Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Guide (2022)

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30 This chapter provides example screening and detailed hydrologic and co-benefits analyses for a hypothetical highway project in the Piney Creek watershed in Colorado. The screening analyses (step 8) are presented first. In step 9 of the decision framework, the state DOT (or other transportation agency) and external partners/stakeholders determine whether the screen- ing analyses are actionable or if more detailed analyses (step 11) are desirable. NCHRP Web- Only Document 333 includes additional screening examples in Washington and Massachusetts. For this example, a hypothetical new highway project is proposed within the Piney Creek watershed. Figure 5.1 shows the Piney Creek watershed near Denver, Colorado. For this example, step 6 of the decision framework determined that out-of-kind mitigation was appropriate for the proposed highway project and that the mitigation techniques to be considered are stream restoration and uplands restoration. Step 7 established that the hydrologic screening tool can be used to assess the needed amount of mitigation and that co-benefits screening was appropriate for the project. As determined in step 1 of the decision framework, the proposed highway project has two components: (1) a new outer beltway and (2) two arterials intersecting the outer beltway. The length of the new outer beltway through the watershed is approximately 16,100 ft with a right- of-way width of 246 ft. The arterials are planned with a length of approximately 34,800 ft and a right-of-way width of 98 ft. The total highway project footprint is 170 acres. The project could be considered a single project implemented at one time or as a series of projects implemented over time. In step 2 of the decision framework, the state DOT determined that the appropriate hydro- logic metrics to be assessed at these locations are the 2- and 100-year peak flow and volume. In step 4, the state DOT and external partners/stakeholders selected two APs for evaluat- ing project impacts and mitigation effectiveness for stormwater quantity. One AP (AP1) is designated downstream of the highway project because this is the first downstream point that captures the entire highway project. The second AP (AP2) is at the downstream end of the Piney Creek watershed where Piney Creek is a tributary to Cherry Creek. The selection of these two APs enables evaluation of mitigation measures downstream and upstream of the highway project. In step 6, the state DOT and external partners/stakeholders selected several mitigation port- folios for consideration. These include the following: • Alternative 1. Forest, uplands, or wetland restoration converts the area in the watershed to one of these landscapes. C H A P T E R 5 Piney Creek Example

Piney Creek Example 31   • Alternative 2. Stream restoration can be used in the less developed upstream areas. Instream enhancement via an increase in sinuosity and reduction in entrenchment increases connectivity with the floodplain—resulting in reduced erosion and flooding following confirmation that stream restoration is necessary in this area. • Alternative 3. Blended stream restoration and forest, uplands, or wetland restoration uses both techniques in alternatives 1 and 2. The locations and magnitudes of these mitigation portfolio alternatives will be determined as part of the hydrologic screening. For step 7, the state DOT (or other transportation agency) and external partners establish agreement on the screening tools applicable to the Piney Creek watershed. For this example, the collaborators agree to apply the hydrologic and co-benefits tools described earlier as part of step 8. 5.1 Screening Analyses (Step 8) This section provides the hydrologic and co-benefits screening analyses for Piney Creek, Colorado. The state DOT and external partners apply the hydrologic screening tools described in Section 3.1 and the co-benefits screening tools described in Section 3.2. 5.1.1 Hydrologic Screening (Step 8A) The process in step 8A assesses the mitigation portfolio with the hydrologic screening tool. Following is a step-by-step application of the tasks in step 8. Figure 5.1. Piney Creek, Colorado, watershed location.

32 Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Guide 5.1.1.1 Collate and Map Information from Previous Steps (Step 8A1) The hypothetical proposed highway project is shown in Figure 5.2. The new outer beltway through the headwaters of the study area and the two arterials intersecting the outer beltway are shown as gray lines. Figure 5.2 also shows the watershed boundary and the two APs selected for this example. The first AP (AP1) is designated at the confluence of subbasins 272 and 274 (small yellow star) because this is the first downstream point that captures the entire hypothetical highway project. The second AP (AP2) is at the downstream end of the Piney Creek watershed (large yellow star) where Piney Creek enters Cherry Creek. The hydrologic metrics to be assessed at these locations are the 2- and 100-year peak flow and volume. These metrics are available as part of the hydrologic screening tool and spreadsheet. 5.1.1.2 Determine Land Covers and Impervious Areas (Step 8A2) Figure 5.2 depicts the NLCD land covers for the watershed and vicinity. The NLCD land covers are used to determine where mitigation opportunities may exist. The hypothetical high- way project impact was calculated as 80% impervious. Therefore, the net increase of impervi- ousness for the highway project is 136 acres. The average annual precipitation is used to determine whether existing pervious areas can be used for mitigation. The average annual precipitation in the watershed of 15 in. is obtained Figure 5.2. The hypothetical proposed highway project.

Piney Creek Example 33   from the local weather station at Centennial Airport, approximately 5 miles west of the Piney Creek watershed. As will be shown in step 8A4, since the annual precipitation is less than 24 in., the determination of the amount of mitigation using pervious areas cannot be determined with the screening tool (see Section 3.1.3). 5.1.1.3 Conduct Prescreening Analysis (Step 8A3) Using the information generated in the previous tasks, step 8A3, part 1, rows A, B, and C of the hydrologic screening tool spreadsheet are populated for AP1 and AP2 (see Figure 5.3). Two columns are used for AP2 because mitigation at this AP will be evaluated both upstream and downstream of the highway project as part of step 8A4. The results are shown in step 8A3, part 2 (see Figure 5.4). Because all the resulting cells in step 8A3, part 2, of the spreadsheet are green, we can proceed using the screening tool. If one or more of the cells is red, the user would evaluate whether that metric at the selected AP has the desired degree of certainty required by the project evaluators. If not, or if the combined “No” metrics for that AP do not have the desired degree of certainty, a detailed mitigation analysis would be recommended. 5.1.1.4 Apply Screening Tool (Step 8A4) Step 8A4, part 1, rows A−E of the hydrologic screening tool spreadsheet are populated and/or selected for AP1 and AP2 (see Figure 5.5). Figure 5.4. Step 8A3, part 2, of the screening spreadsheet. Figure 5.3. Step 8A3, part 1, of the screening spreadsheet.

34 Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Guide 5.1.1.5 Determine Mitigation Quantities (Step 8A5) Step 8A4, part 2, in the hydrologic screening tool spreadsheet provides the specific tables to be used for selecting the mitigation ratios for the project. Figure 5.6 shows that for • AP1 (mitigation upstream) the applicable tables are Table A.2 for forest, uplands, and wetland mitigation and Table A.7 for stream restoration mitigation. • AP2 (mitigation upstream) the applicable tables are Table A.3 for forest, uplands, and wetland mitigation and Table A.7 for stream restoration mitigation. • AP2 (mitigation downstream) the applicable table is Table A.1 for forest, uplands, and wet- land mitigation. Stream restoration mitigation is not feasible to assess via screening. The spreadsheet also shows that pervious mitigation cannot be addressed with the screening tool for this project site because the average annual precipitation is less than 24 in. Pervious mitigation is the conversion of existing pervious landscapes to new landscapes with higher rates of infiltration and vegetation uptake. The following sections describe development of mitigation ratios and quantities for forest, uplands, and wetland restoration of currently impervious areas as well as mitigation ratios and quantities for stream restoration. Forest, Uplands, and Wetland Mitigation. Forest, uplands, or wetland mitigation can take place upstream of the highway project location and AP1 or downstream of the project location but upstream of AP2. If mitigation is placed downstream of the project location, stormwater quantity benefits will not occur at AP1. The project team will need to evaluate whether this Figure 5.6. Step 8A4, part 2, of the screening spreadsheet. Figure 5.5. Step 8A4, part 1, of the screening spreadsheet. D/S: downstream. U/S: upstream.

