Stormwater | Project Profile

Massive flooding in Fort Collins, CO, received national exposure in July 1997 on all major television networks. At the time of the storm, the city had already initiated a project called the Locust Street Outfall to reduce flooding in the Old Town storm drainage basin. The goal of the project was to eliminate the 100-year floodplain surrounding 132 homes and 16 commercial structures. The solution to the flooding was phased reconstruction of nine blocks of residential streets to install a large-diameter precast concrete storm sewer system that discharges into the Poudre River. Because the sewer runs through the 100-year-old neighborhood crammed with conflicting utilities, engineers specified gasketed joints in the storm sewer to limit infiltration and exfiltration of stormwater.

Andrea Faucett, water resources engineer for Sear-Brown, states, "There is no reason to believe that the storm sewer installed in the Locust project will ever be eliminated. Therefore, it had to be built to last." Concrete pipe was specified because of its ease of installation and longevity. Other types of storm sewer material were investigated, but installation would have been more demanding. The simplified installation process allowed the contractor to install an average of eight units of concrete pipe per day, barring any unexpected interruptions.

Construction of the storm sewer began with installation of 108-in.-diameter reinforced concrete pipe (RCP), a 16- x 5-ft. reinforced concrete box (RCB) sewer, an 8- x 5-ft. RCB sewer, and 84-in. and 108-in. storm sewer pipe placed in trenches ranging from 12 to 20 ft. deep.

The intersection of Whedbee and Locust is referred to as "the sump" where the historical drainage of the Old Town basin and the pipeline meet. The main feature in the area is a low street intersection requiring the 8- x 5-ft. RCB and a transition structure to connect downstream to the 16- x 5-ft. RCB sewer and another transition structure farther downstream to the 108-in. pipe. At the lowest point of the intersection, there are eight inlets to intercept flows from the north, east, south, and west sections of the city. In addition to surface flows, two existing storm sewers are intercepted and connected where the 8- x 5-ft. structure transitions to 16- x 5-ft. structure.

The larger RCB reduced the risks associated with installing a cast-in-place conduit and resulted in considerable savings to the City of Fort Collins. The precast option also facilitated moving excavation construction (supported with a trench box). This construction technique allowed vehicular access during construction, residential parking, use of front yards, and a significant acceleration of the construction schedule. The precast option was a major public-relations boost for the project.

Grimm Construction presented the RCB option to Carder Concrete Products Company, which proposed joining the 16- x 5-ft. RCB to a 108-in.-diameter RCP transition and continuing upstream with the 16- x 5-ft. RCB. This approach solved a major hydraulic problem associated with the original design, as it made the transition from the 16- x 5-ft. box sewer to the 108-in. RCP in-line. The in-line transition and the 45 bends considerably improved the hydraulics of the project.

Close coordination among Carder Concrete staff, the contractor, and the engineer were key in acquiring quick approvals from the city. Internet technology also played a role. Using e-mail, Carder Concrete was able to respond to requests quickly and be proactive in regard to the numerous product submittals, quality control, production, and delivery of the various components. Weekly electronic reports tracked how many RCP joints and RCB units were in the yard, outlined which special components were complete and which were in fabrication, and approximated the dates that specials needed farther upstream would be fabricated so that Grimm Construction was never delayed by a lack of pipe or pipe specials.

The $7 million Locust Street Outfall drainage improvements project substantially improved the storm sewer systems in the Old Town drainage basin. In addition to the new infrastructure, citizens of Fort Collins are equipped with a comprehensive flood warning system, which began operating in June 1999. When a major storm hits the city, its effect will be much less of a hardship because of the foresight of the city council in the early '90s to prepare for such an event.

SW November/December 2003

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