**Department of Civil and Hydraulic Engineering, Indian Institute of Science, Bangalore-12.
***Department of Civil Engineering, Birla Institute of Technology, Mesra, Ranchi.
*Extracted from the thesis of the 2nd author submitted in partial fulfilment of the requirements for the award of the M.E. Degree (Hydraulics and Hydraulic Structures) of the I.I. Sc., Bangalore.
This paper presents the results of experimental investigations carried out at the Hydraulics Laboratory of the Indian Institute of Science, Bangalore, on some aspects of composite roughness for steady non-uniform flow in open rectangular channels.
Einstein’s method of evaluating the effect of side friction based on Manning’s flow equation for uniform flow has been applied to the case of steady non-uniform flow which revealed that Einstein’s method can be applied in general and with some error for coarser surfaces. Various existing formulae for evaluating the equivalent V value have been put to test of which Sasikanth and Sarma’s equation appears to be most suitable. On lines similar to those of Strickler’s constant arrived at from actual observations made in Switzerland and using the roughness height as the median sieve size of the material, studies have been made taking the roughness height as the ‘equivalent sand roughness’ based on Nikuradse’s closely packed sand grains and modified to open channel problem by Keulegan. As a result of this analysis, the relation n =6√k/28.52 has been established. Finally the concept of Boyer and others to evaluate Manning’s ‘n’ from velocity measurements for a wide rough channel has been put to test.
