By Marshall Hammond
The Portland Water Bureau (PWB) expects to begin refilling Mt. Tabor’s Reservoir 6 this spring after completing repairs, prompted by the discovery of “voids” beneath the reservoir that left it empty of water since 2021. Repairs on the reservoir are now in the final stages and, if all goes according to plan and weather conditions are favorable, the PWB will begin the week-long process of filling the reservoir in late March.
PWB personnel began to suspect the existence of voids beneath the reservoir after discovering an accumulation of silt in the underdrain system, leading to an inspection during the reservoir’s annual cleaning in 2021.
PWB Chief Engineer Jodie Inman has been overseeing the repair and refilling project. “There had been some silt accumulating in the underdrain over several years and we started to get concerned about the increased volume of material that we were seeing,” said Inman. “What that can mean is that part of the underdrain may not be working, or we may have some sort of a leak, which is what we found.”
The silt accumulation was caused by groundwater seeping beneath the reservoir and washing sand, gravel and other materials through the underdrain system. “As you lose that material you create voids, you’ve washed away the material that’s supporting the bottom of the reservoir and then you no longer have anything holding the bottom of the reservoir up,” said Inman. “It could eventually lead to a failure of the dam if not mitigated.” Such a failure could lead to widespread damage to the surrounding area, according to Inman.
When the reservoir was emptied for a routine cleaning in 2021, the PWB found further evidence of voids. PWB then hired a company called Stantec to scan the area beneath the dam with ground penetrating radar. It was the first time the PWB used the technology on such a large project. The ground penetrating radar located several “significant voids that were of concern.”
A team then drilled through the 12 inches of concrete above the voids to take a core sample, confirming their existence. Portland City Council allotted $250,000 for the use of the ground penetrating radar to detect the voids, and in 2023, an additional $25,000 was allotted for the repairs.
The repairs completed so far consist of removing sections of concrete, repairing a damaged drain line and filling in the voids under the reservoir.
The next phase of the project, pouring the concrete that forms the watertight liner of the reservoir, was scheduled to occur on February 26, weather permitting. Once the concrete is poured, it will take 28 days to cure before water can be added.
When the water, fed by the Bull Run Watershed, starts flowing into Reservoir 6, it will take about a week to reach its full volume of about 75 million gallons and an average depth of about 20 feet.
The Mt. Tabor reservoirs, which are on the National Register of Historic places, were built between 1894 and 1911. The first conduit carrying water from the Bull Run Watershed to the city was completed in 1895.
The whole water system is powered by gravity, with the higher elevation reservoirs at Bull Run feeding water to the lower reservoirs located in the hills around the city in Forest Park, Mt. Tabor and Powell Butte.
After more than a century of use, the Mt. Tabor reservoirs were disconnected from the city drinking water supply in 2015 to comply with federal regulations requiring that reservoirs for drinking water to be covered. Now the reservoirs’ sole function is to enhance the beauty of Mt. Tabor park.
The year they were taken off the city’s drinking water system, City Council passed an ordinance requiring that the reservoirs be preserved for future generations, and allocated $4 million to be spent over the following four years for restoration and maintenance projects.
The reservoirs are maintained by the PWB alongside the Mt. Tabor Neighborhood Association. Current repairs and renovations will not be the last efforts needed to preserve the reservoirs. “They are aging and they are aging gracefully, but we do know that this work will put these reservoirs back in service. We have done some additional inspection and they will continue to require some investment into the future to ensure that they stay watertight and safe,” said Inman.
Once the concrete is poured and a date set for refilling Reservoir 6, a media event will be held to celebrate the refilling. City Commissioner Mingus Mapps has stated that he hopes to attend.
For updates on what’s happening with Reservoir 6, visit portland.gov/water/improvements/mt-tabor-reservoirs-preservation.
Photo by Marshall Hammond.
