CORVALLIS - First, take a wetter than average winter. Saturate the ground and fill streams and reservoirs. Cover the foothills and mountains with a thick blanket of snow. Then chill and freeze the ground. Coat low elevation surfaces with freezing rain.
Then inundate the whole scenario with four days of subtropical rain. The result? A whopper of a flood.
The early February 1996 flood in northern Oregon and southern Washington was caused by the above rare sequence of events, explained George Taylor, state climatologist with the Oregon Climate Service at Oregon State University. Taylor will discuss his flood analyses at the upcoming annual OSU-sponsored James. A. Vomocil water quality conference at OSU's LaSells Stewart Center in Corvallis on Nov. 13.
"When you combine a wet winter with deep snowpack, frozen ground then four days of warm, intense rain, you have a recipe for a flood," said Taylor. "It was such a warm air mass that rain fell in all elevations except the highest Cascade crests."
Taylor is analyzing information from the 1996 flood in order to evaluate its significance for groups such as the Federal Emergency Management Agency (FEMA), land and water management agencies and flood insurance companies. He is trying to compare this flood with all others that have occurred since data recording began, generally in 1928.
By studying precipitation patterns, river flow levels, snow depths, and temperatures, weather station by weather station, county by county, month after month, year after year across the state, Taylor is looking for patterns and record-setting events.
"I am trying to evaluate the likelihood of this kind of thing occurring again," explained Taylor. "Was this a 100-year event? A 20-year event? Many land use and policy decisions get made based on analyses such as these.
"For example, flood insurance kicks in when you reach a certain flood return interval," he continued. "The location of the 100-year flood line in relation to a home or structure can determine whether reimbursement for damages occur. Our analyses help determine more accurately what a 100-year flood actually is."
State government is particularly interested in how often a flood of this magnitude might occur, he said.
"Planning requirements are different for 20-year events than for 100-year events," he said.
Some of Taylor's more interesting findings about the February 1996 flood include:
-The northern Oregon Coast Range area, near Tillamook, had the worst flooding in terms of historical record - probably greater than 100-year events.
-In the south end of the flood area around Eugene, it was probably only a 10-year event.
-Overall throughout northwestern Oregon, the flood was approximately a 25-year event. The flood of 1964 was a little more extreme over a larger area, from northern California through Washington, and eastward to Idaho.
-The Laurel Mountain area, in the Coast Range southwest of Salem, received more than 28 inches of rain in a four-day period. An area close by, west of Valsetz, is considered the rainiest area of Oregon.
-Human land uses can influence flooding patterns. Urbanization in the Tualatin Valley may have augmented flood heights there. Timber harvest activities in the Coast Range may have contributed to landslides that occurred.
Interested citizens, students, land owners, regulators, land use planners and managers and scientists are welcome to register for the water quality conference. For more information, write: Water Quality Conference, Dept. Bioresource Engineering, OSU, Gilmore Hall, Room. 116, Corvallis, OR 97331-3906 or call 541-737-4021.