Digital Landslide Inventory for the Cowlitz County Urban Corridor, Washington by Karl W. Wegmann WASHINGTON DIVISION OF GEOLOGY AND EARTH RESOURCES Report of Investigations 35 version 1.0 May 2006 DISCLAIMER Neither the State of Washington, nor any agency thereof, nor any of their em- ployees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any informa- tion, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or other- wise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the State of Washington or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the State of Washington or any agency thereof. WASHINGTON DEPARTMENT OF NATURAL RESOURCES Doug Sutherland—Commissioner of Public Lands DIVISION OF GEOLOGY AND EARTH RESOURCES Ron Teissere—State Geologist David K. Norman—Assistant State Geologist Washington Department of Natural Resources Division of Geology and Earth Resources PO Box 47007 Olympia, WA 98504-7007 Phone: 360-902-1450 Fax: 360-902-1785 E-mail: [email protected] Website: http://www.dnr.wa.gov/geology/ Published in the United States of America ii iii Contents Abstract .........................................................................................................................................1 Introduction ...................................................................................................................................1 Previous landslide studies .............................................................................................................2 Project history ...............................................................................................................................2 Definition of landslide types..........................................................................................................3 Landslide inventory database ........................................................................................................6 Data collection methods ................................................................................................................7 Aerial photograph reconnaissance ............................................................................................7 Field verification .......................................................................................................................7 Compilation of GIS inventory and database ..........................................................................10 Results .........................................................................................................................................11 Discussion ...................................................................................................................................12 Geologic controls ....................................................................................................................12 Landslide types .......................................................................................................................17 Possible triggering mechanisms .............................................................................................18 Inventory limitations ...................................................................................................................19 Potential uses ...............................................................................................................................19 Planning ..................................................................................................................................19 Hazard map calibration ...........................................................................................................20 Regional analyses ...................................................................................................................20 Summary .....................................................................................................................................20 Acknowledgments .......................................................................................................................20 References cited ..........................................................................................................................21 APPENDIX Bibliography of slope-stability investigations within the Cowlitz County urban corridor .........23 FIGURES Figure 1. Map showing location of the Cowlitz County urban corridor landslide inventory area ..............................................................................................................2 Figure 2. Graph showing the 73-year precipitation record for Longview, Washington .............2 Figure 3. Map of the inventory area showing the distribution of identified deep-seated and shallow landslides and potentially unstable slopes ..............................................3 Figure 4. Map and photographs of the Aldercrest–Banyon landslide, Kelso .............................4 Figure 5. Illustrations showing landslide classification, the anatomy of an idealized complex earth slide–earth flow, and landslide elevations and dimensions used in the inventory database ....................................................................................5 Figure 6. Map and photographs of a large deep-seated landslide along the south side of the Kalama River, illustrating recent shallow-landslide activity within slide body and off slide scarp ......................................................................................6 Figure 7. Illustration of morphological changes to an idealized landslide through time, and corresponding activity-state descriptions used in this study ................................7 Figure 8. Illustration of slope gradients used in the inventory database ..................................10 Figure 9. Generalized stratigraphic correlation diagram for the study area .............................11 ii iii Figure 10. Graph of the relationship between the percent of total landslide area originating from a given geologic unit and the percent of the total study area underlain by that geologic unit ................................................................................11 Figure 11. Photographs of deeply weathered (saprolitic) volcaniclastic sandstone and conglomerate of the Goble Formation exposed in a roadcut along Bodine Road ....12 Figure 12. Photographs of deeply weathered (saprolitic) volcanic tuff of the Goble Formation exposed in roadcut along Bella Vista Road .............................................12 Figure 13. Photographs of cross-bedded and lenticular sandstone of the Sandy River Mudstone...................................................................................................................13 Figure 14. Photographs of a large deep-seated rock slide along the north side of the Kalama River illustrating a large-scale bedrock landslide........................................13 Figure 15. Photograph of the contorted foundation of a house destroyed by a landslide developed within deeply weathered volcaniclastic rocks of the Goble Formation ..................................................................................................................14 Figure 16. Photographs of a small rotational deep-seated landslide in weathered sediments of the Cowlitz Formation along Columbia Heights Road .......................14 Figure 17. Map and photographs of the dormant Beigle Mountain landslide complex, located northeast of Toutle ........................................................................................14 Figure 18. Map and photographs of the dormant–relict deep-seated rotational to translational Columbia Heights landslide .................................................................15 Figure 19. Geologic cross section and photographs illustrating geologic controls on the Aldercrest–Banyon landslide ....................................................................................16 Figure 20. Photographs of the Kelso “K” small deep-seated earth slide–flow, which initiated in fluvial silts and clays of the Troutdale Formation ..................................16 Figure 21. Photograph of a small deep-seated translational earth slide–flow north of Woodland, which initiated in fine-grained Pleistocene Missoula flood deposits ......17 Figure 22. Photographs of a small recent rock fall within a larger dormant–young rock fall–slide located above State Route 4, approximately 0.5 mi east of Stella ....17 Figure 23. Map showing the distribution of active landslides superimposed on the Davis Terrace landslide complex ........................................................................................17 Figure 24. Photograph of a human-induced, small deep-seated rotational earth slide–earth flow north of Kalama along Spencer Creek Road ....................................................18 TABLE Table 1. Database attribute descriptions for the 82 fields found in the ESRI shapefiles ccuc_deep_seated_landslides.shp and ccuc_shallow_landslides.shp ...........................8 iv Digital Landslide Inventory for the Cowlitz County Urban Corridor, Washington by Karl W. Wegmann Washington Division