Crested Butte, Colorado – At the 19^th Annual Colorado Association of Stormwater and Floodplain Managers conference held in Crested Butte, Colorado, September 10-12, 2008, ICON Engineering, Inc. made an entertaining presentation which incorporated a crowd pleasing video starring an animated robot and one of ICON’s own engineers. The presentation was authored by ICON’s own Craig Jacobson, PE, CFM and Steve Brown, PE, CFM. Entitled “When Good Pipes Turn Bad – Evaluation and Inspection of a Major Pipe Outfall System in Aurora, CO,” the presentation focused on the results of ICON ’s initial investigation and evaluation of a very large corrugated metal pipe (CMP) outfall system. The purpose of this project was to evaluate the existing hydraulic capacity and structural integrity of the existing storm sewer system. The study also recommended immediate and future mitigation or replacement needs. In order to accomplish the project tasks and goals, new and innovative methods were utilized throughout the project during analysis of the hydrology and pipe inspection, where a full line of robotics testing was utilized to thoroughly document the internal condition of the pipe system along with other forms of “non-destructive” testing focusing on the various areas of concern.

The City Center outfall system was constructed in 1974 and ranges in size from 84-inches to 120-inches. The overall length of the system is approximately 4,350 feet. The majority of the pipe is CMP with a paved concrete invert. This pipe system serves as the outfall for a large drainage basin within a very prominent area of the City of Aurora. In fact, the pipe system collects runoff from both the Aurora Mall and the Aurora Municipal Center located adjacent to Alameda Avenue. Using SWMM to model the hydrology, it was quickly identified that discharges had the potential to exceed the system capacity. The pipe’s conveyance capacity was further complicated by the complex interaction with the adjacent City Center Detention Facility and at Alameda Avenue where the flows split between the detention pond and outfall system are highly dependent on the tailwater conditions. At the detention pond, the outfall system had the potential to both convey water out of the pond, or “bubble-up” back into the pond. Evaluation of these complex interactions surrounding the outfall system required the development of a dynamic hydrology and storm sewer model utilizing the EPA SWMM 5 program.

Three phases of testing were also completed on the outfall pipe system to evaluate the current conditions and future integrity of the pipes. First, a full line of testing utilizing a Responder Robotic platform was completed to provide: Digital Fiber-Optic Closed Circuit TV Reports (CCTV); Laser Scans for precision measurements of the internal pipe shapes diameters, deformities, and deflections; and Gas Measurements within the pipe. The second phase included comprehensive field inspection and various “non-destructive” testing methods.  This testing focused on discolored and rusted areas that had the potential to be detrimental to the structural integrity of the pipe. Testing under this phase included: Visual inspection; Magnetic Particle and Liquid Penetrant testing; Ultrasonic Measurements to determine wall thicknesses; and Hardness measurements.  Finally, the third phase of testing included the removal and inspection of small samples of CMP material, or “coupons”, in an effort to determine the condition of the CMP material below the concrete invert.  Design of a slip-lining is currently underway.

Corrosion above the concrete invert of the 120-inch CMP City of Aurora City Center Storm Sewer Outfall.