City, National Weather Service collaborate

January 25, 2011

By Warren Kagarise

Forecasters aim to reduce confusion about flood data

Information from the city and the National Weather Service offered a study in contrasts as rain-gorged Issaquah Creek spilled onto city streets in early December.

A shed houses a U.S. Geological Survey flood gauge beside Issaquah Creek at 252nd Avenue Southeast and Southeast 165th Street. By Greg Farrar

Issaquah Creek data from a gauge upstream in Hobart indicated a creek running high, but not enough to cause more than localized flooding. Information from a downstream gauge and a notice from National Weather Service meteorologists, on the other hand, cautioned residents to prepare for widespread flooding in the city.

The arrangement caused some confusion among floodplain residents.

“All of this data that we used to get and analyze and react to is now available for everybody to look at, so people look at that data and draw different conclusions than we do,” Bret Heath, city Public Works Operations and emergency management director, said after the Dec. 12 flood.

The city monitors real-time Issaquah Creek flood data from U.S. Geological Survey flood gauges in Hobart and near the creek mouth in Lake Sammamish State Park. Though emergency planners monitor both gauges, the city uses only the upstream Hobart gauge to determine real-time flood phases in Issaquah.

The city uses creek flow information in order to determine flood response. National Weather Service hydrologists and meteorologists, on the other hand, use the same information — plus topography and numerous other factors — to formulate a forecast.

The agency has created models to forecast flooding on larger waterways, including the Snoqualmie River.

“For Issaquah, the issue for lead time is, we don’t have the model on Issaquah Creek, like we do on our other rivers,” National Weather Service Senior Service Hydrologist Brent Bower said.

Heath, a longtime emergency planner, said the effort to forecast flood phases for Issaquah Creek can pose a challenge.

“Those accuracy rates increase considerably, the closer you get to an actual event,” he said. “So, the forecasts that are out several days have less accuracy level than the ones that are out a day or 12 hours.”

The creek basin includes tributary creeks crisscrossing the region and runoff from the Issaquah Alps. Issaquah Creek has a smaller basin — and more difficult area to predict conditions — than the Green River or Snoqualmie River basins. Conditions change quickly in the Issaquah basin, and 60 minutes can mean the difference between flood phases.

“It is a difficult drainage basin to try and do forecasting for, but they are working on it,” Heath said.

In order to clear up confusion about the gauges and flood information, city planners and National Weather Service forecasters met in December to discuss creek data.

The parties agreed to communicate better in order to alleviate confusion about flood information, but the details remain undefined.

In addition, city spokeswoman Autumn Monahan, the official responsible for updating information during a natural disaster or other emergency, changed the municipal website to present emergency flood information on a single page.

The city offers real-time data, but other sources might not be updated as often, so Heath encouraged residents to use information from the municipal website, radio station and recorded phone line.

The deluge last month also allowed planners to collect data from a gauge installed last year in a “blind spot” along Issaquah Creek.

The gauge should account for runoff from the 15 Mile Creek drainage off Tiger Mountain, but planners need numerous floods in order to calibrate the device.

Warren Kagarise: 392-6434, ext. 234, or wkagarise@isspress.com. Comment at www.issaquahpress.com.

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