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

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41 6.1 Introduction Although various land management practices may influ- ence the types and loads of stormwater pollutants, hydrologic characteristics will determine the relative transport of the contaminants to the BMP system. A number of hydrologic factors may vary the pollutant discharge during a storm event, including rainfall intensity, storm duration, and dura- tion between rainfall events (antecedent dry days). Also, a number of site-specific factors affect the overall loading to a BMP system by influencing the relative partitioning of constituents and the delivery of the constituents to a site. Regional hydrologic influence on performance of BMPs is discussed in Chapter 10 of this report, and a detailed analysis is presented in Chapter 7 and Appendix C of the Guidelines Manual. 6.2 Urban Water Balance As described in the previous chapter, urbanization replaces pervious surfaces that are generally vegetated with impervious surfaces. Furthermore, the process of construction and alter- ation of the land surface changes the nature of the soils and vegetation on the remaining pervious surfaces. The environ- ment of a highway is particularly striking in this regard because of the heavy equipment and imported fill that are usu- ally a part of the construction process. All of these activities drastically alter the vertical water balance among rainfall, infil- tration, ET, and surface runoff. Before construction, native (or agricultural) vegetation may have transpired much of the rainfall, but roadside and other replacement vegetation in the urban setting may not behave similarly. If there is less ET, then more pressure is placed on infiltration to mitigate increased runoff from nearby impervious surfaces. Compacted soils may not easily perform this function (Pitt et al. 1999; 2001), although part of the BMP and LID design process is to enhance infiltration as much as possible. Vegetation that encourages ET (ideally without requiring extensive irrigation to maintain it during the dry season) provides a means to remove water to the atmosphere that might otherwise have to be infiltrated or else simply run off. For instance, experience with Portland’s eco-roof program has shown that vegetated roofs reduce runoff even during the winter, when vegetation is relatively dormant and when there is considerable seasonal rainfall (T. Liptan, Portland Bureau of Environmental Ser- vices, personal communication, 2002). Where does this water go if it cannot infiltrate (into the roof!)? Enhanced ET is the answer, and ET will act to reduce runoff in most facilities for hydrologic source control. With urbanization comes irrigation of planted vegetation, leading to dry-weather flow in areas that are otherwise deserts during most of the year, such as Southern California. This introduced artificial baseflow means that runoff may need to be managed during the whole year, not just during the rainy season. Possible advantages of having runoff during the whole year include the ability to maintain wetlands and wet ponds for treatment. Disadvantages include leaching of previously bound chemicals, such as nitrates and selenium (Strecker et al. 2002). If highway riparian vegetation is irri- gated, this could also be a factor in BMP selection. Although irrigation is unusual along highways, it is not unknown, and drainage of the water table along highway embankments or cuts could have a similar effect (leaching of chemicals by the drainage effluent), at least temporarily, which might require mitigation. 6.3 Hydrologic Site Characterization The discussion in the previous section, coupled with the common and unfortunate lack of constituent treatability data (see Section 4.5), underlines the need for site-specific hydro- logic data for BMP and LID design. If a device relies upon infiltration, then infiltrometer tests should be conducted to C H A P T E R 6 Influence of Hydrologic Characteristics

