Watershed Approach to Mitigating Hydrologic Impacts of Transportation Projects: Conduct of Research Report (2022) / Chapter Skim
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26 Chapter 4. Hydrologic Guidance This chapter outlines the major components of the hydrologic guidance developed through this research project.
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27 • Downstream of the project location a distance corresponding to fixed limit for an increase in drainage area, for example, finding a location downstream that increases the watershed area at the project location by no more than 50 percent. Consider a project site in Oklahoma where a roadway crosses a stream shown in Figure 4.1.
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28 Figure 4.1. Watershed determined by project location (from United States Geological Survey (USGS)
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29 Figure 4.3. Example near a confluence with a larger waterway (from USGS StreamStats)
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30 Figure 4.5. Example near a confluence with a larger waterway moved to just downstream of the next junction (from USGS StreamStats)
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31 Figure 4.6. HUC hierarchy (source: USGS)
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32 4.1.3. Tools for Defining a Watershed Regardless of how the outlet of a watershed is determined, various geographic information system (GIS)
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33 Other tools, such as EPA WATERS (https://www.epa.gov/waterdata/waters-watershedassessment-tracking-environmental-results-system) use the NHDPlus catchments and find the containing catchment and all of the catchments upstream for a given location.
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34 4.2. Hydrologic Impacts, Objectives, and Metrics Identifying the hydrologic objectives, that is, which hydrologic impacts are to be addressed through mitigation is a critical element of the decision framework for mitigating the hydrologic impacts of transportation facilities at a watershed scale.
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35 as sediment erosion, deposition, and transport, water quality, and ecosystems. Richter et al.
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36 Richter et al.
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37 • 4B3. The 4B3 is a biologically-based statistic that indicates an allowable exceedance of a chemical concentration for four consecutive days with an average recurrence frequency of once every 3 years.
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38 core computational code of the analysis components could be accessed in a batch mode if preset computations are needed with little to no user interaction. Figure 4.8.
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39 decision support tools that are scientifically based and practical. Although Bledsoe, et al.
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40 specify the pervious land parameters that control infiltration. Other watershed models use the Natural Resources Conservation Service (NRCS)
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41 In-stream enhancement might include a return to a more natural channel, restoring stream sinuosity for example. This would be represented as a change in channel geometry.
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42 performed to develop the screening tool followed by a description of the development of the screening tool and an explanation of its application.
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43 4.4.2. Detailed Modeling Results for Screening Tool Development Development of the screening tool involved analysis of the results of approximately 1200 HSPF model runs at three locations (Colorado, Georgia, and Minnesota)
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44 Figure 4.9. Geographic location of watershed models.
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45 Table 4.2. Piney Creek, Colorado, hypothetical baseline watershed summary.
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46 Table 4.3. Upper Pine Knot Creek, Georgia, hypothetical baseline watershed summary.
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47 Table 4.4. Bluff Creek, Minnesota, hypothetical baseline watershed summary.
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48 4.4.2.4. Hydrologic Metrics For each model simulation, model output was assessed just below the assumed transportation project location and at the next HUC12 boundary downstream.
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49 In addition to peak and volume statistics, other hydrologic metrics were computed from the simulation model output. These metrics included the 10- and 90-percent exceedance flows from the flow-duration curve computed based on a simple ranking of the daily simulated streamflow values aggregated from the hourly model output.
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50 4.4.2.5.1. Mitigation from Pervious Land Covers The watershed modeling for this research revealed that under some circumstances, mitigation of the impacts of a transportation project could potentially be accomplished through conversion of certain pervious land covers (grasslands, scrub/shrub, urban parks and lawns, and agricultural lands/pasture)
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51 Figure 4.13. Average annual runoff.
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52 The screening tool was developed by testing a range of transportation project sizes ranging from 2 to 20 percent of the tributary drainage area at the AP and on watersheds smaller than 40 square miles. The ratios in this range of conditions were determined by interpolation.
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53 mitigation) or increased stream length (for stream restoration)
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54 conservative estimate (and thus the highest confidence level) , the highest mitigation ratio from among the geographic locations and highway impact sizes was identified and compiled into the final tables.
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55 4.4.3.2. Land Cover Conversion Mitigation Techniques For forest restoration/creation, wetland restoration/creation, and uplands restoration, the screening tool identifies the required mitigation ratio that specifies the number of acres of mitigation to compensate for one acre of highway (development)
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56 • Mitigation site: baseline land cover before mitigation represents an impervious or pervious land cover. For the screening tool: o Impervious land cover is defined as mitigation sites where the existing land is at least 95 percent impervious.
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57 • Area of the watershed between the transportation project and the APs (ADS)
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58 Figure 4.16. HSPF integration with EPA BASINS.
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59 Starting with a baseline model, the modeler modifies the baseline model to represent the altered land cover with the increased impervious area from the new transportation project. Figure 4.18 illustrates the Windows interface to HSPF in BASINS (WinHSPF)
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60 length is extended and the HSPF ‘FTABLE' associated with the stream reach is modified accordingly with a larger surface area and volume. The modeler then runs this new HSPF scenario with the project impact and the mitigation techniques in place.
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61 Figure 4.20. HSPF streamflow mitigation results for the higher flow rates.
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62 4.5.1.2. Cherry Creek Example A second example of adapting an existing HSPF application is based on a model of the Cherry Creek watershed in Colorado as shown in Figure 4.22 within the EPA BASINS system.
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63 precipitation events through rain barrels and similar storage mechanisms for reuse. Water reuse can be modeled in HSPF in a simple hypothetical way by changing the multiplier on the point source discharge, increasing the PERLND/IMPLND retention storage terms, and shifting land area from a developed category back into the forest category.
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64 Figure 4.24. Cherry Creek flow-duration curves.
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65 Figure 4.26. HSPF time series plot.
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