Piney Creek Example 35   outcome is acceptable (i.e., co-benefits may show that this is acceptable), if not, downstream mitigation could be discarded as an option. Table 5.1 summarizes the mitigation ratios for mitigation located upstream of the project location as evaluated at AP1 that the user takes from Table A.2 for the selected hydrologic metrics for this project. Table 5.2 summarizes the estimated mitigation areas determined by multiplying the mitigation ratios by the highway project impervious area of 136 acres. The screening tool results show that grasslands (uplands) restoration mitigation will require almost three to five times the mitigation area as forest restoration or wetland restoration mitiga- tion and may not be feasible as a standalone mitigation technique. In addition, forest restoration mitigation may not be able to achieve a 2-year volume hydrologic metric. In this case, a detailed study would be needed to determine the forest restoration mitigation area needed to meet the 2-year volume hydrologic metric. For any of these mitigation techniques, the largest amount of mitigation area is selected so that all the hydrologic metrics are satisfied. If, as described above for forest mitigation, one or more metrics are not satisfied, or results are unknown, detailed analysis can be conducted to provide quantitative results. For this example, one may select from among the following choices for mitigation upstream of the highway project area and evaluated at AP1: • Forest restoration. Conversion of 218 acres (136 times 1.6) of existing impervious area noting that it is unknown whether this technique will satisfy the 2-year event volume metric. • Uplands (grassland) restoration. Conversion of 966 acres (136 times 7.1) of existing imper- vious area. • Wetland restoration. Conversion of 272 acres (136 times 2.0) of existing impervious area. Mitigation type 100-year peak flow 2-year peak flow 100-year event volume 2-year event volume Forest restoration 1.4 1.6 1.4 N/A Uplands (Grassland) restoration 7.1 2.0 2.5 1.4 Wetland restoration 2.0 1.6 1.6 1.1 N/A: not available as no consistent pattern was detected. Table 5.1. Mitigation ratios for AP1 (area of mitigation per unit area of new impervious highway impact). Mitigation type 100-year peak flow 2-year peak flow 100-year event volume 2-year event volume Forest restoration 190 218 190 N/A Uplands (Grassland) restoration 966 272 340 190 Wetland restoration 272 218 218 150 N/A: not available as no consistent pattern was detected. Table 5.2. Estimated mitigation areas upstream of project for AP1 (acres).

36 Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Guide Table 5.3 summarizes the mitigation ratios for mitigation placed upstream of the highway project location but evaluated at AP2 obtained from Table A.3. The estimated mitigation areas are then determined by multiplying these ratios by the highway project impervious area of 136 acres as shown in Table 5.4. Reviewing these values shows that all three mitigation techniques require similar mitigation areas, but forest and uplands restoration may not be able to achieve a 2-year volume hydrologic metric requirement. In this case, a detailed study would be needed to determine the grassland or forest restoration mitigation area needed to meet the 2-year volume hydrologic metric. As for AP1, the largest amount of mitigation area is selected so that all the hydrologic metrics are satisfied. For this example, one may select from among the following choices for mitigation upstream of the project area but evaluated at AP2: • Forest restoration. Conversion of 204 acres (136 times 1.5) of existing impervious area noting that it is unknown whether this technique will satisfy the 2-year event volume metric. • Uplands (grassland) restoration. Conversion of 299 acres (136 times 2.2) of existing impervi- ous area noting that it is unknown whether this technique will satisfy the 2-year event volume metric. • Wetland restoration. Conversion of 272 acres (136 times 2.0) of existing impervious area. Table 5.5 summarizes the mitigation ratios for mitigation placed downstream of the high- way project location but evaluated at AP2 obtained from Table A.1. The estimated mitigation areas are then determined by multiplying these ratios by the highway project impervious area of 136 acres as shown in Table 5.6. Mitigation type 100-year peak flow 2-year peak flow 100-year event volume 2-year event volume Forest restoration 190 190 204 N/A Uplands (Grassland) restoration 299 218 258 N/A Wetland restoration 190 190 272 150 N/A: not available as no consistent pattern was detected. Table 5.4. Estimated mitigation areas upstream of project for AP2 (acres). Mitigation type 100-year peak flow 2-year peak flow 100-year event volume 2-year event volume Forest restoration 1.4 1.4 1.5 N/A Uplands (Grassland) restoration 2.2 1.6 1.9 N/A Wetland restoration 1.4 1.4 2.0 1.1 N/A: not available as no consistent pattern was detected. Table 5.3. Mitigation ratios for AP2 with mitigation upstream of project (area of mitigation per unit area of new impervious highway impact).

Piney Creek Example 37   Reviewing these values shows that all three mitigation techniques require similar mitigation areas for some metrics and very different areas for others. Forest and wetland restoration may not be able to achieve a 2-year volume hydrologic metric requirement. In this case, a detailed study would be necessary to determine the mitigation area needed to meet the 2-year volume hydrologic metric. The largest amount of mitigation area is selected so that all of the hydrologic metrics are satisfied. For this example, one may select from among the following choices for mitigation downstream of the project area but evaluated at AP2: • Forest restoration. Conversion of 272 acres (136 times 2.0) of existing impervious area noting that it is unknown whether this technique will satisfy the 2-year event volume metric. • Uplands (grassland) restoration. Conversion of 1,400 acres (136 times 10.3) of existing imper- vious area. • Wetland restoration. Conversion of 544 acres (136 times 4.0) of existing impervious area noting that it is unknown whether this technique will satisfy the 2-year event volume metric. The screening results differ at the two APs and with the location of the mitigation (upstream or downstream of the project site). The state DOT and partners need to collaborate on which APs and which hydrologic metrics are the most important and for which there may be flexibility. Stream Restoration. Stream restoration could also be implemented upstream or down- stream of the highway project location. The hydrologic screening tool can be used to estimate how much stream restoration can compensate for the highway project impacts but field validation Mitigation type 100-year peak flow 2-year peak flow 100-year event volume 2-year event volume Forest restoration 272 177 218 N/A Uplands (Grassland) restoration 272 177 1,400 204 Wetland restoration 272 177 544 N/A N/A: not available as no consistent pattern was detected. Table 5.6. Estimated mitigation areas downstream of project for AP2 (acres). Mitigation type 100-year peak flow 2-year peak flow 100-year event volume 2-year event volume Forest restoration 2.0 1.3 1.6 N/A Uplands (Grassland) restoration 2.0 1.3 10.3 1.5 Wetland restoration 2.0 1.3 4.0 N/A N/A: not available as no consistent pattern was detected. Table 5.5. Mitigation ratios for AP2 with mitigation downstream of project (area of mitigation per unit area of new impervious highway impact).

