Scientists Struggle To Measure Latest West Coast Flooding

February 06, 1997

Recent floods in the Pacific Northwest damaged or destroyed about 146 streamflow measuring stations in early January, hampering attempts to provide basic floodflow measurements during the current floods, according to the U.S. Geological Survey.

"We are working to restore our network even as we provide basic measurements of the current flooding to local, state and federal agencies such as the National Weather Service and the Federal Emergency Management Agency (FEMA)," said John Conomos, USGS Regional Hydrologist, Menlo Park, Calif. "We are determined to keep providing the vital information both to make immediate flood predictions and to aid long-term water-management decisions."

The USGS network has about 1,400 stations in California, Nevada, Idaho, Oregon and Washington. The loss of 10 percent of the network due to the New Year floods has hampered the ability of some agencies to anticipate the effects of possible additional flooding in the region.

Record-high streamflows occurred at 39 locations on streams in the five Pacific Northwest states that recently experienced severe flooding, according to the U.S. Geological Survey.

Hundred-year floods or greater, occurred at 18 sites. A hundred-year flood is one that has a one in one hundred chance of occurring during any given year.

"Right now, we are working to restore our streamflow gaging station network, as new storms form in the Pacific," said John Conomos, USGS western region chief for water resources programs. "We are working closely with local and national emergency management officials to be prepared for these and other future events."

The flooding that marked a devastating and wet end to the old year and the start of the new one damaged or destroyed USGS stream gages which feed critical information to the National Weather Service, the U.S. Army Corps of Engineers and state and local emergency management agencies.

Many of the stations are real-time streamflow stations that provide immediate information to emergency officials through satellite telemetry. National and local emergency agencies can tap into the data and get instantaneous readings on the stage or height of river levels. From this data, calculations can quickly be made of the discharge or volume of water flowing in the river, providing vital information to downstream areas. Anticipating the volume of water headed toward a populated area, even with a few hours of warning as to when a stream will overtop its banks, gives local planners and managers the window of safety they need to issue emergency warnings and alerts and to operate dams and strengthen levees.

In the immediate aftermath of flooding, USGS hydrologists quickly work to determine what the peak discharge was at each streamgaging station and then calculate the flood frequency at each site. The comprehensive data provided by the USGS streamgaging network enables hydrologists to use historic data on flow to calculate how often floods of a particular magnitude are likely to occur. This post-flood information is vital to emergency managers in developing preparedness and response plans and in better informing the public of flood risks in a particular area.

The impact of spring rains is a concern as the snow-water content in the eastern Sierra above 7,000 ft is still very high -- as much as 200 percent above the total April 1 'normal' seasonal water content -- already in late January with another series of mid-Pacific storms developing.

Information regarding real-time hydrologic data is available in the Internet by using the following Universal Resource Locator (URL):

* * * USGS * * *

(Note to Editors: For an update on current flood conditions in the western states, call Carl Goodwin in Washington at 206-593-6510; Dennis Lynch in Oregon at 503-251-3265; Jon Nowlin in Nevada at 702-887-7604; Derrill Cowing in Idaho at 208-387-1316; or Mike Shulters in California at 916-278-3026.)

A map is available that shows the locations of USGS stream gages that were destroyed or damaged during recent flooding. Especially hard hit was California, where 79 gages were damaged or destroyed. 3


* California -- The January storms focused their intensity on California's central and northern Sierra Nevada, with flooding in the lower Sacramento and San Joaquin Valleys of California. Initial evaluation of data from USGS streamflow stations shows five new streamflow records, all of which are 100-year flow events or greater. The USGS station on the Merced River in Yosemite Valley had a peak recorded flow of 10,000 cfs on January 2, 1997, a 100-year flow event that exceeds the previous record of 9,860 cfs recorded on December 23, 1955. Facilities in the Yosemite Valley were heavily damaged as a result of the flooding. Other USGS streamflow stations with 100-year or greater events are located in the Tuolumne River basin (2) including the station at Modesto (approximately 70,000 cfs), on the Cosumnes River at Michigan Bar (approximately 93,000 cfs). The South Fork of the American River near Placerville also experienced a new record and a 100-year event with flows of about 70,000 cfs. No serious flooding was associated with this flow. The American River is regulated by Folsom Reservoir above Sacramento. Flooding on the Feather River in the Sacramento Valley and in the San Joaquin Valley involved some 30 levee breaks.

* Nevada -- In Nevada, where 29 gages were damaged and 6 were destroyed, the flood demonstrated that the flood-warning network in the three river affected basins should be expanded and enhanced with real-time data collection platforms. The flood-warning system in that area protects Carson City, Reno, several Indian reservations and many smaller cities and towns.

* Washington -- New records were set in the Puget Sound drainage basins. The most severe flooding was in eastern Washington were Hangman Creek at Spokane reached a new record-high flow of 25,800 cubic feet per second (cfs), which exceeds the old record of 20,600 cfs set in 1963. This new record also exceeds the 100-year flood. Flooding in the state, which was first caused by ice-clogged storm drains, unable to handle the rapidly melting snow, was then exacerbated by the overflow of shallow-storage basins and rising water tables. Dozens of mudslides occurred in the Seattle area, destroying homes and blocking roads. Streams undermined and washed out road embankments, and in urban areas small streams flooded many homes and businesses. Although remarkably little damage was sustained at USGS gaging stations, hydrologists worked through the New Year holiday period to make emergency repairs to satellite telemetry equipment and take immediate post-flood measurements. Downed trees and limbs await removal at dozens of stations, which will need to be removed in order to get stations back to full operational capability to provide information and timely warnings.

* Oregon -- Streams rose quickly to flood levels and the major streams remained above flood levels for several weeks and several streams set new peak record flows. In eastern Oregon, for example, the Imnaha River recorded an estimated peak discharge of 20,000 cfs, which is twice the previous peak of 10,000 cfs. Record keeping began at this site in 1928. Bear Creek, along with many other streams in southern Oregon crested slightly below or equal to previous peaks of record.

* Idaho -- Rock and mudslides triggered by the intense flooding closed several central Idaho highways, including State Highway 95, the main north-south highway connector for the state. Several hundred people were evacuated along the banks of the Payette River, where flows reached and exceeded the 100-year recurrence interval (a 100-year flood is not expected to occur, on average, more than once in 100 years). In Washington County, the Weiser River ran above flood stage for two days and exceeded the 100-year recurrence interval, which forced the evacuation of several towns.

During floods, USGS streamflow information is used by the Federal Emergency Management Agency and many other federal, state and local emergency management agencies as a basis for emergency operations, because the data provide real-time information on floods as they occur. The data are used by the National Weather Service for flood-warning and flood-forecasting purposes.

* * * USGS * * *

January 1997

US Geological Survey

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