Ilwaco plans probe of dam’s weakness

Published 11:19 am Thursday, June 27, 2024

ILWACO — City councilors last week signed off on a state grant that will fund the bulk of a study to analyze the durability of Ilwaco’s dam and set forward a plan to deal with ongoing deficiencies afflicting the structure that provides Ilwaco’s water supply.

The Ilwaco City Council voted unanimously at its June 24 meeting to authorize Mayor Mike Cassinelli to execute a grant agreement between the city and Washington State Department of Ecology. The project that these grant dollars help fund calls for the completion of a topographic survey and seismic stability analysis of Indian Creek Dam, and will also establish an engineering plan to make improvements aimed at preventing erosion of the dam’s embankment.

The cost of the project comes in at about $252,000, of which the funds via Ecology’s High Hazard Potential Dams grant program covers 65% of the total costs with Ilwaco footing the remaining 35% — about $88,000. An additional $19,000 in grant funding from Ecology lowers the city’s financial burden to $69,000, which will come out of Ilwaco’s water fund this year and in 2025.

“Somewhere down the line, we have to do this [work],” said Cassinelli. “The sooner we do it, the cheaper it is.”

Past work

Indian Creek Dam is located about 5.5 miles northeast of Ilwaco and was designed and built to provide the city with a municipal water supply. At full storage, the Indian Creek Reservoir has a capacity of 846 acre-feet — about 275 million gallons. The dam is classified as having a “High” downstream hazard potential, which reflects the general consequences if the dam were to fail and release the reservoir downstream.

Pull Quote

‘In an extreme situation, these deformations may lead to an uncontrolled release of the reservoir contents.’

Washington State Department of Ecology

The dam was inspected in 2018 by Ecology’s Dam Safety Office, which identified several deficiencies that needed to be addressed. In the years since, city water operators have tackled maintenance issues that had been pointed out while other required work has yet to be completed, which this project aims to address.

Ecology’s assessment of the dam’s condition was rated as “Poor,” which means remedial action is necessary. Specifically, the dam’s left groin drainage system was found to have a deficiency that needs to be addressed to ensure erosion of the embankment or the native slope along the left abutment does not occur.

The report from 2018 also found that the dam is likely to crack or have “significant deformations” of the embankment in the event of an earthquake.

“In an extreme situation, these deformations may lead to an uncontrolled release of the reservoir contents,” according to Ecology’s report. “Based on these results the Indian Creek Dam does not meet Ecology’s requirements for stability under seismic loading conditions.”

New tasks

Under the grant agreement with Ecology, the city will contract with an engineering firm to prepare plans and specifications for designing improvements in the dam’s left groin — the area along the contact of the face of the dam with the abutments. An engineering report to document past modifications to the dam’s existing outlet structure will also be prepared, as there are currently no reports on file relating to those changes.

A geotechnical engineering team will also be tasked with conducting a detailed seismic stability analysis of the dam that incorporates the findings of previous seepage analysis and groundwater level monitoring work.

Ilwaco City Administrator Holly Beller said the original cost estimate for the project had not included land disturbance activities that were later identified as necessary for the seismic stability analysis, such as drilling into the Earth. Completing the analysis is required in order to satisfy concerns raised by Ecology in its 2018 report.

The increase in the project’s overall cost and grant match results in a $27,000 hit to Ilwaco’s water fund for this year, and upgrades at two pump stations that had been budgeted for 2024 at a cost of $30,000 are being delayed until next year to accommodate for the unexpected increase. Beller said the city will also have to take a look at its utility rates for its 2025 budget.

“It’s one of the tough choices that we have to make even at relatively high water and sewer rates, but I think this choice has to be made and I commend the staff for bringing this forward in a responsible way,” said Councilor Dave Cundiff. “We cannot have a dam that’s at risk of collapse. We don’t have enough [money] in our tanks to deal with that for any length of time.”