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Suggested Citation:"Chapter 6 - Conceptual BMP Designs." National Academies of Sciences, Engineering, and Medicine. 2014. Measuring and Removing Dissolved Metals from Stormwater in Highly Urbanized Areas. Washington, DC: The National Academies Press. doi: 10.17226/22389.
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Suggested Citation:"Chapter 6 - Conceptual BMP Designs." National Academies of Sciences, Engineering, and Medicine. 2014. Measuring and Removing Dissolved Metals from Stormwater in Highly Urbanized Areas. Washington, DC: The National Academies Press. doi: 10.17226/22389.
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Suggested Citation:"Chapter 6 - Conceptual BMP Designs." National Academies of Sciences, Engineering, and Medicine. 2014. Measuring and Removing Dissolved Metals from Stormwater in Highly Urbanized Areas. Washington, DC: The National Academies Press. doi: 10.17226/22389.
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Page 86
Suggested Citation:"Chapter 6 - Conceptual BMP Designs." National Academies of Sciences, Engineering, and Medicine. 2014. Measuring and Removing Dissolved Metals from Stormwater in Highly Urbanized Areas. Washington, DC: The National Academies Press. doi: 10.17226/22389.
×
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Page 87
Suggested Citation:"Chapter 6 - Conceptual BMP Designs." National Academies of Sciences, Engineering, and Medicine. 2014. Measuring and Removing Dissolved Metals from Stormwater in Highly Urbanized Areas. Washington, DC: The National Academies Press. doi: 10.17226/22389.
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Page 88
Suggested Citation:"Chapter 6 - Conceptual BMP Designs." National Academies of Sciences, Engineering, and Medicine. 2014. Measuring and Removing Dissolved Metals from Stormwater in Highly Urbanized Areas. Washington, DC: The National Academies Press. doi: 10.17226/22389.
×
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Page 89
Suggested Citation:"Chapter 6 - Conceptual BMP Designs." National Academies of Sciences, Engineering, and Medicine. 2014. Measuring and Removing Dissolved Metals from Stormwater in Highly Urbanized Areas. Washington, DC: The National Academies Press. doi: 10.17226/22389.
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83 6.1 Basis of BMP Design Concepts Design of treatment BMPs for highways has received con- siderable investigation for the past 25 years. Accordingly, there exist designs that are relatively proven in operation and can be constructed and maintained with common methods and equipment. Elements of these proven designs were used in the development of the concepts for the GFO media BMP. For example, the Austin Sand Filter has been designed and proven to operate passively in a highway environment and provide good particle removal with a reasonable maintenance interval. The proposed GFO BMP will operate using a similar horizon- tal bed configuration with a pretreatment chamber. The prototype designs can operate passively and with deferred maintenance in the highway environment. The basic design constraints used to guide the development of the con- cept configurations (Caltrans 2004) included: • Obstructions (above-ground obstacles and slopes greater than 4:1) within the clear recovery zone (30 feet from edge of traveled way) were avoided. • Maintenance access with adequate room for maintenance vehicles and equipment must be available outside of the traveled way and shoulder areas. • Passive operation required (no mechanical or powered components). • Inspection is non-destructive, requiring minimum time and training of personnel. • Maintenance intervals are reasonable (actual interval to be refined during prototype testing). • The prototype design is compatible with typical highway drainage systems and requires a minimum amount of phys- ical space. Pretreatment of influent will be highly desirable to remove solids prior to introducing runoff to the GFO media. The typical suspended solids concentration in highway runoff nationally is 132 mg/l (Granato and Cazenas 2009). Pretreat- ment will extend the life of the filter media by reducing par- ticles that would otherwise occlude the system. The prototype designs include a basic pretreatment layer to protect the GFO media but treatment prior to the BMP remains highly recom- mended. Various pretreatment strategies will be discussed. Three configurations have been developed to accommodate the majority of the design scenarios that DOTs will need: highly urban areas, more rural locations with vegetated shoulders or embankments up to a 2:1 slope ratio, and on bridges. The urban area design was developed for highway cross sections that are largely impervious within the ROW. These locations will typi- cally include a paved shoulder between the edge of traveled way and the curb, and pavement between the curb and the ROW. In the urban configuration, runoff is conveyed along the curb to a drain inlet that discharges to a longitudinal drainage system. The drainage system will typically discharge to a cross culvert (transverse to the highway centerline) at a location joining the adjacent municipal storm drain system. A below-grade vault design has been developed for urban locations that are com- patible with the longitudinal drainage system, to preserve sur- face use, and avoid above-ground obstructions. The second design was developed for locations with vege- tated shoulders, with embankments up to a 2:1 slope ratio. Vege- tated shoulders may also have a curbed section and a longitudinal drainage system, but in many cases the paved shoulder abuts a vegetated buffer strip that discharges to a longitudinal vegetated swale. The advantage of this type of design is that runoff may be intercepted in a sheet flow condition and vegetated strips are effective for pretreatment to remove solids in runoff. The proto- type design is configured to work either with a vegetated swale system or with the media filter drain developed by Washington State DOT (WSDOT). The media filter drain (MFD) is con- structed along the edge of shoulder usually following a gravel then vegetated strip, accepting sheet flow from the pavement surface. Both the swale and the MFD designs have the advan- tage of a relatively low hydraulic loading rate with an expected C H A P T E R 6 Conceptual BMP Designs

