Background
In July 2015, the Portland City Council directed the Water Bureau to work with the Mount Tabor Neighborhood Association (MTNA) to manage the quantity and quality of water in the Mount Tabor open reservoirs. Prior to 2016, the reservoirs were connected and in service as part of the potable water system. They were typically drained, cleaned, and refilled twice per year (once in spring and once in fall). There have been many changes to Water Bureau operations at Mount Tabor in the past few years, particularly with the disconnection of the open reservoirs from the water system. A maintenance plan will ensure the reservoirs meet water quality standards.
- Draining: When the reservoirs were connected to the system, instead of draining a large volume of water from the reservoirs directly into the Bureau of Environmental Services (BES) collection system, most of the water in the reservoirs was delivered to customers and to other lower-elevation reservoirs. Now that the reservoirs are disconnected, all of the water will need to be drained into the sewer/stormwater system. We're trying to reduce the amount of water that goes into the BES system and are working with MTNA to develop new protocols for how frequently we drain the reservoirs.
- Cleaning: We no longer have to keep the water clean enough to drink, but we do want to maintain the water at the levels specified by City Council. After disconnecting the reservoirs, we tested the water quality periodically over the first three years to check that algae, chlorophyll, chlorine, pH, temperature, and turbidity were all within expected ranges. Now, we only test as needed.
- Refilling: We want to conserve both staff time and water. To that aim, we're developing protocols that reduce how often we clean the reservoirs.
The cycle of draining, cleaning, and refilling takes several weeks. Also, the weather affects our ability to do this work. If heavy rainfall is expected, we postpone the work so we don’t overburden the sewer system.
Reservoir cleaning schedule
We drain, clean, and refill the Mount Tabor reservoirs on a regular schedule. We do this to protect the quality of the nonpotable water in the reservoirs and to conduct inspections of reservoir conditions. The current schedule for draining, cleaning, and refilling the reservoirs follows.
- Cleanings every second year: We purposefully delayed the annual cleaning of Reservoir 5 in 2018 until March 2019 to learn more about what would happen if the reservoirs were cleaned every two years rather than annually. This reduced the overall amount of water drained and needed to refill the reservoir, and no negative water qualities were observed or reported during that time. Going forward, each reservoir will be cleaned every two years.
- Fewer reservoir refills during dry months: During 2019, we explored what would happen if we stopped refilling the water in Reservoir 5 during the dry summer months, when the City was supplementing its water supply with groundwater from the Columbia South Shore Well Field. We monitored how water evaporation affected the look of the reservoir to make sure that the historic views were not compromised. At its lowest, Reservoir 5 was 71.5 percent full on November 18, 2019, which is within an allowable range.
All three reservoirs were allowed to partially evaporate in 2020.
The Water Bureau, MTNA, Friends of Mt. Tabor and Portland Parks and Recreation are working together to review the management plan annually and make adjustments as needed.
What’s happening now: Spring and summer cleaning
Starting in April 2021, we began the process of draining, cleaning, and refilling all three reservoirs one at a time. The cycle of draining, cleaning, and refilling each reservoir takes three to four weeks. This work is weather dependent. If heavy rainfall is expected, the work will be postponed to avoid overburdening the sewer system.
Reservoir 5: Cleaning is complete.
Reservoir 1: During the cleaning of Reservoir 1, we discovered some concrete that needed repairs. We expect the repairs to be made and the reservoir to be filled by June.
Reservoir 6: Reservoir 6 will be empty from late July until November 2021. For the past year, our plan has been 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 ongoing maintenance, our dam safety group has requested to take advantage of the reservoir draining to use ground penetrating radar (GPR) equipment to inspect for any evidence of leaks below the reservoir. The reservoir needs to be drained in late July so that the GPR investigation can be done when the weather is dry which is required for accurate readings. The balcony repairs will likely take place in the September to October timeframe. Water from Bull Run is not typically abundantly available until November 1, and so Reservoir 6 will remain empty until that time.