38 Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Guide would be needed to confirm that streams in the watershed are candidates for restoration. As indi- cated in Figure 5.6, mitigation with stream restoration downstream of the project site was not considered for this site. Table 5.7 summarizes the stream restoration mitigation ratios obtained from Table A.7 for the AP at the highway location (AP1) with stream restoration upstream. The minimum stream mitigation lengths are then determined by multiplying these ratios by the highway project imper- vious area of 136 acres and presented in Table 5.7. The screening tool results show that stream restoration mitigation cannot meet the 100-year (55,800 ft of additional stream length is not available) or the 2-year volume hydrologic metric requirements. However, a detailed study may be used to verify this result or suggest that some reasonable additional mitigation stream length is available. The use of stream restoration in an alternative strategy with a mix of mitigation techniques is also a possibility but may be more properly evaluated in a detailed study. A detailed study would also better capture types of stream restoration not well represented in the screening tool such as floodplain reconnection. Table 5.8 summarizes stream restoration ratios obtained from Table A.7 for AP2 with the mitigation upstream of the highway project. The minimum stream mitigation lengths are determined by multiplying these ratios by the highway project impervious area of 136 acres as summarized in Table 5.8. The screening tool results show that stream restoration mitigation may not be available to meet the 2-year volume hydrologic metric requirements. However, a detailed study may be used to verify this result or show that some reasonable additional mitigation stream length is available. For most stream restoration mitigation options, designers will need detailed studies to support the design process. The use of stream restoration in an alternative strategy with a mix of mitigation techniques is also a possibility. Quantity 2-year through 100-year peak flow 100-year event volume 2-year event volume Mitigation ratio (ft/acre of new impervious highway impact) 60 60 N/A Estimated stream restoration lengths (ft) 8,160 8,160 N/A N/A: not available as no consistent pattern was detected. Table 5.8. Stream restoration mitigation ratios and estimated restoration lengths for AP2 for mitigation upstream of the project site. Quantity 2-year through 100-year peak flow 100-year event volume 2-year event volume Mitigation ratio (ft/acre of new impervious highway impact) 45 410 N/A Estimated stream restoration lengths (ft) 6,120 55,800 N/A N/A: not available as no consistent pattern was detected. Table 5.7. Stream restoration mitigation ratios and estimated restoration lengths for AP1.

Piney Creek Example 39   The estimated quantities of stream restoration depend on which AP is considered. At a par- ticular AP, the largest amount of mitigation length is selected so that all of the hydrologic metrics are satisfied. For this example, one may consider the following results: • At AP1: A stream restoration length of 6,120 ft is required unless the 100-year event volume is a metric that must be satisfied. It is unknown whether this technique will satisfy the 2-year event volume metric. • At AP2 (with mitigation upstream of the project site): A stream restoration length of 8,160 ft is required. It is unknown whether this technique will satisfy the 2-year event volume metric. Blended Stream Restoration and Forest, Uplands, or Wetland Restoration. From the stream restoration screening results, it appears unlikely highway project mitigation can be achieved with stream mitigation alone. Therefore, a blended mitigation alternative is considered that includes both uplands and stream restoration. In this scenario, a potentially more achievable 2,100 ft of stream restoration is considered. Depending on the AP, this results in the following: • AP1: Based on a mitigation ratio of 45 ft/acre, the stream restoration mitigates 47 acres of impervious highway project impact (2,100/45). Of the original 136 acres of impact to be miti- gated, 89 acres of impact remain to be mitigated with a different technique. • AP2: Based on a mitigation ratio of 60 ft/acre, the stream restoration mitigates 35 acres of impervious highway project impact (2,100/60). Of the original 136 acres of impact to be miti- gated, 101 acres of impact remain to be mitigated with a different technique. Based on an uplands restoration mitigation ratio of 2.2 acres/acre of impact from Table 5.3, this results in an uplands restoration amount ranging from 196 to 222 acres to mitigate the bal- ance of the highway impact. The results from this hydrologic screening (step 8A) provide estimates of the scale of mitiga- tion needed to compensate for the stormwater impacts of the hypothetical proposed highway project(s) for three alternatives. These are used as the basis for the co-benefits screening analysis in step 8B. The combined results of steps 8A and 8B are then used in step 9 of the decision frame- work to evaluate whether screening results are actionable. 5.1.2 Co-Benefits Screening (Step 8B) This section demonstrates steps 8B1, 8B2, and 8B3 of the screening-level analysis of ecosystem service co-benefits described in Section 3.2 for a hypothetical highway project in Piney Creek; a screening assessment of costs (step 8B4) is not included in this example. 5.1.2.1 Establish the Project Context (Step 8B1) This hypothetical highway project involves adding a new highway, in the form of an outer beltway plus two arterials, around Aurora, Colorado, east of Denver. While Piney Creek flows predominantly through the Town of Centennial, much of the surrounding area and the pro- posed sites for each of these mitigation alternatives are in or near Aurora, Colorado, in Arapahoe County. Mitigation alternatives. This screening analysis example compares three mitigation alter- natives that include combinations of stream restoration and uplands restoration. The specific mitigation alternatives are as follows: 1. Uplands restoration. Converting 196 acres in the developed areas back to grass, shrub, or tree cover upstream of the project site. (As described in the hydrologic screening (step 8A), reliance on this mitigation technique alone would require a larger restoration area. The co-benefits analysis can be scaled up for larger areas.)

40 Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Guide 2. Stream restoration. In the less developed upstream areas, 2,100 ft of instream enhance- ment via an increase in sinuosity and reduction in entrenchment increase connectivity with floodplain, resulting in reduced erosion and flooding, following confirmation that stream restoration is necessary in this area. [As described in the hydrologic screening (step 8A), reliance on this mitigation technique alone would require additional stream restoration lengths. The co-benefits analysis can be scaled up for additional length if additional length is feasible.] 3. Blended stream restoration and uplands, forests, or wetlands restoration. Both techniques, as described for alternatives 1 and 2. The hydrologic screening analyses (Section 5.1.1) provided several other possible options. These illustrate the co-benefits of screening. Screening of other options would follow the same procedures. Local objectives. Aurora’s Comprehensive Plan refers to identifying “cost effective solutions for stormwater runoff and water conservation” alongside contributing to a higher quality of life, recreational opportunities, and preserving ecosystems (City of Aurora 2018, p. 12). The plan also describes local concerns for “improving air quality to protect health and the environment, and preserving, enhancing, and connecting open spaces, trails, and waterways” (p. 21). One of the recommended practices outlined in the plan includes “us[ing] green stormwater infra- structure to slow and clean stormwater while providing the benefit of green drainage facilities and corridors” (p. 93). Aurora also maintains an Integrated Water Master Plan (City of Aurora 2017) as well as a Stormwater Master Plan (City of Aurora 2016). In these documents, the city describes a com- mitment to maintaining open drainage systems that serve multiple purposes, including flood protection, stormwater management, recreation, open space, habitat, parks, and trails. This commitment also encouraged the creation of a digital stormwater master plan that combines mitigation efforts with local priorities; gaining access to this plan may help the state DOT better understand the types of mitigation local groups plan to pursue as well as the type of data avail- able for evaluating them for potential co-benefits. Potential collaborators. In addition to the government entities and utilities described in the planning documents and processes outlined previously, several other local groups with broad interests aligned with these mitigation techniques may serve as potential collaborators. A sample of those groups is described, by mitigation technique, in Table 5.9. Identification of potential collaborators is accomplished during step 3 of the decision framework. A search for potential local collaborators also revealed that many similar mitigation efforts have been implemented in the greater Denver metropolitan area. Reviewing any information related to these specific mitigation efforts may provide useful context regarding local goals, Stream restoration Uplands restoration Colorado Stream Restoration Network Trust for Public Land Colorado Riparian Association Great Outdoors Colorado Colorado Water Wise National Resources Conservation Service Colorado Partners for Fish and Wildlife Program Colorado Open Space Alliance Cherry Creek Basin Water Quality Authority Southeast Metropolitan Stormwater Authority Urban Drainage and Flood Control District Table 5.9. Potential partners for Piney Creek mitigation alternatives.

