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Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual (2015)

Chapter: Chapter 4 - Volume Reduction Approaches

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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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Suggested Citation:"Chapter 4 - Volume Reduction Approaches." National Academies of Sciences, Engineering, and Medicine. 2015. Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual. Washington, DC: The National Academies Press. doi: 10.17226/22170.
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49 C H A P T E R 4 This chapter describes a focused menu of primary VRAs that has been developed to provide the user with options for achieving surface runoff volume reduction in the urban highway envi- ronment. Section 4.1 begins with identification of stormwater control measures (SCMs) that are currently found in stormwater guidance and literature that also have volume reduction benefits. This section also introduces recent and ongoing research and pilot projects as part of identifying emerging concepts for volume control. From these sources, the menu of primary VRAs is iden- tified (Section 4.2). Appendix A provides fact sheets for each of the primary VRAs identified in this chapter. These fact sheets provide VRA-specific information about applicability, siting and selection considerations, and conceptual design parameters. Section 4.3 summarizes informa- tion contained in the VRA fact sheets as well as the supporting appendices to provide a relative comparison and overview of VRA attributes and considerations. Section 4.4 provides additional references for VRA design and maintenance information. 4.1 Identification of Potential VRAs from Stormwater Guidance and Literature While the urban highway environment presents unique challenges for achieving volume reduction, experience with SCMs in the highway environment as well as other land uses pro- vides a basis for beginning to compile a menu of potential volume reduction approaches. SCMs with volume reduction benefits have been applied to highways in many jurisdictions (Oregon State University et al., 2006; Geosyntec Consultants et al., 2011), and some have been monitored extensively (www.BMPdatabase.org). Additionally, SCMs based fully or partly on volume reduc- tion processes have been applied in municipal rights-of-way and other municipal land uses for many years; some aspects of these experiences are relevant to the urban highway environment. This section is intended to help the user answer the following questions: • What volume reduction approaches have been applied in the urban highway environment? • How are the volume reduction approaches that have been applied to other land uses applicable to the urban highway environment? 4.1.1 Inventory of SCMs in Highway Guidance and Literature Previous nationwide research by Oregon State University et al. (2006), Geosyntec Consultants et al. (2011), and Strecker et al. (2005) identified a broad menu of SCMs that are applicable to the highway environment and identified the unit operations and processes associated with these SCMs, including volume reduction processes. Table 5 summarizes the SCMs referenced by these documents, identifies whether volume reduction is provided by each SCM, and provides a syn- thesis of the applicability of SCMs in the urban highway environment based on review of these literature sources. Volume Reduction Approaches

50 Volume Reduction of Highway Runoff in Urban Areas The International BMP Database (www.BMPdatabase.org) included 533 studies as of March 2013, of which more than 25% (142 studies) were conducted in the highway environ- ment. Table 6 and Figure 7 provide a summary of the studies contained in the International BMP database that relate to the highway environment. Among these studies, biofilter strips and swales, bioretention, detention basins, and porous pavements have potentially signifi- cant volume reduction benefits. This inventory provides an indication of systems for which empirical performance data exist, although the BMP database does not necessarily represent the relative distribution of SCMs currently in use nor does it necessarily represent the trends in future uses of SCMs. SCM Relative Volume Reduction Potential Potential for Consideration as an Urban Highway VRA Bioretention without underdrains Yes Bioretention with underdrains Yes, unless lined with impermeable barrier Bioslopes/filter strips Yes Capture and use/rainwater harvesting Possible, limited demand for harvested required would limit extent of applicability Catch basin controls N/A No volume reduction benefit Compost/media filters Yes, unless lined with impermeable barrier Constructed wetlands Not typically designed for volume reduction Detention basins/extended detention basins Yes, based on incidental volume reduction benefits Dry wells Possible, risk to groundwater quality maylimit applicability Green roofs No, very limited applicability Gutter filters N/A No volume reduction benefit Hydrodynamic and gross solids removal devices N/A No volume reduction benefit Infiltration basins Yes Infiltration galleries/tanks/vaults Yes Infiltration trenches/strips Yes Oil–water separators N/A No volume reduction benefit Permeable overlays Volume reduction benefit not demonstrated; likely very minor Permeable pavement shoulders Yes Permeable pavement travel lanes Potential, with adequate structural design Pollution prevention/street sweepings N/A No volume reduction benefit Proprietary disinfection, ultraviolet disinfection, flocculation, package plants, etc. N/A No volume reduction benefit Sand filters / Incidental volume loss if unlined/Negligiblevolume reduction if lined Soil amendments in landscaped areas Yes Subsurface flow wetlands Not typically designed for volume reduction Vegetated swales Yes Wet basin/retention ponds Not typically designed for volume reduction Wet vaults N/A Not typically designed for volume reduction : Volume reduction is primary removal process : Significant incidental volume reduction expected; in some cases, may achieve high volume reduction where conditions are favorable; these VRAs also provide treatment processes for conditions when volume reduction capacity is exceeded. : Potential for minor volume reduction in some conditions; primary reliance on other treatment processes for pollutant load reduction N/A: No volume reduction processes present. Table 5. Inventory of SCMs identified by in previous nationwide stormwater guidelines.

Volume Reduction Approaches 51 4.1.2 Relative Frequency of Application of SCMs by State DOTs In a survey of DOT representatives conducted in 2012 (Venner et al., 2013), respondents from all 50 states plus Puerto Rico and the District of Columbia provided rankings of the relative frequency of various SCM types. Table 7 summarizes the findings of this survey and indicates which SCMs provide volume reduction. In descending order, the five most common BMP types are vegetated swales, rock swales, roadside filter strips, dry detention basins, and wet ponds/retention basins. The top 10 are rounded out by infiltration basins and trenches, compost-amended slopes, wetland swales/channels, and oil/water/grit separator vaults. Similar to the inventory of studies extracted from the International BMP Database, this inventory does not necessarily forecast what will be used in future projects; however, it provides an indication of the SCMs that have seen extensive application in the highway environment. Note: “Control” refers to DOT sites that have been monitored without BMPs as control sites. Table 6. Transportation-related study sites in the International BMP Database. Figure 7. Distribution of DOT studies in the International BMP Database (March 2013).

52 Volume Reduction of Highway Runoff in Urban Areas 4.1.3 Recent Research and Emerging Concepts in Volume Reduction Approaches for Urban Highways This section highlights several areas of recent research related to volume reduction and vol- ume reduction approaches to identify new and emerging concepts that may be applicable in achieving volume reduction. International BMP Database Volume Reduction Technical Report The International BMP Database contains over 530 BMP studies, a portion of which include data that relate to the volumetric performance of stormwater controls. An analysis of the volumet- ric data contained in the BMP database can help provide a better understanding of the benefits that BMPs can be expected to provide for volume reduction. Table 8 summarizes category-level analysis of studies in the BMP database to evaluate relative magnitudes of volume reduction performance. The BMP categories considered in this analysis appear to exhibit substantial volume reduction potential. Variability in study performance is relatively high, likely indicative of variability in site conditions, BMP design, monitoring design, climate, and other factors. Nonetheless, this analysis suggests that volume reduction is appreciable in many cases, even when systems are not designed specifically for volume reduction or include underdrains. A 2012 addendum to this analysis evaluated volume reduction performance of bioretention studies in greater detail, including factorial analysis of bioretention performance as a function SCM Name Used in Venner 2013 Survey Frequency Score1 Volume Reduction Provided?2 SCMs used “frequently” and “sometimes” by state DOTs Vegetated swale 86 Rock swale 52 Filter strip 47 Dry detention basin 44 Wet pond/retention basin 31 Infiltration basin 16 Infiltration trench 11 Compost-amended slope 10 Wetland swale/channel 10 Bioretention/rain garden 5 / Wetland basin 3 SCMs used “rarely” and “never,” according to state DOTs Hydrodynamic device -4 N/A Permeable shoulders or parking -7 Catch basin insert -9 N/A Sand filter -10 Permeable (open-graded) friction course overlay for water quality -13 Underground detention vault -13 Bioslope/ecology embankment/filter strip with soil amendment/media filter drain -19 Underground infiltration vault -26 Dry well (class V injection well) -26 MCTT – multi-chamber treatment train (e.g., with tube settlers) -31 N/A Batch detention (real-time automated outlet) -33 1 - Positive numerical factors were tabulated for “frequent” and “sometimes” responses, and negative numerical factors were tabulated for “rarely” and “never” responses. 2 – Ranking based on closest match to inventory of SCMs provided in Table 5. : Volume reduction is primary removal process; : Significant incidental volume reduction expected; other unit processes also provided; : Potential for minor volume reduction in some conditions; N/A: No volume reduction processes present. Table 7. Relative frequency of use of SCMs by state DOTs.

Volume Reduction Approaches 53 of precipitation event size and facility design parameters. Table 9 provides a summary of bio- retention volume reduction performance, including studies divided into those without and with underdrains. This analysis suggests that site factors and design parameters play an important role in volume reduction performance, but that, on average, bioretention tends to achieve high levels of volume reduction. Specifically, it appears that an internal water storage (IWS) zone in bioretention systems with underdrains, created by an elevated outlet from the underdrain sys- tem, shows potential benefits for volume reduction performance. The IWS/elevated underdrain concept can also be incorporated into other VRA designs such as permeable shoulders. Increased Bioretention Usage in Urban DOT Projects Anecdotal evidence suggests that bioretention has perhaps seen the most significant upward trend in terms of usage in highway environments in recent years. For example, in a recent survey conducted by Venner Consulting, Maryland State Highway Administration responded that it is installing 80 permanent BMPs, mostly rain gardens, in one interchange. Two recent urban highway projects constructed by the Oregon DOT (ODOT) in Portland have incor- porated bioretention cells, in one case adjacent to a bridge abutment, and in another within an interchange down gradient of a bridge reconstruction project (Figure 8). A recent project completed by DDOT involved the installation of three bioretention areas within an inter- change (Figure 9). In North Carolina, Luell et al. (2011) evaluated the hydrologic performance of bioretention areas that address bridge deck runoff (Figure 10). This research is relevant to urban highways since it evaluated the performance of undersized systems, similar to those that may be encoun- tered in space-constrained urban conditions. While volume reduction performance declined with smaller size, the decline in performance was non-linear, such that sacrifices in performance were less than the reduction in size. This suggests that substantial benefits can be achieved even when the full sizing criteria cannot be provided. BMP Database Category No. of Monitoring Studies 25th Percentile Median 75th Percentile Average Biofilter – grass strips 16 18% 34% 54% 38% Biofilter – grass swales 13 35% 42% 65% 48% Bioretention (with underdrains) 7 45% 57% 74% 61% Detention basins – surface, grass lined 11 26% 33% 43% 33% Relative volume reduction = (study total inflow volume – study total outflow volume)/(study total inflow volume). Source: Poresky et al., 2011. Table 8. Study total relative percent volume reductions. Analysis Group No. of Studies 25th Percentile Median 75th Percentile Average All studies 20 42% 66% 98% 66% No underdrains 6 85% 99% 100% 89% With underdrains 14 33% 52% 73% 56% Source: Poresky et al., 2011. Notes: Studies with underdrains include systems with elevated underdrains and internal water storage. Analysis included additional data from seven additional studies of bioretention with underdrains that were not available in 2011; therefore, results are not expected to be the same as those shown in Table 8. Table 9. Relative volume reduction statistics for bioretention studies.

