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

strap joints. Most river and stream crossings include concrete encasement over some or most of
the pipeline between the banks of the river or stream.
Major hazards to the pipeline system include earthquakes, floods and landslides, with the
earthquakes and earthquake induced hazards such as liquefaction, surface fault rupture, lateral
spread and strong ground shaking being the most significant.
In general, buried pipelines, such as the SCWA’s aqueducts, are designed for internal pressure
with limited consideration to large relative displacements of ground along its length. Such
pipelines are typically designed with bell and spigot type connections (also known as segmented
pipelines) and do not perform well when subjected to ground failure resulting from earthquakes,
floods and landslides. In the 1964 Anchorage, Alaska earthquake more than 100 water pipe
breaks were reported. In 1971 San Fernando earthquake, the City of San Fernando temporarily
lost water, gas and sewage services due to liquefaction induced lateral spreading along the
eastern and western shores of Upper Van Norman reservoir. For relatively small ground
displacements associated with earthquake ground shaking, the pipelines perform reasonably well
with certain amounts of random damage that can usually be handled as part of emergency repairs
following an earthquake.
The Agency’s pipelines cross many locations where they may be subjected to ground
deformation. The most obvious location is the Rodgers Creek fault crossing of the Santa Rosa
aqueduct. Surface fault rupture displacement of several feet is expected during a major
earthquake on the fault, which the pipeline is not designed for. In addition, the Agency’s
Sonoma aqueduct and the Oakmont pipelines cross the Bennett Valley fault and will likely fail in
a surface rupturing event on this fault. Other vulnerable locations include the Russian River
crossing of the Russian River Cotati intertie and multiple stream crossings as shown in Figure
xx. Depending upon the geometry of the stream bank and the potential for liquefaction, these
locations could have large lateral spread displacements and consequently the pipelines may fail.
The Sonoma aqueduct is also vulnerable to damage from debris impact in a high flood scenario
at three locations where the pipeline is suspended from bridges across creeks.
4.1.4 Storage Facilities
The Agency has a total of 17 flat bottom steel tanks located at eight independent sites. Out of the
17 tanks, seven are anchored while the other 10 are unanchored. The tanks range in size from
0.3 M.G. to 18 M.G. Most of the tanks have overconstrained piping connections that are
potentially vulnerable to damage in an earthquake. Such piping connections consist of the pipe
that is rigidly attached to the tank shell and restrained by burial near the tank or connection to an
adjacent nearby tank. As a result, the piping is unable to accommodate movement of the tank as
32 Local Hazard Mitigation Plan 1-18-08.doc