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

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Chapter 6:Hazard Analysis—Landslides and Rock Falls CHAPTER 6 - LANDSLIDES AND ROCK FALLS 6.1 Description of Hazard Landslides are one of the natural processes of erosion resulting from mass transfer, the movement of material down the side of slopes under the influence of gravity. Mass transfer may also occur as falling, sliding or rolling of individual rocks or rock fragments,washing of loose materials by rain runoff and soil creep. Although gravity is the primary reason for landslides, other naturally occurring factors include: • Erosion by rivers or ocean waves creating over steepened slopes; • Rock and soil slopes that are weakened through saturation by heavy rain and runoff, • Stress created by earthquakes; • High winds uprooting trees along steep slopes. Human made causes of landslides may include grading, terrain cutting and filling and devel- opment on hillsides or excessive slopes. Landslides can occur on any sloping terrain given the right conditions of soil, moisture and the angle of slope. This ongoing process is generally slow and rarely noticeable. Occasionally landslides will occur in a spectacular rush or an abrupt collapse, often triggered by another natural hazard such as an earthquake or flooding. 6.1.1 Hazard Areas Areas that may be considered prone to landslides may include the following: • On existing old landslides • On or at the base of slopes • In or at the base of minor drainage hollows • At the base or top of an old fill slope • At the base or top of a steep cut slope • Developed hillsides where leach field septic systems are used.2t Roadcuts and other altered or excavated areas of slopes are particularly susceptible to debris flows and abrupt collapse. Several areas along the Hamakua Coast are chronic problem areas particularly during periods of heavy rainfall. In addition, the three major gulches Maulua, Laupahoehoe and Kaawalii, which are known for the "horseshoe" turns on State Highway 19, present rock fall problems. The rock fall problems arise during times of heavy rain as well as strong winds which sway the trees along the walls of the gulch back and forth and loosen the dirt and rocks underneath it. Homes that are built along the edge of the Hamakua coast cliffs are also susceptible to abrupt collapse particularly during times of heavy rainfall. These cliffs along the northeast coast of Mauna Kea range in height from 50 to 350 feet. The sea cliffs are eroded through a continu- ous process of wave action at the base of the cliff which cuts a notch and undermines the 21 USGS Fact Sheet FS-071-00 6-1 Hawaii County Multi-Hazard Mitigation Plan