Piney Creek Example 41   potential collaborators, the likelihood and magnitude of co-benefits, and the costs associated with implementing similar mitigation efforts. Some of these efforts might include the following: • Piney Creek Restoration project: https://www.thkassoc.com/piney-creek-restoration • Robinson Gulch Stream Stabilization: https://www.stantec.com/en/projects/united-states- projects/r/robinson-gulch-stream-stabilization • South Platte River Stream Restoration and Habitat Enhancement: https://cpw.state.co.us/ learn/Pages/RA-South-Platte-Restoration.aspx • Westerly Creek Restoration and Water Quality Project: https://dnrweblink.state.co.us/cwcb/ 0/edoc/211232/26i.pdf?searchid=0a1d3ca1-2699-4b2e-b6c5-b5d5159633e5 5.1.2.2 Identify Co-benefits through Established Causal Chains (Step 8B2) Table 5.10 and Table 5.11 describe the likelihood of co-benefits resulting from the uplands restoration and stream restoration, respectively, by evaluating site-specific ecological and socio- economic factors. This relies on basic information about the mitigation site and context to deter- mine which of the causal chain pathways are relevant to the specific mitigation alternatives. 5.1.2.3 Assess Relative Magnitude of Co-benefits (Step 8B3) Based on the findings described in step 8B2, Table 5.12 summarizes the likelihood of each co-benefit and provides some indication of the potential magnitude of the benefits across the three mitigation alternatives. The alternative that includes both stream restoration and uplands restora- tion is likely to result in the greatest co-benefits. Because the stream restoration and uplands resto- ration mitigation alternatives would occur in separate locations without overlap, the magnitude of those co-benefits may be additive. In this hypothetical example, both mitigation sites are proposed within the City of Aurora which, based on a review of local planning documents, endeavors to bolster its water supply and use green infrastructure to support groundwater recharge. These two mitigation techniques, therefore, are likely to align with local planning objectives. Data constraints make it somewhat difficult to provide more granular information to accu- rately characterize the magnitude of co-benefits at a screening level. In this case, a more detailed analysis, as described in Section  5.2.2, may offer additional insights and help decision- makers convey the likely co-benefits of their choices to the public and stakeholders. Moreover, if cost considerations made mitigation alternative 3 infeasible, it would be difficult to choose between the remaining two alternatives with the level of detail provided in the screening analysis. Detailed analysis of co-benefits could add value in this instance. 5.2 Detailed Analyses (Step 11) This section provides an example of hydrologic and co-benefits detailed analyses for Piney Creek, Colorado. Results from the screening analyses (step 8) provide useful initial informa- tion for the detailed analyses. Steps 11A and 11B are the hydrologic and co-benefits analyses, respectively. 5.2.1 Detailed Hydrologic Analysis (Step 11A) For the detailed model analysis of Piney Creek, the process involves three modeling activities: • Baseline model. Obtain and run the baseline model. • Highway project scenario. Modify and run the baseline model to represent the altered land cover with the increased impervious area from the new highway project. • Mitigation scenario(s). Create and run one or more mitigation scenarios, simulating the new highway project plus the mitigation by one or more mitigation techniques.

Co-benefit Site-specific ecological factors that contribute to the provision of ecosystem services Site-specific socioeconomic factors that contribute to the value of ecosystem services Likelihood of co-benefit at mitigation site Water supply maintenance Piney Creek flows into Cherry Creek and on to the Cherry Creek Reservoir. Piney Creek does not appear to directly contribute to water supplies used for public consumption. This co-benefit is unlikely to be associated with this project. Improved drinking water quality Piney Creek flows to Cherry Creek and on to Cherry Creek Reservoir. Cherry Creek Reservoir is not used as a source of public drinking water. This co-benefit is unlikely to be associated with this project given the high baseline water quality and reliance on treatment facilities. Improved human health and welfare See “improved drinking water quality” co-benefit for factors associated with drinking water. See “improved drinking water quality” co-benefit for factors associated with drinking water. This co-benefit is unlikely to be associated with this project. Increased or improved recreational opportunities The state DOT and its collaborators could choose if they want the uplands restoration area to be available to recreators and incorporate desirable recreation elements into the design of the space. Downstream, the uplands site may also create other recreation benefits, including for recreational fishers in connected waterways. Several other recreational sites are near the proposed uplands restoration area, including Cherry Creek State Park, Red-Tailed Hawk Park, Larkspur Park, Golden Eagle Park, Saddle Rock Golf Course, and Piney Creek Trail (City of Aurora 2021a). Several of these areas offer significant tree cover to support forest (or shaded) recreation (City of Aurora 2019a). This co-benefit is likely to be associated with this project. Improved landscape aesthetics The state DOT and its collaborators could choose new plant species and other design characteristics to maximize the potential for aesthetic benefits. Within 1,000 ft of the proposed site, there are 20 residential homes and 5 commercial properties (Arapahoe County Government 2013). The mitigation site will also be adjacent to a highway and potentially attract recreators, both groups that may experience improved views from the mitigation relative to the baseline land cover. This co-benefit is likely to be associated with this project. Non-use and cultural values Improved uplands habitat conditions could support endangered and threatened species in the area, including the whooping crane, piping plover, greenback cutthroat trout, western prairie fringed orchid, and Ute ladies'-tresses (U.S. Fish and Wildlife Service 2021b). Research suggests that the general public holds non-use values for protecting endangered and threatened species [e.g., Richardson and Loomis (2009)]. No tribal lands in the area (U.S. Department of Homeland Security 2020). This co-benefit is likely to be associated with this project. Increased property values See “improved landscape aesthetics” co-benefit for factors associated with viewshed. Within 1,000 ft of the proposed site, there are 20 residential homes and 5 commercial properties (Arapahoe County Government 2013). The property values may benefit from the improved landscape aesthetics. This co-benefit is likely to be associated with this project. Climate stabilization The state DOT and its collaborators could choose vegetation types to maximize the potential for carbon sequestration at the site. The mitigation description mentions grass, shrub, and forest cover. Not applicable This co-benefit is likely to be associated with this project. Climate resiliency The areas adjacent to creeks in Aurora and the surrounding municipalities are generally characterized as floodplains and may be likely to see increased flooding with more extreme precipitation events under climate change (City of Aurora 2020). The state DOT and its collaborators could design the uplands restoration mitigation with resilience against future floods in mind. Uplands restoration efforts that include planting vegetation and slope stabilization slow runoff, reducing the risk of climate-related flooding. Increased vegetation cover and less impervious surface may reduce local temperatures. The population in this area is not particularly socially vulnerable. Relative to the rest of the county, the minority population in this area is small, less than 20% of the population in surrounding census tracts is considered low-income (household income is less than or equal to twice the federal poverty level), and the elderly population is small (U.S. Environmental Protection Agency 2019). This co-benefit is likely to be associated with this project. Table 5.10. Screening assessment of potential co-benefits for uplands restoration at Piney Creek.

