A Watershed Approach to Mitigating Stormwater Impacts (2017)

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36 Background The development of efficient mitigation strategies requires an understanding of the impacts of stormwater management and required mitigation potential of on-site and off-site treatment measures, along with the primary watershed processes that these measures are protecting. This includes the establishment of linkages between stormwater impacts and effective mitigation options that account for key watershed processes and their effect on water quality. Dem- onstrating the linkages between stormwater impacts on watershed processes and possible mitigation measures is critical because of the need to present a range of acceptable and equivalent mitigation options to regulatory and resource agencies when water quality mitiga- tion requirements cannot be met at the project location. A variety of factors can limit on-site mitigation opportunities for highway projects. Examples include limited physical space within the ROW; steep slopes or use restrictions in the clear recovery area; and/or excessive implementation and maintenance costs (e.g., within ultra-urban areas). An approach that uses ecosystem services as a factor in evaluating potential mitigation opportunities can provide an additional lens in which the benefits of each viable mitigation option can be judged. Several approaches are advanced in this chapter in which inclusion of ecosystem services can add to the value of prioritizing potential mitigation. Watershed characterization information (Chapter 3) and existing watershed planning efforts (Chapter 5) should be identified and reviewed when con- sidering off-site mitigation alternatives. Methods for linking project impacts to off-site mitigation options, along with a clearly defined procedure for ranking and prioritizing mitigation opportuni- ties through the use of the WBSMT are presented in Chapter 6. Stormwater Impacts and Pollutant Loads from Impervious Surfaces The stormwater impacts and pollutants associated with state DOT road projects are most often associated with an increase in impervious area. In Chapter 3, several stormwater impacts, pollutants of concern, and hydrologic/hydraulic conditions of concern on which regulatory requirements often focus were identified (see Table 2 and Table 7). Of these, four have been identified as those that would benefit most from a watershed- and ecosystem-based mitigation framework. These include total suspended solids (TSS), nitrogen and phosphorus (eutrophica- tion), runoff volume, and increased stream power/energy. As an example, consider the impact of BMP placement on the treatment of two pollutants: heavy metals and TSS. BMPs designed to meet regulatory requirements for heavy metals typically focus on treating and managing highway runoff. For TSS, better and more cost-effective results may be obtained by thinking Framework for Evaluating Off-Site Mitigation Options C H A P T E R 4 To find the WBSMT, go to the TRB website and search for NCHRP Research Report 840.

Framework for Evaluating Off-Site Mitigation Options 37 beyond the existing project boundary and ROW and within the watershed. The USEPA supports pollutant trading for sediment and nutrients, but does not yet support trading for metals or other toxics (USEPA 2009). Watershed-based mitigation efforts focused on TSS, eutrophication, runoff volumes, and stream power/energy impairments result in more predictable and recog- nizable benefits and there is greater opportunity to mitigate these sources in other watershed locations. In addition, these are common stressors that require mitigation in many watersheds across the United States. The effects of these four stressors are also relatively straightforward to calculate. A signifi- cant body of knowledge and recognized analytical techniques are available to estimate their impacts. They can also be estimated through the use of national datasets and commonly used DOT analytical techniques. The fate and transport of a stressor such as TSS also make it valu- able for approximating the loads of other pollutants that are physically bound or associated with TSS. A brief description of each of the four stressors, their association to roadway systems, and their known stormwater impacts is provided here. • TSS: TSS consists of sediment and particles that are larger than 2 micrometers. Grit, road dirt, sediment from unstabilized vegetated areas (i.e., erosion), and degraded road wearing surfaces (asphalt pavement) are the most common sources of roadway-based TSS loads. These loads are almost always transported in road runoff (Roesner et al. 2007). TSS is associated with many other constituents in highway runoff. TSS can transport or be a source of nutrients, metals, and other pollutants and can settle and be deposited in the stream valley or bed. This can result in impairments to spawning areas, reduced conveyance and flood capacity, and modifications to other instream and overbank area hydrologic and natural resources processes. • Eutrophication: An overabundance of nutrients (primarily nitrogen and phosphorus) in road runoff can stimulate excessive plant and algae growth in receiving waters. As the vegetation dies and decomposes, high levels of organic matter and the decomposing organisms deplete the oxygen, resulting in low dissolved oxygen and potential anoxic conditions that severely affect aquatic species. This process is known as eutrophication. The main sources of nutrients for runoff from roads are atmospheric deposition, soil erosion, decomposing organic debris, animal waste, and fertilizer applied to the road ROW (Leisenring et al. 2014). • Altered Runoff Hydrology: When hardscape and landscape features, such as roadways and sidewalks, are constructed within transportation corridors, there is a disruption of the natu- ral water budget. Increased impervious surfaces can reduce the ability of land cover to infil- trate rainfall and disrupts the natural hydrologic and hydraulic processes. This increases the volume of surface runoff and associated pollutant loads and decreases the delivery of precipitation to groundwater sources. The reduction of vegetation and surface depression storage reduces evaporation and evapotranspiration. The reduction in the replenishment of groundwater results in a reduction of baseflow to streams, which adversely impacts stream ecosystems. • Increased Stream Power/Energy: The introduction of impervious cover in the landscape increases the rate and volume of runoff, while the duration of the high flows and time to peak often decreases. This compressed pulse of water increases the amount of work done on the stream – expressed as stream power. Increases in stream power frequently lead to scouring, downcutting, increased sediment transport, and decreased stream meandering compared to existing stream conditions as well as impacting streambeds, stream banks, and riparian habitat. If left unchecked, increased stream power can lead to reduced stream gradients, which are manifest as head cutting as the stream gradient adjusts to a new quasi-equilibrium condition. This continued process can contribute to increased TSS loads to receiving waters. Stream power values are most often calculated using data and approximations/assumptions

38 A Watershed Approach to Mitigating Stormwater Impacts versus being directly measured using in-stream instrumentation. This is presented in the following equation: Ω = ρ (6)g Q S Where W is stream power, r is density of water (1000 kg/m3), g is gravitational acceleration (9.81 m/sec2), Q is discharge (often proportional to drainage area), and S is slope of channel. Stream power energy is lost by a flowing stream as it performs work on its rock bed, bed- covering sediments, and banks. The impact of increased stream power frequently leads to scouring, downcutting or headcutting (increased bed and bank erosion), increased sediment transport, changes in stream meandering pattern, and changes to the stream gradients. Baseline Physical Watershed Processes and Functions The baseline physical conditions of a watershed are those conditions that can be used in the comparison of mitigation options. The baseline physical functions in the watershed are a result of the hydrologic, hydraulic, physical, chemical, and biological processes. These processes can be separated into upland processes and in-stream or near stream processes (Table 12). Upland processes are those that affect the delivery and routing of water and pollutants to receiving water through hydrologic changes and land development or disturbances in upland areas of the water- shed, whereas in-stream or near stream processes are those which occur within the stream banks and riparian zones. While many of these processes are interrelated, they have been separated into these categories to help relate the processes that may be influenced by mitigation efforts. These processes are defined below to provide a common understanding of how they are involved in altering water quality and quantity. Upland Processes • Interception – The precipitation captured by vegetative covering (leaves, branches, etc.). Generally, the denser the tree canopy in a riparian area, the more interception that occurs. Increased interception includes the wetting of surfaces and slows the rate at which rain impacts the ground, which increases opportunities for infiltration and evaporation. • Infiltration – The ability of soil to transfer precipitation from surface water to shallow and deep groundwater sources. The rate of infiltration is heavily dependent upon factors such as soil type (sand, silt, loam, and clay), soil structure, organic content, vegetative cover, land slope, and intensity of precipitation event. Upland Processes Interception Soil building Infiltration Erosion Evapotranspiration Overland flow Uptake and retention of pollutants by vegetation Plant decay Surface/subsurface storage In-Stream / Near Stream Processes Channel and floodplain storage Erosion and sediment transport Filtration Shading Sedimentation Uptake and retention by riparian vegetation Aeration Hyporheic flow and groundwater discharge Table 12. List of physical watershed processes.