84 advantage in an extended maintenance interval. The sub-drain system from the MFD is connected to the GFO vault for treat- ment. For the vegetated swale configuration, a drain inlet or culvert entrance is used to connect to the treatment vault. The vault spacing is generally dictated by cross-culvert locations to discharge treated effluent. A third configuration has been developed for bridge decks. There may be some instances where dissolved metals removal from bridge deck runoff is desirable. Piping water to the bridge abutment is costly and introduces aesthetic and main- tenance issues. The conceptual design integrates the media into the bridge scupper to treat bridge deck runoff. 6.1.1 Site-Specific Constraints and Design Considerations Each of the three BMP concept configurations has been developed to provide the engineer with flexibility to integrate the units into existing and new highway drainage systems. General considerations for prototype design will include pre- treatment of runoff, pH buffering, physical retention of the media, hydraulic design, and system maintenance. 6.1.1.1 General Constraints and Design Considerations—Pretreatment • Pretreatment of runoff will be desirable upstream of the GFO media to extend the maintenance interval of the unit. The unit is configured as a vault for the urban and rural applications for the reasons discussed earlier, and to mini- mize the amount of media used since it is relatively expen- sive. Pretreatment strategies will reflect the configuration of the highway cross section, whether a longitudinal storm drain system is used, and the availability of ROW. • One of the most flexible pretreatment approaches will be the use of a permeable friction course (PFC) overlay. Eck et al. (2012) have demonstrated that PFC overlays can reduce TSS concentrations in highway runoff to values less than 10 mg/l. PFC has the advantage that it can be placed as an overlay on most pavement sections and is compatible with either new construction or retrofit. PFC also has ancillary advan- tages such as reduced noise, improved visibility during rain events, and improved vehicle stopping. If the use of PFC is not desirable or feasible, pretreatment may be omitted if TSS concentration is relatively low (less than 100 mg/L). Alterna- tively, a pretreatment vault for TSS removal could be used. • Vegetated filter strips may also be a suitable pretreatment strategy where discharge will be collected in open swales prior to treatment or as a part of a system using the WSDOT MFD. NCHRP Report 565 notes that vegetated filter strips can routinely reduce TSS in typical highway runoff to less than 60 mg/l and routinely below 30 mg/l (third quartile). Vegetated strips may only be used in the absence of a curb and gutter system where runoff sheet flows from the paved shoulder. • An alternative to filter strips is the MFD developed by the WSDOT. The MFD has a proven track record for both operation and maintenance and is documented to routinely reduce TSS from typical highway values to less than 20 mg/l (WSDOT 2006). The ‘rural’ area configuration for the con- cept BMP design was developed to be compatible with the MFD design, with vaults located at intervals governed by the desired maximum inflow along the system. The MFD sys- tem has the additional advantage of reducing runoff volume through exfiltration from the system. WSDOT limits the use of the MFD to 3:1 slopes. Prototype testing should include a configuration with the MFD on a 2:1 slope. Alternatively, flow can either be collected by a vegetated swale at the base of the slope, or collected and conveyed in a storm drain sys- tem to the treatment vault (with PFC as pretreatment). • In highly urbanized areas there may not be sufficient space to install filter strips or MFDs and PFC may not be an option acceptable to the DOT. In these cases, the DOT may wish to consider a proprietary, below-grade device for pre- treatment. The most effective of these devices for reducing sediment loads include a filtration process, either using a granular media or membrane. • The conceptual BMP designs include a layer of sand and crushed concrete to cover the GFO media. The purpose of this layer is three-fold: first, the unit weight and the specific gravity of the media is less than water (40 lbs/cf vs. 100 lbs/cf) so the sand/crushed concrete mix is used to dissipate energy of the influent and prevent the GFO from becoming fluidized and/or experiencing media loss. The sand/crushed concrete layer may also modestly increase the pH of acidic runoff to enhance the performance of the media where acid rain conditions are present. The efficacy of this approach can be validated during prototype test- ing. Finally, the sand/crushed concrete layer also serves as a filter to reduce TSS loading to the media to ensure that it does not fail through occlusion prior to the laboratory estimated breakthrough volume for dissolved metals. Pretreatment for some installations may be optional, if sol- ids loading is expected to be relatively low or if a more frequent ‘minor’ maintenance of the sand layer is acceptable. 6.1.1.2 General Constraints and Design Considerations—Runoff Interception and Overflow The conceptual designs for urban and rural areas were developed to accept runoff from a closed conduit system. Highway runoff that is collected in a longitudinal drainage

