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Denver Water Takes a Deep Dive at Gross Dam

When your workers live and work 270 feet underwater for weeks on end, safety first becomes more than just a slogan. Denver Water’s Dam Safety Engineering Manager Douglas Raitt spoke with Hydro Leader about the unique challenges involved in a recent project to replace an underwater trash rack. He also talked about how his organization prioritizes maintenance projects to keep dams— and workers—safe. 

Hydro Leader: Please tell us about your background and how you came to be in your current position. 

Douglas Raitt: I am the engineering manager in the dam safety section of Denver Water. I have been with Denver Water for about 12 years and have more than 35 years of experience in heavy civil construction, working on a variety of large projects in transportation and water resources. For the past 5 or 6 years, I have concentrated on collection systems such as dams, canals, and outlet works. That’s my primary area of interest. Water is the most important resource on earth, and the one most important to the city and county of Denver, so it’s easy to be passionate about this work. 

Hydro Leader: Please tell us about Denver Water. 

Douglas Raitt: Denver Water is a municipal enterprise that is independent of the City and County of Denver. It has its own board of water commissioners and has been a separate entity since the early 1900s. Our principal business is to supply water to our approximately 1½ million customers in the city and county of Denver and in some of the surrounding areas. 

Our collection system covers roughly 4,000 square miles across the state of Colorado, on both sides of the Continental Divide. About half our water comes from the western slope of the Continental Divide over to the eastern side through various canals, diversions, and tunnels. It is a large system with quite a few reservoirs—some large, some very small. There are four treatment plants, a big recycling system, and a large distribution system that extends throughout the area surrounding Denver. The Denver metro area is growing quickly, but our watershed is fixed. Therefore, we have been emphasizing conservation and making sure our system is maintained. That’s quite a challenge with a system that has many components that are over 100 years old. 

Hydro Leader: Please tell us about your background and how you came to be in your current position. 

Douglas Raitt: I am the engineering manager in the dam safety section of Denver Water. I have been with Denver Water for about 12 years and have more than 35 years of experience in heavy civil construction, working on a variety of large projects in transportation and water resources. For the past 5 or 6 years, I have concentrated on collection systems such as dams, canals, and outlet works. That’s my primary area of interest. Water is the most important resource on earth, and the one most important to the city and county of Denver, so it’s easy to be passionate about this work. 

Hydro Leader: Please tell us about Denver Water. 

Douglas Raitt: Denver Water is a municipal enterprise that is independent of the City and County of Denver. It has its own board of water commissioners and has been a separate entity since the early 1900s. Our principal business is to supply water to our approximately 1½ million customers in the city and county of Denver and in some of the surrounding areas. 

Our collection system covers roughly 4,000 square miles across the state of Colorado, on both sides of the Continental Divide. About half our water comes from the western slope of the Continental Divide over to the eastern side through various canals, diversions, and tunnels. It is a large system with quite a few reservoirs—some large, some very small. There are four treatment plants, a big recycling system, and a large distribution system that extends throughout the area surrounding Denver. The Denver metro area is growing quickly, but our watershed is fixed. Therefore, we have been emphasizing conservation and making sure our system is maintained. That’s quite a challenge with a system that has many components that are over 100 years old. 

Hydro Leader: How many of the dams produce hydropower? Do you market the power yourself, or do you sell it to another entity? 

Douglas Raitt: We’ve got seven hydro locations, including dams and transmission tunnels with hydro units. Most of them are small facilities that supply power to the global grid. We generate more electricity than we use. Because our system is supplied from the mountains, most of our distribution system is gravity powered. We have several pump stations in the distribution system, but we rely on gravity to power our transmission conduits and some of the distribution systems. Because topography is in our favor, our energy usage is relatively small compared to that of other systems. 

Hydro Leader: What is Denver Water’s overall approach to dam safety? 

Douglas Raitt: Within the engineering department of Denver Water, which has about 175 people, there is a dam safety section that monitors all our facilities to make sure they comply with current state and federal regulations. Several of our facilities are licensed with the Federal Energy Regulatory Commission (FERC). Also, the Colorado State Engineer’s Office regulates all the dams in our system and subjects them to a regular inspection program. We also carry out continuing maintenance operations and work closely with the operations folks to make sure that maintenance projects are prioritized. Anything we find during inspections that requires attention gets put into our maintenance program, perhaps even as a distinct project if the work is substantial enough to require an engineered design. We work with regulators to make sure that all those projects meet requirements and fit into our capital expenditure program. This ensures that they are funded and executed not only to our satisfaction but to the satisfaction of the regulators. 

