Why is Mount Tabor Reservoir 6 Empty?

Information
Water in Mount Tabor Reservoir 6 reflects the blue sky.
Mount Tabor Reservoir 6 is empty while the Water Bureau investigates structural issues.

Background

City Council passed Resolution #37146 in 2015, establishing the City’s responsibility for preservation activities at the Mount Tabor Reservoirs and setting aside funding for that work. Since then, the Water Bureau and stakeholders have accomplished several preservation tasks and have more to do, which are outlined on our Reservoir Construction Projects page, as well as a maintenance and cleaning schedule outlined on our Reservoir Management page.

During maintenance in the summer of 2021, we cleaned all three reservoirs. During the cleaning process the Water Bureau’s Dam Safety Group used ground penetrating radar (GPR) equipment to inspect for evidence of voids (holes) beneath the concrete on the bottom of Reservoir 6.

What’s Happening Now

Last updated May 18, 2022

During the cleaning process in 2021, the Water Bureau’s Dam Safety Group used ground penetrating radar (GPR) equipment to inspect for evidence of voids (holes) beneath the concrete on the bottom of the reservoir. We did not have the ability to scan the entire reservoir and so a small portion, about five percent, was scanned. This investigation showed that there are voids underneath portions of the reservoir. We are keeping Reservoir 6 empty until we can scan the entire reservoir to evaluate the extent of the voids.

To do the additional work needed to address the voids, the Water Bureau requested funding as part of our budget for the upcoming 2022–2023 fiscal year. In early May, the Mayor proposed to fund $250,000 of the budget request.

The funding included in the budget will allow the Water Bureau to map one cell (half) of Reservoir 6. While not the full amount needed, this funding will allow us to gain valuable information about the extent of the potential damage and to begin developing a scope, schedule, and budget for the needed repairs, including potential GPR on the second cell. This work will provide a basis for a long-term understanding of the asset management needs for Reservoir 6 and help shape future planning for the ongoing maintenance of this historic asset.

Once we know more, we will provide the public with a solid timeline for repair. In the meantime, we will provide updates to this webpage approximately every three months. 

What happened?

All three reservoirs at Mount Tabor were drained for maintenance during the spring of 2021. Our plan was to coordinate the draining and cleaning of Reservoir 6 to coincide with the planned balcony repairs on its inlet and outlet gatehouses.

As part of our ongoing inspection and maintenance program, the Water Bureau’s Dam Safety Group used ground penetrating radar (GPR) equipment to inspect for evidence of voids beneath the concrete on the bottom of the reservoir. We did this because staff had recently noticed sediment (silt or fine dirt) in drainage from pipes that collect water from underneath the reservoir. Seeing that sediment made us concerned that the reservoir’s foundation was washing away.

In late July 2021, we did our GPR scans. We waited until the reservoir was empty and the weather was hot and dry so we could get more accurate readings. We couldn’t scan the entire reservoir, so a small portion, about five percent, was scanned. This investigation showed that there were possible issues underneath the reservoir.

In October 2021, the Water Bureau cored (drilled holes) in those locations of the reservoir floor that were identified with the GPR. The coring confirmed that there are voids in several locations as identified with the scans. In two locations, the voids were significant, with depths of 14 and 22 inches.

Since we found voids in a significant portion of the small area we scanned, we are now planning to scan the rest of the reservoir so we can understand how much will need to be repaired. After this is done, we will evaluate the extent of those issues so that we can provide the public with a solid timeline for repair.

Initial photos of voids underneath Reservoir 6

Underneath the reservoir is a small space with rocks and dirt.

These photos, taken in October 2021, show two of the larger voids underneath Reservoir 6. Ideally there would not be any space between the concrete underside of the reservoir and the ground beneath it. Instead, you can see the concrete of the reservoir at the top of the photo, and the earth below has been washed away so that there is a gap, and nothing is supporting the concrete. The voids pictured here are 14 and 22 inches deep.

Underneath the reservoir is a small space with rocks and dirt.

How does this compare to Reservoirs 1 and 5?

We surveyed Reservoir 1 using the same ground penetrating radar technology and made repairs in 2018. The extent of the voids and the needed repairs at Reservoir 1 were much less because Reservoir 1 is smaller than Reservoir 6. Reservoir 5 has a liner that prevents water from seeping through the concrete and eroding the bedding underneath, so we believe the likelihood of voids beneath it is significantly less.

Why keep Reservoir 6 empty?

The voids confirm that there are areas under the reservoir where nothing is holding the concrete up. With no support, the concrete is much more likely to fail. Since we only inspected a small portion of the reservoir, we don’t know how much of the reservoir could be in the same condition.

In addition, filling the reservoir even a little bit puts weight on the unsupported concrete. Ongoing leakage from the reservoir will continue to wash away the soil under the reservoir, making the condition worse.

What a lot of people don’t know is that the hill they see along the western edge of the reservoir along SE 60th Avenue is an earthen dam. This dam supports the sides of the reservoir and helps hold the water in place. So, when we are concerned about the soil washing away under the reservoir, we are also concerned that the soil could wash away or destabilize the dam.

We’d love to have a magic eight ball to know if, when, and how it could fail. Unfortunately, that is not the case. It could be a slow leak that happens during the day when staff and people are around to take the needed precautions and notify neighbors, or it could happen quickly without noticeable warning. 

We know that failure of Reservoir 6 would impact a large area west and south of the reservoir, including many homes, roads, and businesses. The amount of damage would range depending on how the dam failed, but the risk and cost to homeowners and the community would be very high. It is our responsibility as dam operators to make sure the dam is operated safely, and the community is protected.

When will it be filled?

Since the reservoirs are no longer connected to the water system, unlike the reservoirs at Washington Park, the cost for maintenance and repairs would come out of the City’s budget rather than the funds the bureau collects from ratepayers. The Water Bureau is prepared to undertake the additional investigation and start repairs to be able to fill the reservoir as soon as funding and authority is received from City Council. We cannot start work until then. To facilitate this, the Water Bureau submitted a request for funds as part of its fiscal year 2022–2023 budget. 

Once funded, it will take some time to get a consultant on board and to perform the investigation. We’ve notified the Oregon Water Resources Department about the voids, and they will be required to approve our repair plan. As part of that process and depending on the extent of the repairs, we’ll look at phasing options to try to fill one of the reservoir cells as soon as is reasonable.

What’s happening now?

The Water Bureau submitted the request for funds to do this work as part of its fiscal year 2022–2023 budget. The request can be viewed on page 137 of the bureau’s proposed budget. The City Budget Office runs the budget process.

The Mayor’s proposed budget included $250,000 of funding starting July 1, 2022 to perform GPR of one cell of Reservoir 6. The Water Bureau is in the process of identifying the project manager, initiating the project, and selecting a consultant with capacity to take on this work as soon as possible after July 1st (when the new budget goes into effect and the funding becomes available). We’ll share more details as our plans develop.

Where can I learn more? How will I be informed?

Water Bureau staff and representatives from Friends of Mount Tabor and Mount Tabor Neighborhood Association meet for periodic check-ins related to reservoir management and restoration. Updates will be communicated at these meetings and will also be posted to this webpage.