Co- benefit Site-specific ecological factors that contribute to the provision of ecosystem services Site-specific socioeconomic factors that contribute to the value of ecosystem services Likelihood of co-benefit at the mitigation site Improved drinking water quality Piney Creek flows to Cherry Creek and on to Cherry Creek Reservoir. Cherry Creek Reservoir is not used as a source of public drinking water. This co-benefit is unlikely to be associated with this project. Improved human health and welfare See “improved drinking water quality” co-benefit for factors associated with drinking water. See “improved drinking water quality” co-benefit for factors associated with drinking water. This co-benefit is unlikely to be associated with this project. Water supply maintenance Piney Creek flows into Cherry Creek and on to the Cherry Creek Reservoir. Piney Creek does not appear to directly contribute to water supplies used for public consumption. This co-benefit is unlikely to be associated with this project. Increased or improved recreational opportunities Whether the 2,100 ft of restored stream provides direct recreation opportunities depends on the mitigation alternative design decisions. Downstream recreation benefits may be more likely than on-site recreation benefits. For example, the reservoirs in the immediate area are used for recreational fishing (Colorado Parks and Wildlife 2021). The state DOT and its collaborators may want to determine connectivity of the site with these reservoirs to determine the likelihood of improved recreational fishing opportunities. It is also possible that the restored conditions will provide habitat for several threatened and endangered species with overlapping range that may provide opportunities for wildlife viewing, including the whooping crane, piping plover, greenback cutthroat trout, western prairie fringed orchid, and Ute ladies'-tresses (U.S. Fish and Wildlife Service 2021a). There are several other recreation sites near the proposed stream restoration location, including Cherry Creek State Park and Reservoir, Ponderosa Preserve, and Red-tailed Hawk Park (U.S. Geological Survey 2018). To obtain information on the size of the population potentially benefiting from improved recreational experiences, the state DOT may also consider calling Colorado Parks and Wildlife or the Cherry Creek Basin Water Quality Authority to help characterize the number of recreational fishers that may benefit from increased fishing opportunities downstream. This co-benefit is likely to be associated with this project. Commercial fishing (resource harvesting) The state DOT may want to consult with the Colorado Department of Fish and Game to understand how improved fish habitat in the selected stream may affect the health of downstream fisheries and commercial fishing opportunities. If the stream could support downstream commercial fishing opportunities, then the state DOT may want to characterize the demand for fish and the number of potentially benefiting fishing entities. Cherry Creek Reservoir is used for purposes of harvesting walleye eggs for state fishery propagation efforts. This co-benefit is likely to be associated with this project. Non-use and cultural values Improved stream habitat conditions could support endangered and threatened species in the area, including the whooping crane, piping plover, greenback cutthroat trout, western prairie fringed orchid, and Ute ladies'-tresses (U.S. Fish and Wildlife Service 2021b). Research suggests that the general public holds non-use values for protecting endangered and threatened species [e.g., Richardson and Loomis (2009)]. No tribal lands in the area (U.S. Department of Homeland Security 2020). This co-benefit is likely to be associated with this project. Climate stabilization The state DOT and its collaborators could choose vegetation and other aspects of mitigation design to maximize the potential for carbon sequestration at the site. Not applicable. This co-benefit is likely to be associated with this project. Climate resilience The areas adjacent to creeks in Aurora and the surrounding municipalities are generally characterized as floodplains and may experience increased flooding with more extreme precipitation events under climate change (City of Aurora 2020). The stream restoration mitigation may be designed to improve flows and reduce risks of climate-related flood events at the site. The population in this area is not particularly socially vulnerable. Relative to the rest of the county, the minority population in this area is small, less than 20% of the population in surrounding census tracts are considered low-income (household income is less than or equal to twice the federal poverty level), and the elderly population is small (U.S. Environmental Protection Agency 2019). This co-benefit is likely to be associated with this project. Table 5.11. Screening assessment of potential co-benefits for stream restoration at Piney Creek.

Stream restoration Uplands restoration Stream restoration and uplands restoration Improved human health and welfare ☐ Water supply maintenance Improved drinking water quality Increased landscape aesthetics Within 1,000 ft of the proposed site, there are 20 residential homes and 5 commercial properties. Potential recreators and commuters with views from the highway may also benefit. See uplands restoration Increased or improved recreational opportunities Stream may support recreational fishing in connected reservoirs. State DOT could design mitigation for on-site recreation. Downstream recreational fishing benefits are also possible. See stream and uplands restoration Climate stabilization State DOT could design mitigation to maximize carbon sequestration potential. See stream restoration See stream and uplands restoration Climate resiliency Areas adjacent to waterways near the site are considered floodplains. Climate change is expected to bring more significant extreme precipitation events. See stream restoration See stream and uplands restoration Non-use and cultural values Five endangered and threatened species have ranges in the area. See stream restoration See stream and uplands restoration Increased property values Within 1,000 ft of the proposed site, there are 20 residential homes and 5 commercial properties. See uplands restoration Commercial fishing benefits (resource harvest) Stream may support commercial fishing downstream. See stream restoration Notes: Blue cells highlight the co-benefits that may be anticipated from a particular mitigation technique per the causal chain diagrams presented in Chapter 5 of NCHRP Web- Only Document 333. The checked boxes represent instances where the site-specific ecological and socioeconomic characteristics may support the co-benefit. Unchecked boxes represent instances where site-specific ecological and socioeconomic characteristics were considered and may not support the co-benefit. Table 5.12. Comparison of mitigation alternatives for Piney Creek.

