Live Oak, Calif., needed to overhaul its sewers. Some of the pipes, dating back to 1952, were cracking and collapsing, and the system could not handle even normal flow conditions. With only a limited budget to rehabilitate the infrastructure, the city used a CAD-based sewer model to assess repair alternatives and select the most cost-effective solution to its problem.

The city had determined that rebuilding the entire sewer system would cost $9 million - far more than it could afford. To investigate alternatives to reconstruction, two years ago, the city contracted with Eureka, Calif.-based engineering firm Winzler & Kelly to create a sewer model.

Using Hydra software from Seattle-based Pizer, the model simulated the entire sewer system to illustrate the variations in flow through the system. The main goal was to address the infiltration problem and pinpoint the areas that were most damaged and could not handle the capacity.

The model was created based on an AutoCAD drawing of the existing collection system. That drawing was then updated to include the system pipe network and service areas. Three pump stations and the force mains also were entered into the model. The simulation's flow rates were adjusted to match recorded flows.

An analysis of the model showed that, even under dry weather conditions, several pipes were at a capacity of 85 percent or greater. A second model was created to account for population growth over the next 10 years. That model showed flow surcharges throughout the entire system.

Keeping cost restrictions in mind, city officials used the model to determine the areas in which repair would have the greatest impact, Hickey says. The city has outlined a triage-type approach to repair work, which will include replacing some old pipes, sealing cracks and redesigning some portions of the system to bypass bad sections of pipe.

Modeling work was completed in February 1999. Live Oak officials are applying for federal grants and loans to help fund the repair work.