Framework for Evaluating Off-Site Mitigation Options 39 • Evapotranspiration – The process of water movement from the ground through a plant and its evaporation from aerial parts, such as from leaves, stems, and flowers. • Uptake and retention of pollutants by vegetation – The action of trees, shrubs, and plant roots to accumulate and retain pollutants via their natural metabolic processes. • Soil building – The natural process of organic material decay and the establishment of an organic soil growing medium that is suitable for plant and animal life. Organically rich soils tend to have a high capacity to retain water and adsorb pollutants. • Erosion – The detachment of soil particles from the ground surface, which can occur through raindrop, sheetflow, rill, or gully erosion. This also disrupts local soil formation. • Overland flow – Surface runoff that occurs when precipitation cannot be infiltrated because the soil is saturated or because the precipitation occurs at a rate greater than the soil can infil- trate it, also known as precipitation excess. Overland flow is a natural process and one that highways may or may not alter significantly. Increases in overland flow can increase eutrophi- cation as nutrients accumulate on these surfaces. • Plant decay – The decomposition of organic matter from plants which also enhances soil formation through organic matter supply and microbial food sources. • Surface/subsurface storage – Depressions on the ground surface and pore space in the soils that retain rainfall and runoff. Enhanced also by infiltration (above) and can increase water available for evapotranspiration. In-Stream/Near Stream Processes • Channel and floodplain storage – The ability of adjacent floodplain to reduce the peak stormwater discharge rate through available storage. • Filtration – The ability of vegetation and soils in the floodplain and within the stream banks to trap detritus and filter particulates. • Sedimentation – The settling and accumulation of sediment and associated pollutants on the channel or receiving water bed due to relatively slower velocities, shallower depths, and longer flow paths. • Aeration – The physical process of entraining air and transferring oxygen into the water body through turbulence and increased surface water areas. • Erosion and sediment transport – Bank and channel erosion and the subsequent move- ment of sediment through the action of water or air. Stable streams exhibit a natural balance between sediment supply and downstream transport. • Shading – Tree canopies shade streams and surface areas from solar radiation that can heat surface waters. • Uptake and retention by vegetation – The action of roots of trees and other plants to accu- mulate pollutants via their natural metabolic processes. Linkages of Stormwater Impacts and Physical Watershed Processes Once the baseline physical processes have been defined, the next step is to identify the linkages between physical watershed processes and stormwater impacts. Mitigation efforts often focus on the effects or outcomes to water quality (and quantity) when these processes are altered from their natural state. Table 13, while somewhat subjective, summarizes the physical processes that can be related to each of the stormwater impacts. The table also provides an understanding of whether an alteration in the physical process directly or indirectly contributes to stormwater impacts (net negative effect) or whether, if the physical process is mitigated or improved, there may be a direct or indirect offset (net positive effect) to the magnitude of the stormwater impact.

40 A Watershed Approach to Mitigating Stormwater Impacts The relationship between the impact of upland and in-stream/near stream physical processes identified in Table 13 and each of the four water quality stressors of concern (increased TSS, eutrophication, increased volume, and increased stream power) are identified and discussed here. Upland Processes Affecting Quality or Condition of Receiving Water Interception • TSS: Indirect Positive Effect (+1) – Vegetation loss, which reduces interception, may indi- rectly impact TSS levels through erosion. • Eutrophication: Negligible Effect (0) – Increased interception does not significantly affect eutrophication. • Stream Power/Energy: Indirect Positive Effect (+1) – High rates of tree loss or substantial change in type of vegetation as measured by reduced interception in upland areas (including the project area) may cause higher runoff rates resulting in greater stream power/energy in an impacted watershed. • Volume: Direct Positive Effect (+2) – Reduced interception in upland areas (including the project) can result in greater runoff volumes. Infiltration • TSS: Indirect Positive Effect (+1) – As with interception, increased infiltration can reduce the runoff rate and volume which may reduce erosion and associated sediment loads delivered to the receiving water. • Eutrophication: Indirect Positive Effect (+1) – An increase in infiltration can reduce the amount of surface runoff and nutrient loads (particularly phosphorus) reaching receiving water, thereby reducing eutrophication. • Stream Power/Energy: Direct Positive Effect (+2) – Infiltration has a direct effect on reducing runoff rates and volumes which can reduce typical wet weather flow rates and stream power/ Physical Watershed Processes Stormwater Metrics of Receiving Water Impacts Increased TSS Eutrophication Increased stream power Increased volume Upland Processes Interception +1 0 +1 +2 Infiltration +1 +1 +2 +2 Evapotranspiration +1 +1 +1 +2 Pollutant uptake and retention by upland vegetation 0 +1 0 0 Soil building +1 +1 +1 +1 Upland erosion -2 -2 0 0 Overland flow -1 -1 -2 -2 Surface/subsurface storage +1 +1 +2 +2 In-Stream/Near Stream Channel and floodplain storage +1 0 +2 +2 Filtration +2 +1 +2 0 Sedimentation +2 -1 0 0 Aeration 0 +2 0 0 Erosion and sediment transport -2 -1 0 0 Shading by riparian tree canopy 0 +1 0 0 Pollutant uptake and retention by riparian vegetation 0 +1 0 0 Note: +1 = minor or indirect positive effect, +2 = significant or direct positive effect, 0 = negligible effect, -1 = minor or indirect negative effect, -2 = significant or direct negative effect Table 13. Stormwater impacts on receiving water in relation to identified physical processes.

Framework for Evaluating Off-Site Mitigation Options 41 energy but may also increase base flow during drier periods which may increase stream power/ energy. Increased base flow is likely to represent more natural, undeveloped conditions. • Volume: Direct Positive Effect (+2) – Infiltration has a direct effect on reducing runoff volumes. Evapotranspiration • TSS: Indirect Positive Effect (+1) – As with interception and infiltration, evapotranspiration can reduce the runoff rate and volume which may reduce erosion and associated sediment loads delivered to the receiving water. • Eutrophication: Indirect Positive Effect (+1) – Reduced runoff volumes reduce nutrient loads and therefore the potential for eutrophication. • Stream Power/Energy: Indirect Positive Effect (+1) – Reduced volume of runoff through evapotranspiration may result in a relatively minor reduction in stream power/energy compared to more developed conditions. • Volume: Direct Positive Effect (+2) – Reduced volume of runoff through evapotranspiration may result in a relatively minor reduction in runoff volume. Uptake and Retention of Pollutants by Upland Vegetation • TSS: Negligible Effect (0) – Uptake by vegetation does not affect TSS loads (also see volume and rate of runoff impacts by evapotranspiration). • Eutrophication: Indirect Positive Effect (+1) – Uptake and retention of phosphorus and nitro- gen by upland vegetation reduces these nutrient contributions in upland runoff, which may decrease eutrophication in receiving waters. • Stream Power/Energy: Negligible Effect (0) – No significant relationship between increased stream power/energy and pollutant uptake by vegetation. • Volume: Negligible Effect (0) – No significant relationship between increased pollutant uptake by vegetation and volume of runoff. Soil Building • TSS: Indirect Positive Effect (+1) – Rebuilt soils can infiltrate and hold more precipitation thereby reducing runoff and TSS loads. Soil building can also offset topsoil loss in upland areas and decrease TSS loading in upland runoff. • Eutrophication: Indirect Positive Effect (+1) – Soil building can regenerate upland topsoil loss that may have contributed to upland runoff and increased eutrophication of the receiving water. • Stream Power/Energy: Indirect Positive Effect (+1) – Soil building may improve permeability of soils, thereby reducing upland runoff rates and decreasing stream power/energy. • Volume: Indirect Positive Effect (+1) – Soil building may improve permeability of soils, thereby reducing upland runoff volumes. Upland Erosion • TSS: Direct Negative Effect (-2) – Soil erosion can lead to increased TSS within the stream. • Eutrophication: Direct Negative Effect (-2) – Higher soil erosion can be associated with higher nutrient loading, and therefore higher eutrophication potential. • Stream Power/Energy: Negligible Effect (0) – Upland erosion has an insignificant effect on stream power/energy. • Volume: Negligible Effect (0) – Upland erosion has an insignificant effect on in-stream volumes. An increase in runoff volume may increase erosion, but it is difficult to directly connect the upland process of erosion to an increase in the stream’s discharge volume. Overland Flow • TSS: Indirect Negative Effect (-1) – Overland flow can lead to soil erosion and increased TSS within the stream.

