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Consultant outlines five flood-control options, says mitigation could limit fishery harm
Summary
A state-commissioned hydrology model presented to the Chehalis City Council compared a temporary flow-through dam and a land/levee alternative; the presenter said modeling shows large flood reductions but that SEPA analysis suggests only modest basin‑wide fish declines and argued mitigation measures could offset impacts.
A consultant reported to the Chehalis City Council on April 27 the latest hydrology modeling of five large-scale flood-control alternatives, saying one proposal would temporarily hold roughly 62,000 acre‑feet above the PL site to blunt catastrophic storm peaks while other options rely on levees and land‑based defenses.
Mr. Vanderstu, the presentation lead, said the flow‑through option (option A) models a facility that would operate only during extreme events — between about 13 and 21 days when needed — and otherwise allow normal fish passage. He said the Skookumchuck modification and the large upstream facility together produce most of the modeled flood reduction.
The consultant showed maps and charts comparing a no‑action baseline with alternatives that use a color scale to indicate local increases or decreases in peak flood levels. He said the hydrology team modeled a “hundred‑year plus 22%” event and reported the major reduction in downstream peaks arises from holding significant storage above PL.
On environmental effects, Vanderstu told the council that the state’s SEPA modeling projects declines in Coho, spring/fall Chinook and steelhead when the facility is built, but he said that when the same data are analyzed on a basin‑wide basis the projected impact is small. “This project can’t be permitted unless the state and federal regulatory agencies are convinced that it will not have any negative impact on key environmental issues starting with the fishery,” Vanderstu said, and he described a mitigation package the district has developed that he said could produce a net benefit when measures in the far upper basin are counted.
Council members asked technical questions about the scope of the modeling, impacts on tributaries and the role of the Skookumchuck Dam. Vanderstu said the Skookumchuck change — increasing its outflow capacity and adding fish passage — could add roughly 20,000 acre‑feet of storage above Centralia and that about 60–75% of the modeled benefit came from storage above the PL site.
The presentation emphasized that permitting requires convincing state and federal agencies that fisheries and other environmental values will be protected; Vanderstu said the district’s scientists believe mitigation can prevent net losses. He closed by inviting further questions and noting attendees can review the state documents that were presented at recent SEPA hearings.
Next steps: the modeling and SEPA materials remain part of the public record and are expected to inform any future permitting decisions and local positions about alternatives under consideration.