Piney Creek Example 45   The practitioner first needs to adapt an existing watershed model or develop a new watershed model representing baseline conditions (the baseline model). Results for metrics of interest may then be calculated for comparison with the additional model scenarios developed in the study. Next, the practitioner develops the highway project scenario, which represents the impact of the hypothetical highway project that will be assessed at AP1 and AP2. For the Piney Creek example, the highway project scenario includes 170 acres of new highway with 80% (136 acres) simulated as impervious area. Further detailed GIS analysis identified 7.2 acres of the highway project footprint are already impervious, therefore lowering the new impervious highway area impact to 128.8 acres (this only applies if mitigation is only required for the new impervious area, not the total impervious area). The highway project scenario is implemented in the model by transferring 128.8 acres of pervious natural area (e.g., grass/shrub/barren) to a highly developed impervious area. The remaining project area is added to the pervious component of the highly developed land cover category. The practitioner then develops one or more mitigation scenarios, representing mitigation options of interest. In the hydrologic screening analyses (Section 5.1.1), mitigation needs for forest restoration, uplands restoration, wetland restoration, and stream restoration were devel- oped. Based on these results the state DOT and partners would develop promising mitigation scenarios that could include a single technique (e.g., uplands restoration or a combination of techniques). Different scenarios can consider location of the mitigation upstream or down- stream of the highway project site or both. This example illustrates detailed hydrologic analysis using a blended mitigation scenario incorporating both stream restoration and uplands restoration. The scenario includes 2,100 ft of stream restoration upstream of the highway project site (upper part of the watershed). An upstream location was selected based on examination of stream conditions to determine the most suitable portion of stream as indicated in Figure 5.7. To represent this change in the HSPF model of the Piney Creek watershed, the reach length term (RCHRES:HYDR-PARM2) will be increased from 5.51 to 5.91 miles upstream of the highway project site (subbasin 272 in the model). In addition, a corresponding increase will be made to the area and volume terms of the associated FTABLE, which determines the stage-discharge relationship of the reach within the model. The blended mitigation scenario also includes uplands restoration downstream of the high- way project site where the watershed is more heavily developed (lower portion of the water- shed). Detailed analysis of the lower portion of the watershed as shown in Figure 5.8 indicates an 18-acre developed area that could be a candidate to convert to uplands (grass, shrubs, trees). The same analysis shows at least 2 acres of the site will continue to be impervious roadway. Thus, in the HSPF watershed model, this transition is simulated by removing 16 acres from the highly developed/impervious land cover category and adding those acres to the land cover category used to represent native growth (i.e., grass/shrub/barren). To assess the impacts of the blended mitigation scenarios, the modeler simulates the uplands restoration and stream restoration separately and together to understand the individual and combined effectiveness of the techniques as part of the overall scenario. Table 5.13 reports metrics at the upper AP (i.e., outlet of subbasins 272 and 274). The table includes the results of the baseline run and the effects of the highway project for each of the four metrics. As expected, the highway project results in increases in peak discharge and volume for both the 2-year and 100-year events. The last column contains the results of the stream restoration. Because the uplands restoration component is downstream of the project site (and AP1), it will not affect metrics at AP1.

Figure 5.8. Developed area in the Piney Creek watershed. Figure 5.7. Potential stream restoration area in the Piney Creek watershed.

Piney Creek Example 47   For the two peak discharge metrics, the stream restoration mitigates the effects of the high- way project but not to the baseline (pre-project) levels. The stream restoration has no apparent effect on the event volume metrics. These results are consistent with the screening results reported in Table 5.7 for AP1. The screening indicated that stream restoration would mitigate the peak flows but not for the event volumes. It also indicated that more than 2,100 ft of stream restora- tion would be needed to fully mitigate peak flow project impacts at AP1. Table 5.14 reports metrics for all model simulations at the downstream AP (AP2, outlet of sub- basin 278). The table includes values from the baseline and highway project simulations showing increases caused by the highway project. The remaining three columns summarize the results for the stream restoration upstream of the project site alone, the uplands restoration downstream of the project site alone, and the two acting in combination. For the two peak discharge metrics, the stream restoration upstream of the highway project combined with the uplands restoration located downstream of the project mitigates the effects of the highway project as measured at AP2 but not to the baseline (pre-project) levels. The stream restoration has no apparent effect on the event volume metrics, and the uplands restoration has only a modest effect. Table 5.14 indicates an anomalous result for the 100-year volume metric in the uplands restoration scenario. The calculated value of 926 ac-ft suggests that the mitigation increases the event volume compared with the scenario of the highway project. Each annual Hydrologic metric* Baseline Highway project Upper stream restoration 100-year peak discharge (cfs) 636 713 693 2-year peak discharge (cfs) 223 252 246 100-year event volume (ac-ft) 355 396 396 2-year event volume (ac-ft) 168 195 195 *Peak discharges are based on hourly discharge; event volumes are total discharge from the beginning to the end of a runoff event. cfs: cubic feet per second. ac-ft: acre-feet. Hydrologic metric* Baseline Highway project Upper stream restoration Downstream uplands restoration Upper stream restoration and downstream uplands restoration 100-year peak discharge (cfs) 1,461 1,523 1,508 1,510 1,494 2-year peak discharge (cfs) 502 520 517 516 512 100-year event volume (ac-ft) 892 904 904 926** 900 2-year event volume (ac-ft) 500 526 526 521 523 *Peak discharges are based on hourly discharge; event volumes are total discharge from the beginning to the end of a runoff event. ** See text for explanation of the 100-year volume. cfs: cubic feet per second. ac-ft: acre-ft. Table 5.14. Metrics at the Piney Creek downstream AP (AP2). Table 5.13. Metrics at the Piney Creek upper AP (AP1, subbasins 272 and 274).

48 Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Guide peak in the event volume series decreases with the uplands restoration; however, because some of the smaller events decrease more, this increases the standard deviation of the annual series. This results in a higher estimate of the 100-year volume when fitting to the statistical distribu- tion. Detailed analysis allows deeper investigation into quantitative results to better support decision-making. These results are consistent with the screening results reported in Table 5.6 for AP2. The screening indicated that uplands restoration would mitigate for the peak flows but not for the 100-year event volume without a much larger mitigation area. For the 2-year volume, the screening results are inconclusive, but this detailed analysis shows some mitigative effects of uplands restoration. The detailed analysis reflected in Table 5.14 is based on 16 acres of mitigation, which is much less than indicated in the screening analysis as shown in Table 5.6 for uplands restoration down- stream of the project site. Although the detailed modeling of 16 acres of uplands restoration shows that additional acreage is needed, the results suggest that, in this case, the screening tool conservatively estimated the mitigation amounts needed, which was the intention in design- ing the screening tool [i.e., the tool would be more likely to underestimate the mitigation area needed (to be conservative) than to overestimate the mitigation area needed]. 5.2.2 Detailed Co-Benefits Analysis (Step 11B) This section demonstrates a more detailed analysis of a subset of co-benefits for the uplands restoration out-of-kind mitigation techniques for the Piney Creek case study: • Increased or improved recreation opportunities • Climate stabilization • Increased property values This detailed analysis builds on the findings of the screening analysis in Section 5.1 by pro- viding additional insights regarding the ecological changes and associated ecosystem service co-benefits associated with those changes for this particular mitigation technique. The analysis relies exclusively on secondary data sources (i.e., would not require the state DOT to engage in primary research or data collection) and considers magnitudes of ecological changes and eco- nomic values documented in the literature. 5.2.2.1 Increased or Improved Recreation Opportunities The uplands restoration alternative can result in increased or improved recreation opportuni- ties through two pathways: • Creation of a new public recreation destination on the property itself (that offers hiking or walking trails, wildlife viewing, place space for kids, etc.) • Downstream recreation in the form of increased recreational fishing opportunities How recreation at the upland site may change demand for and the value of recreation in the immediate area is uncertain. An analysis of nearby existing recreation sites reveals several poten- tial substitutes: Red-tailed Hawk Park (35 acres), Larkspur Park (7 acres), Golden Eagle Park (5.2 acres), and Piney Creek Trail (16.1 miles) (City of Aurora 2019b, City of Aurora 2021b, AllTrails 2021). It is uncertain if recreators will view the new uplands site as a substitute or complement to existing alternatives (i.e., it is uncertain whether additional recreational access would increase participation in more recreational activity in the region or simply change the distribution of trips across the available sites; if the latter, this may have the benefit of alleviating any pressure associated with demand for existing sites).