42 A Watershed Approach to Mitigating Stormwater Impacts • Eutrophication: Indirect Negative Effect (-1) – Overland flow can result in stormwater runoff coming into contact with nutrient rich soils that could lead to a higher eutrophication potential. • Stream Power/Energy: Direct Negative Effect (-2) – Increased overland flow can result in increased stream power/energy. • Volume: Direct Negative Effect (-2) – Increased overland flow may result in an increased stream volume. Surface/Subsurface Storage • TSS: Indirect Positive Effect (+1) – Storage in the watershed may reduce the volume of runoff and the potential to transport sediment. • Eutrophication: Indirect Positive Effect (+1) – The addition of upland storage may have a minor effect on receiving water eutrophication by reducing erosional flows. • Stream Power/Energy: Direct Positive Effect (+2) – Storage in upland depressions, soils, and wetlands will have a direct effect on the rate of runoff to the receiving water. • Volume: Direct Positive Effect (+2) – Upland storage will have a direct effect on the water balance of the watershed and the total volume of runoff reaching the receiving water. In-Stream/Near Stream Processes on Water Quality Channel and Floodplain Storage • TSS: Indirect Positive Effect (+1) – Channel and floodplain storage may reduce in-channel flow rate decreasing in and near stream contributions. • Eutrophication: Negligible Effect (0) – Channel and floodplain storage does not have a significant effect on eutrophication. • Stream Power/Energy: Direct Positive Effect (+2) – Channel and floodplain storage may reduce in-channel stream power/energy. • Volume: Direct Positive Effect (+2) – Channel and floodplain storage may reduce receiving water volume. Filtration • TSS: Direct Positive Effect (+2) – Improved natural filtration by riparian soils and vegetation, as well as aquatic vegetation, can reduce TSS concentrations in receiving waters. • Eutrophication: Indirect Positive Effect (+1) – Improved natural filtration by riparian soils and vegetation may result in decreased nitrogen and phosphorus contribution to receiving water, reducing the potential for eutrophication. • Stream Power/Energy: Direct Positive Effect (+2) – Improved natural filtration by riparian soils and vegetation can reduce rate of flow to receiving waters and decrease stream power/energy. • Volume: Negligible Effect (0) – Filtration through floodplain soils can reduce flow rates. Filtration alone may have little effect on volume (but see floodplain storage). Sedimentation • TSS: Direct Positive Effect (+2) – Natural sedimentation in pools, wetland areas, and flood- plains can reduce TSS concentrations in receiving waters. • Eutrophication: Indirect Negative Effect (-1) – Sedimentation within receiving waters may increase nutrient loads resulting in increased eutrophication. For example, phosphorus-rich sediment may significantly impact lakes, reservoirs, and slow moving rivers. Natural sedimen- tation in riparian wetland areas and floodplains can reduce nitrogen and phosphorus contribu- tions to the receiving waters. • Stream Power/Energy: Negligible Effect (0) – Sedimentation within the receiving water may have an indirect positive or indirect negative effect on stream power/energy depending on where it occurs, but in general is considered negligible.

Framework for Evaluating Off-Site Mitigation Options 43 • Volume: Negligible Effect (0) – Sedimentation within receiving waters does not significantly reduce in-stream volumes. Aeration • TSS: Negligible Effect (0) – No significant relationship exists between aeration and increased TSS. • Eutrophication: Direct Positive Effect (+2) – Increased aeration can reduce the potential for eutrophication by increasing biological oxidation. • Stream Power/Energy: Negligible Effect (0) – Aeration does not significantly affect stream power/energy. • Volume: Negligible Effect (0) – Aeration does not significantly affect receiving water volume. Erosion and Sediment Transport • TSS: Direct Negative Effect (-2) – An increase in sediment transport increases TSS loading. • Eutrophication: Indirect Negative Effect (-1) – Higher stream erosion and sediment transport rates can lead to increased nutrient loading, and therefore higher eutrophication potential. • Stream Power/Energy: Negligible Effect (0) – Increased stream erosion and sediment trans- port reduces channel slope and stream power/energy. • Volume: Negligible Effect (0) – Increased stream erosion and transport has an insignificant effect on receiving water volume. Shading by Riparian Tree Canopy • TSS: Negligible Effect (0) – Shading has an insignificant effect on TSS levels (see interception and evapotranspiration for water balance properties of tree canopy). • Eutrophication: Direct Positive Effect (+1) – Shading may have a minor direct (reducing sun- light needed for photosynthesis) and indirect (lower temperatures with shaded areas) effect at reducing eutrophication (e.g., decrease in algae growth). • Stream Power/Energy: Negligible Effect (0) – Shading has an insignificant effect on stream power/energy. • Volume: Negligible Effect (0) – Shading has an insignificant effect on receiving water volume. Uptake and Retention of Pollutants by Riparian Vegetation • TSS: Negligible Effect (0) – Pollutant uptake by riparian vegetation has an insignificant effect on TSS loads. • Eutrophication: Indirect Positive Effect (+1) – Uptake and retention of phosphorus and nitrogen by riparian vegetation reduces these nutrient concentrations, which may decrease eutrophication. • Stream Power/Energy: Negligible Effect (0) – Pollutant uptake by riparian vegetation has an insignificant effect on stream power/energy. • Volume: Negligible Effect (0) – Pollutant uptake by vegetation has an insignificant effect on receiving water volume. Mitigation Options In previous chapters it was noted that mitigation options are often related to where the mitiga- tion can be located. The approach taken has been to describe these mitigation options according to their location (i.e., off-site, on-site) and then by their classification (i.e., in-kind, out-of-kind). Typical mitigation options based on location and the type of mitigation are shown in Figure 4. Location of Mitigation The goal of watershed-based mitigation is to directly address and/or offset the impacts of develop ment on receiving waters by giving the project planner/designer the flexibility to select the

