Cover Solution Extends Reservoir Life

Styles come and go, and sometimes reappear. Take hats, for example. What was fashionable in the ’60s went out in the ’70s, but it may come back a few decades later. However, when it comes to municipal water facilities, yesterday’s model is gone for good, replaced by a more contemporary, sustainable design. 

Just ask the Marin Municipal Water District in northern California, which recently transformed a 120-year-old potable water reservoir into a 4-million-gal tank with a maintenance-free roof structure.

The Problem

The district serves 187,500 customers in central and southern Marin County with a mission “to manage [our] natural resources in a sustainable manner and to provide [our] customers with reliable, high-quality water at a reasonable price.”

One of the district’s seven reservoirs sits atop Forbes Hill in the western section of San Rafael and receives potable water from three different treatment plants.

According to Carl Gowan, the district’s principal engineer, the reservoir’s floating cover and liner were showing signs of age.

“In 2005 we considered building a new tank on the site because we had the available space,” Gowan said. “But a few years later we concluded it would be more efficient to simply replace the liner and cover.”

Gowan cites several issues with the old liner and cover:

• The cover’s uneven surface encouraged pooling of rainwater, increasing the risk of non-potable water leaking into the clean water supply.
• Exposure to sunlight caused the cover to crack, allowing dust and debris to gather.
• The aging structure needed continual maintenance, which came at a cost.

The district brought in consulting engineer Beyaz & Patel, which evaluated a number of options for a replacement cover: steel, aluminum, clear span and column-supported.

“We were looking for a cost effective solution requiring little or no maintenance,” Gowan said. “And it had to be aesthetically pleasing to the community around the reservoir.”

The Solution

After reviewing its options, the district went with the consultant’s recommendation of an aluminum column-supported roof: a custom-crafted OptiDome from CST Covers, a division of CST Industries.

“OptiDome is a next-generation aluminum geodesic dome,” said Hector Moreno, regional sales manager at CST Covers. “Its innovative and optimized design has features that make it an increasingly popular choice for architectural, environmental, municipal and petroleum and other applications.”

Before installation could take place, the project team had to overcome some unique challenges. First, the reservoir replacement project had to be completed within the six-month winter shutdown period. This gave the contractor a relatively narrow timeframe to demolish the existing roof and liner, prepare the reservoir floor for the installation of column footings and install the new roof and liner. Not only was the contractor racing the clock, but it also had to deal with the reservoir’s unusual oval shape and determine the existing conditions by field survey.

“We had to make sure everything was properly surveyed so that the roof could be designed to match the as-built conditions,” said Gary Ho, senior project engineer at Beyaz & Patel. “During preconstruction, the contractor conducted a survey before

CST could start the shop drawing. Most importantly, having a current survey helped us determine where to place the roof supports. Once the supports were set, CST went in and completed the final drawings.”

An important consideration during design was the elevation of the roof to meet seismic standards. An earthquake causes the water to slosh, so special care was taken to place the roof high enough and at a proper slope to mitigate the threat of water coming in contact with the underside of the roof.

“It was a team effort between CST, the contractor and the construction management team,” Ho said. “We all worked together to make sure the survey was properly conducted and the design was done correctly to conform to the reservoir’s as-built condition.”

CST manufactured the panels, battens, fasteners, gaskets and I-beams at its Conroe, Texas, facility and then shipped them to the site, where they were assembled.

“We effectively converted a reservoir into a tank, so we renamed it Forbes Hill Tank,” Gowan said. “And we accomplished it for a fraction of the cost of building a new tank.”

The OptiDome is a versatile solution because it is applicable to a wide variety of shapes and sizes. It is built to withstand extreme conditions and meet stringent performance requirements:

• Its seamless, flush-batten construction increases water shedding and eliminates ponding, and it is watertight.
• Its enclosed gaskets are less susceptible to ultraviolet exposure and degradation.
• Its strength-to-weight ratio supports the weight of work crews and stands up to heavy snow loads and high winds.
• Its lifespan of 50 to 100 years makes it a good investment, with low lifecycle costs.
• It requires virtually no maintenance.

“This was a unique challenge because the reservoir is oval, and to meet the geometrical shape, primary supports had to be outside of the reservoir,” Moreno said. “Our engineering team, along with the consulting engineer, did a fantastic job in coming up with the most optimum and aesthetically pleasing design.”

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The Results

For Marin Municipal Water District – and any water district, for that matter – the value proposition is irresistible.

“The new cover will extend the useful life of the reservoir, provide increased protection for the water supply, improve worker safety and reduce maintenance,” Gowan said.