85 system (storm drain, or vegetated open channel) is introduced to the treatment vault through a flow splitter. The purpose of the flow splitter is to maintain a constant head on the fil- ter media in the treatment vault, bypassing flows that would exceed the hydraulic capacity of the unit. The flow splitter is not necessary in the configuration using a MFD for collection. A conceptual design has not been provided for the flow splitter. There are various public domain systems available that have been developed to facilitate the off-line operation of end of pipe BMPs. The practitioner is free to use an appropri- ate splitter design consistent with local practices and prefer- ences. The splitter should discharge to the site storm drain system for bypass level flows, and to the treatment vault for flows at or below the hydraulic capacity of the media. Care should be taken to ensure that the treatment vault does not surcharge during high flow events. This can be accomplished by designing a splitter system that has a maximum operating water surface for the range of expected inflow. 6.1.1.3 General Constraints and Design Considerations—Maintenance and Estimated Service Life Routine and major maintenance of the treatment vault will be required depending on the volume of inflow, effectiveness or presence of pretreatment, the TSS loading, and the dis- solved metal loading. Routine maintenance tasks will include the following: • Check operation of the flow splitter to ensure the diver- sion conduit and overflow weir are free of obstructions or solids buildup. • Check the sand/crushed concrete filter layer for excessive solids buildup/occlusion. Occlusion is indicated when there is little to no effluent from the vault outlet pipe, indi- cating that most flow is bypassing. • Remove trash/debris from the sedimentation chamber or area above the sand/crushed concrete layer for bridge scuppers. Major maintenance tasks will include the following: • Replace the sand/crushed concrete layer and top layer of geotextile • Replace the GFO media and filter fabric. 6.1.1.4 Configuration #1: Urban Highways The first configuration was developed for urban highway cross sections with a curb and gutter section served by a lon- gitudinal drainage system. The media is held in a cast-in- place vault in a horizontal bed. Flow enters from the top of the media from a weir downstream of a debris chamber and exits through an underdrain below the media bed. The vault is intended to be placed near a discharge point for the high- way longitudinal drainage system, such as at a cross culvert. However, the vaults may be placed at any location along the drainage system and the size of the filter bed is based on the desired flow rate. Figure 6-1 shows the basic configuration from a section view of the urban vault system. The vault must be preceded by a splitter structure to ensure a constant head in the unit during operation. Flow enters the debris/sedimentation chamber in the treatment vault and ponds until it exceeds the height of the filter chamber inlet weir. The purpose of the gravel infiltration area is to elimi- nate standing water between storm events in the debris cham- ber. At the option of the designer, a concrete invert may be substituted, but standing water in the unit will result, and treatment for vectors in the debris chamber may be required. Influent then flows onto the crushed concrete/sand mixture. The concrete should be the same size as the sand and conform to the ASTM C-33 standards for fine aggregate. The crushed/ concrete sand mixture is underlain by a woven filter fabric act- ing as a separation barrier between the crushed concrete/sand mixture and the filter media. The filter fabric should have an apparent opening size of 50 US sieve or smaller, and a flow rate of at least 10 gal/min/sf. Prototype testing should verify the use of a filter fabric as performance has been poor in some biofiltration designs due to clogging. The pretreatment speci- fied for the GFO should reduce this potential, but field studies are required for verification of the final design configuration. 6.1.1.5 Configuration #2: Rural Highways Figure 6-2 shows the vault configuration for rural highways. The configuration is very similar to an urban area, except that the vault is designed to accept input from the WSDOT MFD system or an open swale system transitioned to a closed conduit either through a culvert entrance or a drop inlet. Similar to the urban area, the vault is placed at intervals coincident with cross culvert or other discharge locations for the MFD. Flow into the vault can be limited by an orifice plate at the vault entrance consistent with the capacity of the filter bed. A splitter structure is not needed with the MFD since the sys- tem will not accept discharges that exceed the capacity of the integral sub-drain system. The rural vault configuration can also be used with an open vegetated swale system. Flow from the open system can be directed to the vault via a short length of pipe. A diversion structure must be designed if an open ditch system is used to divert flow that exceeds the capacity of the vault. The vault will generally be placed at the base of the roadway embankment. Embankment slopes of any ratio can be accommodated, since the vault is designed to collect flow from a standard longitudinal drainage system (either open or