44 A Watershed Approach to Mitigating Stormwater Impacts appropriate mix of on-site and off-site practices. On-site options are located on a project site or within the ROW adjacent to the project and directly associated with the project area. The goal of on-site mitigation is to directly mitigate the stormwater runoff impacts of the project (which often includes increased impervious area). Off-site options may be within the ROW at another location (not associated with the project) or outside the ROW but within the same watershed as the project. The goal of off-site mitigation is to provide a functional watershed-based protection or improvement option to stormwater impacts that is cost-effective, addresses priority pollutants, and provides long-term stormwater benefits that are equal to or greater than on-site treatment. Identifying candidate off-site mitigation locations requires a careful review of existing land uses, ownership, drainage systems, outfalls, and impairments. For in-stream mitigation oppor- tunities, an assessment of channel and riparian stability and function is necessary to identify reaches that could be improved or that are susceptible to impacts from increased flows or pol- lutant loadings. Identifying the locally dominant physical processes affecting stream hydrol- ogy, morphology, and sediment dynamics may be necessary to identify locations where stream improvements may be most effective. These types of assessments should be part of watershed planning efforts and may be documented for watersheds that have been studied by resource agencies, watershed councils, and others (see Chapter 5 on Extant Planning). Once candidate off-site mitigation options and locations have been identified, the benefits of each must be considered relative to existing watershed impairments. An important objective of a watershed approach is to ensure that mitigation benefits are sited in locations that are very well suited to provide protection or enhancement of the ecosystem services within the watershed. Where this objective is realized, there is a legitimate basis for suggesting that the use of mitigation ratios for off-site mitigation alternatives should be tempered. Type of Mitigation Options In-kind mitigation uses conventional stormwater management practices and facilities typical of linear projects and are approved for use by state DOTs and appropriate regulatory agencies. These stormwater management facilities are designed to treat a particular design storm, capture a specific percentage of runoff (e.g., capture 80 percent of the average annual runoff volume), and/or address a specific stormwater pollutant, such as TSS. The level of treatment and metric for determining performance provided by off-site, in-kind mitigation is similar or equivalent to that provided by on-site stormwater management. Chapter 3 describes the assessment process to evaluate the potential effectiveness of on-site BMPs. Figure 4. Typical mitigation options. Ex am pl es Ty pe Lo ca tio n On-Site Mitigation In-Kind (Conventional BMPs) Swale, dry pond/detention basin, wetland/ retention pond, bioretention, Media filter,etc. Off-Site Mitigation In-Kind (Conventional BMPs) Swale, dry pond/detention basin, wetland/ retention pond, bioretention, Media filter,etc. Out-Of-Kind Stream restoration, wetland creation, riparin and upland enhancement, in-lieu fee, etc. Combination In-Kind and Out-Of-Kind All of the examples to the left.

Framework for Evaluating Off-Site Mitigation Options 45 Table 14 summarizes in-kind BMPs commonly used by state DOTs. Configuration and siz- ing will vary by state. A description of sizing criteria and effectiveness can be found in several previous NCHRP reports such as NCHRP Reports 444, 565, 728, and 792 (e.g., Epps et al. 2000; Oregon State University et al. 2006; Geosyntec Consultants et al. 2012; and Taylor et al. 2014). Out-of-kind mitigation is designed to provide similar benefits in pollutant reduction and flow control as an alternative to conventional stormwater management approaches. Out-of-kind mitigation may include: • Stream improvement techniques such as physical enhancements that restore natural stream morphology and function, including grading engineered meanders, installing grade control features, planting riparian vegetation, placing large woody debris, and using similar forms of bank and stream channel protection controls • Wetland restoration or creation • Upland restoration • Floodplain reconnection • Reductions in impervious surface connectivity • Infrastructure improvements and maintenance (e.g., removing accumulated legacy pollutants on sediments retained in conveyance systems) • Enhancements to sewage treatment facilities and septic systems, including enhanced nutrient removal systems • Street sweeping • Reducing illicit discharges • Fertilizer reduction • Pet waste management • Land preservation Table 14. Commonly used BMP types, as reported by state DOTs. BMP Number of DOTs that reported using the BMP Percent of DOTs that reported using the BMP Surface detention (Dry ED/wet/infiltration basins, wetlands) 30 81% Vegetated/rock swales 29 78% Hydrodynamic separators 23 62% Oil/water separators 22 59% Infiltration trenches 18 49% Underground detention 17 46% Catch basin inserts 16 43% Low impact development BMPs (e.g., bioretention, amended soils) 16 43% Proprietary media filters (e.g., StormFilter) 15 41% Sand filters 14 38% Filter strips 14 38% Diversion to treatment facilities 10 27% Multi-chambered treatment train systems 7 19% Porous pavements 7 19% Cisterns 3 8% Note: 37 DOTs responded to this survey. Source: Geosyntec Consultants et al. 2012

46 A Watershed Approach to Mitigating Stormwater Impacts Many of the out-of-kind mitigation approaches listed above may already be required by a DOT’s NPDES permit (e.g., reducing illicit discharges, public education, etc.) and therefore may not qualify for mitigation credits. Also, the effectiveness of some approaches may not be very quantifiable, which could affect regulatory acceptance or the mitigation ratio that must be applied. See the following section Developing Mitigation Equivalencies. Table 15 contains a list of out-of-kind mitigation options identified from literature reviews. The table includes a description of the activity, the potential mitigation and pollutant reduction functions of the activity, and key relevant sources that support the use of the option. While this list is not exhaustive, it demonstrates published literature that supports out-of-kind mitigation strategies. While the out-of-kind mitigation strategies identified in Table 15 have value, the scientific level of understanding and/or data availability limit the ability to incorporate many of these into a stormwater mitigation toolbox at this point in time. Four of the out-of-kind mitiga- tion options were selected for further development and consideration: (1) stream improve- ment techniques, (2) upland stabilization, (3) reducing impervious surface connectivity, and (4) wetland restoration/creation. These were selected because of their ability to be reason- ably estimated using typically available datasets. In addition, these mitigation techniques are defensible from a scientific and regulatory perspective, and linkages exist that indicate these mitigation measures enhance or create ecosystem services. Brief definitions of these mitiga- tion techniques are here: • Stream improvement techniques: Mitigation approaches used to improve stream morphology and function, which may include restoring and protecting stream beds and banks; improving connection to floodplains; and dechannelization/restoring the channel morphology. When considering these approaches, the hydrologic issues that may have caused the need for this mitigation type must also be considered. • Upland stabilization: Techniques and treatments to control stormwater pollutant source inputs from the watershed and beyond the riparian corridor through reforestation and permanent erosion and sediment controls that offset hydrologic impacts from increased imperviousness of a project. • Reducing impervious surface connectivity: Techniques and treatments to reduce the runoff from impervious surfaces through the interception of flow to reduce scouring and stream degradation. • Wetland restoration/creation: (1) The rehabilitation or reestablishment of a wetland that has been degraded or destroyed by changes in land use and (2) the construction of a wetland on a site that never was a wetland, both of which can increase storage, provide habitat, and increase nutrient retention and carbon sequestration as well as other ecosystem benefits. Discussions on out-of-kind mitigation options for the remainder of this chapter will be limited to these four mitigation options. Developing Mitigation Equivalencies There are several approaches that can be used to determine whether a mitigation measure presumptively provides benefits to the receiving system. Figure 4 provides several kinds of mit- igation types for on-site and off-site mitigation opportunities. From a regulatory perspective, there must be a demonstration that the benefits of implementing off-site mitigation are equal to or greater than on-site mitigation and that the beneficial uses of the receiving environment are maintained. Stormwater impacts for DOT projects are generally the result of an increase in impervious area and the associated pollutant generation from this surface. Typical on-site