Piney Creek Example 49   Downstream recreational fishing benefits are also possible if the uplands site is well connected with streams that function as tributaries to recreational fishing destinations. While the uplands restoration mitigation site is notably larger than the stream restoration footprint, it remains chal- lenging to map the ecological changes associated with the mitigation to specific fish habitats and fish population benefits. Instead, it is likely the case that the uplands restoration activities will be part of a larger suite of related mitigation efforts in the area that seek to improve water quality conditions in Cherry Creek, which will result in improvements in fish habitat, fish populations, and recreational fishing opportunities. While changes in recreation activity levels associated with the uplands restoration are uncer- tain, if additional data are available on recreational access at a restoration site, state DOTs may wish to estimate the potential increase in recreational trips based on visitation estimates at neighboring sites. Available literature on economic values for recreational activities may then inform the economic value of this activity. For example, Table 5.15 provides context around the value per recreation trip across possibly relevant recreation types using information from the Recreation Use Values Database maintained by Oregon State University. Recreation use values can vary greatly. For example, the value of a mountain biking activity varies based on the loca- tion, duration, and amenities associated with the experience. In addition, different people will value the same experience differently. Analysts may want to consider the potential for increased visitation to the region to generate regional economic activity (i.e., cash flows across trip-related industries in the region). The simplest version of a regional economic impact analysis of this type relies on regional input- output models, such as the commonly used IMPLAN model. An IMPLAN analysis would require information on the change in visitation to the recreation for recreational purposes and average expenditures by type associated with those trips. (Note: Potential sources for trip expenditures may be found by reviewing the economics literature. Additionally, the U.S. Fish and Wildlife Service generates state-level estimates of average expenditures for recreational activities every 5 years as part of its National Survey of Hunting, Fishing, and Wildlife-Related Recreation. Information on this survey is available at https://www.census.gov/programs-surveys/ fhwar.html). Supporting regional businesses may also be a relevant co-benefit for state DOT to consider in addition to the ecosystem services. Activity Average value Freshwater fishing $94.80 Hiking $79.52 Mountain biking $212.69 Picnicking $23.62 Wildlife viewing $84.50 General recreation $39.42 All recreation $83.28 Source: Rosenberger (2016), summarizing literature identified in the Recreation Use Values Database maintained by Oregon State University, available at http://recvaluation.forestry.oregonstate.edu/database. Original values were converted to 2020 using a gross domestic product (GDP) deflator. Table 5.15. Recreation use value per person per day, Western United States (2020 dollars).

50 Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Guide 5.2.2.2 Climate Stabilization Restoring 196 acres of impervious cover to a combination of forest, grassland, and shrubland will increase the ability of the landscape to sequester carbon, resulting in climate stabilization benefits. The benefits of carbon sequestration and storage reflect the avoided marginal climate change-related damages associated with the reduction in atmospheric carbon. That is, in eco- nomic terms, the benefits of reductions in atmospheric carbon dioxide (and other greenhouse gases) reflect the value that people derive from avoiding additional climate change-related impacts (e.g., to crops, human health, infrastructure, etc.). The social cost of carbon is a mea- sure of the public’s willingness to pay to avoid climate change-related impacts. Carbon is valued using estimates of the social cost of carbon (SCC) provided in the Inter-Agency Working Group on Social Cost of Greenhouse Gases (2021) in Table 5.16 and converted from costs per metric ton of carbon dioxide (CO2) to cost per metric ton of carbon (C). The amount of carbon the restored upland area can sequester will depend on the mix of species chosen during mitigation design. Generally, trees sequester more carbon than other vegetative species due to relative size of biomass, although soil carbon sequestration and other site-specific considerations also influence the sequestration and storage potential of a landscape. While forest carbon budget benefits are well researched, grasslands and shrublands sequester more carbon relative to impervious cover. Literature also suggests that not only does the species matter for carbon sequestration rates and potential but also who owns the land and how they choose to manage it (Failey and Dilling 2010). Therefore, while the carbon sequestration benefits of uplands restoration can be estimated given some known attributes of the restoration (e.g., tree species planted), other aspects of management are more difficult to quantify and introduce uncertainty into the overall magnitude of benefits. Nonetheless, for purposes of understand- ing the relative magnitude of the benefits, analysis of forest carbon sequestration is less data- intensive than many other ecosystem service co-benefit categories. To demonstrate how to monetize the carbon sequestration benefits of uplands restoration, this analysis focuses on the uplands area that may be converted to forest specifically. This analysis assumes that about 50 acres will be afforested (of the total 196 acres considered for this alterna- tive) and that four tree species categories will be planted with equal distribution across this area Year 5% 3% 2.5% High impact 2020 $51 $187 $279 $557 2025 $62 $205 $304 $620 2030 $70 $227 $326 $686 2035 $81 $246 $352 $755 2040 $92 $268 $378 $825 2045 $103 $290 $403 $887 2050 $117 $312 $425 $953 Source: Table A-1 of Inter-Agency Working Group on Social Cost of Greenhouse Gases (2021). Note that the values in this table convert guidance based on the value per metric ton of carbon dioxide (CO2) into the value per metric ton of carbon (C). Specifically, the multiplier for translating between mass of CO2 and mass of C is 3.67 (the molecular weight of CO2 divided by the molecular weight of carbon = 44/12 = 3.67). Table 5.16. Social cost of carbon (2020 dollars per metric ton).

