Effects of submerged tropical macrophytes on flow resistance and velocity profiles in open channels

Abstract

In this paper, the results of flume experiments are presented, conducted to investigate the effect of different submerged flexible plant species on flow resistance and flow patterns in open channels. Three common plant types in tropical regions were selected: Cabomba caroliniana, a quite flexible fully submerged species; Echinodorus grandiflorus, a somewhat less flexible, fully submerged species; and Nymphaea rubra, a rooted species with floating leaves. For each plant type, three different densities were tested for various discharges, water depths, and depths of plant submergence. The flow resistance was expressed in terms of the commonly used Manning coefficient n. The results indicate that among the particular plant species investigated here, the floating leaf species N. rubra exhibits the lowest flow resistance (Manning's n = 0.014), viz. less than half the values for C. caroliniana and E. grandiflorus, respectively. A detailed analysis showed that this is due to the particular plant morphology, consisting of relatively large biomass (floating leaves) with only small stem density. It was observed that the flow resistance not only strongly and inversely depends on the Reynolds number, but also on the degree of plant submergence (i.e. water depth/deflected plant height ratio), and on the density of the vegetated plant beds. It appears that the biomass/drag force ratio seems of less importance. From the measured velocity profiles, the influence of the various types of vegetation species could clearly be observed. These factors are important for the design and management of vegetated open channels in tropical urban areas like Singapore.