Tacoma Power

Operational Excellence

Innovative Dam Washer


Removing moss from the face of a dam can be a dangerous and costly process for hydropower plant operators.  To address these issues, Tacoma Power employees at the 462 MW Cowlitz River Hydroelectric Project designed and fabricated a remote-controlled pressure washer to safely and effectively remove moss from the utility’s seven dams.

Before employees invented the dam washer, they removed the moss by hand. It was a long, painstaking, costly, and dangerous process.  Their Innovative Dam Washer has increased employee safety, resulted in faster and less costly moss removal, and has already been used to clean more than 500,000ft2 of concrete on three of its dams – an area larger than 10 football fields.


Tacoma Power owns and operates seven dams on four Western Washington Rivers. In the moist Pacific Northwest, moss tends to grow on the face of the large concrete surfaces. Tacoma Power needs to remove the moss every eight to ten years to facilitate dam safety inspections by the Federal Energy Regulatory Commission.


When an inspector from the Federal Energy Regulatory Commission determined that the moss on Tacoma Power’s Mayfield Dam needed to go, Cowlitz Hydroelectric Project Manager Dean McLeod posed a challenge to his employees to invent a better way to complete the task.

Tacoma Power tackled four issues as it designed and built the dam washer:

  • Increase employee safety
  • Increase efficiency of moss removal
  • Reduce costs of moss removal
  • Overcome design challenges


A hydro project mechanic and electrician combined their skills in design, fabrication and computer programming to create the remote-controlled dam washer from scratch.

A crane is used to lower the dam washer over the edge of the dam and position it. Nozzles shoot water under high pressure back and forth over the concrete and remove the moss with precision. Using the touch screen control unit, employees control the effectiveness of cleaning by adjusting the speed of the carriage that moves the sprayers across the surface of the dam and the vertical travel of the spray nozzles.  The rate of travel of the spray nozzles are slowed down for areas with thicker moss, or sped up for areas with less moss.

Tacoma Power’s invention skillfully combined steel framework, aluminum carriage, two spray nozzles, two motors (1 horsepower each) that move the spray nozzles up and the carriage, touch-screen remote control, wireless computer on dam washer that receives signals from remote control, and variable speed control unit that allows operator to control speed of spray nozzles for more effective cleaning.

Each of Tacoma Power’s seven dams has a different curvature, so the utility is working to customize modifications that will allow the dam washer to eventually fit more of its dams. To date, the device has been used to clean Mayfield Dam on the Cowlitz River, Alder Dam and LaGrande Dam on the Nisqually River, and Wynoochee Dam on the Wynoochee River.


Tacoma Power took the wet, dirty and time-consuming job of removing moss from a dam and turned it into a safe, effective process. Employees’ skills and innovation led to an invention that saves time and money. To date, the washer has cleaned more than 500,000 square feet of concrete on three of Tacoma Power’s dams so far – an area larger than 10 football fields.

  • Increased employee safety: Human safety is the most important benefit of the dam washer. Employees no longer need to be harnessed in a personnel basket and suspended up to 300 feet above a river or concrete below. The former arrangement also required separate life lines and winches for safety.
  • Increased efficiency of moss removal: The dam washer removes moss four times faster than scraping or pressure washing by hand.  It also cleans deeper, which increases the amount of time before the moss grows back.
  • Reduced time and costs of moss removal: Four people were needed to manually clean the concrete. With hand pressure washers, they were able to clean only two vertical bands of concrete four feet wide per day.  Now, two people can perform the task in about half the time.

Stakeholder quotes

“The machine strips the moss off of the dam really well in a five-foot swath. It saves having someone in a Spider basket hanging next to the dam. You’re not spinning around in the basket trying to do the work. It’s amazing… a real time saver.”

– Randy Ketter, NEAS, Contractor for Tacoma Power – Cleaned Wynoochee Dam in two weeks with the dam washer

“Cleaning moss and other organic material off the face of dams has always been a challenge considering the safety risk. The moss cleaning machine removes not only decades of moss, but also most of those safety concerns. It’s quick and effective at exposing the concrete surface for visual inspection.”

– Don Thompson, Tetra Tech, Independent Consultant