Evaluation of Best Management Practices for Highway Runoff Control (2006)

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22 3.1 Introduction Most information about LID facilities and their design is contained in the LID Design Manual developed for this project and available on CRP-CD-63. This chapter only sum- marizes the status of LID and highway systems. State DOTs continue to demonstrate a significant amount of interest and effort in using individual LID techniques. Specific LID tech- nologies such as bioretention and soil amendments are already being incorporated into the roadway design and con- struction programs of several states, including Washington, Texas, Ohio, North Carolina, and Maryland. Much of the basis for the planning, design, and maintenance of these tech- niques for linear projects has been adapted from planning and land-development standards and specifications in state and local government programs or manuals. In addition, many state DOTs’ designs for rural roads include LID aspects, including use of swales and overland flow for drainage. 3.2 General LID Definitions LID is a decentralized source and treatment control strat- egy for stormwater management. The LID site design approach can be used to address planning as well as overall watershed regulatory requirements and resource protection goals. This approach uses an optimal combination of the fol- lowing design and management elements: • Conservation Design. Overall conservation goals—such as wetlands protection, habitat preservation, or aesthetic requirements—are integrated into the design. • Minimizing Development Impacts. Sensitive environmen- tal areas, such as soils with high infiltration rates or potential for erosion and stands of mature vegetation, are preserved by using highly detailed, site-specific design and engineering strategies and techniques. In the highway environment, this may include additional emphasis placed on alignment and realignment of roads. • Maintaining Watershed Hydrologic Timing. Designs goals include preserving runoff patterns and timing peak- runoff rates to approximate existing or predeveloped con- ditions. • Integrated Management Practices (IMPs). IMPs are mul- tifunctional, small-scale, source and treatment control stormwater management practices that can be integrated directly into the infrastructure and landscape. IMPs are an integral component of the highway design process; selec- tion of specific IMPs depends on the alignment and profile conditions. • Pollution Prevention (P2). P2 is the use of management techniques and materials that reduce or eliminate pollu- tion at its source. P2s work in the same way as the non- structural BMP systems discussed in Section 2.4.4. An LID design integrates natural hydrologic functions into the design to replicate the processes of storage, detention, infiltration, evaporation and transpiration, or uptake by plants in order to reduce runoff volumes, attenuate peak- runoff rates, and filter and remove pollutants from runoff. By incorporating controls specifically into upland areas, impacts to wetlands, streams, rivers, lakes, estuaries, and other sensi- tive areas can be reduced or eliminated. 3.3 LID Expands Stormwater Responsibilities The design and management elements listed above allow for LID to meet objectives in addition to replicating prede- velopment conditions and to help expand stormwater man- agement science into community development issues and programs. Some of the new concepts and areas opened to the stormwater planning community by the LID approach include the following: • An emphasis on bringing overall watershed conservation concepts into roadway design, including maintaining C H A P T E R 3 LID in the Highway Environment

