Astoria staff outline dam-safety work plan, warn of downstream risk to homes

Public Works Director Jeff Harrington told the Astoria City Council on Aug. 26 that the city’s three raw-water storage dams are designated “high hazard” because about 114 properties — roughly 72 homes — sit below the reservoirs and could face life-safety risk if a failure occurred. The state’s annual dam inspection letter this year asks the city to provide dates for completing repairs; Harrington said staff will submit a compliance letter and pursue a consent agreement with the Oregon Water Resources Department by the end of the year.

Harrington reviewed a series of prior engineering studies. A 2016 seismic study found a static factor of safety of about 3.2 and a dynamic factor during a Cascadia-type event near 1.5, both above typical minimums, and a 2021 hydrology report modeled a very large probable-maximum-flood event that would overtop the dam by roughly 3 feet. "That analysis showed that that event would pop overtop the dam by 3 feet," Harrington said, and subsequent scour analysis indicated seams in the basalt foundation could be eroded under prolonged overtopping.

Staff described completed and ongoing work: a 24-inch interim drain rehabilitation that allows controlled lowering of the reservoir, leak investigation and repairs that have reduced some losses (including a spring locally called 'Spur 12'), and design studies for a preferred spillway alternative. Harrington noted grants have paid for much of the technical work and that final designs are being advanced to be 'shovel ready' should funding appear. He said, "Our plan is to have a compliance letter put together by the end of the year."

Councilors pressed staff on timing and consequences if the city cannot meet proposed timeframes. Harrington said the state has so far been cooperative and has not indicated punitive measures, but the city must still identify dates for repairs in the consent process. The council’s discussion closed with staff planning to continue studies, pursue eligible federal dam-safety funding, and return with formal timelines and design milestones.

The council recessed later in the meeting and announced it would enter executive session to discuss litigation as allowed under state law.