DYNAMIC MODELING OF A COMPLEX SANITARY SYSTEM IN TOPOGRAPHIC EXTREMES

The Metropolitan Sewerage District of Buncombe County, North Carolina (MSD) operates a sanitary sewer collection and treatment system that includes major components constructed between 50 and 100 years ago. The system includes nearly 800 miles of sewer lines serving the Asheville metropolitan area, with wide-ranging topographic conditions from steep mountainous terrain to extensive lowlands with little relief. During dry weather, this network carries sanitary flows, along with flows resulting from groundwater infiltration. During wet weather, flows from RDII are also introduced. The extraneous flows, extreme sewer line grades, combined with increasing development within the 221-square-mile service area, has worsened reported overflows and lowered service levels. The extensive size of the system, and accelerating maintenance needs from aging structures stressed by widely ranging flows, has lead to extraordinary demands on MSD's staff and capital resources.As MSD sought to improve system management and levels of service, it was concluded that a comprehensive, sequenced rehabilitation program would most cost-effectively evaluate the significant issues affecting system performance and identify optimal improvements. MSD contracted ARCADIS Geraghty & Miller to initiate Phase I of the Major Rehabilitation Program, which was completed in mid-1998. The Phase II Program, concluding in early 2000, includes development of a detailed computer model of the entire MSD system based on the Phase I data and findings. ARCADIS applied the DHI MOUSE model, linked with the MIKE SWMM model, to represent the overall MSD sanitary system. Initial model input was compiled through a direct link with MSD's area-wide GIS. Model results were calibrated to monitoring data collected during a temporary flow monitoring period. Through the modeling effort, improvement alternatives have been identified that include strategic application of sewer rehabilitation and upgrades, off-line storage and additional treatment capacity. These alternatives represent a significant capital savings over improvements identified under traditional evaluations.