Figure 6-1. Urban vault system. Figure 6-2. Rural vault configuration.

87 closed conduit). The GFO treatment vault used downstream of a WSDOT MFD is viewed as a ‘polishing’ step to further reduce the dissolved fraction of metals that are untreated by the MFD. 6.1.1.6 Configuration #3: Bridge Decks The third configuration is designed for bridge deck drain- age systems. The media is housed in a modified bridge scup- per to serve a designated portion of the bridge deck area. Figure 6-3 shows a section of the bridge scupper. The scupper is designed for interception of the ‘water quality’ design flow only, and must be followed downgrade by a scupper designed to intercept the remaining drainage flow to maintain flooded width criteria. The scupper can be fabricated from concrete or steel, or cast integrally with the bridge deck. 6.1.2 Design Procedure The design procedure for the vault or inlet scupper is based on laboratory column tests. A spreadsheet has been developed to assist in sizing the filter bed in the vaults and the bridge deck inlet scupper. The media depth is fixed at a minimum value (10 inches) in the spreadsheet, to achieve a minimum contact time with the media consistent with that obtained during the column testing (3 minutes). Thicker media depths may be used, but the required head should be computed using Darcy’s Law. The permeability of the media (K) was determined to be 0.24 cm/s. Caution should be used in developing designs with head requirements that are rela- tively large since the effective solids loading rate of the media will be higher, resulting in shorter runs between maintenance intervals due to possible media occlusion. User input cells on the spreadsheet are shaded in blue. Out- put cells are shaded in green. The user inputs the area of the tributary highway cross section, the trial vault dimensions, and the spreadsheet computes an estimated vault spacing. Hydrau- lic sizing computations in the spreadsheet are based on a media loading rate of 2 gpm/ft2, a media thickness of 10 inches, and a required head of 8 inches. The media breakthrough time is estimated based on the lab- oratory testing. At a pH 7, the estimated log Kd for the system was determined to be 5. Using the linear isotherm equation: qe = KdCe and a value of 3 µg/L for Ce and a Kd of 105 L/kg, yields a required adsorption capacity of 300 mg removed/kg of GFO. For example, assuming 30 in./yr of rainfall over a one Figure 6-3. Inlet scupper with media.

