Software Used to Model Design Flows for Sewer System Upgrade

Austin, the capital of Texas, is a college town with a population of over 700,000 and a humid sub-tropical climate characterized by hot summers and mild winters, with severe thunderstorms in the spring.

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Austin, the capital of Texas, is a college town with a population of over 700,000 and a humid sub-tropical climate characterized by hot summers and mild winters, with severe thunderstorms in the spring. The city has had problems with sanitary sewer overflows discharging during large storms into the ecologically sensitive Colorado river, which runs through the city. The interceptor sewers, dating from the 1960s and running on either side of the river, had suffered from capacity problems due to high levels of infiltration and inflow (I&I).

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The Colorado River of Texas is the 18th longest river in the United States, yet has both its source and mouth within the state of Texas.
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These issues, plus considerable projected growth for the city, sparked plans to replace the old interceptors with a new, higher-capacity system - the Downtown Tunnel. Austin Water Utility began planning the wastewater interceptor tunnel project with added impetus from the US EPA’s decision to opt for an Administrative Order requiring the central Texas utility to eliminate sanitary sewer overflows by December 2007.

The first part of the new system to be constructed was a tunnel from the South Austin Regional plant to the eastern edge of the city’s downtown area. The old pipes in the city center, with the ongoing population growth, were considerably overloaded.

The Downtown Tunnel will take flows from the interceptors on both sides of the river, running through the downtown area and extending to the Shoal Creek lift station, thereby relieving the capacity problems on both sides of the river.

Modeling

The utility’s engineers had to determine the design flows at the three shafts that connect the new tunnel to the existing system. There were three basic elements to the design flow approach. The first involved examining the basic flow data, inputting that to calibrate the hydraulic model in Wallingford Software’s InfoWorks CS wastewater systems solution, and then adding in the 50-year growth projections.

Dry weather flow data needed to be input, so a great deal of data required examination to determine a representative average dry weather flow. Fortunately the area experienced a storm extremely similar in nature to the proposed five-year design storm, with 4-5 inches of rain, which helped with the level of confidence in the wet weather calibration.

The designers then had to look at the possible future system configuration and where it was thought likely to differ from the existing system, taking into account the 50-year flows and the 2060 results for minimum, average and peak flows. As a third aspect of design flow, consideration had to be given to how three additional factors would affect future flows: the 100 year growth picture, storms larger than the design storm, and long term effectiveness of the utility’s I/I removal program.

Future configurations

Although some areas of the city are experiencing greater growth, the forecast growth rate for the central city 50-year projection was 1.5% annually. The model made exploring potential future system configurations simple. For example, it was able determine the effect on flows of removing a segment of the old 36 inch existing clay pipe system together with the downstream 48 inch North Austin interceptor sewer made obsolete by tunnel construction.

The modelers also examined a 100-year projection, the effectiveness of the Austin Clean Water program in reducing unsatisfactory intermittent discharges, and the issue of storms larger than the design storm.

The 100-year projection suggested that further wastewater system capacity might be needed if growth rates continue at the same pace as today and centralized collection is still used, and if recycling and new water sources are introduced. The 1% projected growth rate translates to a 50% increase in flows between 2060 and 2110.

The success of the $200 million clean water program will be achieved by embracing an entirely new way of running the wastewater utility. This will involve running the utility with more attention and more resources, rather than the old ‘build and forget’ approach common in the past. Past I&I reduction efforts had little success, but it is hoped that current initiatives and improved utility operations will bring success and at a minimum enable the city to prevent increases in I&I levels.

Service levels will also be improved, and when the current round of projects is completed, flow monitoring will be undertaken to gauge the success in reducing unintended discharges.

Further work

Modeling a 10-year storm showed a 10% to 15% increase in flows in the network, so the final tunnel and shaft sizing process, which is due to be undertaken in the near future, will factor this in as a ‘capacity cushion’ as well as other considerations such as the cost impacts of larger or smaller capacity tunnels. Looking at large storms and getting a better understanding of system response is a modeling challenge for the future.

Modeling based on current system flow characteristics has been found to yield a good prediction of 50-year flows, which are the minimum that need to be accommodated. Taking the 100-year growth picture and larger storms into consideration suggests a need to provide greater capacity. It is hoped that the investment to be made in 2010 will be seen as a wise investment in 2060 and 2110.

Editor’s note:

This article was adapted from a paper by Tom Ellison, Principal Engineer, Austin Water, and Joe Smith, PE, Austin Clean Water Program, presented to the 2007 Wallingford Software North American User Conference.

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