54 Volume Reduction of Highway Runoff in Urban Areas Figure 8. Examples of ODOT bioretention installations. Left: Highway 99E Viaduct, Portland, OR; Right: I-5 Exit 298, Portland, OR. Source: Geosyntec Consultants. Figure 9. Design plans and constructed bioretention retrofit in DDOT interchange. Source: Geosyntec Consultants et al., 2011. Adaptation of Bioretention and Other Stormwater Controls for Low-Permeability Environments Various recent research has been conducted related to the performance of bioretention in mar- ginal soil conditions. Selbig and Balster (2010) studied rain gardens in clay soils in Wisconsin and found that a high level of volume reduction (nearly 100%) was achievable with appropriate siz- ing factors and a shallow design profile commensurate to site infiltration rates. Brown and Hunt (2011) reported on various experience using an internal water storage underdrain configuration (Figure 11) to enhance volume reduction in areas where soils were not adequately permeable to allow the installation of bioretention without underdrains. Underdrains can be configured in various ways to provide a storage zone while providing a supplemental pathway for water to discharge when infiltration capacity is exceeded. Recent guidance in California (Orange County Public Works, 2011; Ventura County Watershed Protection Department, 2011) has identified

Volume Reduction Approaches 55 bioretention with internal water storage (i.e., elevated underdrain) as a preferred approach for maximizing volume reduction when infiltration rates are measureable but are not adequate to support a full infiltration design without underdrains. A similar concept is applicable to perme- able pavements, including permeable shoulders. Advancement in Guidance for Washington State DOT Media Filter Drain Media filter drains (formerly referred to as an “ecology embankments”) were originally pio- neered and demonstrated by Washington State DOT (WSDOT). A technology evaluation report prepared for ecology embankments for WSDOT (Herrera Environmental Consultants, 2006) showed both significant volume and load reductions up to, in some cases, 100%. Lessons learned from early applications of this technology led to revisions and refinement of design criteria, including various configurations. The WSDOT Highway Runoff Manual includes detailed design criteria for three configurations (side slope with underdrain, median configuration with under- drain, and side slope application without underdrain), and, at the time of this writing, WSDOT was in the process of developing guidance for additional configurations that was due to be pub- lished in 2014 with updates to WSDOT’s Highway Runoff Manual (Personal communication, Figure 10. Bioretention cell in North Carolina DOT right-of-way researched by Luell et al. (2011). Source: Stacy Luell, Geosyntec Consultants. Figure 11. Schematic of a bioretention cell with an internal water storage layer. Source: Brown, 2009.

56 Volume Reduction of Highway Runoff in Urban Areas Mark Maurer, April 18, 2013). Figure 12 shows an example implementation of a media filter drain, and Figure 13 shows an example schematic cross-section extracted from recent design guidance. Adaptation of Permeable Pavements to High-Volume Roadways Permeable friction course (PFC) overlays are not considered to be VRAs. However, research and development related to PFCs for highway applications has led to additional durability improvements to the permeable pavement wearing surface through the use of additives and higher-performance–grade binders. These additives have the potential to broaden application of permeable surfaces to highways with increased load/traffic demands, including full-depth permeable pavements that can achieve significant volume reduction. Additionally, recent proj- ect experiences and research have established empirical evidence for the potential suitability of Figure 12. Media filter drain along SR 14 in Clark County, WA. Source: Washington State DOT, 2011. Figure 13. Media filter drain design guidance. Source: Washington State DOT, 2011.

Volume Reduction Approaches 57 permeable surfaces within high-volume roadways with heavy loadings (Maine DOT, 2010; see also Appendix F, which is part of NCHRP Web-Only Document 209). In 2009, the Maine DOT constructed the first state DOT permeable-asphalt roadway in the northeast United States. The project includes a full-depth permeable pavement for 1,500 ft of a four-lane highway reconstruction for the Maine Mall Road in South Portland. In 2008 the average annual daily traffic for 2008 was 16,750 vehicles, including heavy truck traffic. Four years after construction, the durability is considered to be exceptional and perme- ability has been adequate, although maintenance (via vacuum sweeping) has been required to address track-on of sediments from adjacent roadways. Key advancements that allowed this project to support heavy loads were high-strength asphalt binders used in the surface wearing course, an asphalt-treated permeable base, and at least 21 in. of structural subbase material (Figure 14). The University of California Pavement Research Center, in cooperation with Caltrans, conducted laboratory and modeling investigations to evaluate the structural and hydraulic performance of permeable pavement as highway shoulders that can be subjected to heavy loads (Wang et al., 2010; Jones et al., 2010; Chai et al., 2012). These studies found that the retrofit of roadways with permeable shoulders is technically feasible and is economically advantageous in freeways when compared to conventional stormwater structure installation (Chai et al., 2012, summarized by Kayhanian, 2012). Further, this research suggested that permeable shoulders could be effective for subgrade permeability of as low as 10-5 cm/s (0.014 in./hr). However, field-scale evaluation was not conducted, and as noted by Kayhanian, “any simulated design must be constructed and tested before implementation in highway or road environ- ments.” Permeable concrete applications are also being advanced through research sponsored by FHWA and others. At the time of this writing, Cackler et al. (2012) were evaluating a full- depth permeable concrete shoulder with photo-catalytic cement binder for water quality as well as air quality benefits in a 46,000 average annual daily traffic urban freeway in St. Louis. This pilot included development of full-depth permeable concrete shoulder designs capable of withstanding highway loadings. Appendix F provides more in-depth discussion about the emerging applicability of permeable pavements as part of achieving volume reduction in the urban highway environment. Figure 14. Structural profile of Maine Mall Road project. Source: Maine DOT, 2010.

58 Volume Reduction of Highway Runoff in Urban Areas Development of Technical Guidance for Retain On-Site Requirements The introduction of “retain on-site” requirements in non-DOT environments—for exam- ple, EISA legislation as well as various MS4 permits (see Section 3.1.1)—has introduced new considerations and challenges into the project design process in these areas. Thus far, these requirements do not apply to highways; however, the technical guidance associated with these requirements is transferrable, in part, to highway environments. Requirements to retain storm- water to the MEP based on “rigorous feasibility analysis” (Orange County Public Works, 2011), or similar language, have led to the development of accompanying technical guidance for project proponents and reviewing agencies. For example, the U.S. EPA (2009) issued Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects Under Section 438 of the Energy Independence and Security Act, which provides guidance on how to compute requirements as well as how to evaluate feasibility. Orange County developed its Tech- nical Guidance Document for Preparation of Water Quality Management Plans, which provides a comprehensive set of criteria as well as a stepwise process for determining the feasible level of stormwater retention that can be provided on-site as well as criteria for meeting requirements in off-site locations (Orange County Public Works, 2011). While these efforts have not specifically focused on urban highways, many of the concepts developed out of these efforts are applicable to identifying feasible volume reduction approaches for reducing urban highway runoff. These efforts include feasibility criteria related to infiltration rates, water balance and groundwater quality, geotechnical issues, harvested-water demand, site planning, and other factors. They also identify the key questions that need to be asked at different phases of project development, from planning through design. For example, as part of project planning, the use of regional feasibil- ity maps (see example in Figure 15) can help identify constraints that may exist in certain areas. Regional maps developed by local jurisdictions can serve as resources for evaluating infiltration feasibility for highway projects. Figure 15. Example screening exhibit identifying mapped contaminant plumes in Orange County. Source: Orange County Public Works, 2011.

Volume Reduction Approaches 59 Research in Active Real-time Controls Most current efforts in the field of stormwater engineering are focused on analyzing and developing designs that passively achieve target goals (e.g., peak attenuation, volume reduc- tion, water balance, pollutant removal targets); however, passive systems rarely represent optimal solutions. Dynamic systems are particularly well suited for complex situations where timing, duration, peak control, volume reduction, use and reuse, or water quality are critically impor- tant. Recent advances in information technology infrastructure as well as hardware systems and software solutions are providing opportunities to achieve higher performance from stormwater controls through real-time, dynamic operation. The Water Environment Research Foundation (WERF) initiated a project in 2011 to evalu- ate the use of highly distributed real-time control technologies for green infrastructure (e.g., advanced rainwater harvesting systems, actively controlled green roofs, wet detention basins, and underdrain bioretention systems) and demonstrate the role that these technolo- gies can play in improving the functions of urban green infrastructure (Water Environment Research Foundation, 2011). As this research shows, active controls provide an opportunity to enhance volume reduction and flow-control performance of traditional controls by operating the outlet of the system based on precipitation forecasts, the status of system storage, or other real-time parameters. Through the use of logic algorithms, passive systems could be operated to provide longer hold times of captured water when new rainfall is not expected, thereby increasing the opportunity for infiltration and ET losses to occur. Real-time controls can also allow volume reduction practices to be operated in more marginal environments (such as those with low soil infiltration rates or sensitive water balance) where the system may need to be adaptively operated to balance volume reduction goals with issues associated with extended periods of standing water (e.g., plant health, vectors) and other factors. These benefits can sometimes be gained with relatively minimal infrastructure investments. Figure 16 shows an example outlet structure retrofit of a dry detention basin to improve volume reduction and flow-control performance. These systems have the potential to change operations, maintenance, and monitoring regimes compared to passive systems. The outlet of the system is intended to be operated autonomously via the real-time data feeds and custom logic established as part of design; therefore, this type of system is not expected to increase operational burden substantially. However, the systems can also be observed by a manager via an online dashboard and operated via manual override if needed. Active feeds from these systems, such as of water levels and periodic photographs, can help determine needs for maintenance without routine site visits. Active feeds can also serve the role of long-term monitoring of system performance, with considerably reduced costs compared to traditional monitoring approaches. Creative Storage and Discharge Approaches The volume reduction performance of SCMs is primarily a function of how much runoff the system can store and the rate at which stored water can be infiltrated, evapotranspired, and/ or used. By distilling the design of VRAs to these fundamental elements, there is potential to improve the volume reduction performance of existing or proposed VRAs through creative design adaptations. Opportunities to increase storage potentially include: • Selecting VRA treatment media with higher porosity, • Using check dams to create ponding, and • Using pore space in the subbase layers below traditional pavement surfaces as a storage reservoir.

