MAPPING AND ANALYSIS OF THE RIO CHAMA LANDSLIDE AND EVALUATION OF REGIONAL LANDSLIDE SUSCEPTIBILITY, ARCHULETA COUNTY, COLORADO by Cole D. Rosenbaum 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: _______________________ Cole D. Rosenbaum 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 ABSTRACT Recent landslides, such as the West Salt Creek landslide in Colorado and the Oso landslide in Washington, have brought to light the need for more extensive landslide evaluations in order to prevent disasters in the U.S.. The goal of this research is to characterize and map the Rio Chama landslide, evaluate conditions at failure, predict future behavior, and apply these findings to create a regional susceptibility model for similar failures. Based on the classification scheme proposed by Cruden and Varnes (1996), the Rio Chama landslide is an active multiple rotational debris slide and flow complex with observed activity since 1952, located near the headwaters of the Rio Chama River in south-central Colorado. Site reconnaissance was conducted in 2015 and 2016 and coupled with laboratory testing of samples and limit equilibrium stability analysis. A hierarchical heuristic model using an analytic hierarchy process was applied to evaluate the susceptibility of the region to failures similar to the Rio Chama landslide. Weights were assigned to parameters based on their influence on landslide susceptibility, and weighted parameters were combined to produce a regional susceptibility map. The causative factors in order of most to least contribution to susceptibility are: slope angle, lithological class, surface waterbody density, stream density, slope aspect, profile curvature, and land use. The regional model accurately and consistently identifies zones as having low, moderate, and high susceptibility to failures similar to the Rio Chama landslide. Discussion of the causative factors and their impact on susceptibility, as well as the implications of each susceptibility zone is presented. The regional model is intended to identify areas susceptible to failures similar to the Rio Chama landslide, allowing appropriate preventative action to be taken. iii TABLE OF CONTENTS ABSTRACT……………………………………………………………………………………...iii LIST τF FIGURES………………………………………………………………..……......…...vii LIST OF TABLES…………………………………………………………………….………….ix ACKστWLEDGMEσTS………………………………………………………….….…………xi CHAPTER 1 IσTRτDUCTIτσ…………………………………………………..………1 1.1 Background………………………………………………………….………1 1.2 Purpose and Scope……………………………………………………..……2 CHAPTER 2 PRIOR RESEARCH……………………………………………………...…6 2.1 Geologic Setting and Regional Landslide Behavior……………………...…6 CHAPTER 3 METHODOLOGY………………………………………...………………10 3.1 Data Collection…………………………………………….………………10 3.1.1 Historical Imagery…………………………………………………10 3.1.2 Geologic Information………………………………………………10 3.1.3 Existing Landslide Inventories……………………….……………11 3.1.4 Hydrologic Information……………………………………………14 3.1.5 Topographic Information………………………………..…………15 3.1.6 Land Cover Information……………………………...……………15 3.1.7 Precipitation Information……………………………..……………15 3.2 Evaluation of Recent Landslide Activity………………………..…………16 3.3 Site Investigation and Sampling………………………...…………………16 3.3.1 Failure Mode Characteristics………………………………………16 3.3.2 Groundwater Conditions………………………………...…………17 3.4 Laboratory Testing of Material Properties…………………………………17 3.5 Slope Stability Analysis……………………………………………………18 3.5.1 Backward Analysis…………………………………...……………18 3.5.2 Forward Analysis………………………………………..…………20 3.5.3 Sensitivity Analysis……………………………………………..…20 3.6 Regional Landslide Susceptibility Analysis……………………….………20 3.6.1 Selection and Evaluation of Causative Factors……………….……21 iv 3.6.2 Analytic Hierarchy Process…………………………………...……21 3.6.3 Map Validation…………………………………………….………25 3.6.4 Map Post-Processing……………………………………….………26 CHAPTER 4 RESULTS……………………………………………………….…………28 4.1 Data Collection………………………………………………….…………28 4.1.1 Geologic Information………………………………………………28 4.1.2 Existing Landslide Inventories…………………….………………29 4.1.3 Hydrologic Information……………………………………………30 4.1.4 Topographic Information…………………………………..………32 4.1.5 Land Cover Information……………………………………...……32 4.1.6 Precipitation Information……………………………..……………32 4.2 Evaluation of Recent Landslide Activity……………………..……………34 4.3 Site Investigation and Sampling…………………………………...………36 4.3.1 Material Properties…………………………………………………36 4.3.2 Failure Mode Characteristics………………………………………39 4.3.3 Groundwater Conditions……………...……………………………47 4.4 Laboratory Testing of Material Properties…………………………………48 4.4.1 Soil Classification………………………………….