2. The applicable rain gage, (3) North Central – Ft Worth Meacham, is also selected for this site. Defaults for the remaining values are used for this example. Several parameters are provided on the Project Options tab that affects the BMP costs. Table B-4 provides a summary of the primary parameters used for this example. The Project Design tab includes data that may be common across multiple BMPs, as well as data that are unique to each BMP. In this example, the common data for the north portion of the bridge are the tributary area, the percent impervious- ness, and the soil type grouping. The first two of these were previously noted as 23,804 ft2 (0.546 acres) and 86% imper- viousness. The soil type group is silt loam (Type B). Bioretention The design parameters on the Project Design tab of the Bio- retention Evaluation Tool are Storage Volume and the pres- ence of an underdrain as is shown in Figure B-8. In order to achieve the required 50% reduction in the annual loading of nitrate and dissolved phosphorus, a storage volume of 1,000 ft3 is needed leaving all other design inputs at their default values. Underdrains are not needed because the soil type is hydrologic Group B. Fifty percent removal of the annual loading of nitrates and dissolved phosphorus is accomplished because this BMP captures and infiltrates 50% of the stormwater inflow. The area required to site this BMP is at least 760 ft2, which rep- resents the footprint of the BMP at the top of the freeboard level. Additional area will be needed for grading, access, and appurtenances. Using the parameters summarized in Figure B-8 and the default components for bioretention, the total estimated capital cost of this BMP including design fees is $15,400. Considering capital and maintenance costs, the net pres- ent value of the whole life cycle costs is $61,000. Table B-5 summarizes the result of the analysis for the bioretention treatment BMP. Dry Detention The design parameters on the Project Design tab of the Dry Detention Evaluation Tool are Storage Volume, the pres- ence of an impermeable liner, and water quality depth as is shown in Figure B-9. In order to achieve the required 50% reduction in the annual loading of both nitrate and dissolved phosphorus, a storage volume of 1,500 ft3 is needed while lowering the water quality depth to 1.1 ft to promote infil- tration. An impermeable liner is not used so that infiltration may occur. Fifty percent removal of the annual loading of dissolved phosphorus is achieved while 57% of the nitrates are removed. The area required to site this BMP is at least 2,180 ft2, which represents the footprint of the BMP at the top of the free- board level. Additional area will be needed for grading, access, and appurtenances. Using the parameters summarized in Figure B-9 and the default components for dry detention, the total estimated capital cost of this BMP including design fees is $29,500. Considering capital and maintenance costs, the net present value cost over the whole lifecycle is $59,000. PFC The design parameters on the Project Design tab of the PFC Evaluation Tool are permeable friction course surface area and depth as is shown in Figure B-4. In order to achieve the required 50% reduction in the annual loading of both nitrate and dissolved phosphorus the entire bridge deck area of 17,204 ft2 is used with a 3-inch course thickness. According to the tool, PFC removes 66% of the dissolved phosphorus, exceeding the requirement, but none of the nitrates, falling short of the requirement of reducing both constituents by 50%. Because this BMP is applied on the bridge deck itself, no additional area is required for its implementation. Using the parameters summarized in Figure B-10 and the default components for PFC, the total estimated capital cost of this BMP including design fees is $24,500. Considering capital and maintenance costs, the net present value of the whole lifecycle cost is $52,900. Parameter Value Location Adjustment Factor 100 Design Life 25 years Discount Rate 5 percent Inflation 3 percent Expected Level of Maintenance Medium Table B-4. Project options cost parameters.

B-14 Figure B-8. Bioretention design parameter screen shot. BMP Capital Cost ($) Net Present Value (Whole Lifecycle) ($) Nitrate Removal (%) Dissolved Phosphorus Removal (%) Minimum Footprint (ft2) Bioretention $15,400 $61,000 50 50 760 Dry Detention $29,500 $59,000 57 50 2,180 PFC $24,500 $52,900 0 66 0 Sand Filter not applicable not applicable 0 0 not applicable Swale $37,500 $70,000 50 50 1,260 Table B-5. BMP cost and performance comparison. Sand Filter The sand filter does not provide removals of nitrates or dissolved phosphorus according to the Sand Filter Tool. Therefore, this BMP is not applicable for this site. Swale The design parameters on the Project Design tab of the Swale Evaluation Tool are the swale bottom width, bottom length, and effective amended soil depth as is shown in Fig- ure B-11. In order to achieve the required 50% reduction in the annual loading of both nitrate and dissolved phosphorus, a sufficient area for infiltration (swale bottom width multi- plied by length) combined with an adequate amended soil depth are needed. Conveyance capacity for storm flows and flow from other drainage areas must also be considered. In this case, it has been assumed that stormwater from other drain- age areas will bypass in a separate conveyance. The resulting design parameters are shown in the figure. Fifty percent removal of the annual loading of both nitrates and dissolved phosphorus are achieved with this design accord- ing to the tool. The area required to site this BMP is at least 1,260 ft2, which represents the footprint of the BMP at the swale bottom. Additional area will be needed for grading the side slopes, access, and appurtenances. Using the parameters summarized in Figure B-11 and the default components for the swale, the costs were relatively high. One of the default components in this tool is a metal

B-15 Figure B-9. Dry detention design parameter screen shot. Figure B-10. PFC design parameter screen shot. beam guardrail. This was removed from the costing because it would already be required for roadside safety with or without this BMP. In other words, it is not part of the cost required to achieve the pollutant removals. Without this component, the total estimated capital cost of this BMP including design fees is $37,500. Considering capital and maintenance costs, the net present value of the whole lifecycle cost is $70,000. Conveyance An additional consideration for the implementation of each of the treatment BMPs, except for the PFC, is the potential need for collection and conveyance from the deck to the BMP. The cost for any such conveyance is added to the cost of the BMP for estimating the full cost of compliance.

