Effects of Roughness Density on the Determination of Flow Resistance in Spatially Averaged Vegetated Open Channel Flow

Abstract

The problem of prediction of resistance to flow in open channels has attracted the attention of hydraulic engineers for a long time. Over the last few decades, the Nikuradse’s concept of an equivalent sand grain roughness size has been successfully used in a wide variety of problems involving hydraulic resistance to flow. However, when it comes to practical implications such as vegetated flows, where the relative spacing, size and shape of the roughness elements are particularly important features, then the use of an equivalent roughness size becomes problematic because of the non uniformity of the boundary roughness. To overcome this problem, many researchers have investigated extensively using artificial roughnesses on the channel boundary, to predict the friction factor in rigid boundary open channel flow. By drawing analogies between skin friction and form drag, this study proposes a new friction factor function based on the roughness density. This new formula has been verified using data acquired from several flume experiments, carried out for flow fields with vegetation simulated by artificial cylinders. Results indicate that, for a given geometrical shape, the maximum friction occurs at a particular roughness density, when the bed is fully covered by large eddies. After this the flow resistance starts to decrease irrespective of the increase in roughness density. This variation should be quite similar in all the cases of geometrical shapes of roughness elements. Further this method provides a useful basis for practicing engineers to calculate the friction factor in vegetated flows, flow around bridge piers, etc. It also assists in the determination of when the maximum friction would occur, developing a significant step towards the better understanding of the resistance mechanism.