Hydraulic and Geomorphic Assessment of the Merced River and Historic Bridges in Eastern Yosemite Valley, Yosemite National Park, California Open-File Report 2013–1016 U.S. Department of the Interior U.S. Geological Survey Hydraulic and Geomorphic Assessment of the Merced River and Historic Bridges in Eastern Yosemite Valley, Yosemite National Park, California By J. Toby Minear and Scott A. Wright Open-File Report 2013–1016 U.S. Department of the Interior U.S. Geological Survey ii U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2013 Revised and reprinted: 2013 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit http://www.usgs.gov or call 1–888–ASK–USGS For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order this and other USGS information products, visit http://store.usgs.gov Suggested citation: Minear, J.Toby, Wright, Scott A., 2013, Hydraulic and Geomorphic Assessment of the Merced River and Historic Bridges in Eastern Yosemite Valley, Yosemite National Park, California: Sacramento, California, United States Geological Survey, Open-File Report 2013-1016, 74 p. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. iii Contents Abstract ........................................................................................................................................................................... 1 Introduction ...................................................................................................................................................................... 3 Purpose and Scope ......................................................................................................................................................... 4 Site Description ............................................................................................................................................................... 4 Methods ........................................................................................................................................................................... 7 Scenario development ................................................................................................................................................. 7 Data Collection ............................................................................................................................................................ 7 Present-day topography ........................................................................................................................................... 7 Hydrologic data ........................................................................................................................................................ 9 Water-surface elevations ....................................................................................................................................... 10 Wood influences on stage data .......................................................................................................................... 11 Present-day surface grain-size data ...................................................................................................................... 11 FaSTMECH 2D hydrodynamic model ........................................................................................................................ 12 Development of the present-day topographic surface model ................................................................................. 12 Surface generation ................................................................................................................................................. 13 Development of scenario topography .................................................................................................................... 14 Development of historical surface .......................................................................................................................... 14 Mesh configuration ................................................................................................................................................. 15 Roughness ............................................................................................................................................................. 16 Boundary conditions .............................................................................................................................................. 20 FaSTMECH solution parameters ........................................................................................................................... 20 FaSTMECH model evaluation................................................................................................................................ 21 Test of FaSTMECH mesh configuration ................................................................................................................ 22 Uncertainty and limitations of the hydraulic modeling ............................................................................................ 22 Geomorphic assessment ....................................................................................................................................... 24 Results .......................................................................................................................................................................... 25 Results of FaSTMECH hydrodynamic model ............................................................................................................ 25 Results of geomorphic assessment ........................................................................................................................... 27 Discussion ..................................................................................................................................................................... 28 Summary ....................................................................................................................................................................... 29 Acknowledgements ....................................................................................................................................................... 30 References cited ........................................................................................................................................................... 31 iv Figures Figure 1. Overview map of the watersheds of eastern Yosemite Valley and the project site. .................................. 34 Figure 2. Photograph of ground-based LiDAR unit on river left streambank approximately 100 m downstream of Ahwahnee Bridge at site of the previous Lower Pines Campground.. .......................................................................... 35 Figure 3. Map of bridge locations in eastern Yosemite Valley.. ................................................................................ 36 Figure 4. Longitudinal profile of the Merced River through Yosemite Valley (modified from Eagan 1998)............... 37 Figure 5. Digital elevation model of the Merced River study reach, with evaluation cross-sections marked. ........... 38 Figure 6. Comparison of flow data for the historical gage on Tenaya Creek (USGS gage 11265000) and Merced River at Happy Isles (USGS gage 11264500).. ............................................................................................................. 39 Figure 7. Photograph of flow through river right equestrian tunnel at Clark’s Bridge, taken looking upstream. Figures 13 and 14 show the main bridge spans for Clark’s and Ahwahnee Bridges..................................................... 40 Figure 8. Map of Merced River in eastern Yosemite Valley, showing the locations of study cross-sections established by the YNP as part of an ongoing streambank monitoring program. ......................................................... 41 Figure 9. Photograph of the side channel and the ground-based LiDAR unit, Maptek I-Site 4400, looking upstream into the side channel between Sugar Pine and Ahwahnee Bridges. ............................................................................. 42 Figure 10. Map of the project site on the Merced River in eastern Yosemite Valley.. ............................................ 43 Figure 11. Stage-discharge relation developed by YNP for Tenaya Creek from discharge measurements collected by Balance Hydrologics in WY2006 and YNP in WY2009-2010. .................................................................................. 44 Figure 12. Flood probability plotted for the Merced River, using peak discharge data collected at the USGS Merced River at Happy Isles gage, 11264500.. ............................................................................................................ 45 Figure 13. Photograph of a “tape-down” measurement of river stage during WY2010 on the downstream side of Clark’s Bridge.. .............................................................................................................................................................