88 acre drainage area yields a volume of stormwater runoff of approximately 3,000 m3 per year. For an influent copper con- centration in the stormwater runoff of 10 µg/L and desired effluent concentration of 3 µg/L, the required mass of copper adsorbed per year is approximately 22 g. Dividing the copper adsorbed per year by 300 mg removed/kg of GFO yields a GFO demand of 72 kg/yr. The unit weight of GFO was mea- sured as 40 lb/ft3 or 640.74 kg/m3. The spreadsheet also provides capital and O&M and whole life cost estimates using user provided unit prices. The unit cost of GFO provided in the spreadsheet as a default is $15/lb. This cost is likely to decrease for GFO purchased in bulk quan- tity, and represents the cost of the material obtained for the laboratory trials. The user can overwrite the default unit prices for GFO, poured in place concrete, and the cost to remove and dispose of spent filter media on a per vault basis. 6.1.2.1 General Sizing and Hydraulic Loading The laboratory testing was completed using columns with a cross sectional area of less than a square centimeter. Con- sequently, the hydraulic sizing procedure provided in the spreadsheet must be validated during prototype testing. It is likely that the field permeability of the media will vary from that tested in the laboratory, and the permeability will change over time as solids accumulate. The crushed concrete/sand filter layer is used to reduce the solids loading to the media and extend the media life. Therefore, the anticipated change in permeability for the media over the operating life of the filter should be relatively small. The crushed concrete/sand layer and geotextile must be removed and replaced whenever the head requirements for the design flow become unacceptably high. Pretreatment of flow is recommended to maximize the maintenance interval of the top layer and geotextile fabric. Oversizing the vault will also increase the maintenance interval, but the cost of the GFO media likely makes this option less desirable than a more effective pretreatment system. In summary, the prototype test- ing program should assess a range of filter bed sizes based on the variables of solids loading, copper loading, flow, media thickness and contact time, hydraulic loading rate, and time to breakthrough. The initial values for these parameters in the spreadsheet tool that can also be used to develop the prototype designs (measured data where appropriate is preferable) are: • Solids loading: 100 mg/l • Copper loading: 10 µg/l • Media thickness: 10 inches • Media contact time: 3 minutes • Hydraulic loading rage: 2 gpm/ft2 (based on laboratory testing) • Adsorption: 300 mg (copper in influent)/kg (GFO media) The study team found that there are a variety of grain sizes commercially available for GFO. The study team recom- mends that the prototype testing evaluate the efficiency of the media for metals removal as well as the time to breakthrough and hydraulic performance. 6.1.2.2 Regional Considerations for General Sizing Criteria Locations with high metals loading rates and high average annual rainfall will require larger amounts of media and more frequent maintenance intervals than those areas with lower metals loading and lower annual rainfall. Annual loading rates for TSS and metals will be the most important considerations when sizing the vault or bridge scupper. Retention of metals on the media in areas with acid rain may vary compared to those areas with more pH neutral rainfall. Ultimately, regional testing of the design to assess variability in performance and operation will be desirable. The design is likely optimized by providing storage upstream of the vault to reduce the filter bed size. The design water qual- ity volume or flow rate will be dictated in most cases by state criteria. In absence of a required flow or volume design stan- dard, the guidelines published in the ASCE Manual of Practice No. 87, Design of Urban Stormwater Controls, is recommended. This publication provides a method to compute a localized treatment volume based on a maximized detention volume concept to capture the average annual volume whereby beyond this value relatively larger increases in incremental storage vol- ume are required for diminishing increases in average annual percent capture. This type of procedure is recommended as one approach to normalize BMP sizing based on local rainfall and approximate a maximum extent practicable (MEP) standard. 6.1.3 Description of Construction Details of Preferred Concept Alternatives The prototype design was developed to be simple to con- struct using locally available components such as standard manhole rings and covers. Cast-in-place concrete is recom- mended to allow vaults and bridge scuppers of various sizes to be used, though precast components are a viable option. Struc- tural design may be optimized from that shown based on soil unit weight and expected lateral earth pressure and location of the water table for the design depth though a single standard will likely be sufficient for the vast majority of installations. Manhole access is recommended to be 36″ without a taper to allow extra room for vactor operations to remove spent media and place new media. Confined space procedures will be required. The perforated outflow pipe should be connected to a receiving storm drain system for discharge. Access should be provided at the connection location, and the length of the

