Coastal North Bay Including Point Reyes National Seashore Severe Watches & Warnings NOAA Weather Radio

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: http://www.Weather.Gov/mtr

Local river forecast center: http://www.Cnrfc.NOAA.Gov

Nws: http://www.Weather.Gov/

NWS mobile: http://Mobile.Weather.Gov

California flood preparedness: http://www.Water.CA.Gov/floodsafe/CA-flood-preparedness/fpw_home.Cfm

Fema: http://www.Fema.Gov/fema/

Map service center: https://MSC.Fema.Gov

US Army corps of engineers: http://www.Usace.Army.Mil/