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Characterization and Analysis of a Large Rockfall Along the Rio Chama, Archuleta County, Colorado

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Characterization and Analysis of a Large Rockfall Along the Rio Chama, Archuleta County, Colorado CHARACTERIZATION AND ANALYSIS OF A LARGE ROCKFALL ALONG THE RIO CHAMA, ARCHULETA COUNTY, COLORADO by Robert L. Duran A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Masters of Science (Geological Engineering). Golden, Colorado Date_______ Signed_______________________ Robert L Duran Signed_______________________ Dr. Wendy Zhou Thesis Advisor Signed_______________________ Dr. Paul Santi Thesis Advisor Golden, Colorado Date_______ Signed_______________________ Dr. M Stephen Enders Professor and Interim Head Department of Geology and Geological Engineering ! ii! TABLE OF CONTENTS ABSTRACT ....................................................................................................................... vi LIST OF FIGURES .......................................................................................................... vii LIST OF TABLES .............................................................................................................. x ACKNOWLEDGMENTS ................................................................................................. xi CHAPTER 1: INTRODUCTION ....................................................................................... 1 1.1 Background ............................................................................................................... 3 1.2 Geologic Setting ........................................................................................................ 4 1.3 Purpose and Scope .................................................................................................... 5 CHAPTER: 2 METHODS .................................................................................................. 6 2.1 Data Collection and Building Geodatabase .............................................................. 6 2.2 Field Investigation Preparation and Field Investigation ........................................... 7 2.3 Remote Sensing Analyses ......................................................................................... 7 2.3.1 Narrowing the Time Window - LANDSAT 5 Thematic Mapper (TM) ............ 8 2.4 Examination of Geometric Opportunities for Failure ............................................... 9 2.4.1 Planar Failure ................................................................................................... 10 2.4.2 Wedge Failure .................................................................................................. 10 2.4.3 Toppling Failure ............................................................................................... 11 2.5 GIS Analysis ........................................................................................................... 12 2.5.1 Selection of Causative Factors ......................................................................... 14 2.5.2. Digitization of Rock Type .............................................................................. 14 2.5.3 Slope and Aspect .............................................................................................. 15 2.5.4 Identify Areas of Unstable Slope, Aspect, and Rock Type ............................. 15 !iii! 2.5.5 Identify Large Cliffs ........................................................................................ 16 2.5.6 Combination of Factors .................................................................................... 16 2.6 Colorado Rockfall Simulation Program Analysis ................................................... 18 2.6.1 CRSP Calibration with the Rio Chama Rockfall (Jones et al., 2000) .............. 19 2.6.2 CRSP Assessment of Identified Highest Susceptibility Areas ........................ 19 CHAPTER: 3 RESULTS AND ANALYSIS ................................................................... 21 3.1 Site Characterization ............................................................................................... 21 3.1.1 Field Investigation ........................................................................................... 22 3.1.2 Source Zone ..................................................................................................... 26 3.1.3 Rubble Zone ..................................................................................................... 29 3.1.4 Slope Zone ....................................................................................................... 33 3.1.5 Toe ................................................................................................................... 34 3.2 Previous Events ....................................................................................................... 37 3.3 Date of Occurrence ................................................................................................. 39 3.4 Triggering Mechanisms .......................................................................................... 41 3.5 Is the Rio Chama Rockfall a Sturzstrom? ............................................................... 43 3.6 Source Zone Identification Map ............................................................................. 44 3.6.1 Rock Type, Regional Fractures, and Kinematic Analysis ............................... 44 3.6.2 Combination of Factors .................................................................................... 48 3.7 CRSP Calibration and Application to Highest Susceptibility Areas ................... 51 CHAPTER 4: DISCUSSION ............................................................................................ 60 4.1 Initial Failure of The Rio Chama Rockfall ............................................................. 60 4.2 Geometric Opportunities for Failure ....................................................................... 60 ! iv! 4.3 Cliff Height Selection ............................................................................................. 60 4.4 Extent of Map Coverage ......................................................................................... 61 4.5 Source Zone Identification vs. Hazard Map ........................................................... 61 CHAPTER 5: CONCLUSIONS ....................................................................................... 62 REFERENCES ................................................................................................................. 65 APPENDIX A: CRSP ANALYSIS .................................................................................. 69 APPENDIX B: CATALOGUE TREE OF FINAL MAP ................................................. 77 APPENDIX C: SUPPLEMENTAL DATA..………………………………...…………. 79 ! v! ABSTRACT The Rio Chama area experienced a natural rockfall in spring 2004. The focus of this study is to quantify and characterize the event, to investigate the possible trigger(s) and failure mechanisms, to identify possible similar events nearby, and to produce a rockfall Source Zone Identification Map of the surrounding area. An approach that included field mapping, Geographic Information System (GIS) based analysis, and a variety of remote sensing methods was used to investigate the Rio Chama Rockfall. Field mapping provided firsthand information about the extent and character of the event. GIS based analysis assisted in narrowing the time period of occurrence by providing a platform for viewing and manipulating remote sensing data. GIS was also used to organize, process, and compile data in order to produce a Source Zone Identification Map of the surrounding area to identify source areas where similar rockfalls could occur. Optical remote sensing assisted in narrowing the time window of occurrence as well as in delineating the boundary of the slide based on damaged and undamaged vegetation. The volume of the slide was estimated from mapped boundaries and estimated thicknesses. The Rio Chama Rockfall was divided into 4 zones, the Source Zone, the Rubble Zone, the Slope Zone, and the Toe. The zones are based on type of material, size of material, amount of material, type of movement, and gradient of the slope. The Rubble Zone was further divided into individual lobes based on the lobe’s relative elevation. The trigger of the Rio Chama Rockfall appears to be runoff from snowmelt and precipitation. No similar style failures were observed in the immediate area. A map of the area was produced identifying source areas susceptible similar types of failures. ! vi! LIST OF FIGURES Figure 1.1 Location of the Rio Chama Rockfall ................................................................. 3 Figure 2.1. Flowchart of the GIS Process ......................................................................... 13 Figure 2.2. Workflow for Generating the Source Zone Identification Map ..................... 18 Figure 3.1. Glacial Trimline (indicated by the red line) ................................................... 21 Figure 3.2. Gently Sloping Valley (red line indicates approximate glacial trimline) ....... 22 Figure 3.3. The Four Zones of the Rio Chama Rockfall .................................................. 23 Figure 3.4. Map of the Rio Chama Rockfall, With Individual Lobes Delineated Based on Field Observations .................................................................................... 25 ! Figure 3.5. The Source Zone of the Rio Chama Rockfall. Note lighter rock to the right, which is where the rockfall block detached from before falling and disintegrating ................................................................................................
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