Unidirectional Flushing Program
City of Tracy, California

West Yost developed a new unidirectional flushing program for the City to assist in reducing customer complaints, improving water quality, and extending the longevity of their water system infrastructure. The project included developing an optimized flushing program using the City’s hydraulic model, creating a flushing sector map and flushing loops for the distribution system, developing field crew mapbooks, conducting and evaluating results of a pilot test on select areas, and developing a system-wide flushing program.

Prior to implementing a full scale program, West Yost worked with the City to establish a pilot program for flushing that would determine the cost per mile of pipeline flushed, and provide metrics for postponing and/or eliminating capital improvement program facilities as a result of improved system performance and longevity from flushing. The pilot program incorporated both worst case and best case loops (or system areas). Testing in worst case areas provided information on challenges and allowed the team to assess the extent of any critical repairs. Testing in best case areas allowed field crews to “road test” the mapbook procedures. In addition to providing needed information for gauging full scale system flushing costs, benefits, and staff requirements, the pilot program in Tracy also identified the need for additional time to exercise valves, and that preforming pre-flushing field reconnaissance was critical. Pre-flushing field reconnaissance allowed crews to mark hydrants, clean valve boxes, exercise valves, and identify any potential problems, including valves and/or hydrants that showed up on the system maps, but could not be located in the field.

West Yost developed seasonal diurnal curves to determine the normal flow paths and direction of water movement in the system under typical seasonal demands. Having accurate diurnal curves effectively puts the “uni” in unidirectional flushing, assuring that the flushing program is moving clean water into dirty areas (and not vice versa). The City’s system was first divided into sectors that sequenced the movement of water from clean to dirty areas. These sectors were then divided into loops that were sized for completion by a flushing crew in one day. Field notebooks were developed for staff providing step-by-step specifics for each loop, including color coded maps and test sheets which were valuable in providing feedback, so common issues could be identified and modifications could be made.

Following the initial flushing program, West Yost also assisted the City in incorporating GIS information through their InfoWater® model. This new tool was instrumental in simplifying the administration of the program and reducing the time required to plan, administer and track program results. The model provides automatic selection of which isolation valves to close, runs sequences and calculates results, prints location specific graphics, and provides integrated and detailed field reporting sheets virtually with a push of a button.