Landslide Susceptibility

#1 Least Susceptible#2 Marginally Susceptible#3 Generally Susceptible#4 Most Susceptible

Higher Numbers = Increasing Landslide Susceptibility

AREA 1 - LEAST SUSCEPTIBLE AREA. Landslide and other features related to slope instability are very rare to non-existent within this area. Included within this area are topographically low-lying valley bottoms and alluviated floodplains. Most areas have slope angles of less than 5 degrees. Part of the area may be underlain by material that lacks the strength to support steep slopes (such as unconsolidated alluvium) but is relatively stable due to the flatness of the slope and lack of potential energy. Land within Area 1 will probably remain relatively stable unless the topography is radically modified. The area may contain local unstable slopes, such as steep banks of erosion gullies and slopes of cut and fill material modified by grading.

[Recommended Application: Engineering geological investigations are generally not necessary for proposed development except where mass grading creates and/or encounters local slopes, such as deep cuts and drainage channel banks.]

AREA 2 - MARGINALLY SUSCEPTIBLE AREA. This area includes gentle to moderate slopes (with slope angles up tho 25 degrees) underlain by relatively competent material or colluvium that is considered unlikely to remobilize under natural conditions. Also includes broad ridgetops and spur crests that are underlain by relatively competent material but flanked by steep, potentially unstable slopes. The stability of slopes within Area 2 may change radically in response to modification of the adjacent terrain.

[Recommended Application: Due to the variability of terrain,engineering geological investigations may be advisable, locally, prior to development.]

AREA 3 - GENERALLY SUSCEPTIBLE AREA. This area contains two subareas: (1) SUBAREA 3-1: Slopes within this area are at or near their stability limits due to a combination of weaker materials and steeper slopes (many slope angles exceed 25 degrees). Although most slopes within subarea 3-1 do not currently contain landslide deposits, they can be expected to fail, locally, when modified. Debris flows may originate within subarea 3-1 and flow downslope, sometimes at high speeds, to impact land included within Areas 1, 2, 3, or 4. (2) SUBAREA 3-2 generally occupies steeper and higher slopes which are less stable and more susceptible to landsliding, rockfall, debris flow, other slope failures and erosion. Most slopes have angles exceeding 35 degrees and heights exceeding 500 feet.

[Recommended Application: Detailed engineering geological investigations should be required for proposed development because slopes in Area 3 are at or near their stability limits.]

AREA 4 - MOST SUSCEPTIBLE AREA. This area is characterized by steep slopes and includes most landslides (whether apparently active at present or not) and slopes upon which there is substantial evidence of downslope creep of surface materials. Slopes within Area 4 should be considered naturally unstable, subject to failure even in the absence of the activities of man. This area is subdivided into two subareas: (1) SUBAREA 4-1, generally located outside mapped landslide areas, contains unstable slopes underlain by both weak materials and adverse geologic structure (dip slopes/daylighted bedding planes). It also includes questionable landslides. (2) SUBAREA 4-2 includes the landslides mapped in the present study and nearby unstable areas.

[Recommended Application: Detailed, comprehensive engineering geological investigations, including specific slope-hazard mitigation measures, are necessary for any proposed development in Area 4 because slopes are unstable. Special Note: When determining land-use suitability, due to the highly unstable nature of terrain in Area 4, it is advisable to consider choosing one of two extreme alternatives and avoid intermediate approaches. For example, either the land should remain undeveloped or minimally developed and used as open space (nature preserve, park, golf course,etc.) or high density/high value development should be permitted that justifies the high cost of mitigation and occupies sufficient area to allow use of proper stabilization measures.]

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