On the Job: Stormwater Modeling System Tackles Slippery Slopes

14 Nov, 2004 By: Cadalyst Staff Cadalyst

The city of Manizales, at an elevation of 7,050 ft in the Colombian Andes, benefits from having perfect conditions for growing world-class coffee. The region's steep slopes (figure 1) allow for average daily sunlight to strike the coffee plants at a near 90-degree angle, improving the growth of the plants. That same geography, however, makes it quite difficult to construct a combined sewer system.

Figure 1. The same steep hillsides that allow Manizales, Colombia, to grow great coffee present major obstacles when developing a combined sewer system.

More than half the city's sewer system contains pipes at slopes greater than 10%, and in some extreme cases in excess of 50%. This topology, combined with the heaviest rainfall intensities in Colombia, frequent seismic activity, and small budgets, challenges local engineers to look for innovative solutions.

University Students Assist Project
Aguas de Manizales, the local water utility, is working with the National University of Colombia to develop a storm sewer model, using StormCAD from Bentley's Haestad Methods product line (figure 2). Students are using StormCAD to analyze the city's combined sewer system and to evaluate low-cost rehabilitation strategies to reduce high-flow velocities and constant overflows in the system.

Figure 2. StormCAD displays storm drains in Manizales that have grades exceeding 50%.

"Aguas de Manizales supports the university, allowing students to work on real-world projects to broaden their learning experience," said Carlos Vasquez, the director of the project for Aguas de Manizales. "StormCAD is a powerful tool that is easily picked up by the students, and their projects allow us to build a complete combined sewer model of the city."

In the San Luis creek basin, heavy rainfall results in frequent CSOs (combined sewer overflows) that exceed Colombian sanitary regulations. Pipes that collapse due to seismic activity further aggravate the situation. The nearly constant CSOs have generated a permanent cesspool, with all the associated negative environmental and social impacts. Andres Hoyos, a student at the National University of Colombia, was assigned to develop a StormCAD model of the San Luis creek basin to study this problem and develop a remedial plan.

Aguas de Manizales possesses an extensive geospatial mapping of the water and drainage systems of the city, so Hoyos could complete the model-building process quickly using StormCAD's shapefile connection utility. Aguas de Manizales also has a wealth of good field data obtained during a recent monitoring project. Using this data, Hoyos was able to calibrate the model under various loading conditions. "Excellent agreement between modeled and observed results helped to reassure the city that sound decisions could be made based on the model," Hoyos says.

Identifying Problems
A series of simulations using the calibrated StormCAD model identified that the system was under capacity, even for a five-year event. These simulations also revealed that this poor performance was not only due to low-capacity pipes but was compounded by the poor design of a diversion structure, which separates flow between two adjacent subcatchments.

Once the problem areas were identified, scenarios were developed in StormCAD to investigate various proposals for the rehabilitation of the main collecting pipes. Because of the difficult terrain and geological conditions, slopes of the existing system could not be easily modified without a major construction investment. For this reason, special restrictions were created for StormCAD's automated design utility, maintaining the existing structure depths and pipe slopes and only allowing changes to the pipe diameters in certain areas during the optimization runs.

"Our geological and topographical conditions make deep excavations cost-prohibitive," said Hoyos. "The flexibility of StormCAD allowed us to investigate a wide range of alternatives, thus guaranteeing the most economic option for the rehabilitation." Through detailed analysis, optimized design runs, and easy scenario comparison features in StormCAD, the city of Manizales is well on its way to overcoming its difficult local conditions for combined sewer systems, and returning to the business of simply being a city that produces some of the best coffee in the world.

Haestad Methods

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Cadalyst Staff

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