Multi-Hazard Mitigation Plan: 06. Landslides And Rock Falls

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Chapter 6:Hazard Analysis—Landslides and Rock Falls 6.3 Probability of Occurrence Landslides and rock falls typically occur as a result of another hazard or force of nature, and due to the multitude of forces that may cause landslides and rock falls assigning or calculating the return period of events of a destructive magnitude would be extremely difficult. However, it is possible to determine areas that have a higher potential of risk for such events as was done in a study was conducted by URS. In this study a conceptual level slope risk map was prepared, using an adaptation of the slope hazard methodology given in the FEMA 2007 HAZUS-MH MR3 Technical Manual. The approach involved the interactions of three primary slope hazard input criteria simplified to low, medium and high hazard susceptibility, described below: Topography: A high resolution slope map of the entire island is composed of recent aerial, lidar and satellite 10m DEM topographic survey data from USGS and FEMA sources. Slope criteria are categorized as: 1. <20degreees (low hazard susceptibility) 2. <20-40 degrees (medium hazard susceptibility) 3. >40 degrees (high hazard susceptibility) Geologic groups: Soil and rock categories assigned based upon strength and susceptibility to landslide was derived from existing UDSA NCRS surface maps and USGS geologic maps. Adapted geologic group assignments for the Island of Hawaii include: 1. shallow rock, fresh volcanics (low hazard susceptibility) 2. clay surficial soils,weathered rock (medium hazard susceptibility) 3. weak soft soils, ash deposits, mapped historic slide talus (high hazard susceptibility) In addition, URS interpreted historic Hawaii DOT landslide locations, as medium hazard for locations with preliminary FHWA Rockfall Hazard Rating System (RHRS) of less than 200 and high hazard for locations with preliminary RHRS values greater than 200. 6-4 Hawaii County Multi-Hazard Mitigation Plan