Piney Creek Example 51   Table 5.17. Carbon density and sequestration rates by tree species (metric tons per acre). (13 acres each of Douglas fir, fir-spruce-mountain hemlock, lodgepole pine, and ponderosa pine). (Note: The tree mix at the mitigation site may also include deciduous trees like maple, hawthorn, and oak; however, for demonstration purposes, this analysis only considers coniferous trees.) Carbon sequestration rates of these tree species in the Rocky Mountains (South) region are based on Smith et al. 2006 (see Table 5.17). (Note: For more in-depth analysis, other resources for estimating carbon sequestration rates to tree resources are available at https://www.fia.fs.fed. us/forestcarbon/#CarbonScience.) This analysis assumes all trees will be planted at the same time (age 0) and mature together. While trees continue to sequester carbon over their lifetime, this analysis considers benefits during the first 30 years following their planting (2020−2050). Over the first 30 years following completion of the uplands restoration activity and based on the values in Table 5.16, the total present value benefit of sequestered carbon from trees ranges from approximately $15,000 to $190,000 (2020 dollars), depending on the discount rate and impact (average or high) assumptions. On an annualized basis, these benefits range from $900 to $9,100 (2020 dollars). In addition to these quantified benefits, other planted vegetation— including grasses and shrubs—will sequester carbon and result in economic benefits, albeit at lower rates. These estimates, therefore, reflect a lower bound relative to the total carbon seques- tration benefits across the uplands restoration site. 5.2.2.3 Increased Property Values Inhabitants of properties within view of a restored upland area may experience improved aesthetic experiences. One way in which improved aesthetics result in economic benefits is through increased property values. Both the total number of properties that may benefit from these property value increases as well as the amount these property values increase on average are necessary determinants of the total economic benefit associated with property value increases. GIS analysis of the surrounding area identified 20 residential homes and 5 commercial prop- erties within 1,000 ft of the proposed mitigation site (Arapahoe County Government 2013). Based on publicly available data, home values in the surrounding area range from $320,000 to $820,000. These property values represent baseline (pre-project) conditions. Tree age Douglas fir Fir-spruce-mountain hemlock Lodgepole pine Ponderosa pine Carbon density Sequestration rate Carbon density Sequestration rate Carbon density Sequestration rate Carbon density Sequestration rate 0 0.0 0.00 0.0 0.00 0.0 0.00 0.0 0.00 5 1.1 0.22 0.7 0.14 0.9 0.18 0.7 0.14 15 2.9 0.18 1.6 0.09 1.7 0.08 1.5 0.08 25 8.0 0.51 4.8 0.32 3.7 0.20 3.8 0.23 35 15.0 0.70 9.9 0.51 6.8 0.31 7.5 0.37 45 22.1 0.71 14.8 0.49 10.5 0.37 11.7 0.42 55 29.0 0.69 19.7 0.49 13.8 0.33 15.5 0.38 65 34.8 0.58 23.7 0.40 17.0 0.32 19.1 0.36 75 40.0 0.52 27.4 0.37 20.0 0.30 22.4 0.33 85 44.7 0.47 30.8 0.34 22.8 0.28 25.6 0.32 95 48.8 0.41 34.0 0.32 25.4 0.26 28.5 0.29 105 52.4 0.36 36.9 0.29 27.8 0.24 31.2 0.27 115 55.6 0.32 39.6 0.27 29.8 0.20 33.7 0.25 125 58.5 0.29 42.0 0.24 31.7 0.19 35.9 0.22 Source: Carbon density figured directly from Appendix B of Smith et al. (2006). Sequestration rates were calculated by the authors. All values reflect afforestation of live trees in the Rocky Mountain (South) region.

52 Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Guide The economics literature identifies several important findings about property values when greenspace is made available. In a meta-analysis of 35 studies that valued general open space in the United States specific to low-impact development practices, Mazzotta et al. (2014) isolated characteristics of open space projects and nearby homes that lead to increases in housing prices. The authors found that open space projects that included trees and that are described as “protected, dispersed, and not recreational” lead to the largest property value increases. This indicates that the state DOT should consider that there may be tradeoffs in the types of ecosystem services provided by uplands restoration that prioritize recreational opportunities and those that confer property value benefits. Further, homes located nearest to the open space, that had lower baseline values, and that were located in more densely populated areas saw the largest gains. Another study on the value of open space found considerable variation across studies and noted that many attributes that result in economic benefits may be site- and context-specific (McConnell and Walls 2005). While Mazzotta et al. (2014) offer a functional form that could be used in benefit transfer, applying the function would require mitigation design details and more specific information about individual properties. However, to illustrate the analysis, Table 5.18 describes the variables in the model, the estimated coefficients, and the level of statistical significance from Mazzotta et al. The table also presents the input assumptions used for this case study. For example, this exercise assumes that open space will be increased by 30% within 250 m of the mitigation, 10% within 250 m to 500 m of the mitigation, and 1.9% across the watershed (where population density is greater than 800 people per square mile, and there are currently about 8,000 acres of open space in Aurora). Moreover, the example assumes the mitigation will include both trees and riparian buffer and be defined as contiguous, recreational, and protected, resulting in values of “1” for each of those variables. Using the home price data previously mentioned, the midpoint property value is currently $570,000, and it is assumed that nearby properties are about 0.5 acres each (the natural log of 0.5 is −0.69). Each of these assumptions is multiplied by the relevant model coefficients, then the result- ing values are summed to calculate the total percent change in property values. As presented in Table 5.18, this model predicts property value increases of approximately 5.1% within 250 m Variable in model from Mazzotta et al. (2014) Model coefficient from Mazzotta et al. Statistical significance of coefficient from Mazzotta et al. Benefit transfer input assumptions from case study Output of benefit transfer (250 m) Output of benefit transfer (250 m to 500 m) Intercept 0.039 0.039 0.039 Percent increase in open space, within 250 m of mitigation 0.169 0.001 level 30 5.07 N/A Percent increase in open space, within 250 m to 500 m of mitigation 0.102 0.001 level 10 N/A 1.02 Percent increase in open space if watershed has >=800 people per square mile -0.063 0.01 level 1.9 -0.1197 -0.1197 Binary, if there is riparian buffer area 0.252 0.05 level 0.252 0.252 Binary, if there is wetland area -0.013 0 0 Binary, if there are trees at site 0.245 0.001 level 0.245 0.245 Binary, protected, dispersed, and not recreational 0.392 0.001 level 0 0 Binary, contiguous, recreational, and/or protected 0.081 1 1 0 1 0 1 0.081 0.081 Ln (lot size) -0.018 -0.69 0.012477 0.012477 Home price ($ thousands) -0.0009 0.001 level 570 -0.513 -0.513 Percent change in property values 5.1 1.0 Source: Stylized example using the benefit transfer function provided in Mazzotta et al. (2014). See main text for assumptions used for model input. Table 5.18. Property value increases using benefit transfer.

Piney Creek Example 53   of the mitigation area and 1.0% between 250 m to 500 m of the mitigation area. For reference, a study of the property value benefits associated with an open space increase of 15,000 acres between 1981 and 1995 in Boulder, Colorado, found residential prices rose by 3.75% over that interval (Riddel 2001). 5.2.2.4 Summary These analyses offer insight into the types of methods and data that would be useful for a more in-depth analysis of the magnitude and value of co-benefits stemming from particular mitigation techniques. As demonstrated, each type of analysis may differ in method, available data sources, and level of detail. A detailed analysis, providing additional analytical details and a comparison of multiple mitigation alternatives, can be found in Chapter 7 of NCHRP Web-Only Document 333. The detailed co-benefits analysis demonstrates how additional information about the mitiga- tion design can offer opportunities to convey the various benefits related to ecosystem services. For instance, the screening analysis identified the number of commercial and residential prop- erties that could benefit from increases in value; through detailed analysis, the state DOT could use existing literature and mitigation specifications to predict the likelihood and magnitude of those increases in property values. Similarly, for carbon sequestration, the screening analysis indicates that carbon sequestration benefits are likely, particularly when trees are part of the mitigation design. The assessment included the number of acres of trees and the tree species types to establish carbon sequestration rates and social cost of carbon values to monetize these benefits. Both of these examples signal the importance of having mitigation specifications in hand when engaging in detailed co-benefits analysis. Beyond a more thorough assessment of individual co-benefits, the detailed analysis also pro- vides additional insight relative to the screening analysis to guide decision-making between mitigation alternatives. Chapter 7 of NCHRP Web-Only Document 333 offers a more complete detailed analysis. Overall, the full analysis identifies that, for this example, uplands restoration may have greater co-benefits than the stream restoration mitigation, in part due to its much larger geographic footprint and the more significant change in landscape-level land cover (from imper- vious surface to natural landscape). The screening analysis, however, identified which categories of co-benefits were likely for each mitigation technique given site-specific characteristics; in this case, the detailed analysis allowed for quantification and valuation of some co-benefits to facilitate comparison.