LANDSLIDE HAZARD ZONATION MAPS FOR RAPID CITY, SOUTH DAKOTA

Rapid City is located on the eastern flank of the Black Hills in Paleozoic and Mesozoic stratigraphy including limestone, shale, and sandstone units dipping approximately 12° to the east. Swelling clay, gypsum, friable sandstone, and structural fabric add to a high natural instability. Rapid development in this hilly region makes landslide awareness a critical concern for landowners. Landslides have occurred in Rapid City since the earliest development in the 1880s. Most have occurred due to development in inappropriate areas, unstable areas without mitigation, and areas of swelling soils or having dissolution potential. Lack of reliable information has been the largest contributing factor to slope instabilities. We provide a first attempt at developing a usable tool for development and planning that consists of three thematic maps; geology, landuse, and slope. Ranks assigned to each layer were based on its potential to cause, or influence, landslide activity and ranged between 5 (high) to 1 (low). Ranked maps were overlain using a weighted overlay to produce a landslide hazard zonation map. The resulting map contains five zones; very high, high, moderate, low, and very low. About 15% of the region had very high susceptibility (slopes >55⁰) and high susceptible areas occurred in isolated patches. Both areas represent favorable development properties. These products may provide information for developers to conduct risk assessment during development.