Excess nutrient pollution and eutrophication are widespread problems that must be solved at watershed scales, and stream restoration is increasingly implemented as a solution. Yet few studies evaluate the cumulative effects of multiple individual restoration projects on watershed-scale nutrient loading. We constructed a HEC-RAS model of stream restoration implemented throughout a generic 4th order watershed typical of the Piedmont physiographic province of the eastern USA. We simulated restoration of hyporheic exchange as one increasingly popular technique that receives dissolved nitrate-nitrogen (NO3--N) mitigation credit under the Chesapeake Bay TMDL. We populated the model with hyporheic exchange (0.3% of surface flow per hyporheic-exchange inducing in-stream restoration structure) and NO3--N removal (supply-limited denitrification removes all NO3--N that enters the hyporheic zone) values from prior literature on in-stream structures and related restoration techniques. We then varied the percentage of stream channels in the watershed in which restoration occurred. For watersheds with less than 100% of stream channels restored, we also varied where in the watershed (i.e. stream order) that restoration occurred. We found that hyporheic restoration in our 4th order watersheds has the potential to reduce NO3--N loading to downstream waterbodies by up to 83%, but that a maximum of <100% reduction exists given certain watershed characteristics. Model results revealed a nonlinear relationship between percent of stream channels restored and percent NO3--N loading reduction that occurred at the watershed outlet. This indicates that the effects of individual projects are not linearly additive, and must be evaluated in the context of how much of the watershed has already been restored. We also found that restoration was more effective at reducing NO3--N loading when it occurred in higher order streams (e.g., 3rd and 4th order), yielding load reductions upward of 30% compared to < 10% in lower order streams (e.g., 1st and 2nd order). Thus, the location of an individual restoration project within a watershed is important in determining its effect on NO3--N. Overall, our results indicate that hyporheic restoration can have significant effects on watershed NO3--N loading to downstream waterbodies, yet the watershed must be viewed as a whole to understand the potential impacts of any particular project under consideration. / Master of Science / Nutrient pollution and harmful algal blooms are widespread problems that must be solved at watershed scales, and stream restoration is increasingly implemented as a solution. Yet few studies evaluate the cumulative effects of multiple individual restoration projects on watershed-scale nutrient loading. We constructed a HEC-RAS model of stream restoration implemented throughout a generic watershed typical of the mid-Atlantic USA. We simulated restoration of nutrient-reducing groundwater flow cells along a stream corridor (hyporheic exchange) as one increasingly popular technique that is emphasized under the Chesapeake Bay TMDL. We populated the model with hyporheic exchange and nitrate-nitrogen (NO3--N) removal values from prior literature on in-stream structures and related restoration techniques. We then varied the percentage of stream channels in the watershed in which restoration occurred. For watersheds with less than 100% of stream channels restored, we also varied where in the watershed (i.e. stream order) that restoration occurred. We found that hyporheic restoration in our watershed has the potential to reduce NO3--N loading to downstream waterbodies by up to 83%, but that a maximum of less than 100% reduction exists given certain watershed characteristics. Model results revealed a nonlinear relationship between percent of stream channels restored and percent NO3--N load reduction that occurred at the watershed outlet. This indicates that the effects of individual projects are not linearly additive, and must be evaluated in the context of how much of the watershed has already been restored. We also found that restoration was more effective at reducing NO3--N loading when it occurred in larger streams, yielding load reductions upward of 30% compared to less than 10% in smaller streams. Thus, the location of an individual restoration project within a watershed is important in determining its effect on NO3--N. Understanding the maximum possible degree of NO3--N reducing hyporheic exchange is an important step for practitioners and policy-makers in choosing the most effective location for a stream restoration based on a project's goals, and cannot be done without analyzing the watershed as a whole. With more watershed-scale planning and a better understanding of certain physical characteristics, we can choose restoration locations and strategies that will ultimately work more efficiently toward reaching a nutrient reduction goal.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/96566 |
| Date | 23 January 2020 |
| Creators | Calfe, Michael Louis |
| Contributors | Civil and Environmental Engineering, Hester, Erich T., Scott, Durelle T., Strom, Kyle Brent |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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