60 Volume Reduction of Highway Runoff in Urban Areas Opportunities to increase infiltration discharge rates potentially include: • Piping pretreated discharge to deeper wells below compacted fills, • Directional drilling to convey pretreated runoff to areas with better infiltration, • Breaking up lower-permeability materials using ripping or blasting, and • Using plant palettes with high ET rates (and capable of withstanding drought). In each case, site-specific factors should be considered to ensure that using creative (and potentially unproven) methods does not result in unintended consequences. 4.2 Menu of Volume Reduction Approaches Based on the information presented in Section 4.1, a focused menu of primary VRAs was identified. Section 4.2.1 provides an introduction to each primary VRA while Section 4.2.2 introduces other volume reduction concepts and approaches that can potentially supplement or support the primary menu of VRAs. The VRA fact sheets in Appendix A serve as an in-depth resource for each primary VRA, including volume reduction processes, applicability, safety con- siderations, and conceptual design schematics and parameter guidelines. Finally, Section 4.2.3 Figure 16. Active control outlet structure retrofit of dry detention basin to increase volume reduction and flow-control benefits, Pflugerville, TX. Source: Geosyntec Consultants.

Volume Reduction Approaches 61 introduces site planning approaches that are integral to reducing runoff volumes and providing opportunities to incorporate VRAs. This chapter and the accompanying fact sheets in Appendix A are intended to help the user answer the following questions: • What is the menu of volume reduction approaches available for my project? • What are the system components, key processes, and key design parameters for these approaches? • How are these approaches applicable to the urban highway environment? • How are these approaches applicable across climate zones and watershed characteristics (e.g., soil types, topography, groundwater elevations)? 4.2.1 Primary Volume Reduction Approaches Table 10 provides an introduction to the primary volume reduction approaches that have been identified for the urban highway environment and provides a simple graphical schematic associated with each VRA. Each of these VRAs is supported by a fact sheet that can be found in Appendix A. 4.2.2 Other Potential Volume Reduction Concepts and Approaches Several additional volume reduction concepts and approaches have been identified from guid- ance manuals and literature that may have applicability in some urban highway projects as part of a volume reduction strategy. Harvest and Use and Land Application Harvest and use consists of capturing runoff in storage features and providing a distribu- tion system to use the captured water to help meet non-potable water demands in the project footprint or the project vicinity. Storage can be provided in an open pond or reservoir, an underground tank or vault, or in another storage feature. Demands for harvested water within the urban highway environment may include landscape irrigation, wash water for maintenance vehicles (at maintenance yards), toilet flushing (at rest areas or maintenance yards), or other uses potentially associated with the project type or with adjacent land uses. The performance of harvest and use is strongly dependent on how much demand is present for the water, par- ticularly during the times of year when precipitation or melt occurs; therefore, the applicability of harvest-and-use systems is highly site specific and is likely to be very limited in general for urban highway environments. Land application is a variation of harvest and use that involves capture of water and applica- tion of this water to pervious areas at a rate that exceeds the agronomic demand (i.e., rate at which plants actually use water). Water is generally applied at a rate that is informed by the infiltration rate of the underlying soil such that land application does not result in surface run- off. Because excess water beyond agronomic rates is primarily infiltrated, geotechnical factors associated with infiltration as well as issues related to water balance should be considered in determining whether land application is feasible for a given project. Incidental Volume Reduction in Other SCMs Beyond the VRAs identified in Section 4.2.1, other SCMs may have the potential to achieve some incidental volume reduction even if not designed specifically for this purpose. For exam- ple, recent analysis of the International BMP Database (Poresky et al., 2011) suggests that dry detention basins with permeable bottoms can achieve an average of approximately 30% volume reduction. Volume reduction may also be important in some wetland and wet pond systems that drain down periodically or have permeable side slopes that when flooded absorb water,

62 Volume Reduction of Highway Runoff in Urban Areas VRA 01 – Vegetated Conveyance This category includes engineered vegetated swales and other vegetated drainage features that serve the purpose of conveying stormwater runoff and can also provide significant reduction of stormwater runoff volume. Variations on this approach include an amended soil or stone storage layer to increase storage capacity and promote infiltration. A critical element of this VRA is that it must be designed to sustain robust plant growth so that infiltration rates are maintained and regenerated via root structure, and the conveyance system itself does not contribute to sediment loading from scour. VRA 02 – Dispersion This category consists of the dispersion of runoff toward existing or restored pervious areas for the purpose of reducing stormwater runoff volumes and achieving incidental treatment, and includes road shoulders amended with compost and additional materials such as sand (if needed), designed to convey runoff as sheet flow over the surface or as shallow subsurface flow through amended soil layers. Dispersion reduces overall runoff volume by means of infiltration and evapotranspiration. Volume reduction performance can be improved with the use of flow spreaders, shallow slopes, and soil amendments. A critical element to this VRA is ensuring that dispersion areas support robust vegetative growth to stabilize the surface and maintain good infiltration rates. VRA 03 – Media Filter Drain This VRA consists of a stone no-vegetation zone, a grass strip, a storage reservoir filled with specialized media, and a conveyance system for flows leaving the reservoir. This conveyance system usually consists of a gravel-filled underdrain trench or a layer of crushed surfacing base course. The stone no-vegetation zone produces sheet flow, which is pretreated as it flows across the grass strip, and is then captured by the storage reservoir, where it infiltrates into the subsoil or is discharged through the underdrain. This VRA is typically installed between the road surface and a ditch or other conveyance located downslope. This VRA is based specifically on designs developed and applied by WSDOT. VRA 04 – Permeable Shoulders with Stone Reservoirs This VRA includes use of a permeable pavement surface course (asphalt, concrete, or interlocking pavers) along the shoulders of a roadway, underlain by a stone reservoir. Precipitation falling on the permeable pavement as well as stormwater flowing onto permeable pavement from adjacent travel lanes infiltrates through the permeable pavement top course into the stone reservoir, from which it infiltrates into the subsoil and/or is discharged through an underdrain and outlet control structure. Through the use of an underdrain and flow-control outlet to augment infiltration capacity, permeable shoulders can be applied in a wide range of soil conditions. Table 10. Introduction to primary menu of VRAs.

Volume Reduction Approaches 63 VRA 07 – Infiltration Trench This category of VRA consists of a stone-filled trench that provides subsurface storage of stormwater runoff and allows water to infiltrate through the bottom and walls of the trench into subsoils. Pretreatment for infiltration trenches is commonly provided via a vegetated conveyance such as swales or filter strips. Infiltration trenches tend to be well suited to the linear highway environment as they are generally constructed in a linear configuration and their surface tends to be nearly flush to the existing grade. VRA 08 – Infiltration Basin Infiltration basins are relatively large, shallow basins that discharge water primarily via infiltration. Their contours appear similar to those of detention basins but do not have a surface discharge point below their overflow elevation. Infiltration basins are typically located in relatively permeable soils. Infiltration basins can be designed with detention surcharge above the infiltration volume to provide a combination of volume reduction and peak flow mitigation. Infiltration basins are differentiated from bioretention basins because they are typically built at a larger scale and typically do not include an engineered soil medium. Vegetative cover may also be different. VRA 09 – Infiltration Gallery Underground infiltration systems include a broad class of VRAs that consist of storage reservoirs located below ground preceded by pretreatment systems. Water is pretreated, routed into the systems, and infiltrates into subsoil. A range of potential options are available for providing storage, including use of open-graded stone or a variety of engineered storage chambers (concrete, plastic, or metal). There are also a range of potential locations where underground infiltration systems can be placed, including below parking areas, below access roads, or below travel lanes. VRA 05 – Bioretention Without Underdrains Bioretention consists of a shallow surface ponding area underlain by porous soil media storage reservoirs and an optional porous stone storage layer. Captured runoff is directed to the bioretention area, where it infiltrates into an engineered soil medium and then infiltrates into the subsoil. Engineered soil media is a central element of bioretention design and typically includes a mixture of sand, soils, and/or organic elements that are designed to provide permeability, promote plant growth, and provide treatment. When infiltration is exceeded, water is conveyed to a surface discharge via an overflow riser or via an overland flow pathway. VRA 06 – Bioretention with Underdrains This VRA is similar to VRA 05, but includes an underdrain system to supplement infiltration discharge. Where soil infiltration rates permit, volume reduction can be enhanced by installing a stone reservoir beneath the underdrain discharge elevation. An upturned elbow or outlet structure can be used to create a retention storage zone (i.e., internal water storage zone). This category of VRA is suitable for a wider range of conditions than bioretention without an underdrain and can potentially be used to mimic natural base flows via careful control of discharges from the underdrain. Table 10. (Continued).

64 Volume Reduction of Highway Runoff in Urban Areas media filters with permeable bottoms, and other controls not specifically identified as VRAs. Site-specific design information or monitoring data can be used to help estimate the potential incidental volume losses that may occur. Real-Time Control of Outlets for Enhanced Volume Reduction Performance or Performance Monitoring As introduced in Section 4.1.3, recent advancements and research in distributed real-time controls provide opportunities to improve the volume reduction and flow-control performance of traditional passive systems. For example, the use of active controls on the outlets from bio- retention or other infiltration systems could allow water to be stored and infiltrated for longer periods when additional rainfall is not forecasted, thereby increasing volume reduction com- pared to a passive underdrained systems, but still allow the system to drain via a supplemental outlet when rainfall is forecasted (as determined from an Internet feed) or when the period of inundation (detected by a depth sensor) has exceeded a duration that would be of concern for plant health or vector issues. Another example configuration is retrofitting dry extended deten- tion basins so that water can be retained for longer periods and enhanced infiltration can occur. This requires a relatively minor retrofit of the existing facility outlet structure (see WERF pilot project example in Figures 16 and 17). Real-time control systems can have the added benefit of providing continuous monitoring datasets, which can be used to identify needed maintenance or to evaluate long-term performance. Hydrologically Referenced Discharge to Mimic Natural Hydrology As introduced in Chapter 3, “hydrologically referenced discharge” refers to the controlled release of stored water in such a way that flow rates and timing of discharge mimic the natural hydrologic response, considering surface runoff and elevated base flow, associated with “shallow interflow” that follows rainfall events in some watersheds. DeBusk et al. (2011) hypothesize that the discharge from some stormwater controls can approximate shallow interflow response in North Carolina; it follows that potential exists to adapt discharge rates to local watershed response in other areas. Figure 17. Example schematic of real-time–control bioretention outlet. Source: Geosyntec Consultants.

Volume Reduction Approaches 65 Hydrologically referenced discharge is a recent concept that has not seen extensive applica- tion in addressing volume reduction requirements; however, it may be an important concept for balancing volume reduction goals with physical constraints such as shallow groundwater, low soil infiltration rates, and sensitive water balance conditions. In these cases, infiltration could have negative consequences. Additionally, a system with a controlled release may actually better mimic the natural hydrologic response than a system that only provides infiltration. Hydrologically referenced discharge could be incorporated into any VRA that has an under- drain by providing a flow-control orifice or an active outlet control. A challenging aspect of this approach would be the determination of what range of flow rates would be considered to be equivalent to base flow as part of quantifying which portions of the discharge hydrograph should be considered “volume reduction.” This would require a region-specific, watershed-specific, or site-specific analysis, such as DeBusk’s assessment in North Carolina (Figure 18). Additionally, some discharge of pollutants would be inherent in the treated discharge. 4.2.3 Site Planning Approaches to Reduce Runoff Volume Site planning is a fundamental tenant of LID guidance for municipal land uses. The Bay Area Stormwater Management Agencies Association guidance entitled Start at the Source (1999), as well as many other similar publications, have provided guidelines for development engineers and planners in laying out a development project to reduce impacts, including reducing runoff volume. Site planning in the urban highway context has different considerations and is not typi- cally covered by existing guidance. Given the need to interface with existing infrastructure, meet the transportation and safety functions inherent in roadway projects, and work within a limited right-of-way, there tends to be less flexibility to incorporate many of the recommended site plan- ning principles for stormwater management. However, if volume reduction goals are considered at an early planning phase, it may be possible to incorporate certain site planning approaches Figure 18. Example stream-flow hydrograph exhibiting shallow interflow response. Source: DeBusk et al., 2011.