B-16 The Deck Drain Tool is used for this purpose. The param- eters governing the design storm event and bridge configura- tion are entered into the Tool. The bridge deck length, width, longitudinal slope and transverse slope describe the drainage area. A runoff coefficient of 1.0 is used for the bridge deck and the design rainfall intensity is required for use in the Rational Method. For this example, an intensity of 4.0 inches per hour is assumed. These inputs are shown in Figure B-12. Using the Rational Method, the design flow (on each side of the deck) is: Q CCiA 1.0 4.0 391 22 43,560 0.79 cfs( )( )= = × = Figure B-11. Swale design parameter screen shot. Figure B-12. Deck drain design parameter screen shot.

B-17 Since this is less than the gutter capacity of 1.84 cfs calcu- lated by the Tool and shown in Figure B-12, the design flow can be carried on the bridge deck within the 10 ft shoulder without any need of a collection and pipe conveyance system. If the shoulder width is reduced in the future as a result of changing the lane configuration, this must be reevaluated. Discussion The final component of completing this analysis is to develop the most cost-effective strategy for meeting the water quality requirements. Of the BMP treatment alternatives summarized in Table B-5, PFC and the sand filter do not pro- vide sufficient nitrate and/or dissolved phosphorus removals to satisfy the minimum requirements of 50% removals. Of the remaining alternatives, bioretention offers the lowest capital cost and only slightly higher net present value whole lifecycle cost compared with dry detention. Although consid- eration of the lifecycle costs signals a slight cost advantage for dry detention, the operations and maintenance costs are uncertain compared to the capital cost. Given the pressure on roadway budgets, it may be difficult to justify doubling the upfront costs in the anticipation of small long-term savings. In addition, bioretention has a smaller footprint than dry detention, reducing the easement acquisition requirements, which have not been included in the cost comparison. How- ever, dry detention has the advantage of exceeding the stan- dard for nitrate removal. The remaining alternative, the swale, is more expensive (capital and lifecycle) than bioreten- tion and dry detention. Therefore, bioretention is the recom- mended treatment BMP for this example. Another alternative that may be considered is treating runoff from an adjacent section of highway in the same watershed in lieu of treating the bridge deck stormwater runoff. This alter- native would be an example of offsite treatment. If this example had resulted in requiring a collection and conveyance system on the bridge, then treating a comparable highway section would eliminate the need for and cost of such a system. In addition, treating a comparable highway section would also eliminate the challenges of locating a treatment BMP at or near a bridge abutment where the terrain may not be conducive to siting a BMP. This example provided a sample analysis of the recom- mended assessment steps for the north section of the FM 56 bridge over the North Bosque River in Texas. The same pro- cedure should be followed for the smaller south section to consider the full project. Overall the procedure must include consideration of the following: • BMP costs and effectiveness • The need for collection and conveyance on the bridge • Comparison with treatment of an alternative roadway section References Bluestein, H. (2009). The Formation and Early Evolution of the Greensburg, Kansas, Tornadic Supercell on 4 May 2007. Weather and Forecasting, 24: 899-920. Kane, R. C. (1987). Precipitation Caracteristics of Mesoscale Convective Weather Systems. J. Climate Appl. Meteor., 26: 1345-1357. Malina, J., et al. (2005). Characterization of Stormwater Runoff from a Bridge Deck and Approach Highway, Effects on Receiving Water Qual- ity. Center for Transportation Research, University of Texas at Austin. Morin, E. D. (2006). Spatial Patterns in Thunderstorm Rainfall Events and Their Coupling with Watershed Hydrological Response. Adv. Water Resources, 29: 843-860. Owens MK, Lyons RK. (2004). Evaporation and Interception Water Loss from Juniper Communities on the Edwards Aquifer Recharge Area. Report to the Upper Guadalupe River Authority, Texas A&M University System. Pomeroy, Christine (2009). BMP and LID Whole Life Cost Models: Version 2.0, Water Environment Research Foundation Project SW2R08, Alexandria, VA. Smith, J. M. (2001). Extreme Rainfall and Flooding From Supercell Thunderstorms. J. Hydromet., 2: 469-489. TRB (2004). NCHRP Report 474: Assessing the impacts of bridge deck runoff contaminants in receiving waters, Vol. 2: Practitioner’s Hand- book. Transportation Research Board of the National Academies, Washington, DC. URS Corporation (2010a). Stormwater runoff from Bridges. Final Report to Joint Legislation Transportation Oversight Committee, North Carolina Department of Transportation – Raleigh. URS Corporation (2010b). Targeted Aggressive Street Sweeping Pilot Pro- gram: Phase III Median Sweeping Study, prepared for the City of San Diego Stormwater Department, San Diego, CA.