VARIATION OF THE ACCURACY OF MANNING'S FORMULA WITH PIPE EQUIVALENT ROUGHNESS HEIGHT

Abstract

The accuracy of simplified friction loss relationships such as Manning's formula depends on the pipe material but the relationship has so far not been obtained. This paper develops and then presents in tabular and graphical form, the variation of the accuracy of head loss, pipe diameter and discharge estimates obtained by the Manning's formula, with pipe equivalent roughness height. Thus, once engineers in practice, have an idea of the type of pipe material to be used, they will be able to determine if accuracy of the Manning's formula will be suitable for the application they have on hand.

The results show that within the range of flow of practical interest in the water supply industry, accuracy of head losses, diameters and discharges estimated with the Manning's formula improves with increasing roughness. They can be estimated within factors of 3.71. 1.28 and 1.93 respectively for equivalent roughness height of 1 µm. These factors decline to 1.22, 1.04 and 1.10 respectively for equivalent roughness height of 1 cm.