Hydro Leader: Please tell us about the Gross Dam expansion project. 

Douglas Raitt: Gross Dam was put in service in 1954. Many of the systems that were put in place in the 1950s worked fine then, but over the years they have aged, and the regulations and requirements for these types of facilities have evolved. Over the years, there have also been some modifications to Gross Dam. Our latest projects were maintenance oriented and aimed to bring us into compliance with current regulations. An upstream control slide gate was added to the primary outlet works intake in 2003, and a 7.6‑megawatt hydroelectric powerhouse was added to the system in 2007. Last winter, we replaced a section of penstock pipe that was in the original configuration, with the inlet control downstream of the dam. 

Our recent trash rack project was necessary to comply with the current seismic regulations, which have changed since the 1950s. We had a 70‑year-old structure that worked fine as a trash rack, but like the outlet works, it was not in compliance with current regulations, specifically seismic requirements. As we were studying the dam for a future dam-raise project, we identified the immediate need to bring the trash rack into regulatory compliance. 

Hydro Leader: Absent changing regulations, would you still need to replace the trash rack after a certain number of years? 

Douglas Raitt: Gross Reservoir has a relatively small upstream watershed of about 80 square miles. Sediment is an issue for dams and intakes, but runoff from our 80‑square-mile watershed has a limited sediment load and has not caused excessive sediment accumulation. There was some sediment around the trash rack, but no appreciable effect on operations. With the flows into the outlet works, it was, to some extent, self-scouring. Although the structure itself was a concrete frame with steel grating and no corrosion control, it was holding up quite well because it was in a noncorrosive environment. Really, there was nothing operationally wrong with the trash rack. The need to comply with current seismic requirements is what drove us to design a replacement, take out the old trash rack, and replace it with a new and compliant structure. 

Hydro Leader: Please tell us about the process of that replacement. 

Douglas Raitt: AECOM was the principal designer for this feature. It teamed up with Stantec on the site for various other improvement projects. The design team looked at the existing structure as it was designed in the 1950s. We used a remotely operated vehicle to inspect the structure and look at the sediment. We had last inspected it 18 years ago, in 2003, when we installed a gate on the outlet works and made some modifications to the trash rack for the slide gate for upstream control. The design team had a good amount of information to start with. The team created a new design for the trash rack. We put a lot of thought into the design to make it simple to install. When you’re working with divers at that depth, simplicity is important. We had one of the qualified contractors involved in a constructability review during the design process and got good input on how to arrange the connections and allow for existing concrete surface irregularities. We worked with our regulators, FERC and the Colorado State Engineers’ Office, to get through the design process. We put proposals out to a small number of contractors that we trust for this kind of work. Once we got proposals back, we made decisions based primarily on the qualifications of the team and its planned approach. The last criterion was cost. Both candidates were competitive—both teams had a good understanding of the work and the pricing was fair. We ended up choosing Global Diving & Salvage. 

Hydro Leader: Have you worked with Global on previous projects? 

Douglas Raitt: I have not personally worked with Global, but the company was on our shortlist because of its previous work with Denver Water on some other facilities. The company actually installed the gate on the Gross Dam primary outlet works intake in 2003, so the staff had experience at this location. That gave them a leg up when it came to their understanding and ability to describe a work plan. 

Hydro Leader: Would you talk more about the logistics of what the divers did during this project? 

Douglas Raitt: With conventional air diving, once you get down to 100 feet, your work time is only about a half an hour due to the decompression time required for coming back to the surface. This work required the divers to work continuously for many weeks at a depth of 270 feet. The only way to accomplish this was with saturation diving. Saturation diving is like going to space. Global put four divers into a pressurized environment in which they were constantly breathing a special combination of helium and oxygen called heliox. They worked, ate, slept, and showered down there. They have chambers that allow them to bring in fresh food from the contractor’s floating galley, but to some extent, the life of a saturation diver is like that of an astronaut. 

The new penstock Denver Water installed downstream of the trash rack in the Gross Dam primary outlet works system.

Two divers work a double shift while the other two rest. The two divers who are working go down in a diving bell to the work zone with all their tools. The first works half a shift with the underwater chainsaw, chisels, drills, and the other tools it takes to disassemble a reinforced concrete frame structure. A crane supporting the underwater work is positioned on the barge and hoists all the heavy loads in and out. The diver puts shackles together and wrestles with all the rigging at depth. 

