AWWA JAW60541

In "Modeling for Hydraulic Capacity" (Boxall et al, April 2004) the authors' key finding is that growth of roughness elements in pipes affects pipes in two ways: it increases the equivalent sand grain roughness; and, decreases the pipe diameter. As stated by the authors, most methods used to account for roughness growth adjust the roughness coefficients alone and do not also decrease the pipe diameter. The authors are indeed correct in stating that both should be adjusted; however, how this effect is quantified should be examined. The relationship depends on the type of flow, the flow velocity, and the diameter of the pipe. In general, a 1-mm (0.04-in.) increase in roughness has a much greater effect on head loss than a 2-mm (0.08-in.) decrease in diameter. Although a relationship between roughness (or Cfactor) and diameter reduction exists, there are an infinite number of pairs of roughness and diameter values that can reproduce observed head loss. It would take a great deal of testing for a modeler to determine a specific point in any pipe. Such extra work can only be justified on rare occasions. Includes figures.