• Abstract and Figures
• Public Full-text

Incision of Steepland Valleys by Debris Flows by Jonathan David Stock B.S. (University of California, Santa Cruz) 1992 M.S. (University of Washington) 1996 A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Earth and Planetary Science in the GRADUATE DIVISION of the UNIVERSITY OF CALIFORNIA, BERKELEY Committee in charge: Professor William E. Dietrich, Chair Professor Roland Bürgmann Professor T. M. Narasimhan Fall 2003 This dissertation of Jonathan David Stock is approved. ___________________________________________________________ Chair Date ___________________________________________________________ Date ___________________________________________________________ Date University of California, Berkeley Fall, 2003 Incision of Steepland Valleys by Debris Flows Copyright © 2003 by Jonathan David Stock ABSTRACT Incision of Steepland Valleys by Debris Flows by Jonathan David Stock Doctor of Philosophy in Earth and Planetary Science University of California, Berkeley Professor William E. Dietrich, Chair Steepland valleys are prone to episodic debris flows, which are flowing mixtures of rock and water. Debate continues about whether debris flow valley incision is adequately represented by modified fluvial incision laws (e.g., stream power law) that predict power laws of drainage area against valley slope. Using a wide range of topography from debris flow-prone temperate steeplands in the U.S and around the world, I find that inverse fluvial power laws (straight lines on log-log plots) rarely extend to valley slopes greater than ~ 0.03 to 0.10, values below which debris flows rarely travel. Instead, with decreasing drainage area the rate of increase in slope declines, leading to a curved relationship on a log-log plot of slope against drainage area. This curved relation is found along recent debris flow runouts in the U.S. with extensive evidence for bedrock lowering by the impact of large particles entrained in the debris flow, and along field-mapped runouts of older debris flows in the western U.S. and Taiwan. By contrast, downvalley from terminal debris flow 1 deposits, where strath terraces often begin, area-slope data follow fluvial power laws. Valleys cut by debris flows have long-profiles different from those cut by rivers. To measure bedrock lowering rates in both places, I installed hundreds of erosion pins in the rock floors of steep valleys recently eroded by debris flows in Oregon, and in bedrock rivers in Washington, Oregon, California and Taiwan. I monitored these sites for 1 - 7 years. Pins in valleys scoured by debris flows have been buried by colluvium, indicating a lack of fluvial incision. By contrast, pins in riverbeds (with power law area-slope plots) have lowered at rates up to cm’s per year, at values that are proportional to the square of bedrock tensile strength. Cultural artifacts in the fluvial deposits of adjoining strath terraces in Washington and Taiwan rivers indicate at least several decades of lowering at these extreme rates following historic exposure of bedrock. Observed lowering rates at most sites far exceed estimated long-term rock uplift rates, so the observed reaches of these rivers cannot be adjusted to either bedrock hardness or rock uplift rate in the manner predicted by the stream power law. Although power law plots of area versus slope may be consistent with regions of fluvial incision, they need not reflect the stream power bedrock river incision law. To explore why area-slope data is curved for valleys cut by debris flows, and to construct a debris flow incision law, I visited recent debris flow sites around the western U.S. I found that rock damage by impacts accounted for the majority of lowering. Field measurements of debris flow valley slope and bedrock weathering at these sites show a tendency for both to increase abruptly above tributaries that 2 contribute throughgoing debris flows. Indirect measurements indicate that debris flow length and long-term frequency increase downvalley as individual flows gain material, and as tributaries with more debris flow sources join the mainstem. From these field observations, I propose that long-term debris flow incision rate is proportional to the integral of solid inertial normal stresses from particle impacts along the length of a flow of unit width, and the number of upvalley debris flow sources. I construct a model which predicts that downvalley increases in flow length and frequency are balanced by reductions in inertial normal stress from reduced slopes and less weathered bedrock, leading towards equilibrium lowering. I hypothesize that reductions in valley slope compensates for gains in debris flow frequency and length and leads to observed non-power law plots of slope against drainage area. On the basis of this fieldwork and modeling, I propose that steepland valleys above 3-10% slope are predominately cut by debris flows, whose topographic signature is an area-slope plot that curves in log-log space. These valleys are both extensive by length (>80% of large steepland basins) and comprise large fractions of mainstem valley relief (25-100%), so valleys carved by debris flows, not rivers, bound most hillslopes in unglaciated steeplands. Debris flow scour of these valleys appears to limit the height of some mountains to substantially lower elevations than river incision laws would predict, an effect absent in current landscape evolution models. Forward modeling indicates that stream power laws poorly capture steepland 3 valley long-profiles, and that an incision law particular to debris flows is required to evolve most unglaciated steeplands. 