Page 14 - Yucaipa Valley Water District
P. 14
Many of these unengineered levees have only a few feet of freeboard (distance from
water level to top of levee) at high tide. A breach of a single segment within a levee system
will inundate the interior island. The vast open space within the delta islands exceeds the
immediately available volume of fresh water.
Some delta levees are ... vulnerable to liquefaction, and as a result the levees could settle,
slump and spread, probably leading to a catastrophic breach.-
Accordingly, simultaneous flooding of multiple islands would draw in saline water from
San Francisco Bay, contaminating the fresh water supply for California's water projects.
With insufficient fresh water reserves to flush the salt out of the delta during our historic
drought, delta water could remain salty for years.
The findings of engineering consultants and university research show that this
catastrophic scenario might very well have occurred had Sunday's earthquake struck
near or beneath the delta.
Strong shaking from a nearby earthquake can cause saturated, loose, sandy soils to
behave temporarily like a liquid (a process called liquefaction). Some delta levees are
built of, or are founded on, sandy soils vulnerable to liquefaction, and as a result the
levees could settle, slump and spread, probably leading to a catastrophic breach.
Exacerbating the risk, many delta levees are founded on peat, soil that is composed of
organic material. Peat is highly compressible and soft, and it is known to be capable of
shaking strongly during earthquakes. Peat does not experience strength loss like
liquefiable sand, but recent research shows that it is vulnerable to settlement after strong
shaking, which would reduce freeboard and contribute to the risk of breach.
This is the subject of ongoing research because it is important for engineers to understand
the diverse array of hazards that threaten our levees.
Seismically induced failures of levees are not the product of imaginative speculation. They
have been observed elsewhere around the world (mainly Japan) where earthquakes,
often close to the magnitude of the Napa event, have occurred near major levee systems.
We ignore the potential for seismic failure of levees in California at our peril. We must do
all we can to prevent these catastrophic failures from affecting our water supply. Let's
heed the warning that Sunday's earthquake provides by adopting a long-term solution to
the water conveyance infrastructure problem in the delta.
Scott J. Brandenberg and Jonathan P. Stewart are the vice chairman and chairman,
respectively, of the UCLA Department of Civil and Environmental Engineering.
Yucaipa Valley Water District Board Meeting - September 3, 2014 - Page 14 of 51
water level to top of levee) at high tide. A breach of a single segment within a levee system
will inundate the interior island. The vast open space within the delta islands exceeds the
immediately available volume of fresh water.
Some delta levees are ... vulnerable to liquefaction, and as a result the levees could settle,
slump and spread, probably leading to a catastrophic breach.-
Accordingly, simultaneous flooding of multiple islands would draw in saline water from
San Francisco Bay, contaminating the fresh water supply for California's water projects.
With insufficient fresh water reserves to flush the salt out of the delta during our historic
drought, delta water could remain salty for years.
The findings of engineering consultants and university research show that this
catastrophic scenario might very well have occurred had Sunday's earthquake struck
near or beneath the delta.
Strong shaking from a nearby earthquake can cause saturated, loose, sandy soils to
behave temporarily like a liquid (a process called liquefaction). Some delta levees are
built of, or are founded on, sandy soils vulnerable to liquefaction, and as a result the
levees could settle, slump and spread, probably leading to a catastrophic breach.
Exacerbating the risk, many delta levees are founded on peat, soil that is composed of
organic material. Peat is highly compressible and soft, and it is known to be capable of
shaking strongly during earthquakes. Peat does not experience strength loss like
liquefiable sand, but recent research shows that it is vulnerable to settlement after strong
shaking, which would reduce freeboard and contribute to the risk of breach.
This is the subject of ongoing research because it is important for engineers to understand
the diverse array of hazards that threaten our levees.
Seismically induced failures of levees are not the product of imaginative speculation. They
have been observed elsewhere around the world (mainly Japan) where earthquakes,
often close to the magnitude of the Napa event, have occurred near major levee systems.
We ignore the potential for seismic failure of levees in California at our peril. We must do
all we can to prevent these catastrophic failures from affecting our water supply. Let's
heed the warning that Sunday's earthquake provides by adopting a long-term solution to
the water conveyance infrastructure problem in the delta.
Scott J. Brandenberg and Jonathan P. Stewart are the vice chairman and chairman,
respectively, of the UCLA Department of Civil and Environmental Engineering.
Yucaipa Valley Water District Board Meeting - September 3, 2014 - Page 14 of 51
