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Hazards

North Coast Floods and Tsunamis

The low-lying areas of Oregon's coast are vulnerable to inundation by high waters. Heavy, winter rains, rapid snow melt, high tides and strong winds can all contribute to coastal flooding. Offshore earthquakes and ocean landslide can trigger tsunamis. The causes may be very different, yet the effects can be quite similar - water logged land, shoreline erosion, habitat destruction and economic loss

Floods

Some areas of the North Coast are more prone to flooding than others. For example, parts of Tillamook County are extremely vulnerable. Winter rainfalls coupled with high tides and fierce storms are ideal conditions for flooding as the increased volume of water cannot flow out to the Pacific, so backs up into Tillamook's wetlands, pastures and even city streets. These spectacular "100 year floods" could happen every year as the term actually means that there is 1 in 100 chance of a large flood reaching a certain elevation. More often, there is persistent flooding of the lowest elevation throughout the wet season. Even these smaller events cause erosion and slow destruction of shore-side habitat.

Coastal residents should be aware of the flood plain designations and plan development accordingly. For example, Tillamook County provides extensive information on how to and respond to floods. It is not always feasible to not live and work in the flood plain. But care can be taken to prevent serious loss of property and destruction of the natural environment.

Tsunamis

In a tsunami, the water comes from the sea and not the land. It appears as a series of waves that cover sandy beaches, overtop sand spits and wash in and out of bays. A tsunami on the Oregon coast would be generated by one of two types of events:

  • a distant earthquake where run-up can be negligible to significant (10 to 15 feet) depending on the distance from the event, its location and the level of tide - The 1964 Alaska Earthquake is an example of this type of tsunami triggering event
  • a rupture or quake on the Cascadia Subduction Zone which could generate a series of waves 20 feet or higher and move quickly (e.g. 16 knots) - The last of these major tsunamis was recorded in Japanese records in 1700 as well as documented in estuarine sediments on the Oregon and Washington coast

On the North Coast, communities plan for these potentially disastrous events through community awareness, designation of evacuation routes, and training. Visualizations and maps are ways to help people understand the mechanics and impact of a tsunami. Communities also can plan where they build so as to avoid future problems.

For data about coastal hazards use the Hazards Reporter. From the "Maps" tab either enter an address or place name, or use the map to select the appropriate map layer.

Sources - Floods

Dinicola, Karen. The 100-Year Flood. U.S. Geological Survey Fact Sheet 229-96. 2005

Federal Emergency Management Agency. Map Service Center. [Accessed March 2007]

Tillamook County, Office of Community Development. Flood Information. Tillamook, OR; The County. 2007

Sources - Tsunamis

Atwater, Brian, Musumi-Rokkaku Satoko, Satake Kenji, Tsuiji Yoshinobu, Ueda Kazue & David K. Yamaguchi. The Orphan Tsunami of 1700 - Japanese Clues to a Parent Earthquke in North America. US Geological Survey Professional Paper 1707.

Olmstead, Dennis. Development in Oregon's Tsunami Inundation Zone: Information Guide for Developers and Local Government. Portland, OR: Oregon Department of Geology and Mineral Industries. Open File Report OFR-03-05, 2003.

Oregon Department of Geology and Mineral Industries. Portland, OR.

Priest, George. Explanation of Mapping Methods and Use of Tsunami Hazard Maps of the Oregon Coast.

Geologic hazards on the Oregon Coast: An Introduction to Tsunamis. Portland: OR: Oregon Department of Geology and Mineral Industries. Open-File Report O-95-67. 1995.

Sokolowski, Thomas J. The Great Alaskan Earthquake and Tsunamis of 1964. Palmer, AK: West Coast & Alaska Tsunami Warning Center. 1990.

Compiled by Janet Webster, Head Librarian, Guin Library