4 ACKNOWLEDGEMENTS I would like to thank those who labored in the field with me, in alphabetical order: Simon Cardinale, Mauro Cassidei, Tegan Churcher, Alex Geddes-Osborne, Meng-Long Hsieh, the Kapor family, Taylor Perron, N. P. Peterson, Cliff Riebe, Josh Roering, Joel Rowland, Kevin Schmidt, Leonard Sklar, Adam Varat, and Elowyn Yager. Others labored in the lab, including Douglas Allen and Dino Bellugi who taught me what GIS skills I have, Charlie Paffenberger our systems administrator, and Chad Pedriolli, who did a substantial portion of the map area-slope analysis. Their generosity and patience resulted in much of the data presented hereafter. Many have contributed intellectual ideas through reviews or offhand comments. These include Michael Singer, Greg Tucker and Stephen Lancaster who improved Chapter 1 considerably, and Jim Kirchner who helped with the analysis of curved vs. linear area-slope data. Others have contributed data, including Stephen Lancaster who shared 10-m DEM's of the Elliot State Forest and Tracy Allen who provided stage records for the Eel River. I thank Leonard Sklar for rock tensile data in Chapter 2. I thank the following grant for funding: NASA grant NAG 59629. Those who educated me can take credit for any lasting contributions from this work. I would not have begun my studies without the wise and gentle encouragement of Robert Garrison and Leo Laporte of U. C. Santa Cruz Earth Sciences. They taught me to think of myself as a young professional, and I am still trying to live up to their high ethical ideals. To Robert Anderson of U.C. Santa Cruz I owe much of my initial love of geomorphology, for he taught me both the breadth and depth of landform i analysis. I thank the Earth Science department at Santa Cruz for an education rich in technical and humanistic ideas. Other faculties could benefit greatly from their example. During my M.S. work at the Department of Geology, University of Washington, I benefited greatly from the wise guidance of David Montgomery and Tom Dunne, two extraordinary teachers and scientists. They have had a profound effect on my professional life as examples of people who were both great researchers and mentors, alive to the possibilities of students and new ideas. Their mentoring has stood me in good stead during many rough times, and I cannot imagine being a geomorphologist without their guidance. In my time at Berkeley I have found great comfort from my colleagues starting with Douglas Allen, Dino Bellugi, Josh Roering and Ray “FBI” Torres, and continuing with Elowyn Yager. I would like to thank Michael Singer for many joyful hours in the field, and for reawakening much of my enjoyment of geomorphology. I owe a debt to Professors Narasimhan and Bürgmann for their cheerful support on my committee. My advisor William Dietrich has truly shaped me as a professional, for it is his voice that I hear whenever I am confronted with the unknown in the field. I have tried to adopt his extraordinary analytical skills, and his ability to simplify problems to a set of measurements to be made and analyzed. Throughout many trying times, he has consistently supported this debris flow research, sometimes from his own pocket I think. Thank you Bill. The last and greatest debt I owe to my immediate family and to Ana Kapor. From my family came the love of learning, from the times I spent on Mom’s lap ii reading, to digging for dinosaur fossils with Dad in the desert. They never flagged in their support, both spiritual and material. At last I can start returning the material support! To Ana I owe the energy and confidence to make it through the final lap. You stood by me and gave me strength, thank you. iii “It exceeds my powers but not my zeal” J. J. Rousseau (from the private papers of James Boswell) iv Jonathan David Stock-Vitae Personal Born March 16, 1970 in San Francisco Education: Jesuit High School, Carmichael, CA B.S., 1992, University of California, Santa Cruz M.S., 1996, University of Washington Present Position: Ph. D. candidate, Earth & Planetary Sciences, U.C. Berkeley Experience: 1989-1991 Field and soils lab technician, Environmental Impact Report writer, Raney Geotechnical 1990 X-Ray Fluorescence Lab technician, U.C. Santa Cruz.

Load more

  243

Copy Link