66 Volume Reduction of Highway Runoff in Urban Areas to reduce the runoff volume from the site and potentially achieve other benefits. The following sections provide guidance on planning principles, with specific emphasis on how these can be incorporated into urban roadway designs. Early Identification of VRA Opportunity Locations Opportunities for VRAs can be identified at an early stage of the project by considering the preliminary roadway alignment, preliminary infiltration rate screening, preliminary geotechni- cal screening, and preliminary groundwater quality/water balance screening. Guidance for a phased assessment of these last three factors is provided in Appendices C, D, and E, respectively. Early identification of areas within the project footprint where VRAs may be feasible can help ensure that these opportunities are made available for volume reduction purposes as each phase of designs is developed. For example, as various site layout alternatives are developed and consid- ered, opportunities for VRAs can be identified, and the opportunities for VRAs can be included as a factor in the ranking and scoring of various alternative layouts. For example, at the time of development of this manual, Illinois DOT was developing potential alternative designs for the “Circle” interchange in urban Chicago (http://www.circleinterchange. org). At the scale of planning shown in Figure 19, preliminary assessment of infiltration feasibility Legend Hypothetical opportunities for infiltration Note: full screening not conducted – for illustration purposes only Figure 19. Example project layout alternative for the Circle interchange project in Chicago, IL. Source: Background image—Illinois DOT, http://www.circleinterchange. org. Illustration of hypothetical opportunities was added by the research team.

Volume Reduction Approaches 67 could be conducted, and if results were favorable, opportunity areas could begin to be iden- tified for placement of VRAs such as bioretention areas, infiltration basins, and infiltration galleries. Working around critical infrastructure such as the light-rail tunnel, various utilities, and other constraints, project designers could attempt to reserve opportunity areas for VRA features. As designs progressed, other infrastructure could be designed to avoid these areas to the extent possible. Develop Drainage, Grading, and Utility Configurations to Accommodate VRA Opportunity Locations In addition to early identification of VRA opportunity locations, the development of drainage and grading configurations can be important in determining which VRA opportunities can be utilized. For example, looped interchanges typically represent one of the most significant oppor- tunities for VRAs within the urban environment and have the potential space to support a wide variety of approaches, ranging from dispersion to bioretention areas to infiltration basins. How- ever, using these locations for VRAs requires that water can be routed to these locations—this is a function of the alignment of storm drains, the road longitudinal profile, and the finished grade of the interchange island relative to the roadway surfaces. Where these factors can be adjusted within the site constraints to use the interchange area as a VRA, the overall outlook for achieving volume reduction performance can be significantly improved. Figure 20 illustrates an example of project drainage and grading configuration developed to facilitate the use of available space for an infiltration basin. As another example, the design of permeable pavement shoulders requires that a supple- mental drainage pathway be provided for runoff exceeding the storage and infiltration capacity of underlying soils. This is typically achieved via an underdrain system with connections to a roadside conveyance swale or an underground pipe. In the latter case, the alignment of the storm drain relative to the alignment of the permeable shoulder (and relative to the alignment of other utilities) can significantly influence the practicality of connecting underdrains to the storm drain at regular intervals. Figure 20. Example of project drainage and grading configuration developed to use available space at interchange, Tumwater, WA. Source: Google Earth Professional.

68 Volume Reduction of Highway Runoff in Urban Areas Limit Footprint of Disturbance Limiting the footprint of disturbance is a general principle that can potentially be incor- porated into the grading design of some roadway projects as well as the construction-phase plans for equipment access and stockpiling of materials. During grading design, footprints of disturbance can be limited by attempting to leave areas at their natural grade, without cut or fill, when possible. Plans can identify limits of disturbance and include directions for clearing and grubbing and remedial grading activities not to be performed outside of these areas. Plans and specifications can also dictate that vehicle traffic access and stockpiling be prohibited in these areas. Limiting the footprint of disturbance is especially important for areas where dispersion (VRA 02) is proposed since it may be costly or impossible to fully restore a dispersion area if it is affected via compaction (incidental or intentional), topsoil removal, or other activities during construction. Similarly, the footprint of proposed infil- tration VRAs should be protected from disturbance, to the extent possible, to help preserve infiltration rates and avoid costly efforts to restore infiltration capacity after other construc- tion activities are complete. Minimize Non-Essential Impervious Surfaces Hardened surfaces are an essential element of transportation corridors. However, imper- vious surfaces that do not support an essential transportation function can add cost to a project and increase its runoff potential. The decision to pave or otherwise harden a surface that does not serve a transportation purpose may be informed by maintenance consider- ations associated with landscaping or the desire to prevent pathways for infiltration in the roadway prism. For example, decorative rock may have limited uses due to maintenance problems. However, opportunities may exist to use other approaches that are pervious or partially pervious, such as permeable pavement (designed for foot traffic only), decomposed granite, or native grasses that can have a minimal maintenance burden and may be favorable compared to pavement in terms of capital costs. Such an approach would help reduce runoff and reduce project costs. Potential opportunities to use pervious or partially pervious land covers include: • Gravel, grassed, or permeable pavement medians (see example in Figure 21), • Permeable pavement or gravel access roads, • Degraded granite or permeable multi-use pathways, • Vegetated or stone-lined ditches rather than concrete-lined conveyances (see example in Figure 21), and • Directing access road or path runoff to adjacent permeable surfaces to help disconnect runoff. Conserve or Amend Topsoil In addition to the use of amended soil in VRAs (such as VRA 01—Vegetated Conveyance and VRA 02—Dispersion), soil amendments can be added to native soils or compacted shoulders to reduce the amount of runoff that originates from pervious areas themselves. Alternatively, stockpiling topsoil during clearing and grubbing activities and placing it over pervious areas at the end of the project can help reduce the cost of importing materials to amend soils. Compost amendments have been demonstrated successfully in the roadway environment as an initial treatment following construction to promote stabilization and vegetation growth as well as to reduce runoff volumes from pervious areas over the long-term (U.S. EPA, 2013c; Connecticut DOT, 1999; Black et al., 1999; Glanville et al., 2003; Washington Department of Ecology, 2011; Persyn et al., 2004). Figure 22 illustrates a healthy soil profile from guidance developed by the Washington State Department of Ecology (2011).

Volume Reduction Approaches 69 4.3 Summary of VRA Attributes and Considerations Each VRA has a distinct set of attributes and considerations that are inherent in its design and function. Fact sheets provided in Attachment A provide an extended summary of each of the primary VRAs. This section provides summaries of key attributes of each primary VRA to facili- tate comparison as well as to serve as a concise reference. This section serves as the foundation for the prioritization and selection process that is described in Chapter 5. Example of hardened surface without apparent transportation function Outlet from Storm Drain Inlet to Storm Drain Example use of pervious swale (stone lined) rather than concrete-lined ditches in median to reduce runoff Figure 21. Example of opportunity to minimize impervious surface in project design (CA-680 at Olympic Blvd., Walnut Creek, CA). Source: Google Earth Professional. Figure 22. Example healthy soil profile. Source: Soils for Salmon, Washington Department of Ecology, http://www.soilsfor salmon.org/pdf/Soil_BMP_ Manual.pdf.

70 Volume Reduction of Highway Runoff in Urban Areas 4.3.1 Summary of Relative Volume Reduction Mechanisms and Potential Water Balance Issues by VRA The amount of volume reduction achieved by a VRA, as well as the proportional split between deeper infiltration and ET that occurs in a VRA, is a function of the underlying infiltration rate of site soils, soil moisture-retention properties, plant root depths, rainfall intensity, and facility design characteristics, specifically the footprint and the depth of the BMP. When shallow BMPs with larger surface areas are used, the level of ET tends to increase due to the additional retained moisture content in the top layer of soils in closer contact with the atmosphere (Strecker and Poresky, 2009). In contrast, when deeper BMPs with smaller footprints are used, or when BMPs do not contain amended soil and vegetation elements, a greater portion of the water balance is associated with deeper infiltration, and ET plays a more minor role. In some cases, the presence of an underdrain can increase airflow through the soils, which may also increase ET potential in more porous soils/media. While site-specific conditions are understood to influence performance, the inherent design attributes and spatial scales associated with each VRA can be used to establish the relative amounts of volume reduction and the approximate proportional split between deeper percolation and ET that can be expected. Based on the total volume reduction potential and the split between deeper percolation and ET, the relative potential to infiltrate a greater volume than natural can be esti- mated. Finally, inherent aspects of the design determine whether it is possible to design the VRA with an outlet control to provide base-flow–mimicking discharge in cases where this option is via- ble. Table 11 provides a relative comparison of the nine primary VRAs with respect to these factors. 4.3.2 Summary of Geometric Siting Opportunities and Footprint Requirements by VRA Siting of VRAs within the urban highway environment is governed, in part, by the inherent geometric characteristics (i.e., shape) and sizing requirements (i.e., footprint) of each VRA and how these characteristics match the geometries and space constraints associated with the urban highway environment. In the case of some VRAs, geometric configuration is critical for providing the intended function. For example, a vegetated conveyance is inherently linear, while dispersion VRA Total Relative Volume Reduction Potential Relative Portion of Losses to Deeper Percolation in Typical Conditions Relative Portion of Losses to ET in Typical Conditions Relative Potential to Infiltrate More than Natural Possible to Design for Base-Flow– Mimicking Discharge? VRA 01 – Vegetated Conveyance L/M L/M M L/M No VRA 02 – Dispersion M/H L/M M/H L/M No VRA 03 – Media Filter Drain M/H L/M M L/M No VRA 04 – Permeable Shoulders M/H M/H L M/H Yes VRA 05 – Bioretention w/o Underdrain H M/H M M/H No VRA 06 – Bioretention w/Underdrain M/H M/H M L/M Yes VRA 07 – Infiltration Trench H H L M/H Yes (w/underdrain) VRA 08 – Infiltration Basin H H L M/H Yes (w/underdrain) VRA 09 – Infiltration Gallery H H L M/H Yes (w/underdrain) H: high; M: medium; L: low Where dual rankings are shown, this indicates that ranking is significantly influenced by site conditions. Table 11. Summary of relative role of volume reduction mechanisms by VRA.