All the while, the diver is in communication with his dive tender, who is up top in the dive shack. A 20‑foot-long container contains all the systems required to monitor the health and safety of these divers working at depth. The water is around 40 degrees, so the divers wear special wetsuits with a hose that provides hot water to keep them comfortable. The divers have a breathing gas supply line and a communications link that sends video from their diving helmets to the umbilical connected to the barge. The people up top are with them 100 percent of the time. One person is watching breathing gas and another is watching the operations through the helmet camera. The communication requires an acquired ear, because the heliox mix makes the divers’ voices high pitched, like Donald Duck’s. 

The divers went into the saturation chamber on September 22, 2021, and got out on October 29. The last 4 days were for decompression. It’s a tough job that requires specialized skills. 

Hydro Leader: What does Denver Water have to do to ensure that this is all done safely? 

Douglas Raitt: As a facility owner, we want to make sure that the work is done as safely as possible. To accomplish that, we hire the right people. There are only two companies that we would trust with this work. Everything that can be done to make it a safe operation is done, but it is still risky. Our precautions don’t eliminate the anxiousness, though. If something goes wrong at 270 feet, there are ways to deal with it, but you don’t want to ever have to get there. 

Hydro Leader: Now that the divers have come out of the saturation chamber, is the project done? 

Douglas Raitt: The underwater work is done, but the paperwork and demobilization remain. Part of the successful completion of a project like this is gathering all your records. We have video, we have measurements, and we have an accurate quality record of what was constructed that shows that it was done correctly. Now, we are compiling all that and preparing reports for FERC and the state engineer, because we don’t want anyone to have to go back down there for another 70 years. When this project was designed, we were relying on documents from the 1950s. We want to leave an even better record for those who follow us, because every component of this dam system requires maintenance. 

Hydro Leader: The Gross Dam project ties into a lot of regulations. What is your message to the federal and state governments and to relevant agencies? 

Douglas Raitt: Dam safety regulations have evolved over time based on things that haven’t gone well, including seismic events, great rainfall events, and occasional dam failures. There have not been many failures in the United States, but it can happen when heavy rain hits a dam that hasn’t been maintained as it should. Dam owners, state regulators, and federal agencies all have the same goal: perfect dam safety. Not many new dams are being built, and the ones that are in place need to be maintained. This trash rack project or the previous outlet works project are all about maintenance. Owners have to be willing to set aside enough budget to maintain these facilities, especially high-hazard dams. That’s our job as dam owners and operators. Requirements must be realistic. Owners and operators provide quite a bit of feedback to the regulators to make sure that the rules make sense and are achievable. We have an excellent relationship with the Colorado State Engineer’s Office and the FERC staff in San Francisco and Washington, DC, who we report to on this project. It’s a good system, and it’s there to protect the public while we’re delivering water. 

Hydro Leader: What is your vision for the future of Denver Water? 

Douglas Raitt: We have a storage project in front of us at Gross Reservoir. We’re raising the height of the existing dam to increase its storage capacity. The permitting has been in progress since 2003, and I think we’re going to be able to start work in 2022. We’re going to increase the reservoir’s storage capacity from 42,000 acre-feet to just over 118,000 acre-feet. We want to meet the needs of our growing population and to be prepared for the uncertainty that comes with dry periods, whether you call it climate change, a drought, or a dry spell. Rainfall and snowpack fluctuate, and we need to be prepared for the worst. If you run out of your water supply, you have failed your customers. 

Increasing storage is one of the flags I like to wave. Raising a dam is a new approach that we can take instead of building an entirely new dam. It’s a way to solve the problem of water storage with fewer environmental consequences. I think we will need more storage in the future, but new storage facilities are getting harder to permit, and it is getting harder to find sites, because all the easy ones have already been taken. Building new facilities is becoming more challenging and more costly. The storage we’re building now is much more costly than it was 50 years ago. That affects rates and customers’ bills, but at the same time, everyone expects water to come out of their faucet when they turn the handle. That’s the challenge going forward. 

Douglas Raitt is the engineering manager of the dam safety section of Denver Water. He can be reached at douglas.raitt@denverwater.org. For more information about the Gross Reservoir expansion, visit grossreservoir.org.