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California, Mountains Of San Benito County And Interior Monterey County Including Pinnacles National Monument

Public Information Statement

Statement as of 8:00 AM PDT on October 23, 2014

October 20th through 24th is California flood preparedness week!
The National Weather Service forecast office for the San
Francisco and Monterey Bay areas will feature a different
educational topic each day during the preparedness week.

Today's topic: coastal flooding

Californias coastline is approximately 850 miles long, making it
one of the longest state coastlines in America (only Alaska's and
Florida's coastlines are longer). When tidal areas, estuaries,
and deltas are included, californias coastline grows to
approximately 3,425 miles.

When most people think about coastal hazards, it is most likely
that tides, waves, and rip currents come to mind or maybe even
tsunamis. While these are important coastal hazards to be aware
of, coastal flooding is typically a result of how rivers interact
with the estuaries and lagoons that they fill at the coast. As
such, coastal flooding is an ever-present hazard that can occur
year-round and can extend well inland. Rivers along the often
mountainous, terraced coastline of California may be entrenched in
canyons for much of their length, limiting flood threat during
winter storms to only roadways and structures in the valley floor.
But they often open into floodplains before entering the sea,
distributing flood threat across a more expansive area.
Floodplains in these coastal settings can be inundated with
variable frequency by riverine and estuarine waters. Particularly
important and common along the California coast are river mouths
that are controlled, in part, by beach-barrier bars that form
seasonally. Beach-barrier bars episodically and naturally close
river mouths, typically during the Spring, Summer, and fall when
river flows are insufficient to keep the river mouth clear of
alongshore sediment delivery and accumulation from the ocean. When
river flows wane and beach bars block the surface connection of
the river with the ocean, estuaries can fill up with river waters
that continue to trickle in, creating backwater coastal lagoons
that can extend miles inland, depending on the configuration of
the valley. Reconnection and draining of these lagoons occurs
naturally, although in many locations drainage occurs at a stage
above low-elevation infrastructure. Artificial drainage of these
lagoons below critical flood levels is the primary and historical
tactic employed by local water management agencies; however, this
practice has fallen out of favor because of the threat this
imposes on environmental resources and habitat. Much research and
planning is currently being directed towards developing strategies
to manage water levels in estuaries and lagoons that reduce flood
threat while simultaneously managing the ecosystem in a
sustainable, healthy manner.

Most large and small creeks/rivers that meet the coastline in
California exhibit beach-barrier bars and seasonally-impounded
lagoons. Examples in our area include the Russian River estuary
at Jenner, the San Lorenzo river at the beach boardwalk, the
Pajaro river and the Pajaro dunes area, and the Carmel river and
its associated lagoon.


A tsunami is a series of long-period waves (on the order
of tens of minutes) that are usually generated by an impulsive
disturbance that displaces massive amounts of water, such as an
earthquake occurring on or near the sea floor. Underwater volcanic
eruptions and landslides can also cause tsunami. The resultant
waves are much the same as waves propagating in a calm pond after
a rock is tossed. While traveling in the deep oceans, tsunami have
extremely long wavelengths, often exceeding 50 nm, with small
amplitudes (a few tens of centimeters) and negligible wave
steepness, which in the open ocean would cause nothing more than a
gentle rise and fall for most vessels, and possibly go unnoticed.
Tsunami travel at very high speeds, sometimes in excess of 400
knots. Across the open oceans, these high-speed waves lose very
little energy. As tsunami reach the shallow waters near the coast,
they begin to slow down while gradually growing steeper, due to
the decreasing water depth. The building walls of water can become
extremely large in height, reaching tens of meters (30 feet or
more) as they reach the shoreline. The effects can be further
amplified where a Bay, Harbor, or lagoon funnels the waves as they
move inland. Large tsunami have been known to rise to over 100
feet! The amount of water and energy contained in tsunami can have
devastating effects on coastal areas.

Coastal floodplains and estuaries are commonly used for
residential and Industrial development and/or recreational
activities. People inhabiting these areas, as well as those
conducting business or recreational pursuits there should be
vigilant of The Hazards that may develop. Stay alert, and listen
for warnings and other notifications of coastal flooding other
hazards along the coast from your local Weather Service office and
local emergency management personnel.

Join US tomorrow for information on flood safety and flood
information tools.


Important flood websites

Local NWS office:

Local river forecast center:


NWS mobile:

California flood preparedness:


Map service center:

US Army corps of engineers:

Weather Severe Map
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