RAINFALL UPDATED 08/19/2017 14:29 PST
LA CONCHITA HISTORICAL EVENT TRIGGERS
The following historical triggers were encountered in LA Conchita and help to serve as indicators of a potential geological event:
- 15” of rainfall in 30 days or less.
- The 1995 La Conchita landslide (deep seated) occurred two months after the area received approximately 15-inches of rainfall over a 30 day period. Ground cracks, bulging and sloughing at the toe of slope were also observed several months prior to this failure.
- 8” of rainfall in any two-week period.
- The 2005 landslide (shallower) in contrast occurred at the culmination of a wet 2–week period where approximately 8 inches of rain fell.
- 1” of rainfall per hour or greater carries the potential to trigger flooding, landslides or debris flows at any location.
- A small landslide impacted one property in December 2010 following a storm that produced approximately 1” of rain in 24 hour period.
Landslide: The term “landslide” describes a wide variety of processes that result in the downward and outward movement of slope-forming materials including rock, soil, artificial fill, or a combination of these. The materials may move by falling, toppling, sliding, spreading, or flowing.
Debris Flow: A debris flow is a form of rapid mass movement in which a combination of loose soil, rock, organic matter, air, and water mobilize as slurry that flows downslope. Debris flows are commonly caused by intense surface-water flow, due to heavy precipitation or rapid snowmelt that erodes and mobilizes loose soil or rock on steep slopes. Debris flows also commonly mobilize from other types of landslides that occur on steep slopes, are nearly saturated, and consist of a large proportion of silt- and sand-sized material.
MANDATORY EVACUATION CRITERIA
In the event any one or a combination of conditions listed below are observed, a mandatory evacuation order will be issued to La Conchita residents:
- Observation of significant slope failure.
- Observation of surface conditions that a failure is occurring including bulging, cracking or sloughing of soil from any portion of the La Conchita hillside.