YANG Kai-lin, GUO Yong-xin, FU Hui, GUO Xin-lei (China Institute of Water Resources & Hydropower Research, State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, Beijing 100038, China)

Abstract: The average roughness height k of pipe is a key parameter in hydraulic calculation of pipe systems while it is also an important index for evaluating the manufacturing qualities of the same type of pipes made of different manufacturers. For a new type of pipes, the first thing from the view of point of hydraulics is to calibrate the value of k in hydraulic labs precisely to predict the capacity of water conveyance in the design of urban water-supply and drainage systems. The authors of this paper did the systematical experiment to calibrate the values of k for the three types of pipes. The results illustrate that the value of k found by the Colebrook equation varied significantly with the change of Reynolds number Re. This is a contradiction with the fact that the value of k should be a constant for the given pipe within a short time. One of the reasons is that there are the uncertainties in the measurement parameters. This paper presents the equations for calculating the uncertainties of the roughness coefficient, average roughness height and the flow rate of suppressed sharp-crested weirs, on the basis of the uncertainties of the measurements of pipe diameter, length and head loss or piezometer head as well as width, height and head above crest level of weir for flow rate. Then, how to calibrate k reasonably is investigated by the systematical analysis of the uncertainties for the measured parameters.

Key words: pipe, roughness height, calibration and uncertainty

Published in: Journal of Hydraulic Engineering, Vol. 43, No. 12, 2012