Table 15. Out-of-kind BMP mitigation tools and associated pollutant reductions. Mitigation Tools Literature Source WATERSHED ENHANCEMENT Stream improvement techniques Stream improvements can reduce sediment loads to streams, maintain capacity of the stream channel, and improve aquatic habitat. In the West Fork White River Watershed Assessment, it was estimated that the annual sediment load to the west fork of the White River was 36,000 tons per year, 66% of which was from accelerated stream bank erosion. Another study of the Blossom Way Branch found that the annual sediment load is estimated to be 1,700 tons/year, 11% of which is from accelerated stream bank erosion. Michigan United Conservation Clubs 1997 WCRC 2011, 2012 ADEQ 2004 Hubbard et al. 2003 Upland restoration and land preservation Soil bioengineering can reduce erosion and reduce TSS loading to water bodies. Upland stabilization can also involve tree planting, grass planting, etc. A study of three soil bioengineering slope stability projects indicated that all three projects showed an “improvement in stability for erosion control.” The controls were also effective in slide stabilization. The sites were modified to support 43-95% vegetative cover. Land preservation in upland areas can have similar benefits but may require greater land area, depending on current land use and condition. Lewis and Salisbury 2001 Reducing impervious surface connectivity Recent studies indicate that directly connected impervious area (DCIA), rather than total impervious area, is responsible for the majority of stream alteration in urban areas. Reducing DCIA provides interception of flow to reduce scouring and stream degradation. The USEPA (2014) outlined some ways of reducing impervious surface connectivity and the percent of volume reduction achieved. The removal of pavement provides significant volume reduction, redirection of runoff from rooftops to infiltration areas provides an 85% reduction, permeable pavement and other swales provide 75% reduction, and infiltration trenches and basins provide 13-100% depending upon site infiltration rates and runoff depth captured. Roy and Shuster 2009 USEPA 2014 Wetland restoration/creation Wetlands provide habitat for wildlife but they also provide water quality improvement for nutrients, metals, etc., and flood attenuation. Wetlands can provide additional treatment for wastewater, stormwater, acid mine drainage, or agricultural runoff causing impairment to water bodies. Stormwater wetlands typically remove 45% of total nitrogen. Wetlands can also remove heavy metals 36-80%, phosphorus 40-100%, lead 80-95%, TSS 90-100%, BOD5 80-100%, bacteria 60-80%, and copper 80-95%. Jurries 2003 Kentula 2008 Lee et al. 2009 NRC 2011 Federal Interagency Wetland Mitigation Action Plan Committee 2004 Buffers grass/meadow and forest Buffers can provide stormwater management benefits such as interception of water and associated nutrients, and they can also act as barriers between pollution hazards and water bodies. Removal efficiency of TSS for vegetative filter strips and vegetated swales are 85% and 70%, respectively. USEPA 2012b Description / Associated Pollutant Reduction Outfall stabilization Outfall stabilization can prevent erosion near the outfall, which also prevents sediment deposition. USEPA 1992 Floodplain reconnection Floodplain reconnection can be a means to provide temporary storage of floodwaters, moderate peak flows, improve water quality during floods, recharge groundwater, and prevent erosion. The focus of floodplain reconnection in this effort is in bank reshaping to allow for hydrologic restoration. Chagrin River Watershed Partners, Inc. 2009 (continued on next page)

EDUCATION Nutrient management Reducing the excess amount of fertilizer applied in public places and agricultural areas can reduce nitrogen loading to water bodies. Chesapeake Stormwater Network 2014 Branosky et al. 2011 NRC 2011 Education State and local agencies have developed a number of public education programs to address fertilizer use, manure management for hobby farms, septic tank maintenance, pesticide use, and waste disposal by homeowners and the general public. These programs can result in reductions in nutrients and toxin loadings in urban and rural environments. Maryland Department of the Environment 2011 USEPA 1999a Pet waste management Laws and regulations requiring the appropriate collection and disposal of pet wastes can reduce the levels of fecal coliforms and associated disease causing bacteria and parasites in stormwater discharges. USEPA 2012a Mitigation Tools Literature SourceDescription / Associated Pollutant Reduction Sub-soiling Sub-soiling is the physical loosening of the soil with specific tilling action to treat compaction and increase infiltration rates without significant disruption of surface layers, and is often combined with soil amendments such as compost to increase soil water retention and promote microbial growth that improves soil structure. Increases in retention and infiltration can reduce volumes discharged to receiving waters and reduce pollutant loads. Pennsylvania Department of Environmental Protection 2006 Tree planting Trees affect stormwater through three processes: interception, transpiration, and infiltration. In the Pacific Northwest, the percentage of precipitation intercepted by conifers is 18-51%, and the percentage of annual precipitation intercepted by deciduous trees is 8-13%. The percentage of annual precipitation transpired by conifers is 10-12%, and the percentage of precipitation transpired by deciduous trees is 25%. The percentage of annual runoff increase after deforestation of conifers was 32% and 23-32% for deforestation of deciduous trees. Measured average annual stormwater runoff from conifer and deciduous forested areas was 44-45% and 39% respectively. Herrera 2008 Coder 1996 INFRASTRUCTURE IMPROVEMENT & MAINTENANCE Enhancements to water pollution control facilities / sewage treatment plants Enhancements to water pollution control facilities/sewage treatment plants can be an effective way to achieve watershed nutrient reduction targets. On an annual load basis, the cost for nutrient reduction by enhancing treatment plant processes is several orders of magnitude below the cost of most stormwater practices, for example, as viewed on a unit $/kg of nitrogen basis. Chesapeake Bay Program 2012 USEPA 2000a, 2000b Septic system upgrades or pumping or connection to WWTP An upgraded, nitrogen-removing septic system can reduce the system’s nitrogen load by half. Traditional septic systems do not address nitrogen removal, and thus deliver about 24.32 lbs/year of nitrogen per system to groundwater or nearby water bodies. Upgraded nitrogen-removing septic systems can reduce nitrogen loading by half. Maryland Department of the Environment 2014 Street sweeping Street sweeping programs reduce the amount of debris entering, and thereby clogging, stormwater infrastructure. Another benefit of street sweeping is the removal of metals, TSS, and other hazardous wastes from the streets, preventing metals from entering receiving water bodies. The City of San Diego estimates that motorized sweeping removes a significant amount of solids and debris prior to entering into the storm drain system. City of San Diego Storm Water Division 2014 Sutherland 2009 Table 15. (Continued).

