This green-roofed reservoir supplements drinking water supplies with gravity-fed rainwater, important because Logan, Utah, has no secondary water source. Photo: Stanley Consultants
Utah's State Drinking Water Board had demanded that Logan city officials up reservoir capacity by 1.5 million gallons to keep up with rapid population growth. The city's existing reservoirs were ancient and leaking badly. Rather than buy land for a new reservoir, the city opted to revamp the existing one. Unfortunately, the public works staff didn't have the time or expertise to perform the work themselves.
The city's request for proposals outlined three principal challenges:
- The shape and size of the site—600 feet long and 80 feet wide
- The site's layout—steep mountains on the east, a private golf and country club to the west and north, and storage tanks on the south
- The need to store water from spring flow, supplemented by well flows when necessary.
Muscatine, Iowa-based Stanley Consultants presented a plan that boosted capacity, saved space and money, resulted in virtually no downtime, and beautified the site.
The design team minimized the footprint by incorporating turbulent jet mixing in the reservoir's four tank cells, minimizing “dead spots” where sediments and chlorine concentrations could accumulate and increasing the cells' flexibility. Designed by Stanley Consultants project manager Jeff Masters, the mixing system includes a nozzle at the end of inlet piping to create turbulent flow, allowing for turnover of stored water, reducing chlorine requirements, and improving water quality. A newly designed pipe system connects all four cells so operators can use one cell, all four at once, or any of the cells in combination. Throughout construction and installation, the existing cells had to remain in service.
Opting for precast double-T concrete beams to support the roofing structure eliminated the need for about 50 support columns. Also, by deciding against pour-in-place concrete, they avoided having to take time to erect support scaffolding and wait for the material to cure.
The city also placed a great deal of importance on aesthetics. Reservoirs are not known for their overwhelming beauty, and the proximity to both a trail and a golf course dictated that the structure be as “pretty” as possible. Stanley's design team incorporated a green-roof structure on the reservoir's deck. The roof consists of a 60-mil ethylene propylene diene monomer membrane from Indianapolis-based Firestone Building Products; protection board; geotextile fabric from Fullterton, Calif.-based Triumph Geo-Synthetics Inc.; topsoil; and wildflowers. Also, the west face of the concrete structure features a patterned, textured surface to better complement the natural surroundings.
For financing, the city used capital improvement funds and a construction bond from a state revolving fund administered by the State Division of Drinking Water. Even with the challenging design features and the need to keep the old tanks online during the process, the project managed to come in on time and within 3% of budget.
Any concerns about the project's success disappeared as soon as the old water tanks were disconnected from the new structures. The amount of water delivered by the springs to the tank site compared to the amount of water in the system, known as “delivery efficiency,” jumped from 54% to 89%. This proved that the old tanks had been leaking an astounding 3 mgd.
Project: Logan Replacement Reservoir
AEC firm: Stanley Consultants
Cost: $6.2 million
Project delivery method: Design-bid-build
Size: 5.65 million gallons
Wow factor: The green roof helps the structure blend in with the neighboring country club and nature trails
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