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 intwo ways: it increases the equivalent sand grain roughness; and,decreases the pipe diameter. As stated by the authors, most methods usedto account for roughness growth adjust the roughness coefficients alone anddo not also decrease the pipe diameter. The authors are indeed correct in statingthat both should be adjusted; however, how this effect is quantified should beexamined. 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 infinitenumber of pairs of roughness and diameter valuesthat can reproduce observed head loss. It would take agreat deal of testing for a modeler to determine a specificpoint in any pipe. Such extra work can only be justifiedon rare occasions. Includes figures.

Product Details

Edition:

Vol. 96 - No. 10

Published:

10/01/2004

Number of Pages:

6

File Size:

1 file , 190 KB