………………48 4.4.2 Unit Weight…………………………………………………...……49 4.4.3 Moisture Content……………………..……………………………49 4.4.4 Jar Slake Test………………………………………………………49 4.4.5 Direct Shear Test………………………………………...…………50 4.5 Slope Stability Analysis……………………………………………………53 4.5.1 Backward Analysis………………………...………………………53 4.5.2 Forward Analysis………………………..…………………………53 4.5.3 Sensitivity Analysis……………………………………..…………53 4.6 Regional Landslide Susceptibility Analysis…………………….…………58 4.6.1 Properties of Susceptibility Zones…………………………………62 CHAPTER 5 DISCUSSION…………………………………………………...…………66 5.1 Failure Mechanics…………………………………………………….……66 5.2 Factors Contributing to Failure……………………………………….……68 v 5.3 Current and Future Stability…………………………………………..……71 5.4 Regional Susceptibility……………………………….……………………73 CHAPTER 6 CONCLUSIONS………………………………………..…………………76 REFEREσCES…………………………………………………………………………………..78 APPENDIX A FIELD OBSERVATION MAPS……………………………..……………82 APPENDIX B CAUSATIVE FACTOR MAPS………………………………...…………84 APPENDIX C LABORATORY MATERIAL PROPERTIES………………….…………88 APPENDIX D PRELIMINARY PAIRWISE COMPARISON MATRICES……...………99 APPENDIX E FINAL PAIRWISE COMPARISON MATRICES EVALUATED USING MANUAL INTERPRETATION…………………………...……104 APPENDIX F FINAL PAIRWISE COMPARISON MATRIX AND WEIGHTS FOR FACTORS…………………………………………………………..……109 APPENDIX G HISTORICAL REMOTE SENSING……………………………….……110 APPENDIX H SENSITIVITY ANALYSIS……………………………………...………114 APPENDIX I EXTENT OF RIO CHAMA LANDSLIDE FAILURE TYPES………….118 APPENDIX J MτDIFIED REGIτσAL SUSCEPTIBILITY AσALYSIS……..………119 vi LIST OF FIGURES Figure 1.1 Site location map………………………………….…………………………3 Figure 1.2 Topographic map of recent landslide extent at Rio Chama landslide………4 Figure 2.1 Geologic map of the area surrounding the Rio Chama landslide……...……7 Figure 3.1 Rio Chama landslide source zone…………………………………….……12 Figure 3.2 Box and whisker plot of the slope angle for different subsets of the landslide inventory…………………………………………………………13 Figure 3.3 Location of two cross-sections selected for stability and sensitivity analysis………………………………………………………..……………19 Figure 3.4 Analytic hierarchy for the susceptibility of the study area to landslides similar to the Rio Chama landslide……………………...…………………22 Figure 3.5 Histogram of the susceptibility scores from the final susceptibility map….27 Figure 4.1 Map showing the distribution of lithological classes and HUC-12 watersheds throughout the study area…………………...…………………29 Figure 4.2 Map of the study area showing the location of landslides from the Colorado Landslide Inventory and the map of landslides in New Mexico...30 Figure 4.3 Maps showing surface water features in the study area……………………31 Figure 4.4 Moving mean annual precipitation data from the Cumbres Trestle weather station from 8/6/1980 to 6/7/2017…………………………...……35 Figure 4.5 Monthly observed and average precipitation data from the Cumbres Trestle weather station from 6/15/1986 to 6/22/1978 during which the landslide with the longest travel distance was observed in aerial photos….36 Figure 4.6 Delineated landslide events based on time-series aerial photographs showing the relative dates landslide features formed………………...……37 Figure 4.7 Photograph of recent head scarp on the southeast portion of the landslide..38 Figure 4.8 Photograph of exposed Conejos Formation showing desiccation cracks, rutting, and gravel lens…………………………………………………..…38 Figure 4.9 Optical photograph with lobes identified.…………………………….……40 Figure 4.10 Back rotated block on the northeast side of the Rio Chama landslide area as indicated by tilted trees angled toward the head scarp…….………42 Figure 4.11 Photograph of a sag pond forming on the southeast side of the recent landslide area………………………………………………………………42 Figure 4.12 Photographs of tilted trees (A) and pistol-butted trees (B) caused by back-rotated blocks near the recent head scarp…………………….………43 vii Figure 4.13 Photographs of damage to tree bark and limbs along the margins of lobe six……………………………………………………………..………43 Figure 4.14 Location and extent of landslide lobe that developed between October 2015 and August 2016………………………………………..……………44 Figure 4.15 Photograph of landslide lobe 8 that developed between October 2015 and August 2016.…………….…………………………….………………45 Figure 4.16 Photograph of graben and tension crack upslope of the head scarp….……46 Figure 4.17 Photograph showing the exposed Blanco Basin Formation overlain by older landslide deposits in a stream in the southern landslide area……..…47 Figure 4.18 Photograph taken near the center of the landslide area showing the contact between Blanco Basin Formation and the recent landslide