Volume Reduction Approaches 71 areas should have a relatively long flow path perpendicular to the roadway. VRAs such as infiltra- tion basins are inherently larger and involved ponding of water within an impoundment such that they are practical only where there is a larger contiguous space with a relatively flat profile. Additionally, the amount of space required is a function of the VRA type, as well as the climatic and soil conditions and the volume reduction goals. To provide a general perspective on the space needed for VRAs to achieve meaningful volume reduction, a case study analysis was conducted using the Volume Performance Tool. Design infiltration rates representative of hydrologic soil groups A and C (0.8 and 0.2 in./hr, respectively) were used for this assessment, and analyses were conducted on the Portland (OR) airport precipitation gage and the New Orleans (LA) airport precipitation gage. These gages represent distinctly different climates; Portland gets less precipitation on average (37 in. per year) than New Orleans (61 in. per year) and has an 85th-percentile, 24-hour storm depth (0.6 in.) that is less than half as large as that of New Orleans (1.4 in.). Additionally, Portland’s precipitation is characterized by mostly frontal, low-intensity precipitation events, while New Orleans’ precipitation is characterized primarily by convective events (i.e., thunderstorms and tropical storms). While many other combinations of climate characteristics exist across the United States, and many other soil conditions could be encountered, these scenarios provide a reasonable basis for developing an approximate range of potential size requirements. The primary purpose of this analysis was to demonstrate the potential impacts of VRA type, climate, soil type, and volume reduction goals on the required VRA footprint. Table 12 summarizes the results of this analysis. Several key findings from the analysis can be used to inform a general understanding of how VRA selection, design goals, and site and climate conditions influence the sizes that may be required: • In general, sizing was a function of location, design infiltration rate, and the volume reduc- tion target. Location and Hypothetical Volume Reduction Target Portland, OR New Orleans, LA 40% long-term capture 80% long-term capture 40% long-term capture 80% long-term capture Design Infiltration Rate 0.8in./hr 0.2 in./hr 0.8 in./hr 0.2 in./hr 0.8 in./hr 0.2 in./hr 0.8 in./hr 0.2 in./hr General VRA Type Approximate Required Tributary Area Ratios (VRA Area as Percent of Tributary Area) Characteristic shallow-flow VRAs VRA 01• • • • • • • • • – Vegetated Conveyance VRA 02 – Dispersion VRA 03 – Media Filter Drain Assuming 6-in. amended soil depth 1% 6% 4% 16% 8% 17% 30% 90% Characteristic shallow-ponding VRAs VRA 04 – Permeable Shoulders VRA 05 – Bioretention w/o Underdrains VRA 06 – Bioretention with Underdrains and IWS Assuming 1-ft storage 1% 3% 4% 8% 4% 7% 15% 25% Characteristic deeper-ponding VRAs VRA 07 – Infiltration Trench VRA 08 – Infiltration Basin VRA 09 – Infiltration Gallery Assuming 3-ft ponding <1% 1% 2% 3% 1% 2% 4% 6% Table 12. Case study assessment of general space requirements for VRAs.

72 Volume Reduction of Highway Runoff in Urban Areas • Major increases in sizing tend to be required to improve volume reduction performance from 40% to 80% capture, suggesting that partial volume reduction is likely to be significantly more practicable in space-constrained highway environments. • Soil infiltration rates were most sensitive in the performance of shallow-flow VRAs and least sensitive for VRAs with deeper ponding depths. However, it should be noted that deeper VRAs would take longer to drain, and much of the storage may need to be provided below ground to avoid vector issues if implemented in soils with lower permeability. • In general, as the depth of storage in VRAs increased, the amount of space required for VRAs decreased. Based on the relative space requirements interpreted from this analysis and based on a review of typical sizing guidance, the various geometric siting opportunities within the urban highway environment can be rated in terms of their suitability for each VRA. Table 13 provides a summary of the relative suitability of the various VRAs for different urban highway opportunities, considering the typical space associated with each opportu- nity as well the inherent shape of the opportunity. Figure 23 provides an example illustra- tion of common geometric siting opportunities that may be present in the urban highway environment. 4.3.3 Summary of Potential Geotechnical Impacts Associated with Classes of VRAs Geotechnical impacts may include impacts on utilities, slope stability, settlement/volume change, impacts on retaining walls or foundations, and impacts on pavement strength and durability. While a wide variety of factors contribute to the potential for geotechnical impacts (as discussed in Appendix E), the inherent location of certain VRAs within the urban highway Geometric Siting Opportunity V R A 01 – V eg et at ed C on ve ya nc e VR A 02 – D is pe rs io n VR A 03 – M ed ia F ilt er D ra in VR A 04 – Pe rm ea bl e Sh ou ld er s VR A 05 – B io re te nt io n w /o U nd er dr ai ns VR A 06 – B io re te nt io n w ith U nd er dr ai ns VR A 07 – In fil tra tio n Tr en ch VR A 08 – In fil tra tio n Ba si n VR A 09 – In fil tra tio n G al le ry Medians X X X X X X Shoulders, including breakdown lane and area within clear zone (less than approx.15% or 6H:1V) X X X X X X Shoulders, outside of clear zone (less than approx.15% or 6H:1V) X X X X X X X Moderately steeper shoulders (steeper than approx.15% or 6H:1V but less than approximately 25% or 4:1) X ROW locations with limited uses (e.g., wide spots, irregular geometries) X X X X X X X X Adjacent natural areas X Looped interchange medians X X X X X X X X Diamond interchange medians X X X X X X X X Low traffic areas – maintenance yards, etc. X X X 1 X X X X X 1 – Permeable pavement in general; shoulders not present. Table 13. Summary of geometric siting opportunities by VRA.

Volume Reduction Approaches 73 environment and the inherent unit processes provided by these VRAs (see Table 11) can be used to provide a relative assessment of potential geotechnical issues associated with different types of VRAs (Table 14). 4.3.4 Summary of Relative Potential Risk of Groundwater Quality Impacts Associated with VRAs Groundwater quality impacts may include mobilization of contaminants in soil or ground- water, introduction of contaminants from stormwater runoff, and introduction of contam- inants from spills. The potential for groundwater quality impacts are a function of many factors (as described in Appendix D), and the choice of VRA type has inherent implications for potential risks to groundwater quality. Constituents of concern in highway runoff may include nutrients, pesticides, organic compounds, pathogenic microorganisms, heavy metals, and salts. Table 15 discusses factors related to VRA design that influence relative potential for ground- water quality impacts. “Pretreatment” is a general term that refers to providing an initial level of treatment that is provided to stormwater before it enters a VRA, such as filtering through grass, settling, centrifugal separators, and media filters. Table 16 provides a synthesis of relative risk posed by each of the nine primary VRAs based on the discussion provided in Table 15. Note that a higher relative ranking does not neces- sarily imply that the VRA should not be used; however, it may be less favorable where other site conditions suggest a higher potential for groundwater quality issues. See Appendix D and Section 5.2 for guidance in evaluating overall feasibility of VRAs relative to groundwater quality issues. Looped Interchanges Diamond Interchanges Irregular ROW Shoulder Outside of Clear Zone Shoulder Inside of Clear Zone or Breakdown Lane Median or Inside Breakdown Lane Figure 23. Key to common geometric opportunities within urban highway environments (hypothetical opportunities shown).

Category of VRA Characteristic Properties Example Opportunities and Constraints Related to Geotechnical Issues1 Opportunities Constraints Direct infiltration into roadway subgrade Example: VRA 04• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • – Permeable Shoulders VRA 09 – Infiltration Gallery Broad footprint; may only receive direct rainfall or equivalent Road subgrade has important structural considerations, particularly for flexible pavement design Broad footprint may allow infiltration in relatively dense soils Standard roadway designs typically account for wetting of subgrade Rigid pavement design (i.e., concrete) less sensitive to strength of subgrade Utilities in the right-of-way (ROW) Settlement and volume change processes (i.e., consolidation, frost heave, swelling, liquefaction) Reduction in strength of subgrade material from increase in moisture content Infiltration in breakdown lane and near shoulders Example: VRA 03 – Media Filter Drain VRA 04 – Permeable Shoulders Outside of main travel lanes; significantly less loading Smaller footprint; more concentrated zone of infiltration Shoulders designed to accommodate less traffic loading than travel lanes Well-distributed inflow Can have moderate to high tributary area ratio2 Linear configuration less susceptible to groundwater mounding than basin configurations Underdrain with outlet can control amount of water infiltrated Typically shoulder must be compacted to same degree as mainline roadway Potential for water to migrate laterally into mainline subgrade rock or nearby development Settlement or volume change Potential reduction in slope stability for embankment or depressed sections Infiltration and Surface Dispersion in Clear Zone Example: VRA 02 – Dispersion VRA 03 – Media Filter Drain Allows incidental infiltration over relatively broad area; also provides ET Typically coupled with a vegetated conveyance at toe of filter strip Drainage over shoulder is a typical design feature Compost amended results in relatively limited increase in infiltration compared to standard design Higher proportion of losses to ET than other VRAs May lead to erosion issues if applied on slopes that are too steep Slopes may need to be compacted to same degree as mainline roadway In some cases, settling or volume change could damage roadway. Subject to frozen ground issues Channels, trenches, and other linear depressions offset parallel to roadway Example: VRA 01– Vegetated Conveyance VRA 05/06 – Linear Bioretention Variations VRA 07 – Infiltration Trench Tends to be located 10 or more feet from travel lanes Typically effective water storage depth is between 6 in. and 36 in. Tributary area ratio may be low May be fully or partially infiltrated Channels with positive grade are common drainage features; have relatively limited increase in risk. Due to horizontal separation, features have less potential to damage roadway. Some settlement may be tolerable. Deeper designs may avoid frost impacts. Greater potential for impacts out of ROW due to proximity to ROW line Greater potential for mounding due to concentration of infiltrating footprint May reduce stability of slopes if located near top or toe Basins and localized depressions Example: VRA 05/06 – Bioretention (more centralized variation) VRA 08 – Infiltration Basin Typically located in more centralized locations Typically have a relatively low tributary area ratio Typically effective water storage depth is between 12 in. and 60 in. Centralized areas, such as wide spots in ROW or interchanges, may allow ample setbacks from foundations, slopes, and structural fill. May be possible to preserve natural soil infiltration rates through construction Impacts of potential settlement may be minor. Deeper ponding depths may result in substantial groundwater mounding and lateral water migration; greater setbacks may be needed than would be applied for more distributed systems. Surface systems subject to frozen ground issues 1 – Examples are provided to identify typical opportunities and constraints of the infiltration design feature. Additional opportunities and constraints may be present based on site-specific conditions. More information regarding risk indicators, design implicatio ns, and potential mitigation measures is provided in Chapter 3. 2 – “Tributary area ratio” refers to the ratio of the infiltrating surface to the total tributary area. A high tributary area ratio indicates that the infiltrating footprint makes up a large portion of the total tributary area, and vice versa. Table 14. Summary of relative geotechnical opportunities and constraints for specific categories of VRAs.

