After Howard Hanson Dam’s reservoir reached record levels in January 2009, the U.S. Army Corps of Engineers discovered seepage in the right abutment of the dam, leading to efforts to mitigate a potential flood. Improved drainage works were completed in October 2011, and the dam was declared safe to maintain its maximum reservoir level. REPORTER FILE PHOTO

Study shows potential dam failure could impact more than 25,000 residents

  • Wed Sep 20th, 2017 1:42pm
  • News

A study commissioned by the King County Flood Control District in the wake of the emergency evacuation of communities near Northern California’s Oroville Dam in February shows the impact that a potential dam failure could have on the region.

The report recommends updating emergency action plans and the development of “inundation maps” showing the potential areas that could be underwater if a dam fails or water needs to be released from the dam.

“This study and its recommendations provide steps forward to strengthen our evacuation plans, coordination between jurisdictions and planning for potential flood events,” said Reagan Dunn, chair of the Flood Control District’s Board of Supervisors. “The close call at the Oroville Dam was a good opportunity to update our region’s emergency action plans and find ways to improve.”

“Upwards of 22,000 people and over $4 billion in economic activity would be disrupted in the Kent Valley alone in the event of a failure at the Howard Hanson Dam,” said Dave Upthegrove, vice chair of the Flood Control District’s Board of Supervisors. “The recommendations in this study will help the county achieve a critical level of preparedness in the event of a dam failure.”

The study, done in collaboration with the King County Office of Emergency Management (KCOEM), and King County Water and Land Resources Division (WLR), was presented to the Flood Control District’s Executive Committee at its Sept. 18 meeting.

The report illustrates that the Emergency Action Plans (EAPs), which the state Department of Ecology requires for the 82 dams in King County, show gaps that could impact their readiness and ability to respond to a dam failure emergency. Less than a third of the EAPs have been updated in the last year, with the majority having no update in the last five years.

Sixty of the EAPs either did not have, or had no current, inundation map, which would show emergency responders the areas that would be impacted by either a dam failure or the need to release water to prevent the failure of a dam.

The study also stated the need to develop a countywide dam safety education program. With more than 25,000 people potentially required to evacuate in the case of a dam failure, the study calls for the development of a comprehensive program that educates municipalities and residents on the steps they would need to safely and effectively evacuate from areas at risk.

Along with the creation of an education program, there is also a need to assess the capability of regional entities to shelter and care for those who must evacuate because of a dam failure. The study calls for an assessment of what may be needed versus what is available to identify any potential gaps relative to a dam failure event.