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Long-term fix to Howard Hanson Dam may be a few seasons off
Residents up and down the Green River have been watching and waiting since the announcement in January that Howard Hanson Dam – located just a few miles northeast of Enumclaw – had sustained damage and that the dam would have to operate at a diminished capacity.
The Seattle District of the U.S. Army Corps of Engineers is evaluating two depressions on the right abutment at the dam, which provides flood risk reduction and water storage on the Green River. Working to characterize the problem, so they can develop and implement an interim and long-term fix, Corps technical experts believe they are one step closer to final assessment.
To date, engineers have excavated the small and large depressions, have installed additional monitoring equipment and are conducting tests as the traditional summer conservation pool is stored at the dam.
“What we have found so far indicates that we may have isolated the reasons the two depressions formed, but we still need to evaluate the extent and effects of the seepage issues within the abutment, particularly in areas not protected by the existing grout curtain,” said Mark Ohlstrom, chief of engineering-construction at Seattle District.
The Corps cautions that a long-term solution is still perhaps a few flood seasons away.
“What we have found so far hasn’t yet fully addressed the seepage issues within the abutment or increased our confidence in using the full flood storage capacity of the dam,” Ohlstrom said. “While the dam is not in immediate danger of failing, there is an increased risk to the downstream communities for higher flood levels until such time that the seepage issues with the right abutment have been resolved.”
In the interim, the Corps has been working closely with King County and the downstream cities to prepare for flood season, should higher-than-standard flows be necessary from the dam.