This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government ARC/INFO coverages and a PDF file for this report are available at http://geology.cr.usgs.gov/greenwood-pubs.html Contents Page Introduction . .1 Purpose and Approach . .1 Methods . .2 Limitations . .3 Hurricane Mitch Rainfall . .4 Jinotega/San Rafael del Norte . .5 El Sauce/San Nicolas . .7 Sébaco/San Nicolas . .8 La Fundadora . .10 Matagalpa . .11 Dipilto . .13 Cinco Piños . .15 Summary, Conclusions, and Recommendations . .15 Acknowledgments . .16 References . .17 Illustrations Figure 1 Map showing locations of seven study areas in Nicaragua . 2 2 Map of rainfall isohyets from Hurricane Mitch and the principal meteorological stations in Nicaragua . .4 3 Boulder deposit at Quebrada Grande in the El Sauce/San Nicolas study area . .7 4 Source area and deposits of debris flow near La Caña . .9 5 Structures destroyed by debris flows behind hospital at La Trinidad . .9 6 Landslides along the road to Jinotega in the La Fundadora study area showing broad scale of response . .11 7 Deep-seated, slow-moving earth flow on road to Jinotega . .12 8 Headscarp of landslide that produced a debris flow that traveled through the village of San Francisco . .13 9 Headscarp of landslide in deeply weathered materials in northern Matagalpa study area . .13 10 Toe of the earth flow at San Simón de Palcila . .14 11 View up slope of earth flow at San Simón de Palcila . .14 i Plate 1 Map of landslides and related downchannel effects triggered by Hurricane Mitch in parts of Jinotega and San Rafael del Norte quadrangles, Nicaragua 2 Map of landslides and related downchannel effects triggered by Hurricane Mitch in parts of El Sauce and San Nicolas quadrangles, Nicaragua 3 Map of landslides and related downchannel effects triggered by Hurricane Mitch in parts of Sébaco and San Nicolas and quadrangles, Nicaragua 4 Map of landslides and related downchannel effects triggered by Hurricane Mitch in part of La Fundadora quadrangle, Nicaragua 5 Map of landslides and related downchannel effects triggered by Hurricane Mitch in part of Matagalpa quadrangle, Nicaragua 6 Map of landslides and related downchannel effects triggered by Hurricane Mitch in part of Dipilto quadrangle, Nicaragua 7 Map of landslides and related downchannel effects triggered by Hurricane Mitch in part of Cinco Piños quadrangle, Nicaragua Table 1 For each study area, the lithology and areal extent of each geologic unit, the number of landslide initiation locations within each geologic unit, and the geologic susceptibility index . .6 Cover Photos: Hurricane Mitch approaching Central America on 26 October 1998, 13:15 UTC (07:15 local time). The hurricane is over the northern half of Nicaragua at this time. The image is derived from satellite data that has been color-enhanced and processed to yield a perspective rendering of the hurricane. Modified from image produced by Hal Pierce, Laboratory for Atmospheres, NASA Goddard Space Flight Center. Landslide triggered by Hurricane Mitch on hillslopes near Matagalpa, Nicaragua. Photograph by Eugene Schweig, 25 September 1999. ii Landslide Response to Hurricane Mitch Rainfall in Seven Study Areas in Nicaragua By Susan H. Cannon, Kathleen M. Haller, Ingrid Ekstrom, Eugene S. Schweig III, Graziella Devoli, David W. Moore, Sharon A. Rafferty, and Arthur C. Tarr Abstract: We present maps of seven study areas in susceptible to landslide activity. The Coyol Inferior Group Nicaragua that show the impact from landslides and dacite was most susceptible to landslide activity in the El related downslope effects triggered by the passage of Sauce/San Nicolas study area. The Coyol Superior Group Hurricane Mitch, 21 to 31 October 1998. Most (perhaps ignimbrite was most susceptible in the Sébaco/San Nicolas 95%) of the landslides mapped were debris flows. The study area, unit Tmmi-b (lithology unknown) was most sus­ maps show debris-flow source areas as small as a few ceptible in the La Fundadora study area, the Coyol Superior square meters and only a few meters deep that produced Group basalt and andesite and Quaternary colluvium were debris flows that traveled only tens of meters downslope, most susceptible in the Matagalpa study area, decom­ to landslides that covered approximately 80,000 m 2 , and posed granite and granodiorite were most susceptible in produced debris flows that traveled at least 3 km down the Dipilto area, and both decomposed granite and gran­ channels. The remaining 5% of mapped landslides were odiorite and volcanic materials in the undifferentiated slow-moving earth flows that started moving during Matagalpa and Inferior Coyol Groups showed high suscep­ Hurricane Mitch and continued to move for at least 1 year. tibilities in the Cinco Piños area. Landslides most common­ The study areas of El Sauce/San Nicolas, Dipilto, and ly developed in areas of weathered and/or hydrothermally Cinco Piños, and