Flood facts

Weather service explains dangers for peninsula area

Posted: Friday, October 10, 2003

On an appropriately rainy day, government officials and emergency service providers gathered for a Fall Flood Workshop sponsored by the Kenai Peninsula Borough Office of Emergency Management on Thursday.

As his agency issued minor flood warnings for the Kenai River at Cooper Landing and Kenai Lake through Saturday, National Weather Service hydrologist Larry Rundquist told workshop attendees about the many causes of flooding in Alaska, explained how the weather service predicts floods and defined various levels of flood alert messages.

Forty people representing the cities of Seward, Homer and Kenai, the Kenai Peninsula Bo-rough, the Alaska Department of Fish and Game, the Department of Natural Resources, Central Penin-sula General Hospital and the American Red Cross met in the day-long workshop at the Pacific Rim Institute for Safety Manage-ment to learn the dynamics of floods, their causes and methods of predicting them.

Fall flooding on the Kenai Peninsula one year ago washed out roads and bridges and caused extensive property damage particularly in the Seward and Ninilchik areas toward the south end of the peninsula.

"We are monitoring on a regular basis and putting out forecasts during the entire open-water season," Rundquist said.

He described the season as May through October, and somewhat earlier or later depending on weather.

He described nine ways in which floods could be caused in Alaska including freeze-up flooding, snow melt, glacier melt, rainfall, geologic control floods, dam breaks, tsunamis, coastal flooding and jokulhlaups -- an Icelandic term for the periodic release of water when glacier lakes drain.

"Freeze-up flooding occurs on the Kenai River, especially here in town, when frazil ice creates frazil jams," Rundquist said. Describing frazil ice as that which forms as ice crystals in the river, he said the frazil jam eventually meets the ice sheet near the mouth of the river, blocking water and creating flooding.

"Snow melt flooding typically is not as big a problem on the Kenai (Peninsula)," he said.

"First, you need the entire basin to be covered and then melt and produce runoff all at the same time. It's more common further north because of the quick warm-ups there," he said.

He did tell the group that snow-melt flooding could be a problem in this area if temperatures remained cold during March and April and then there was a sudden warm-up.

"Some of the greatest floods in Alaska have been from rainfall events," he said. He explained that flooding from rainfall was more typical on the east side of the Kenai Peninsula than the west due to the movement of airflow over the peninsula.

Weather systems coming in from the Gulf of Alaska have a lifting effect when they reach coastal areas.

"Lifting on the east side causes rain to fall from the clouds," Rundquist said.

"The downward flow as the clouds cross the peninsula stabilizes the water in the clouds. Then, when the clouds reach the other side of the (Cook) Inlet, lifting once again causes rainfall," he said.

"In October of 1986, the system got stuck on the east side and Seward got 15 inches of rain in a day."

He said rainfall-caused flooding is forecast by using a calibrated hydrologic model that tells forecasters how much rain falls at measuring stations across the peninsula, how much water is in the soil and in rivers and streams and the frequency of periods of precipitation.

Rundquist announced to the group that his office recently received funding to add two weather stations on the Kenai Peninsula -- one in the Anchor River or Ninilchik area and one on top of the Harding Ice Field.

Another type of flooding that could occur on the peninsula was described by Rundquist as geologic control flooding.

The causes include natural lakes dumping out due to geologic events such as earthquakes, and earth and snow avalanches that tumble into lakes or that block streams, he said.

He also defined various National Weather Service terms such as flood stage, the water level at a gauge that causes flooding downstream; minor flooding, flooding that causes minimal or no damage; moderate flooding, which causes some structural damage and road inundation; and major flooding, which causes extensive inundation of structures and roads.

Rundquist said the weather service forecast office in Anchorage wants to hear about flooding problems when they occur, and he told workshop participants they also could receive weather forecast information by calling (800) 472-0391 or the Alaska-Pacific River Forecast Center at (800) 847-1739.

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