Framework for Evaluating Off-Site Mitigation Options 49 (and in-kind) mitigation measures for these stormwater impacts as shown in Figure 4 would be selected based on site characteristics. Sizing is determined through various methods such as cal- culations and modeling processes to determine the necessary size of mitigation measure that can achieve the stormwater objectives. These stormwater objectives are frequently expressed as cap- ture of a percentage of stormwater runoff, runoff volume reduced, or the pollutant load reduced from the stormwater runoff. This is important as it can determine the volume of runoff mitigated or pollutant load reduction required to meet the stormwater objectives. The metrics used to size or evaluate on-site stormwater controls (real or conceptual) may be used as a potential equivalent metric for evaluating and sizing off-site mitigation options. For example, these metrics can be used to determine whether an off-site project would achieve volume or load reductions equal to or greater than an on-site project. In addition, a combined approach is also acceptable. If only a portion of on-site, in-kind mitigation can occur, the remaining balance can be used to determine the required off-site mitigation quantity, although an off-site mitigation ratio may need to be applied and the location of the off-site mitiga- tion may be used to influence this ratio (e.g., a higher ratio may be used for upstream versus downstream mitigation or if the mitigation location is outside of the subbasin relative to the point of impact). Therefore this on-site, in-kind calculation, even if only conceptual (e.g., no on-site opportunities exist or are desired), provides an understanding of what may be needed for equivalent off-site benefits. The advantage to this approach is that a load reduction equivalency is relatively sim- ple to determine because it directly addresses the pollutants of concern in highway runoff. The results are easy to communicate to regulatory agencies. The main disadvantage of this approach is that it does not address the environmental impacts of stormwater runoff beyond the pollutant load. Neither watershed functions nor the mitigation option that would provide the most benefit for the watershed are considered, so the effort may do little to address larger watershed issues. While this approach may seem somewhat novel, the establishment, use, and acceptance of equivalency metrics by regulators and the user community are the basis for many off-site mitigation approaches. Mitigation ratios, a form of determining equivalent rates of application that account for uncertainty in success are a common practice for estab- lishing equivalent mitigation in other fields such as mitigating for wetland impacts. Other equivalency metrics are also used. One metric being explored in the Chesapeake Bay region compares mitigation of project impervious surfaces to mitigation of nonproject (and non DOT) impervious area or the purchase of available credits through mitigation banks and in lieu fee programs that quantify the amount (area or cost) using the metric of “impervious area treated.” There are also other approaches that can include watershed benefits as a part of deter- mining equivalency in projects for meeting stormwater objectives. One in particular is employing ecosystem services valuation. The USEPA’s 2003 Water Quality Trading Policy states that the USEPA supports implementation of water quality trading by states, interstate agencies and tribes where trading [c]ombines ecological services to achieve multiple environmental and economic benefits, such as wetland restoration or the implementation of management practices that improve water quality and habitat. (USEPA 2003) This opens the door to the use of ecosystem services approaches to help achieve CWA goals. If an NPDES permit does not explicitly allow for water quality trading, there could be sig- nificant impediments to the use of ecosystem services approaches. For example, NPDES MS4 permits are required to include provisions that achieve the standard to reduce pollutants to the maximum extent practicable. In such situations, at least some on-site treatment of stormwater would likely be required.

50 A Watershed Approach to Mitigating Stormwater Impacts Equivalency Using Ecosystem Services Ecosystem services can be evaluated through a combination of quantitative and qualita- tive approaches to demonstrate how the impacts to watershed functions at one location in a watershed can be offset by providing improvements to the watershed functions in another part of the watershed. One approach is to characterize how society uses and values the services provided by the watershed and then identify improvements that can enhance those services. The broad array of traditional in-kind BMPs as well as out-of-kind techniques, such as stream and wetland restoration and impervious surface reduction and disconnection discussed earlier are all possible means of effecting ecosystem services production in a watershed. The USEPA’s EnviroAtlas ecosystem mapping tool includes an ecosystem services analysis tool that calculates an index based on a simple sum or a user-defined weighted sum of the relevant indicators of each category of ecosystem services, including food, fuel, and materials; biodiversity conser- vation; clean and plentiful water; climate stabilization; natural hazard mitigation; recreation, culture, and aesthetics; and clean air (Neale 2014). The USEPA has indicated that this tool is planned for use in many future watershed and system analyses. It will also be maintained and enhanced to include more statistically robust means to calculate some of the indices within the tool (Neale 2014). Table 16 describes the ecosystem services used in USEPA’s EnviroAtlas ecosystem mapping tool. A second ecosystem services–based approach to establish equivalences between mitiga- tion options and to rank and prioritize mitigation options in the project watershed is a Stormwater Impact/Ecosystem Services Framework (as based on the information reviewed in López-Marrero and Hermansen-Báez 2011; Lundy and Wade 2011; Malinga et al. 2013; Maynard et al. 2010; Maynard et al. 2011; Moore and Hunt 2012; Petter et al. 2013; Scholz and Uzomah 2013; The SEQ Ecosystem Services Framework 2014). This framework is based on the physical processes by which project–generated stormwater runoff affects environ- mental quality. It assesses the impacts of stormwater on environmental indicators and how these indicators are related to ecosystem services through sets of the physical processes involved in providing each type of ecosystem service. These same physical processes are Table 16. Ecosystem services descriptions. Service Description Food, fuel, and materials Food, fuel, and materials (such as timber) are products derived from ecosystems. As populations increase, the demand for food, fuel, and materials increases also. Healthy ecosystems are essential for providing these services. Biodiversity conservation Biodiversity is essential for proper functioning of all ecosystems. Biodiversity is considered the variety of all forms of life. Clean and plentiful water Lakes, rivers, and streams provide about 80% of usable freshwater. Clean water is key to the health of people, the environment, and the economy. Climate stabilization Healthy ecosystems (including soils, oceans, and vegetation) can capture and store carbon, which reduces or stabilizes the amount of carbon dioxide in the atmosphere. Increases in carbon dioxide in the atmosphere heat the atmosphere, causing climate change. Natural hazard mitigation Vegetated land cover intercepts and infiltrates water, which helps to reduce flooding. Vegetated stream buffers also absorb water along streams and rivers, which can hold excess water and reduce flooding. Recreation, culture and aesthetics People find value in ecosystems such as National Parks, National Forests, National Wild and Scenic Rivers, etc. Recreation and tourism plays a large role in the economy. Clean air Wetlands, trees, and soil filter and store pollutants such as carbon monoxide, carbon dioxide, ozone, and nitrogen oxides. This helps to produce clean air.

Framework for Evaluating Off-Site Mitigation Options 51 the mechanisms through which the mitigation options contribute to the improvement of environmental conditions. Once these physical processes for stormwater impacts, mitiga- tion options, and ecosystem services have been identified and agreed upon with the regu- latory agencies and other stakeholders, they can be used to assign weighted links between stormwater impacts, mitigation options, and ecosystem services (see The SEQ Ecosys- tem Services Framework 2014). These weighted links can be summed to become the basis by which mitigation options can be determined as equal in order to establish a ranking and prioritization system for selecting the preferred watershed-based mitigation options for a specific transportation project. The process for establishing these weighted links is demonstrated in Chapter 6. The advantage of using ecosystem services as a stormwater impact mitigation approach is the consideration of watershed-specific solutions. By considering more holistic mitigation alternatives, a more beneficial outcome may be identified that is not included within the traditional in-kind approach. Also, through the identification and weighting of the physical processes involved, a currency that can be understood and applied when evaluating mitiga- tion options can be developed. The focus on broader watershed issues such as the relation- ships between receiving water impairments, beneficial uses, and ecosystem services helps to ensure that the mitigation efforts address a broader and more strategic range of environmen- tal concerns. The relationship between designated beneficial uses and ecosystem services is discussed below. Designated Beneficial Uses and Ecosystem Services Beneficial uses and how they relate to ecosystem services may be thought of as “functions and/or activities that are supported by a level of water quality” (USEPA 2014). Over 200 forms of beneficial uses have been established by the various states. Careful examination shows that many of these are variants on common themes. For the purposes of developing watershed miti- gation approaches for stormwater related impacts, a basic set of beneficial uses were identified for consideration by DOTs in evaluating water resources impairment in project watersheds and their associated ecosystem service (Table 17). Table 17. Primary designated beneficial uses for consideration when evaluating watershed-based mitigation. Ecosystem Services Beneficial Uses BiodiversityConservation Clean and Plentiful Water Food, Fuel, and Materials Natural Hazard Mitigation Recreation, Culture, and Aesthetics Aquatic life/warm and cold habitats D D S S Drinking water supply D Primary and secondary contact recreation D S Fish consumption (edible seafood) D S S Aesthetics S D Industrial uses D D Wildlife/terrestrial life D D S S Rare and endangered species D D S Wetland habitat D D S S D - Beneficial use depends on ecosystem service S - Beneficial use supports ecosystem service