parts of Sébaco/San Nicolas and Mata­ altered materials. In addition, depth of scars appeared to galpa showed particularly extensive landslide activity; be controlled by depth of weathered mantle. landslides covered up to 80% of the terrain in places. We have provided a record of landslide activity in Study areas La Fundadora and Jinotega/San Rafael del response to the storm of record over areas of diverse geol­ Norte, and parts of Sébaco/San Nicolas and Matagalpa, on ogy, geomorphology, microclimates, and vegetation. When the other hand, showed significantly lower concentrations combined with information on the physical properties of of landslides. hillslope-mantling materials (including the depth of weath­ Susceptibility indices identify those geologic units ering), hillslope form and gradient, rainfall characteristics, that produced the most landslides in each study area in and travel distance, these inventories could provide a response to the rainfall conditions specific to Hurricane foundation for the development of accurate and meaning­ Mitch. In the Jinotega/San Rafael del Norte study area, ful susceptibility maps. Quaternary-age hillslope-mantling colluvium was the most Introduction Purpose and Approach: The passage of describe the physiographic, geologic, and cultural Hurricane Mitch through Central America in settings of each study area, and provide informa­ October and November of 1998 caused wide- tion on rainfall totals measured from 22 to 31 spread and devastating flooding and landsliding. October 1998, in or nearby each study area. For In this work we document the landslide response the study areas for which it is available we present to the rainfall that accompanied Hurricane Mitch the deviation of Hurricane Mitch rainfall totals in Nicaragua. We generated 1:50,000-scale maps from the normal rainfall for the last 10 days of for seven study areas located throughout October. Finally, we present the results of our Nicaragua that show the areas impacted by land- landslide mapping, including the magnitude and slides and related downchannel effects (fig. 1). form of the landslide response, and briefly explore In what follows we first briefly summarize the relation between geologic units and landslide what we have learned about the rainfall produced susceptibility for each study area. by Hurricane Mitch in Nicaragua. We then 1 87°30' 85°30' 14°00' Dipilto HondurasH o n d u r a s NICARAN I C A R AGUG UA Cinco Piños Jinotega/ San Rafael del Norte La Fundadora El Sauce/ Matagalpa San Nicolas Sébaco/ San Nicolas 12°30' GUATEMALA HONDURAS Figure 1. Map showing locations of seven EL SALVADOR study areas in Nicaragua. NICARAGUA Funding for this study was provided by U.S. in Nicaragua during Hurricane Mitch, we selected Agency for International Development (USAID). these study areas based on their differing geologi­ The work was performed in cooperation with the cal conditions, proximity to populated areas, and Instituto Nicaragüense de Estudios Territoriales availability of aerial photography. Aerial photo- (INETER) and the U.S. Geological Survey graphs taken between 4 and 19 December 1998, International Geology group. under the Open Skies program of the Defense Threat Reduction Agency (http://www.mitchnts1. Methods: Landslides and related effects in cr.usgs.gov.data.aerial.html), were used to map the and adjacent to downstream drainages that landslides shown on the maps. The aerial photo- occurred in response to the torrential rainfall that graphs are at 1:60,000 and 1:30,000 scale; addi­ accompanied Hurricane Mitch during its 22 to 31 tional aerial photographs were available for parts October 1998 journey over Nicaragua were of the Matagalpa and Jinotega/San Rafael del mapped for seven study areas: Jinotega/San Rafael Norte study areas. Digital Raster Graphics (DRG) del Norte; El Sauce/San Nicolas; Sébaco/San scanned images of 1:50,000-scale quadrangles Nicolas; La Fundadora; Matagalpa; Dipilto; and projected to Transverse Mercator were used as Cinco Piños (fig. 1, and accompanying plates). base maps for the mapping. We mapped landslides Although landslide activity occurred in other areas and related downstream effects by first identifying 2 e use the term "landslide" to describe all types of able destruction. As a result of these characteristics, Wslope failures, including slow-moving earth mitigation of debris-flow hazards can be more difficult flows, rotational and translational slides (Cruden and than mitigation of flood hazards. Most of the landslide- Varnes, 1996; Varnes, 1978), and fast-moving debris related damage and deaths that occurred during flows. Most (perhaps 95%) of the landslides that we Hurricane Mitch were a result of debris flows. mapped are debris flows. A debris flow is a rapid Slow-moving earth flows are the other type of land- downslope movement of viscous slurry consisting of up slide described in this report.