Flume experiment for evaluation of effect of different bed conditions on Manning's roughness coefficient in open channel flow

Abstract

The direct measurement of stream bed surface roughness in terms of roughness coefficients remains a difficult task. The variation in Manning's roughness coefficient (n) occurs due to many contributing factors, which shows necessity to determine n for different bed materials. This study was carried out with objective to determine Manning's roughness coefficient for different sizes of bed materials using selected discharges and bed slopes. Experiments were conducted in a hydraulic flume by creating different bed conditions under controlled conditions in the laboratory. Discharges were varied from 0.088 m3s-1m-1 to 0.038 m3s-1m-1 with varying bed slopes from 0.25% to 2%. The value of Manning's roughness coefficient was found to be increasing with the increasing size of bed material and bed slope for every discharge. A mathematical relationship was developed in terms of bed slope, discharge and gravel size with help of the experimental findings. Different types of performance indicators viz., coefficient of determina- tion (r2) and coefficient of efficiency (CE) were used to evaluate the accuracy of the developed model. The results indicate better agreement between observed data and predicted results for the newly developed parameterized equation.