52 A Watershed Approach to Mitigating Stormwater Impacts Identifying Candidate Off-Site Mitigation Options A project planner/designer has the flexibility to select a mix of on-site and/or off-site practices that best or most cost-efficiently achieve the stormwater management goals. The mitigation opportunities identified in Figure 4 designate off-site mitigation options as in-kind (using typical DOT stormwater management BMPs) and out-of-kind mitigation options (using watershed management opportunities). Watershed characterization information, as indicated in Chapter 3, can be used to identify locations within the watershed that would benefit most from mitigation to improve overall watershed health, beneficial use, and functions. As a part of the watershed characterization, consideration of ecosystem services as well as those mitigation opportunities that maintain or improve ecosystem services can be a way to prioritize the most effective mitigation options. Off-Site, In-Kind Mitigation Off-site, in-kind mitigation is rather simplistic in that it is very similar to the current prac- tices and approaches of most DOTs. A mitigation ratio or factor may be required to account for uncertainties in the ability of the off-site project to have either the same or proportional mitigation benefits. The process is broken into these steps shown in Figure 5 and outlined below: • Step 1: Determine project impacts and mitigation targets – Establish mitigation targets (e.g., volume and load reduction targets) based on potential project impacts and regulatory require- ments. Impacts and mitigation targets may be estimated using tools such as the NCHRP BMP Evaluation Tools (Taylor et al. 2014), the SELDM (Granato 2013), or a variety of other models, tools, or approaches. Figure 5. Determining off-site in-kind mitigation options. STEP 1 Determine Project Impacts and Mitigation Targets STEP 2 Identify Off-Site BMP Locations and BMP Type STEP 3 Summarize Watershed Characteristics STEP 4 Determine Average Annual Loads to Off-Site BMP STEP 5 Estimate Size of Off-Site BMP

Framework for Evaluating Off-Site Mitigation Options 53 • Step 2: Identify off-site BMP locations and BMP type – One or more candidate off-site locations and stormwater BMP types are selected based on evaluation of physical water- shed characteristics, stormwater improvement needs, or existing watershed plans. • Step 3: Summarize watershed land use characteristics – Land use characteristics for the tribu- tary drainage area are then determined for the proposed off-site BMP using available spatial datasets. • Step 4: Determine average annual loads to off-site BMP – The load (as appropriate for average annual or single event determination) to off-site BMPs can then be determined by applying the average pollutant concentrations in runoff from each of the land uses using either data summarized from the HRDB (Granato and Cazenas 2009) and the NSQD (Pitt 2008) or from models, such as SELDM (Granato 2013), or other assessment tools, such as the Simple Method (Schueler 1987). • Step 5: Estimate preliminary size of off-site BMP based on load reduction target deter- mined by conceptual on-site alternative – The results from the analysis of the On-Site BMP Assessment Process can be used to determine the target volume and load reduction for off-site BMPs. These targets along with the estimated loads and volumes from the drainage area can be used to compute the needed percent capture of the runoff or the runoff volume for a particular off-site BMP type and location. This percent capture can then be used to estimate the design volume and footprint of the off-site BMP using a continuous simula- tion model such as USEPA SWMM or the NCHRP BMP Evaluation Tools developed by Taylor et al. (2014). Off-Site, Out-of-Kind BMP Options The process for off-site, out-of-kind systems may then proceed as follows: • Step 1: Determine project impacts and conceptual on-site BMP performance – Complete analysis using project data to determine the conceptual on-site BMP size that provides the target volume and/or load reduction that would be required for off-site BMPs. • Step 2: Consult watershed information and extant planning to identify off-site, out-of-kind mitigation options – Identify beneficial uses, impairments, and baseline ecosystem services of the watershed and evaluate how out-of-kind mitigation options may help to improve these qualitative watershed health indicators. • Step 3: Summarize watershed characteristics – Use available data from the USEPA EnviroAtlas or other source to summarize annual rainfall, imperviousness, and land use distribution of the watershed. • Step 4: Determine average annual loads from watershed that may be mitigated by off-site, out- of-kind mitigation – Use rainfall data, imperviousness, soils information, and characteristic land use concentrations to estimate average annual loads per unit impervious area from the watershed. • Step 5: Estimate quantity of off-site mitigation based on load reduction targets determined by conceptual on-site alternative – Using the unit area loads from the watershed, estimate the equivalent impervious area requiring mitigation to meet load reduction targets and then translate this equivalent area into the quantity of off-site, out-of-kind mitigation based on a mitigation ratio. Refer to Chapter 6 for more detail on these steps and how they are incorporated into the WBSMT. Mitigation for Ecosystem Services The goal of the ecosystem services approach to stormwater mitigation is to select off-site mitigation measures that offset stormwater impacts by improving the watershed processes. In

54 A Watershed Approach to Mitigating Stormwater Impacts evaluating the ecosystem services that can be linked to stormwater impacts of a DOT project, five groups of ecosystem services were identified for consideration: (1) biodiversity conservation; (2) clean and plentiful water; (3) food, fuel, and materials; (4) natural hazard mitigation (e.g., flood protection); and (5) recreation, culture, and aesthetics. These five were selected because (1) most have a regulatory context or requirement; (2) they include representative conditions, or services, that are of concern for most DOTs and communities; and (3) they are affected by many impacts in watersheds in which transportation projects occur. In addition, these address critical elements of the fishable/swimmable goals of the CWA, which are themselves essentially statements of ecosystem services. Described here are the physical process linkages of the four specific mitigation options for each of the five ecosystem services. The following types of assumptions and logic were applied to support the development of the linkages in Table 18: • Increases in TSS loads can be offset by mitigation options that build soil quality, reduce pol- lutant loadings (sediment, nutrients, organic, and inorganic), reduce soil erosion, and reduce sediment transport. • Increases in eutrophication/phosphorus loads can be offset by mitigation options that improve water aeration, reduce pollutant loadings [nutrients, biochemical oxygen demand (BOD), chemical oxygen demand (COD), organic, and inorganic], and increase shading. • Increases in stream power/energy can be offset by mitigation options that improve in- stream energy dissipation; enhance upland hydrologic processes of interception, evapo- transpiration, water retention, and infiltration; improve nature filtration by riparian soils and vegetation; and reconnect floodplains and restore natural (or appropriate) stream channel geometry. • Increases in runoff volumes can be offset by mitigation options that enhance upland hydrologic processes of interception, evapotranspiration, water retention, and infiltra- tion; reconnect floodplains and restore natural (or appropriate) stream channel geometry; reduce in-stream flows to improve natural filtration and sedimentation; and restore or create wetlands. Each of these physical processes can be enhanced by implementing one or more of the miti- gation options (stream improvement techniques, upland stabilization, reducing impervious surface connectivity, and wetland restoration, creation, and enhancement). For example, a mit- igation project employing upland stabilization as a mitigation option can enhance ecosystem services through building soil quality, controlling pollutant sources, enhancing water intercep- tion and retention, increasing infiltration, and reducing sediment loading and transport. These reductions and enhancements, in turn, would improve the quality of the receiving environment waters with concomitant increases in the quality of the water supply, biodiversity and associated food provisioning, improved flood management, enhanced navigability, and improved oppor- tunities for recreation (fishing/swimming). Table 19 summarizes the linkages between the mitigation options that are expected to improve each physical watershed process and the relevant ecosystem services that are affected. Recommended Approach to Developing a Watershed-Based Stormwater Mitigation Framework The recommended approach to developing a national scale watershed-based stormwater miti- gation framework is to use existing national datasets (e.g., USEPA’s EnviroAtlas) to provide as much watershed baseline information as feasible to ensure that data at the HUC-12 scale are avail- able during the watershed characterization phase. Because the resolution and representativeness of available national datasets is limited, the framework must have the flexibility to include user input