Volume Reduction Approaches 75 Risk Factor Discussion Lower Risk Indicators Higher Risk Indicators Footprint/tributary area ratio The relative footprint of the system influences the pollutant loading per unit area and the potential for natural assimilative capacity to be overwhelmed. Systems with broader, shallower footprint such as dispersion Systems with deeper profiles and smaller footprints, such as infiltration trenches Strata at which infiltration occurs When infiltration occurs below the strata of organic soil or closer to the groundwater table, there tends to be less assimilative capacity. Systems infiltrating near the surface where soils have higher organic content and biologic activity Systems infiltrating below the organic strata and not providing a treatment layer, such as imported amended soil (or other pretreatment) Amount of infiltration occurring In systems where a higher portion of losses occur to infiltration, the potential for groundwater impacts tends to be higher. Systems with less infiltration, such as vegetated conveyances Systems with more infiltration, such as underground infiltration systems Potential for pretreatment or treatment within the VRA Pretreatment is important to reduce potential for clogging as well as to address groundwater quality. Systems providing a treatment layer such as an engineered soil media layer or an amended soil layer Systems where pretreatment cannot be practically provided and treatment processes within and below the VRA are limited, such as permeable pavement Table 15. Summary of relative VRA-related risk factors for groundwater quality impacts. VRA Relative Risk of Groundwater Quality Impacts1 Key Characteristics Influencing Ranking VRA 01 – Vegetated Conveyance L More limited infiltration, shallower ponding, and soil filtration of infiltrating runoff VRA 02 – Dispersion L Shallower ponding; high soil contact ratio; amended/organic/biologically active soils VRA 03 – Media Filter Drain L Shallow ponding; specialized media with high treatment capacity VRA 04 – Permeable Shoulders M/H Can have a relatively small footprint area; some pretreatment provided in base material but additional pretreatment not practical; can infiltrate water below organic soil strata VRA 05 – Bioretention w/o Underdrains L/M Provide treatment for most constituents within media; can have relatively small footprint and deeper infiltrating surface VRA 06 – Bioretention with Underdrains L Provide treatment for most constituents within media; infiltrate less water than bioretention w/o underdrain VRA 07 – Infiltration Trench M/H Deeper profile typically below surface soil strata; pretreatment options may be limited VRA 08 – Infiltration Basin M Deeper profile and typically smaller tributary area ratio, but soil can be amended to improve water quality VRA 09 – Infiltration Gallery M/H Deeper profile typically below surface soil strata; pretreatment may not address all pollutants of concern 1 Rankings are relative to other VRAs, not a complete ranking of total risk, which is also a function of pollutant sources and site conditions. H: high; M: medium; L: low Table 16. Relative ranking of potential groundwater quality risk by VRA.

76 Volume Reduction of Highway Runoff in Urban Areas 4.3.5 Summary of Safety Considerations by VRA A number of key safety considerations that may relate to the siting and design of VRAs include: • Limitations on grading and structures within the “clear zone” along the road shoulders to allow errant vehicle recovery and reduce collision hazards, • Vegetation management to maintain line-of-site requirements as well as to eliminate collision hazards within the clear zone, • Adequate supplemental drainage, as needed to avoid flooding of travel lanes, and • Lane closures to facilitate low-speed maintenance activities within the right-of-way. Based on its respective location within the highway environment and its inherent design attri- butes, each VRA has a different suite of factors that should be considered in design. Safety con- siderations that may apply to specific VRAs are discussed in the respective VRA fact sheets and are summarized in Table 17. These factors do not necessarily result in VRAs being considered infeasible but should be considered in selection, siting, and design. 4.3.6 Summary of Maintenance Activities by VRA Maintenance activities of VRAs range from regular highway maintenance activities, such as trash control or vegetation management, which may be done regardless of whether a VRA is in place, to more VRA-specific maintenance activities that are needed to maintain the intended function of the systems. These activities can be categorized into routine maintenance, which includes normally scheduled inspections and activities that are needed on a regular basis, and corrective maintenance, which includes as-needed activities triggered by observations of dam- age, failure, pending issues, or other factors that require action to be taken to return the facility to its intended function. Tables 18 and 19 provide an inventory of routine maintenance activities Potential Safety Considerations VR A 01 – V eg et at ed C on ve ya nc e VR A 02 – D is pe rs io n VR A 03 – M ed ia F ilt er D ra in VR A 04 – P er m ea bl e Sh ou ld er s VR A 05 – B io re te nt io n w /o U nd er dr ai ns VR A 06 – B io re te nt io n w ith U nd er dr ai ns VR A 07 – In fil tra tio n Tr en ch VR A 08 – In fil tra tio n Ba si n VR A 09 – In fil tra tio n G al le ry Limitations on side slopes and berms within the clear zone, including check dams, etc. X X X X X X X Limitations on drainage structure design within the clear zone (i.e., pipe inlets/outlets flush to slope) X X X X X X Stability of soil within clear zone, particularly if compost amended X X X X X Vegetation management to remove collision hazards Vegetation management to maintain line of site X X X X Supplemental drainage to ensure free drainage of travel lanes in the event of clogging X Low-speed vehicle maintenance activities or lane closures X X Table 17. Summary of potential safety considerations by VRA.

Volume Reduction Approaches 77 Routine Maintenance Activities VR A 01 – V eg et at ed C on ve ya nc e VR A 02 – D is pe rs io n VR A 03 – M ed ia F ilt er D ra in VR A 04 – Pe rm ea bl e Sh ou ld er s VR A 05 – Bi or et en tio n w /o U nd er dr ai ns VR A 06 – Bi or et en tio n w ith U nd er dr ai ns VR A 07 – In fil tra tio n Tr en ch VR A 08 – In fil tra tio n Ba si n VR A 09 – In fil tra tio n G al le ry Mowing Maintain level spreading functions Landscaping and weeding Routine woody vegetation management Sediment removal/management Vacuum sweeping Trash and debris removal Erosion repair Rodent hole or beaver dam repair Fence or access repair Key: Primary maintenance activity; Minor maintenance activity; may not apply in some cases or may be limited; Not usually applicable. Table 18. Summary of potential routine maintenance activities by VRA. Corrective Maintenance Activities V R A 01 – V eg et at ed C on ve ya nc e VR A 02 – D is pe rs io n VR A 03 – M ed ia F ilt er D ra in VR A 04 – P er m ea bl e Sh ou ld er s VR A 05 – B io re te nt io n w /o u nd er dr ai ns VR A 06 – B io re te nt io n w ith U nd er dr ai ns VR A 07 – In fil tra tio n Tr en ch VR A 08 – In fil tra tio n Ba si n VR A 09 – In fil tra tio n G al le ry Regrading to maintain level spread function X X Regrading to address sediment deposition or erosion X X X X Repairing of berms, inlets/outlets, or other structures X X X X X X X X Cleaning of underdrain pipes X X X Decompaction/re-amendment X X X X Partial removal of surface material to remediate clogged surfaces X X X X X X Complete replacement of system components (full depth) to remediate clogged surfaces X X X X X X Reseeding to provide needed coverage X X X X Significant revegetation to provide needed coverage X X X X X Remediate contamination from acute or chronic loadings (oil, gas, or other contaminants) X X X X X X X X X Table 19. Summary of potential corrective maintenance activities by VRA.

78 Volume Reduction of Highway Runoff in Urban Areas and corrective maintenance activities, respectively, that may apply to each of the primary VRAs. These tables were developed based on review of guidance manuals and interviews with DOT maintenance staff. However, it is important to note that information on maintenance require- ments of VRAs in the highway environment is still limited to inform decision making. NCHRP Report 792: Long-Term Performance and Life-Cycle Costs of Stormwater Best Management Prac- tices (Taylor et al., 2014a) evaluates maintenance needs and whole life-cycle cost estimating tools for a variety of stormwater control measures, including VRAs. 4.3.7 Summary of Relative Whole Life-Cycle Costs by VRA The initial cost of a VRA is only a part of the total expenditure needed to keep the system operational into perpetuity. Whole life-cycle costs (WLCs) include up-front capital costs as well as maintenance costs through the life span of the facility and the cost of replacing or restoring the facility at the end of its usable life. Future costs are typically expressed in terms of their net present value (NPV), where future costs are discounted based on when they occur and their assumed discount factor. The use of WLC methods to compare VRAs is strongly recommended. Focusing only on capi- tal costs can lead to a focus on minimizing the initial costs, which can lead to design decisions that result in greater life-cycle costs (Lampe, 2005). The maintenance, repair, and unplanned or premature failure of a component, either as a result of normal conditions, short-sighted design decisions (e.g., neglecting to provide pretreatment; lack of adequate maintenance access or safety), or unanticipated conditions, can account for a significant proportion of the overall costs associated with a VRA. Thus, when planning the construction of a VRA, the designer should develop relative WLC estimates, both to compare between different VRAs and to inform deci- sions (i.e., the cost of more robust pretreatment versus the cost of more frequent maintenance/ replacement). The WLC of VRAs can vary widely as the result of site-specific and regional differences (Wash- ington State DOT, 2011). While conducting an economic appraisal of the VRA, a major difficulty is assessing the relative benefits and risks of technologies and projects. The same type of technol- ogy implemented at one location could have significantly different costs at another location as a result of different site conditions, project types, and regional factors (Lampe, 2005). A significant source of variability in VRA costs is the baseline for comparison of costs. The baseline for estimating the cost of VRAs is the minimum set of design attributes that would have been required for the project. For example, if the project was still required to convey and treat runoff (regardless of the use of VRAs), then the costs of this scenario form the baseline; the net or incremental cost of VRAs should then account for additional costs as well as any avoidance of baseline conveyance or treatment costs that could be accrued. For example, if the use of a vegetated conveyance can reduce the amount of storm drains that must be con- structed and the volume of water that must be treated, both of these costs could be subtracted from the capital cost of constructing the vegetated conveyance when comparing the VRA’s net project cost. The type of project being constructed is also important in terms of determining the costs that would be considered incremental to the project as a result of using VRAs. For example, in new roadway construction projects or major lane additions, bulk grading is typically considered to be an expense accrued by the overall project—project plans are often developed to balance cut and fill, where feasible, to avoid the expense of hauling material on or off site. Therefore, costs of excavation and hauling for construction of VRAs is typically shared by the overall project, with little incremental cost attributable to VRAs. Similarly, a new project would typically have been required to provide some type of vegetation or surface stabilization in the locations where VRAs

