Experimental investigation of Slope Change Impact on Manning’s n in a tilting bed flume

Abstract

This paper presents the findings of laboratory investigations on a tilting bed flume to determine the impact of bed slope on Manning’s roughness coefficient, n (Manning’s n). Correct estimation of Manning’s n is crucial in the relatively accurate approximations of velocity and discharge. While various studies have examined factors affecting Manning’s n, limited research has assessed the direct impact of bed slope.  In this study, a tilting bed flume with a uniform graded 12.5 mm gravel bed was used to investigate the variation of water depth along the flume length at various slopes. Using the discharge computed by the volumetric method at the collection container and water depth measured at three points within the flume, the cross-sectional area, hydraulic radius, velocity, and Manning’s n were computed at three different slopes: 0.04, 0.09 and 0.13. It was found that the average water depth, cross-sectional area, and hydraulic radius decreased, whereas the velocity and discharge increased as the bed slope increased. The results revealed that Manning’s n increases by 50% as the slope increases from 0.04 to 0.09. However, the value of Manning’s n decreased when further increasing the slope to 0.13. These findings revealed conclusively that Manning’s n is not independent of the bed slope. Therefore, a factor of slope should be incorporated alongside other aspects to determine a more accurate value of Manning’s n.