Framework for Evaluating Off-Site Mitigation Options 55 Table 18. Linkages between mitigation options and ecosystem services based on upland physical processes. Mitigation Options Effect ofMitigation Option on Physical Processes Effect of Physical Process on Enhancing Ecosystem Services Rationale St re am im pr ov em en t te ch ni qu es R ed uc in g im pe rv io us su rfa ce c on ne ct ivi ty W et la nd re st or at io n/ cr e a tio n Cl ea n an d pl en tif ul w at er Bi od ive rs ity co ns er va tio n Fo od , f ue l, an d m a te ria ls N at ur al h az ar d m iti ga tio n R ec re at io n, c ul tu re , a n d ae st he tic s Increases interception Increases infiltration Enhances evapotranspiration Upland stabilization increases vegetative cover, resulting in enhanced interception of surface runoff. Decreased surface runoff reduces peak flows in and the transport of pollutants to receiving environments, resulting in cleaner water and a greater number of aquatic species and individuals. Cleaner water and more species lead to more food availability and greater recreational opportunities. Stream improvements (floodplain reconnection), upland stabilization, reduced connectivity, and wetland restoration/creation lead to greater water retention and opportunities for stormwater infiltration. Increased infiltration reduces the volume of water and amounts of pollutants transported to adjacent receiving environments, resulting in cleaner water and greater number of aquatic species. Stream improvements (floodplain reconnection), upland stabilization, reduced connectivity, and wetland restoration/creation lead to enhanced evapotranspiration, which reduces the volume of water and pollutants transported to receiving environments, leading to improvements in clean water, species survivorship and individual numbers, food and fuel species, and enhanced recreational opportunities. individual organisms. properties. services, as noted above. flow Up la nd s ta bi liz at io n Enhances pollutant uptake and retention by upland vegetation Builds soil quality Reduces upland erosion Reduces overland Increase surface/subsurface storage Upland stabilization, reduced impervious surface connectivity, and wetland restoration/creation lead to greater productivity of the plant community. An increased number of individual plants and plant species increase the opportunity for the uptake and retention of both organic and inorganic pollutants, reducing the loading of these materials to adjacent receiving environments. Reduced pollutant loads result in cleaner water and a greater number of species and Upland stabilization and reduced impervious surface connectivity contributes to improved soil quality. Higher quality soils lead to more complex plant communities with increased retention of stormwater, and resulting infiltration and evapotranspiration, leading to the improvements in ecosystem services noted above for these physical Upland stabilization and reduced impervious surface connectivity reduce upland erosion, which reduces the transport of sediments and particle associated pollutants to adjacent receiving environments. This improves ecosystem Upland stabilization and reduced impervious surface connectivity reduce overland flow and the associated transport of pollutants and increased discharges to receiving waters, which leads to cleaner water and reduced flooding. Stabilizing upland areas, reducing impervious surface connectivity, and creating and restoring upland wetlands increases the amount of water storage in the watershed and reduces runoff, enhances water quality, and provides water for beneficial upland plants, animals, insects, and microbes.

56 A Watershed Approach to Mitigating Stormwater Impacts Table 19. Linkages between mitigation options and ecosystem services based on in-stream/near stream physical processes. Mitigation Options Rationale Effect of Physical Process on Enhancing Ecosystem Services S tr ea m im pr ov em en t te ch ni qu es U pl an d st ab ili za tio n R ed uc in g im pe rv io us su rf ac e co nn ec tiv ity W et la nd r es to ra tio n/ cr ea tio n C le an a nd p le nt ifu l w at er B io di ve rs ity co ns er va tio n F oo d, fu el , a nd m at er ia ls N at ur al h az ar d m iti ga tio n R ec re at io n, c ul tu re , an d ae st he tic s Stream improvements and wetland restoration/creation create more storage in the channel, riparian areas, and flood plain thereby reducing velocities and providing more opportunities for infiltration, filtration, sedimentation, and plant establishment. Natural water features provide habitat and passive recreational opportunities. Stream improvements and wetland restoration/creation result in increased filtration of dissolved and particle borne pollutants. Reduced pollutant loads to receiving environments enhance water quality which increases species survivorship and numbers, which leads to increased services of food and recreational opportunities. Stream improvements and wetland restoration/creation result in increased sedimentation (e.g., within the floodplain), which can clarify water and reduce in-stream concentrations of sediment-bound pollutants. Stream restoration and riparian enhancements increase the aeration of flowing water by incorporating pools and riffles, placement of woody debris, and channel drop structures, which leads to higher dissolved oxygen concentrations. Higher Dissolved Oxygen levels contribute to cleaner water, increased species numbers (e.g. food species and plant growth), and more recreational opportunities. Stream stabilization, upland stabilization, reduced connectivity, and wetland restoration/creation decrease stream erosion and sediment transport through reduced peak flows and hydromodification. More naturally functioning rivers and streams result in cleaner, less turbid water and more natural stream morphology. This increases species numbers, food and fuel species, and recreational opportunities. Reduced stream erosion and sediment transport decreases the movement and remobilization of particle borne hazardous materials. Stream improvement techniques and wetland restoration/creation result in increased riparian establishment and growth, leading in turn to increased shading and subsequent reductions in stream and river temperatures. Moderated stream temperatures increase species survivorship and numbers, which leads to increased services of food and recreational opportunities. Mitigation Option on Physical Processes Effect of Channel and Floodplain Storage Increases filtration Increases sedimentation Increases aeration Reduces in-stream erosion and sediment transport Increases shading Enhances pollutant uptake and retention by riparian vegetation Stream improvements and wetland restoration/creation lead to greater productivity of the riparian plant community. An increased number of individual plants and plant species increase the opportunity for the uptake and retention of both organic and inorganic pollutants. Reduced pollutant concentrations result in cleaner water and a greater number of species and individual organisms.

Framework for Evaluating Off-Site Mitigation Options 57 information that can incorporate data from local and targeted watershed stakeholders and public agencies. Information on how to collect this data to customize and refine the scale or local interest within the tool is discussed in Chapter 5. The recommended mitigation approach is based on identifying equivalent stormwater miti- gation options within the watershed and/or using the concept of watershed processes to connect mitigation options to beneficial uses, ecosystem services, and stormwater impacts and mitiga- tion priorities. The first step in any watershed-based mitigation approach is to first identify and evaluate on-site stormwater management goals and opportunities for a project. If on-site oppor- tunities are limited or not feasible, then the off-site opportunities can serve as the mitigation or partial mitigation for the project. All mitigation measures are expected to achieve watershed benefits that match or exceed the mitigation provided by on-site, in-kind practices. By including baseline beneficial use and ecosystem services metrics for the watershed, rank- ing of out-of-kind mitigation measures is possible by weighting these watershed metrics. While weighting factors may be based on the desirability of improving an ecosystem service, the ulti- mate selection of an out-of-kind mitigation measure is expected to be driven by DOT require- ments, regulatory agency acceptance, and watershed needs and opportunities. As candidate off-site mitigation alternatives are identified, they can be evaluated in achieving project mitiga- tion needs. Methods to link project mitigation needs to off-site mitigation alternatives within the WBSMT are presented in Chapter 6 followed by worked hypothetical case study examples presented in Chapter 7.