Volume Reduction Approaches 79 are proposed. Therefore, the cost of vegetating a VRA would only be the incremental cost of the proposed VRA vegetation versus the baseline vegetation or stabilization plans had the VRA not been used. In contrast, projects conducted solely to retrofit roadways with VRAs have different considerations because the VRA improvements may be the only improvements being made as part of the project. In most cases, this results in a greater cost attributable to the VRA construc- tion. For example, the cost of the VRA could include expenses such traffic control, utility reloca- tion, hauling, clearing and grubbing, vegetation reestablishment, and other costs that would have typically been considered incidental to the overall project if the VRA had been constructed as part of a new project or a major lane addition. Yet another case is when a retrofit project is done simply to improve the volume reduction potential of an existing BMP, such as by modifying the outlet structure of an extended detention basin or by adding check dams to a vegetated ditch to improve infiltration. These types of retrofits can accrue relatively minor costs since they involve limited physical modifications. In summary, many factors influence the cost that should be attributed to a VRA. The purpose of this section is to introduce the qualitative framework for comparing the relative costs of VRAs and understanding how site-specific factors may influence these costs. Section 5.4.5 provides more specific detail for developing VRA costs for a specific site once conceptual designs have been developed. Whole Life-Cycle Cost Framework Terminology This manual introduces a specific framework for calculating the various costs of a VRA. Key terms and concepts associated with this framework are introduced in the following: Capital cost. A fixed-value, one-time expense caused by purchasing labor, land, materials, or equipment needed to bring a stormwater technology to an operable status. Appraisal and planning costs of the VRA, engineering design costs, site investigation costs, and the initial con- struction costs are all capital costs. Baseline cost scenario. An alternative cost scenario to which the incremental (net) cost of a VRA is compared. Depending on regulatory conditions and project type, this could be a case where traditional technology is used—for example, a piped stormwater conveyance system to conventional treatment systems—or could be defined as the “no-action” option. This should be defined because it is necessary to calculate the incremental whole life-cycle cost of a VRA. Net capital cost. This term is defined as the capital cost of the chosen VRA scenario less the capital cost of the baseline cost scenario. O&M cost. The cost needed to preserve a technology’s water quality, volume reduction, and conveyance function, and in some cases its aesthetics. This includes any labor, materials, and structural repairs over the life span of the technology. Functional maintenance costs help pre- serve the performance and safety of the VRA, whereas aesthetic maintenance costs help provide public acceptance of the technology and might reduce the need for functional maintenance. Net O&M cost. The maintenance cost of a selected VRA less the maintenance cost of the baseline cost scenario. For example, if vegetation and trash management would be required for a road shoulder in the baseline case, this unit cost should be subtracted from the costs for vegeta- tion and trash management for the VRA areas. Life span. The amount of time before the VRA needs to be replaced or substantially recon- structed. A period that begins when the stormwater technology becomes functional and ending when that technology is decommissioned and disposed of or reconstructed. Understanding the life span of a VRA is important for computing WLCs and planning for future O&M costs.

80 Volume Reduction of Highway Runoff in Urban Areas Replacement cost. This is the cost to install a new VRA at the end of the current VRA’s life span or substantially reconstruct the VRA. Some VRAs, such as dispersion, have a low replace- ment cost, primarily involving minor regrading and decompaction, whereas others, such as per- meable shoulders, could have high replacement costs if excavation down to subgrade material is needed to regenerate the infiltration capacity of the system. Discount factor. This is a factor used to compute the present value of future costs. It is typi- cally calculated as the rate of return that could be obtained for cash on the open market minus the rate of increase in the cost of goods and services (i.e., inflation). If goods and services are expected to increase at the same rate as the growth of money invested in the open market, then the discount rate would be zero—implying that goods would cost the same in the future (in terms of present- day value) as they would today. Typically, it is assumed that goods and services will increase at a rate less than the rate of return on the open market; therefore, a positive discount rate is typically used. This implies that future costs are discounted in terms of present value. This factor is impor- tant for O&M and future purchases. Whole life-cycle cost. The total cost of a stormwater technology throughout its life span; includes all capital costs, O&M costs, and disposal/decommissioning costs that are associated directly with the VRA, normalized to net present value. WLC is typically represented in terms of cost per year, and in present-day dollars. Net whole life-cycle cost. The whole life-cycle cost of a VRA less the whole life-cycle cost of the baseline cost scenario. The net whole life-cycle cost is important because different technolo- gies will have cost reductions in different stages of development (e.g., capital costs vs. O&M costs vs. disposal costs). Computations of WLCs are discussed further in Section 5.4.5. Site-Specific Influences on VRA Whole Life-Cycle Costs The actual costs of a VRA depend on a large number of site-specific factors and are thus dif- ficult to estimate and especially difficult to generalize categorically. In addition, regional costs, such as permitting costs, regulatory requirements, and the state’s construction economy, can cause cost difference between two similar sites. Managers should develop their own site-specific frameworks to calculate the cost of a VRA based on the considerations presented in the following paragraphs (Taylor et al., 2014b). The factors discussed in the following paragraphs represent important sources of variability. Retrofits versus redevelopment versus new construction. The type of project greatly influ- ences the net costs of a VRA. As introduced previously, implementing a VRA in a new construc- tion project or major redevelopment project could result in limited increases in capital costs and possibly reduced net costs if baseline requirements, such as piped infrastructure and conven- tional treatment, can be reduced or avoided. The costs of building a VRA in a retrofit project are often the greatest. Most costs are associated with the VRA itself because the project is centered around the VRA, and thus costs like grading and soil removal are part of the VRA cost. For ex- ample, permeable shoulders can cost much less for new roadways (where the use of a permeable shoulder offsets the need for traditional pavement and base material) than retrofit applications (where costs include demolishing current shoulders and hauling existing base material away while installing new base material and the permeable top course). Land allocation and land use costs. The cost of land is likely to create one of the largest dif- ferences in project costs. In some cases, DOTs could have a surplus of land in the ROW suitable for a VRA, while in other cases, the implementation of the VRA might require additional land purchases. In the latter case the cost of land can be extremely variable by location, depending on surrounding land use (Strassler et al., 1999) and location.

Volume Reduction Approaches 81 Project-scale and unit costs. Larger projects with fewer, large-scale VRAs can potentially be built at lower costs than smaller-scale projects or those that have many distributed controls. Larger-scale projects can have reduced managed costs per unit area. Additionally, each feature may have fixed costs (for example, inlet and outlet structure) regardless of size; therefore, the more elements, the greater costs may be. An exception would be if using more distributed con- trols helps avoid significant amounts or sizes of conventional drainage infrastructure either as a part of the actual road project or required downstream improvements. Flexibility in site selection and site suitability. Site-specific parameters that influence costs include the ability to configure the site to allow efficient drainage design, the availability and accessibility of the work area, traffic control (in retrofit projects), site contamination, and existing infrastructure. Regional performance-related parameters. The average annual rainfall, storm shape and characteristics, catchment area runoff characteristics, and climate will affect the VRA sizing and costs to meet a given objective. See Table 12 for an example of the influence of local rainfall pat- terns on sizing requirements to meet given design goals. Soil type and groundwater vulnerability. These factors will determine if infiltration meth- ods can be used and what kind of infiltration methods, including storage, are applicable to the site. In some cases, in low infiltration rate situations, additional required storage or attenuation will raise the cost of the VRA. An example case study is assessed in Table 12 regarding the influ- ence of infiltration rates on the ability to meet project goals. Planting. The availability of suitable plants for the site and region and the level of planting needed for a particular VRA or pollutant reduction goal will influence cost. Additionally, plant- ing influences the maintenance costs and could add additional costs, such as for irrigation. Level of experience of the designers and contractors. Design and construction costs gener- ally vary across regions. However, traditional cost methods often overlook experience as a factor. Some areas in the United States have required stormwater water quality and volume controls for 20 years; in these areas, local design/engineering agencies and local contractors will more typically have the skills to design and build the VRAs more effectively. Such experience will help reduce unexpected initial costs and/or O&M costs. Regulatory requirements. Different regions require varying water quality treatment and volume control limits. Differences in the acceptable limit for water quality constituent concen- trations or volumes to be controlled will create different costs across VRAs due to design and construction costs. In addition, permitting costs will vary across regions and with project size. An example case study is assessed in Table 12 regarding the influence of design goals on the sizing requirements of VRAs. Time of construction. Depending on the region, construction costs or delays due to weather or other seasonal construction considerations might influence the cost of implementing a VRA. Summary of Typical Relative Costs of VRAs The average or typical whole life-cycle cost of a VRA is difficult to estimate because of the vari- ability in design and construction due to site-specific factors, as introduced previously. Addition- ally, information on whole life-cycle costs and life span to inform decision making is still limited. Table 20 represents the relative costs of select VRAs based on a typical application, with notes to identify key site-specific factors that may influence these rankings. Because relative capital costs can be significantly different in new roadway projects and lane additions versus retrofit projects,

82 Volume Reduction of Highway Runoff in Urban Areas VRA Capital Costs – New Roadway or Major Redevelopment1 Capital Costs – Retrofits or Minor Redevelopment O&M and Replacement/ Reconstruction Costs Effective Life Span 2 VRA 01 Vegetated Conveyance Low to Moderate Can typically be easily incorporated into grading plans for non–ultra-urban settings Provides conveyance function that can offset pipes and structures Low to Moderate Modifications to existing swales to improve volume reduction may be inexpensive. Can add significant cost if regrading and rerouting must be done to accommodate VRA Low to Moderate Requires more frequent maintenance of debris removal and vegetation upkeep than typical vegetated or concrete ditch without water quality functions Erosion/scour must be addressed. 20 to 50 Years Regrading of conveyance Decompact underlying soils, potentially add new amendments Correct major erosion VRA 02 Dispersion Low Assuming no acquisition costs for the ROW; if land acquisition is needed, it can render this option cost- prohibitive. Provides conveyance function that can offset pipes and structures Low to Moderate Assuming no acquisition costs for the ROW; if land acquisition is needed, it can render this option cost-prohibitive. Depends on extent of routing and grading improvements needed to utilize dispersion area Low Requires minimal maintenance of vegetation that would be similar to vegetated ROW Reconstruction costs are typically lower than those with original construction 50 to 100 Years Regrade level spreader Decompact underlying soils, potentially add new amendments Correct major erosion Longer time period than for VRA 01 since water is spread over larger area VRA 03 Media Filter Drain Low to Moderate Assumes no acquisition costs for land Assumes shared grading/excavation costs with project Moderate Requires minor excavation and removal of soil May require modifications to drainage patterns Can fit on existing shoulders Low to Moderate Requires infrequent maintenance to remove sediment and maintain conveyance if tributary watershed is stabilized Periodic maintenance possibly needed to replace media 5 to 20 Years3 Regrade level spreader Replace media if exhausted Shorter time period than for VRA 01 and 02 because footprint tends to be smaller and more specialized media are used VRA 04 Permeable Shoulders with Stone Reservoirs Low to Moderate Assumes new development or lane additions where permeable pavement net cost is cost over and above traditional pavement cost. High Requires excavation and hauling of previous roadway, import of new material Equipment, labor, and installation costs are directly associated with VRA. Moderate to High Requires regular vacuum sweeping of shoulder to maintain permeability Surface replacement may be required more frequently than for traditional pavement. Full-depth replacement may cost more than initial construction. If water is routed directly to subbase via inlets, sweeping is not needed, but earlier clogging of the subbase layer may occur. 15 to 25 Years4 Replace top course of permeable pavement due to structural wear Fully excavate to restore infiltration capacity of subgrade Dependent on sediment loading, traffic loading, and other factors; not well established VRA 05/06 Bioretention Moderate Specialized planting and soil, so net cost increase should be considered over areas that would have been planted. Assumes grading and conveyance is performed in conjunction with overall project Costs can increase, and volume performance declines with use of an underdrain. Moderate to High Cost depends on cost of rerouting flows to specific areas. Some aspects of site investigation and construction are not shared with overall project. Possibility of additional land acquisition Moderate Regular maintenance of vegetation and trash needed, similar to baseline landscape maintenance. May require restoration of surface infiltration capacity and replanting at regular intervals 5 to 12 Years (Partial) Dependent on effectiveness of pretreatment Partial reconstruction involves restoration of surface infiltration capacity and replanting. Intervals may be longer if vegetation is robust. 25 to 50 Years (Complete) Complete reconstruction involves replacement of media/ structures/piping at less frequent intervals. VRA 07 Infiltration Trench Moderate to High Requires several additional construction materials Assumes no land acquisition Can be incorporated into excavation plans High Cost depends on cost of rerouting flows to specific areas. Some aspects of site investigation and construction are not shared with overall project. Possibility of additional land acquisition High Requires maintenance of debris and sediment removal to maintain infiltration Failures have been common Replacement cost can be similar to new construction cost because infiltration surface is not exposed. 5 to 15 Years Dependent on effectiveness of pretreatment Excavate rock and rework trench to maintain infiltration rates; backfill with existing rock after removing fines May only be able to restore capacity a limited number of times before moving the facility location. Table 20. Relative comparison of typical VRA costs per volume of stormwater managed (adapted from Washington State DOT, 2011).

