_______________________________________________________________________ OPEN-FILE REPORT 595 _______________________________________________________________________ ROCKFALL SUSCEPTIBILITY MAPS FOR NEW MEXICO Daniel J. Koning and Mark Mansell New Mexico Bureau of Geology & Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 (575) 835-6950 (Daniel Koning office phone) http://geoinfo.nmt.edu 2017 http://geoinfo.nmt.edu/publications/openfile/details.cfml?Volume=595 This project was initiated in order to better evaluate the potential hazard posed by rockfalls in the state of New Mexico. Maps showing rockfall susceptibility (i.e., where rockfalls could be expected to occur) could serve as a tool to identify regions in the state where rockfall risk should be considered by public and private planners, as well as the NM Department of Transportation. Such maps would also be essential precursors for complete hazard maps, which would incorporate other rockfall parameters such as magnitude (intensity) and frequency of occurrence. These hazard maps could then be folded into risk maps, which incorporate potential costs and societal impacts. The rockfall susceptibility maps presented in this report will also assist the NM Department of Homeland Security in preparing for their 2018 update for the N.M. State Hazard Mitigation plan for FEMA. Funding for this project was obtained via a sub-grant through the NM Department of Homeland Security and Emergency Management (FEMA-4152-DR-NM-020). Two maps were made that pertain to rockfall susceptibility (Plates 1 and 2). The accompanying report gives a background of rockfall hazard, details of the methodology by which the two maps were made, and uses and limitations of these maps. Plate 1 shows the point densities of the mapped rockfalls of Cardinali et al. (1990), which are contoured using the kernel function. Plate 2 is a product of a method defining slope-angle classes to map rockfall susceptibility for the entire state of New Mexico. The method relates mapped rockfalls (Cardinali et al., 1990) to nearby slope values. Using a 28 m DEM in ARC GIS, a slope map is created. We capture the maximum slope around a mapped rock fall point using a 300 m-radius window, which corresponds to the median of the error range in the mapped rockfall points of Cardinali et al. (1990). The average and maximum value of the slope within this window was obtained, but the distribution curve for the average value is heavily skewed to low values, probably because most of New Mexico is relatively flat and spatial errors would result in a rockfall being on low-sloping ground. However, the maximum value within the window gave a quasi-normal distribution centered on a mean value of 29° and having a standard deviation of 12°. We chose to use these maximum value within the 300 m-radius window, with the assumption that most rock falls tend to accumulate on relatively steep talus slopes. Using the mean and standard deviations calculated from the distribution of these maximum slopes, we categorize the aforementioned slope map into three bins that relate to rockfall susceptibility. "Likely susceptible" zones correspond to slopes lying at or above the mean-less-one standard deviation (17°). These higher slopes likely contain local areas (over distances of several hundred meters to about a kilometer) that correspond to rockfall sources, transport paths, or accumulation zones. "Potentially susceptible" zones correspond to slopes in the range of 8-17°, bracketed by the mean-less-one standard deviation and the 5th percentile of the aforementioned maximum-slope value distribution. In the "potentially susceptible" zone, there could possibly be rockfall-producing ledges not captured by the 28 m DEM. This susceptibility zone also includes a 470 m buffer extending downslope (on 5-17°sloping topographic surfaces) of high susceptibility areas, designed to capture rockfalls that have sufficient momentum to travel notably downhill of ?17°slopes. The 470 m distance value corresponds to the 90th percentile (excluding outliers) of mapped rockfall distances from the high-susceptibility zone. "Unlikely susceptible" zones include very low slopes (<8°) lying outside of the aforementioned buffer. Final processing steps consisted of down-sampling to 500 m grid-size consistent with a 1:750,000 final map scale. The two susceptibility maps (Plates 1 and 2) should be considered as a first-order approximation of rockfall susceptibility. Rockfall, as used here, includes rock topple. In natural hazard assessments, the term 'susceptibility' is the