23 runoff and recharge patterns that preserve the overall watershed resource protection goals. This encourages designers to consider impacts on habitats and sensitive environmental areas rather than just trying to meet a stan- dard discharge rate. • Consideration of the entire range of frequencies and dura- tions of storm events during the course of any given time period. Many conventional approaches target control for only a few specific storm events (e.g., 2-yr 24-hr or 1 in. over 24 hrs). By using natural processes of storage, ET, and infiltration, the runoff from smaller but much more fre- quent microscale storms can be controlled or completely eliminated. These microstorms typically constitute 70 to 90% of the total annual precipitation (Wright and Heaney 2001). Larger storms can be controlled by adding sufficient storage volume distributed throughout the site or in more centralized detention facilities. • Use of minimization techniques. For instance, reducing lane and paved shoulder widths can reduce the volume of stormwater and required area for treatment. This reduction in infrastructure could potentially save capital and increase the area of developable land (Thurston et al. 2003). • Designing stormwater controls for targeted pollutant issues and using a treatment train approach when multiple opportunities exist for treatment by a variety of techniques along the flow path. A high-efficiency filter path may now be possible. • Use of P2 to reduce pollutant loads by the selection of road and associated infrastructure materials and operational pro- cedures that minimize sources of pollution. Pollutants can be treated more closely to their source rather than conveyed and allowed to potentially escape into the environment. • Integration of stormwater controls into the roadway, land- scaping, and/or infrastructure, including adjacent systems. This creates opportunities to share costs for construction, construct facilities incrementally, and create aesthetically pleasing landscapes and building components that can manage stormwater (NAHBRC 2003). Historically, the approach to managing stormwater quality on highways has been to collect and convey stormwater through swales or pipes to a centralized end-of-pipe dis- charge point or treatment system. However, the rolling topog- raphy in piedmont areas and the flat slopes in coastal zones limit the length and extent of any centralized system. The existing highway runoff infrastructure in the United States is inherently “distributed” and may provide unintentional, but likely significant, water-quality and -quantity benefits. There- fore, a key difference between conventional and LID designs for roads is the scale of the distribution of runoff and the inte- gration of water-quality controls into and throughout the drainage network. Any application of LID in the highway environment must recognize opportunities for runoff con- trol resulting from the inherent characteristic of many small catchments distributed over long, linear roadways. Note that the LID approach is not a significant departure from current rural road design practices, in which curb- and-gutter systems are not typically used. The difference is that, in the LID approach, flows are specifically designed not to be concentrated or transported for long distances. The LID concept does provide a formal framework in which to select appropriate surface drainage, landscaping, and infiltration designs. 3.4 Microstorm Management Management of microstorms (frequently occurring storms with high or low intensity and short duration) is one of the key strategies of LID. LID emphasizes treatment of small storms by taking advantage of multiple, distributed, small-scale stor- age, detention, infiltration, and evaporation functions within the site. Large events can be controlled by increasing the deten- tion and retention volume of more centralized facilities. Wright and Heaney (2001) suggest the following guiding prin- ciples for the control of microscale storms: • Minimize directly connected impervious area (DCIA). • Increase flow paths and times of concentration. • Increase ET and infiltration, but not at the expense of nuisance flooding. 3.5 LID Stormwater Management Framework LID as a form of BMP in a highway system may be inte- grated into the overall stormwater management components of the highway. The characteristics of highway systems that favor integration of LID can be summarized as follows: • LID favors the use of decentralized source control systems. Linear highway systems are typically decentralized already because the only available controllable drainage area is the right-of-way. • The ET of microstorm runoff is a key strategy for reducing runoff volumes regardless of the infiltration potential at a site. • Over 85% of state DOT roads are “rural”(see Table 1-2). As most rural roads use open drainage systems, it is reasonable to assume that the vast majority of them already control some portions of microstorms by soil soaking and ET and/or infiltration on the adjacent right-of-way. The major exceptions would be roads where there are steep slopes or other geotechnical elements necessitating a curb and gutter or where the roadside soils have low permeability.

24 Consequently, swale drainage or overland flow/dispersal tends to be the dominant BMP for the majority of high- ways. Many of the swale systems have not been optimized for water retention and water-quality treatment. A compo- nent of LID for these types of systems would be to develop design principles to maximize water retention, reduce runoff rates, and improve treatment. • The 14.3% of state DOT roads that are not rural have curbs and gutters and associated closed drainage systems. Argu- ments have been advanced to reduce or eliminate curbs and gutters on low-use access roads and low-use parking areas (Li et al. 1998; Heaney et al. 2003). Curb-and-gutter drainage is much more essential in high-use state DOT transportation networks. Control of microstorms in these facilities must be developed as part of a closed drainage system. • Unlike urban developers, who can view land use as a deci- sion variable to reduce wet-weather impacts, state highway designers do not typically have the option of changing road locations. State highway designers can, and already do, use trading approaches (as applied to some NPDES and total maximum daily load [TMDL] permitting, for instance) to mitigate wet-weather impacts by more intensive controls elsewhere. Applications of LID technology to the highway environ- ment presented in this project incorporate the characteristics listed above. Detailed discussions of specific BMP and LID facilities are contained in the Guidelines Manual and LID Design Manual, respectively. The hydrologic analysis described in Chapter 10 of this report relies strongly on continuous simulation to evaluate the relative impact of microstorms. Additional review of the LID literature, back- ground information, and design guidance are contained in the LID Design Manual.