Volume Reduction Approaches 83 VRA Capital Costs – New Roadway or Major Redevelopment1 Capital Costs – Retrofits or Minor Redevelopment O&M and Replacement/ Reconstruction Costs Effective Life Span 2 VRA 08 Infiltration Basin Moderate Assumes no acquisition costs for land Assumes potential additional excavation and infrastructure to convey water to centralized location Basins can offset pipes or reduce size of downstream conveyance. High Cost depends on cost of rerouting flows to specific areas. Aspects of site investigation and construction not shared with overall project. Possibility that additional land acquisition may be needed Costs can be low if existing detention basin can be converted to infiltration. Moderate Requires debris and sediment removal to maintain infiltration. Maintenance of any conveyance systems 5 to 10 Years (Partial) Dependent on effectiveness of pretreatment Partial restoration involves restoration of surface infiltration capacity; can be longer if deep- rooted plants are used. May only be able to restore capacity a limited number of times before moving the facility location 25 to 50 Years (Complete) Complete restoration involves replacement of structures/piping and deep restoration of subgrade at less frequent intervals (25 to 50 years); eventually may need to move facility location if possible. VRA 09 Infiltration Gallery Moderate to High Excavation and piping can be incorporated into construction plans. Assumes robust pretreatment system. High Cost depends on cost of rerouting flows to specific areas. Aspects of site investigation and construction are not shared with overall project. Assumes robust pretreatment system High Below grade, difficult to maintain Requires debris and sediment removal to maintain infiltration Requires regular maintenance of pretreatment system 10 to 25 Years5 Rough estimate, assuming robust pretreatment; could be much less without pretreatment If gallery is accessible, it may be possible to restore capacity a limited number of times before reconstruction. 1 Bullets provide explanation of and qualifications for ranking provided in table. 2 Bullets summarize the activities associated with reconstruction at the end of the effective life span and provide explanation of basis for estimated life span. 3 Based on WSDOT best professional judgment; systems have not been in place for full life cycle. 4 Not provided by WSDOT; estimated from Ballestero et al., 2007; Low Impact Development Center, 2005. 5 Best professional judgment; highly site specific and dependent on pretreatment methods used. Table 20. (Continued). a separate column is provided for these two categories. This table is intended to be used for planning purposes only. A site-specific cost analysis is recommended to help select VRAs once conceptual designs have been developed (see Section 5.4.5 for guidance). NCHRP Report 792: Long-Term Performance and Life-Cycle Costs of Stormwater Best Management Practices (Taylor et al., 2014a) evaluates whole life-cycle costs and develops whole life-cycle cost estimating tools for a variety of stormwater control measures, including VRAs. 4.4 Additional References for VRA Design and Maintenance Information The VRA fact sheets contain references to selected design and maintenance manuals that may be useful in providing information to support more detailed design efforts for specific VRAs as well as maintenance planning. Selected references are also listed in this section to provide a resource for users seeking additional information to support VRA design and maintenance planning. 4.4.1 Selected Nationwide Guidance The following nationwide guidance manuals may serve a supporting role in developing detailed VRA designs and developing maintenance plans: • Oregon State University et al. (2006). NCHRP Report 565: Evaluation of Best Management Practices for Highway Runoff Control. Transportation Research Board of the National Academies. http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_565.pdf.

84 Volume Reduction of Highway Runoff in Urban Areas • Geosyntec Consultants et al. (2011). NCHRP Report 728: Guidelines for Evaluating and Selecting Modifications to Existing Roadway Drainage Infrastructure to Improve Water Quality in Ultra- Urban Areas. Transportation Research Board of the National Academies. http://onlinepubs.trb. org/onlinepubs/nchrp/nchrp_rpt_728.pdf. • Taylor et al. (2014a). NCHRP Report 792: Long-Term Performance and Life-Cycle Costs of Storm- water Best Management Practices. Transportation Research Board of the National Academies. • WEF. (2012). Design of Urban Stormwater Controls. WEF and ASCE Manual of Practice 23. WEF Press. https://www.e-wef.org/Default.aspx?TabId=192&ProductId=18172. • Strecker et al. (2005). Critical Assessment of Stormwater Treatment Controls and Control Selec- tion Issues. 02-SW-01. Alexandria, VA: Water Environment Research Foundation; London: IWA Publishing. • Decentralized Stormwater Controls for Urban Retrofit and Combined Sewer Overflow (CSO) Reduction (WERF, Report 03-SW-3) http://www.iwapublishing.com/template.cfm? name=isbn184339748x. • Low Impact Development Design Manual (3-210-10) (United States Navy) http://www.wbdg. org/ccb/DOD/UFC/ufc_3_210_10.pdf. 4.4.2 Selected State-Specific DOT Guidance For reference purposes, Table 21 provides a selected inventory of recent state-specific DOT stormwater guidance manuals that may serve a supporting role in developing detailed VRA designs and developing maintenance plans. Users should adhere to local guidance and criteria, as applicable.

Volume Reduction Approaches 85 State Year Published Publication Title AZ 2009 ADOT Post-Construction Best Management Practices Manual For Highway Design and Construction http://www.azdot.gov/Inside_ADOT/OES/Water_Quality/Stormwater/PDF/adot_post_con struction_bmp_manual.pdf 1993 Highway Drainage Design Manual – Hydrology, http://www.azdot.gov/Highways/Roadway_Engineering/Drainage_Design/PDF/ADOTHig hwayDrainageDesignManual_Hydrology.pdf 2007 Highway Drainage Design Manual – Hydraulics, http://www.azdot.gov/Highways/Roadway_Engineering/Drainage_Design/PDF/ADOTHig hwayDrainageDesignManual_Hydraulics.pdf CA 2010 Caltrans Storm Water Quality Handbooks – Project Planning and Design Guide, http://www.dot.ca.gov/hq/oppd/stormwtr/ppdg/swdr2012/PPDG-May-2012.pdf 2012 Caltrans Highway Design Manual, http://www.dot.ca.gov/hq/oppd/hdm/hdmtoc.htm GA 2001 Georgia Stormwater Management Manual Volume 2, http://documents.atlantaregional.com/gastormwater/GSMMVol2.pdf MA 2004 Storm Water Handbook For Highways and Bridges, http://www.mhd.state.ma.us/downloads/projDev/2009/MHD_Stormwater_Handbook.pdf 2006 Massachusetts Project Development & Design Guide, http://www.mhd.state.ma.us/default.asp?pgid=content/designguide&sid=about MD 2000 2000 Maryland Stormwater Design Manual – Volumes I and II, http://www.mde.state.md.us/programs/Water/StormwaterManagementProgram/SoilErosi onandSedimentControl/Documents/MD%20SWM%20Volume%201.pdf MN 2000 MnDOT Drainage Manual, http://www.dot.state.mn.us/bridge/hydraulics/drainagemanual/ 2009 Stormwater Maintenance BMP Resource Guide http://www.lrrb.org/media/reports/2009RIC12.pdf NJ 2004 New Jersey Stormwater Best Management Practices Manual, http://nj.gov/dep/stormwater/bmp_manual2.htm NV 2006 Storm Water Quality Manuals – Planning and Design Guide, http://www.nevadadot.com/uploadedFiles/NDOT/About_NDOT/NDOT_Divisions/Engine ering/Hydraulics/2006_PlanningAndDesignGuide.pdf NY 2010 New York State Stormwater Management Design Manual, http://www.dec.ny.gov/chemical/29072.html, http://www.dec.ny.gov/docs/water_pdf/swdm2010entire.pdf OH 2012 Location & Design Manual, Volume 2 Drainage Design, http://www.dot.state.oh.us/Divisions/Engineering/Hydraulic/LandD/Pages/TableofConten ts.aspx OR 2011 Hydraulics Manual, ftp://ftp.odot.state.or.us/techserv/geo- environmental/Hydraulics/Hydraulics Manual/Table_of_Contents_rev_Nav.pdf PA 2010 PennDOT Drainage Manual, ftp://ftp.dot.state.pa.us/public/bureaus/design/PUB584/ RI 2010 Rhode Island Stormwater Design and Installation Standards Manual, http://www.dem.state.ri.us/programs/benviron/water/permits/ripdes/stwater/t4guide/desm an.htm SD 2011 Drainage Manual, http://sddot.com/business/design/forms/drainage/default.aspx TX 2011 Hydraulic Design Manual, http://onlinemanuals.txdot.gov/txdotmanuals/hyd/hyd.pdf WA 2011 Highway Runoff Manual, http://www.wsdot.wa.gov/publications/manuals/fulltext/m31- 16/Chapter5.pdf (updates expected in 2014) 2010 Hydraulics Manual, http://www.wsdot.wa.gov/publications/manuals/fulltext/M23- 03/HydraulicsManual.pdf 2012 Stormwater Manual for Western Washington, https://fortress.wa.gov/ecy/publications/summarypages/1210030.html Table 21. Selected state-specific DOT manuals related to stormwater management.

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Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 802: Volume Reduction of Highway Runoff in Urban Areas: Guidance Manual explores practices for the reduction of stormwater volumes in urban highway environments. The report outlines a five-step process for the identification, evaluation, and design of solutions for runoff volume reduction based on site-specific conditions. The manual also includes a set of volume reduction approach fact sheets and a user guide for the Volume Performance Tool.

NCHRP Web Only Document 209: Volume Reduction of Highway Runoff in Urban Areas: Final Report and NCHRP Report 802 Appendices C through F explores the research developed for this report to help achieve surface runoff volume reduction of highway runoff in urban areas.

The report is accompanied by a CD-ROM that contains a tool to estimate the performance of volume reduction. The CD-ROM is also available for download from TRB’s website as an ISO image. Links to the ISO image and instructions for burning a CD-ROM from an ISO image are provided below.

Help on Burning an .ISO CD-ROM Image

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CD-ROM 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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