natural propensity of the landscape to produce a given hazard (in this case, rockfall). In other words, this map presents a first-order approximation of the likelihood that a rockfall event will occur in a specified area based on local terrain conditions, given adequate driving forces or destabilizing phenomena. As described in the associated report, three classes of susceptibility are mapped using statistically defined boundaries of slope values; these boundaries are obtained by comparing the distribution of mapped rock falls with modern-day slopes. These mapped rockfalls (Cardinali et al., 1990) have occurred over the course of thousands of years (perhaps up to hundreds of thousands of years), under conditions that are not necessarily comparable to today. This map does not contain information on the potential for adequate driving forces to occur, nor does it contain information on the frequency at which adequate driving forces may occur, and hence is not a complete hazard map. This map is intended for use at a spatial scale of 1:750,000 for regional planning purposes and for determining where more detailed studies may be warranted. In the absence of other data regarding rockfall intensity (i.e., kinetic energy) and event frequency, this map is neither intended nor adequate for risk assessment, nor is the map intended or adequate for site-specific assessments of rockfall susceptibility. The New Mexico Bureau of Geology and Mineral Resources shall not be liable under any circumstances for any direct, indirect, special, incidental, or consequential damages with respect to claims by users of this product.The views and conclusions presented here should not be interpreted as necessarily representing the official views or policies, either expressed or implied, of the State of New Mexico. ----------------------------------------------------------------------- SYSTEM REQUIREMENTS: A computer capable of running some or all of the following software: Adobe Acrobat (v.9 or later) to view the Plates (*.pdf) -- freeware: http://get.adobe.com/reader ArcGIS (v. 10.0 or later) to open GIS projects/data (*.mpk) ArcReader (v. 10) to view GIS data contained in the map packages -- freeware: http://www.esri.com/software/arcgis/arcreader ----------------------------------------------------------------------- FILES OF INTEREST: Note: some of the ArcGIS map packages are quite large and may take a long time to download. This report is also available on DVD for a nominal fee. Report pertaining to Rockfall Susceptibility Maps for New Mexico (PDF file). Appendix A. Workflow procedure for slope-based susceptibility map (Plate 2) Appendix A. Procedure to make rockfall susceptibility map for New Mexico using slope-angle criteria (PDF file) Table A-1. Maximum slope values within a 600 x 600 m rectangle around a given raster grid cell (EXCEL tables) Table A-2. Table A-2. Mean slope values within a 600 x 600 m rectangle around a given raster grid cell (EXCEL table) Table A-3. Euclidian distances from >=17 degree slopes (EXCEL table) Table A-4. Validation of results (EXCEL table). Appendix B. Input ARC data for slope-based susceptibility map (Plate 2) ARC map package called AppendixB_InputData.mpk. Appendix C. Interim ARC rasters ARC map package called RockfallInterimRasters.mpk. Appendix D. Final product ARC material Four ARC map packages called: Plate_1_Density_RocktoppleSusceptibility_Plot.mpk Plate_2_Hillshade_slopebased_RocktoppleSusceptibility_Plot.mpk RockTopple_Denisty_pageSize.mpk SlopeCriteria_Rockfall_pageSize.mpk Two Plates: Plate 1: Rockfall susceptibility map of New Mexico using density contouring (PDF file) Plate 2: Rockfall susceptibility map of New Mexico using slope criteria (PDF file) Note: In order to reduce file size and download time, a lower-resolution hillshade relief image was used in the ARC map packages than was used to construct Plate 1 and Plate 2 PDF files. ----------------------------------------------------------------------- METADATA: Detailed metadata are included with all ARC products. These products include MXD files, geodatabases, ARC tables, and rasters. Metadata items include: Tags, Summary, Description, Credits, Use limitations, Extent, and Citation Contacts. ----------------------------------------------------------------------- SUGGESTED CITATION: Koning, D.J., and Mansell, M., 2017, Rockfall susceptibility maps for New Mexico, New Mexico Bureau of Geology and Mineral Resources Open-file Report 595, 41 p., 4 digital appendices, 2 plates. < http://geoinfo.nmt.edu/publications/openfile/details.cfml?Volume=595 >