quantify infiltration rates, especially for areas with disturbed soils (e.g., near highways). If ET will be encouraged (e.g., through vegetation), good local estimates of ET should be obtained (e.g., the U.S. Bureau of Reclamation’s AgriMet web site for western states: http://www.usbr.gov/pn/agrimet/index. html). A characterization of soils in the catchment and par- ticulates from the roadway will aid in estimating the effec- tiveness of sorption in reducing constituents such as heavy metals. Is a baseflow present that might leach chemicals from soils in the catchment and/or lead to a dewatering of soils in the highway embankment? What seasonality is exhibited in the precipitation records? All these factors (and more) affect BMP/LID design. A science-based approach to design includ- ing measurement of key parameters such as infiltration rates is recommended. 6.4 First-Flush Phenomenon The first flush generally refers to the delivery of a dispro- portionately large load of constituents during the early part of a runoff hydrograph (Sansalone et al. 1997); this phenomenon has already been discussed in conjunction with specific pollu- tants in Section 4.4 and is recapped here. A number of juris- dictions base system sizing criteria on the treatment of the first flush, maximizing efficiency by using the system to achieve an overall net reduction of pollutants without treating the relatively clean water that discharges during the later stages of a storm event (Minton 2005). First-flush phenomena are dis- cussed in numerous studies (Barrett et al. 1995a; Driscoll et al. 1990; Field et al. 2000; Glenn et al. 2002; Hoffman et al. 1985; Hoffman and Quinn 1987; O’Shea et al. 2002; Sansalone et al. 1997; Sansalone et al. 1998), and it has been noted that a first flush often has a variety of stormwater constituents such as hydrocarbons, metals, sediment, and nutrients. The extent to which a first flush is observed has been found to vary according to the flow event and the relative partitioning of constituents between particulate and dissolved phases. A mass-to-volume curve (see Figure 4-5) is one method of displaying the relationship between mass transport and the storm flow event, in order to predict the strength of a first flush. Normalized with respect to time and based on the dura- tion of the storm event, the mass and volume curves for the event are plotted simultaneously. The first flush occurs when the mass curve is above the flow volume curve (Sansalone et al. 1997). The ratio of the mass curve area to the flow curve area represents the strength of the first flush, and the relative slope of the mass curve represents the mass mobilization rate (Sansalone et al. 1997). The first flush may or may not appear at a given site for a given constituent. For instance, solids may erode more read- ily later in a storm event, when soils are saturated (Sutherland and Jelen 2003). Generalizations cannot be made about whether or not there will “always” be a first flush for con- stituents in particulate, dissolved, or mixed forms; Christina and Sansalone (2002) emphasize the need to characterize any possible first-flush phenomenon on the basis of the particle size distribution of the constituent. But most importantly, BMP effectiveness can rarely be achieved with a system based on capturing just the early part of the storm event, even though this leads to more economical treatment (because a lower volume needs to be controlled). Better practice—or at least, more demonstrably justifiable practice—will usually be based on capture of a specified volume for all storms, as deter- mined by a continuous simulation. This method is illustrated in Section 10.4. Clearly, the variability of concentration and flow during a storm event strongly influences the selection of a BMP. High- rate devices, such as filters and some proprietary devices, are very susceptible to inefficiencies created by possible elevated concentrations late in a storm, whereas devices that typically store runoff from more than one event (e.g., ponds, wetlands) are less susceptible to these issues. 6.5 Pavement Residence Time Pavement residence time generally refers to the lag time between rainfall and runoff increments and influences the partitioning of constituents and the loading dynamics for a BMP at a particular site. Generally, the pavement residence time is characterized as either the average or initial residence time. As described in Sansalone et al. 1997 and Sansalone et al. 1998, the initial pavement residence time (IPRT) refers to the lag time between the initial rainfall and initial runoff and is indicative of the time required to wet the pavement sur- face and fill depression storage before pavement runoff occurs. Generally, the IPRT has been found to be lowest for higher-intensity events, although studies have found that the IPRT is fairly consistent for most levels of intensity, only dif- fering by a factor of 2 (Sansalone et al. 1997; Sansalone et al. 1998). The APRT is one measure of the time of concentration for a particular event, and, using hydrographs and hyeto- graphs, APRT can be determined by calculating the mean time differential between each hydrograph and hyetograph centroid during a storm event (Sansalone et al. 1997). Gener- ally, the lower the APRT, the higher the intensity of the event because of the shorter time associated with lateral sheet flow over the site. For the 0.07-acre site used in Sansalone et al. (1998), IPRT generally ranged from 3 to 14 min, with a mean time of 8 min, whereas the APRT ranged from 1.5 to 15 min, with a mean time of 5.2 min. Residence time is an important hydrologic characteristic to consider when determining BMP loading from a runoff vol- ume perspective and when sizing a volume-based BMP sys- tem to maintain a certain holding time. From a water-quality 42

perspective, residence time has been shown to be important with regard to the partitioning of constituents, specifically for metals, sediment, and hydrocarbons. A longer APRT gener- ally yields a higher dissolved fraction of sediment and metals (Sansalone et al. 1997; Sansalone et al. 1998), and, given that removal of dissolved constituents with BMP systems is generally a much more difficult process, this hydrologic characteristic should be considered in the BMP selection process. For example, largely pervious areas with long APRT (e.g., subdivisions and low-density residential areas) are often treated using a wet pond or other sedimentation facility because of aesthetic benefits, available land, lower relative cost, etc. However, sedimentation facilities are rarely able to remove dissolved pollutants via their primary FPC (sedimen- tation), and therefore other BMP alternatives for these types of areas should possibly be explored. 6.6 Flow Rate The transport (or load or flux) of constituents off a surface is the product of concentration times discharge and thus is a function of catchment characteristics that influence discharge, including cover, slope, porosity, roughness, depressions, and the nature of the rainfall (Hoffman and Quinn 1987). The flow rate may be indicative of the amount of remaining mass avail- able for discharge after the storm event because as the flow rate increases so does the mass of constituents that are able to be transported at a given rate (Hoffman and Quinn 1987). In general, BMP pollutant-removal efficiency, flow reduction capabilities, and overall effectiveness depend on the flow into and out of the system. Flow rate and intensity influence the particle size distribu- tion of sediment in a sample, in addition to the total mass carried during the event (Minton 2005). As mentioned in Section 4.4.3 Sansalone et al. (1997) found that particles smaller than 8 µm were rapidly transported during high-flow events, and a larger proportion of total mass for the site was transported. Low-flow events generally do not wash off as large a proportion of total mass, indicated by a presence of both total and dissolved mass still available for delivery (Sansalone et al. 1997). Generally, however, as flow rate increases, the proportion of larger particles also increases. Transport of pollutants such as metals and hydrocarbons is highly influenced by the sediment content in a sample and thereby also affected by flow rate. Hoffman et al. (1985) found that large peaks in loading rates of pollutants (TSS, hydro- carbons, lead, iron, copper, and chromium) are synonymous with higher flow rates. 43