|Cherokee County, in Georgia USA, is growing fast. Prospering from the economic strength of nearby Atlanta, the rapid population and business growth of the last decade places strains on all utilities, including water distribution and wastewater collection. The Cherokee County Water and Sewerage Authority (CCWSA) currently serves 50,000 water customers and 21,000 sewer customers, and the numbers are increasing.
|.||CCWSA is a recent user of hydraulic modeling software, but they have been tracking suitable packages for a number of years. Jeff Hooper, GPS/GIS and Sewer Model Manager at CCWSA explained: “We've known for a few years that the time would come when we needed models to evaluate and plan any additions required to our infrastructure as a result of the frequent building developments in Cherokee County. With this in mind we've tracked the products at conferences and exhibitions and decided our best options were InfoWorks WS for water distribution and InfoWorks CS for wastewater collection.”|
Before starting to build the models, CCWSA is very meticulous in mapping their physical network as accurately as possible. Their surveys of the distribution system and collection system use advanced technology with four fixed GPS reference stations from which roving GPS receivers can locate network assets to within 1 centimeter accuracy over the entire 430 square miles (1100 sq. kilometers) of the county. This level of accuracy, higher than that typically used for GIS location data, gives CCWSA an added level of confidence in their models.
Further accuracy comes from using SCADA measurement of the reality of what is happening in the network, rather than simply the listed rating of assets. For example, the model uses SCADA data on the actual throughput of water at a pump rather than the rating of the pump, eliminating potential errors if the pump works above or below its stated level.
With confidence that the model of the network is very close to reality, there is little need to manipulate calibration factors to match the model to observed network performance. InfoWorks has standard roughness factors for pipes based on material and age of pipe, and at Cherokee Country these have almost always produced good model results.
Jeff Hooper is a firm believer in putting this high level of accuracy into the model build. “The alternative is to build a model that is a little removed from reality – maybe the falls on the pipes are not quite accurate, or a pump rated at 900 gallons per minute is working at 850. Correcting these inaccuracies with fallacious roughness factors, trying to impose a fit that does not really exist, is clearly compounding errors and moving the model even further from reality.”
An example of the value of modeling came when a local developer applied for permission to build a school, commercial development and 300 new homes. The initial GIS output showed that the development would have little impact, but Jeff Hooper's team undertook a GPS survey of the outfall lines at the location and input these details to the InfoWorks CS model. Within half an hour it was evident that the development would have a significant impact, and that raw sewage would overflow under certain conditions. The model also helped in presenting the results visually to the CCWSA Board, who saw the need to re-locate and increase the size of the sanitary sewer line.
At present both the InfoWorks models are under construction. Approximately 10% of the water distribution network has been input into an InfoWorks WS model, and the CS model is further ahead, containing 25% of Cherokee County's 20,000 manholes in its sanitary sewer network, including pipes down to 8 inches (20cm). The models are already producing valuable results for the areas covered. “We're pleased with our progress on both models”, states Jeff Hooper “and even in these early days they are delivering very useful results. We have a lot of proposed developments, both homes and businesses, and the evaluation of their impact used to be a laborious process. The models undertake these evaluations with great accuracy in a fraction of the time.”