89 discharge pipe should be limited to about 100 feet without intermediate access. 6.1.3.1 Configurations #1 and #2: Urban and Rural Highways The concept prototype design shown in Figure 6-1 accepts flow from a splitter structure and introduces it to a storage chamber. The primary purpose of the storage chamber is to reduce the size of the filter bed. The dimensions of this chamber can be optimized based on the estimated mainte- nance interval of the filter bed using the spreadsheet tool. The storage can also be varied to meet physical constraints of the longitudinal drainage system such as available discharge loca- tions. The structure should be designed with the minimum depth possible for ease of construction. The dimensions of the filter chamber will be based on the hydraulic loading rate. The chamber may be constructed to any dimension, though in the concept prototype design, the width has been shown as 3.5 feet to be compatible with the manhole access and limit the sub-drain to a single outlet pipe. Wider filter chambers would require multiple outlet pipes and a manifold. The designer may want to consider sloping the filter chamber invert slab for designs that become longer or wider than about 5 feet to improve drainage of the media. The outlet pipe should rest on the invert slab with perforations along the entire circum- ference. The outlet pipe should not be raised off of the invert, since positive drainage is needed to completely drain the filter chamber of the vault. 6.1.3.2 Configuration #3: Bridge Decks The scupper prototype configuration will be cast integrally with the bridge deck for concrete box girder and slab designs. Other design configurations will be required for steel bridges. Solids loading on bridge decks is generally consistent with at grade roadways. Due to the relatively confined dimensions of the scupper inlet, pretreatment for solids removal on a bridge deck would be a prerequisite. To reduce solids loading to the bridge scupper, a PFC overlay is recommended. As previously indicated, PFC overlays have been shown to be effective in reducing TSS in highway runoff. A PFC overlay can be used on a bridge deck to achieve solids reduction. Specific design will be based on local conditions, but the overlay should probably be discontinued at a distance from the bridge rail- ing coincident with the edge of the scupper inlet to allow the flow within the overlay to collect in an effective ‘gutter’ area. It is also recommended that a second scupper inlet is provided downstream of the treatment inlet in the event that the treat- ment inlet becomes blocked with solids.

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 767: Measuring and Removing Dissolved Metals from Stormwater in Highly Urbanized Areas presents prototype best management practices (BMPs) for the removal of dissolved metals in stormwater runoff.

The report presents three conceptual configurations in detail: two vault system configurations for urban and rural settings, and an inlet scupper with media for bridge deck drainage systems.

The report also includes standard protocols to accurately measure the levels of dissolved metals in stormwater. Practical guidance on the use of these protocols is provided in an appendix to the final report. The report is accompanied by an Excel spreadsheet on CD designed to assist in sizing the filter bed in the vaults and the bridge deck inlet scupper.

The CD is also available for download from TRB’s website as an ISO image. Links to the ISO image and instructions for burning a CD from an ISO image are provided below.

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CD Disclaimer - This software is offered as is, without warranty or promise of support of any kind either expressed or implied. Under no circumstance will the National Academy of Sciences or the Transportation Research Board (collectively "TRB") be liable for any loss or damage caused by the installation or operation of this product. TRB makes no representation or warranty of any kind, expressed or implied, in fact or in law, including without limitation, the warranty of merchantability or the warranty of fitness for a particular purpose, and shall not in any case be liable for any consequential or special damages.

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