Local Hazard Mitigation Plan.pdf - Sonoma County Water Agency ...

the County are exposed to long duration peak ground accelerations in excess of 0.15g (Figure 8).
To assess the liquefaction and lateral spread hazard to the Agency’s facilities, potentially
liquefiable soils that consist of young alluvial deposits and artificial fill present within the
Agency’s service area are overlain on the Agency’s water supply system as shown in Figure 9.
The figure also shows locations where the pipelines cross streams and open slope faces. Such
stream crossing locations coupled with high liquefaction potential have a very high likelihood of
lateral spread and resulting pipeline damage.
All of the Agency facilities that lie in areas marked as moderate, high and very high will likely
experience liquefaction because the estimated ground acceleration at all Agency facilities is
greater than 0.3g, the triggering threshold for a moderate susceptibility rating. As shown in
Figure 9, significant portions of the Agency’s system are vulnerable to liquefaction. Areas of
high and very high liquefaction potential exist at collector sites, Mirabel well-field and Ely
booster station. A high susceptibility to liquefaction exists along the transmission lines, the
Wohler Intertie, most of the Santa Rosa aqueduct, significant portions of Petaluma
aqueduct and localized areas of the Cotati and Sonoma aqueduct. Creek crossings along
these portions of the transmission lines, as shown in Figure 9, have a very high potential for
damage due to the potential for lateral spread. The main power line from the Wohler
substation to the collectors is also located in an area of very high liquefaction potential.
3.1.5 Earthquake Induced Landslides
Earthquake-induced slope failures or landslides commonly occur over wide areas on hill slopes
during large (magnitude 6.5 or larger) earthquakes and can produce significant damage. The
most common earthquake-induced failures are rockfalls, rock and soil slides, and soil
avalanches, slumps and flows. Rockfalls, avalanches, and flow-type failures are especially
hazardous because they often occur rapidly and travel great distances from the point of initiation.
These types of rapid failures present significant impact to structures sited on slopes or valley
areas downhill from the initiation site, and can distort or break shallow-buried pipelines crossing
the sliding plane of the slope failure.
The opportunity for seismically induced slope failure is dependent on the potential for
appropriately high levels of ground shaking to initiate movement. The susceptibility for failure
is based on conditions that predispose the slope to failure including static stability, local geology,
slope inclination, groundwater conditions, rock strength, and the duration and intensity of
shaking. The potential for landslides is higher during seasonal wet periods when hill slopes are
saturated with water.
16 Local Hazard Mitigation Plan 1-18-08.doc