Effectiveness of trees, shrubs and grasses in reducing flow velocity near riverbanks in humid tropics.

Abstract

The aim of this research project was to study and assess comparatively the effectiveness of riparian vegetations in reducing flow velocity near riverbanks in the humid tropics. To achieve this goal, six vegetation treatments (Salix Purpurea and Sesbania sesban) as shrubs; (Pennisetum Purpureum and Pennisetum Macrourum) as grasses and finally (Syzigium guineense and Saccharum officinarum) as trees. The Saccharum Officinarum was taken as a tree because of its diameter and rigidity but normally, it is classified as a grass. One bare bank treatment was to serve as a control treatment. This assessment was achieved by measuring velocity profile data points in three sections: one section at 1m upstream of the vegetated section, second in it and the third at downstream of it 1m away. In each section, nine profiles were collected: 3 perpendiculars to the bed and six slanted profiles perpendicular to the bank of the channel. The study was conducted in field artificial trapezoidal flume (25m long, 2.9 m top wide, 0.8 m bottom wide, bed slope of 0.0003 and 1.2 m deep). The flume got water from diverted natural river “Meti” by means of a weir dam. Data were collected with help of Son Tek’s HorizonADV software (v1.20) installed in a computer and a 10MHz ADV, instrument manufactured by SonTek Inc.. They were post processed with WinADV32 (v2.027) software of Bureau of Reclamation (US. D.I) and post processed results were manipulated using Excel. From post-processing, several parameters were calculated from velocity measurements to facilitate results judgement. These include two main variables: Turbulent kinetic energy (TKE) of the turbulent part of the flow and Reynolds shear stresses. All treatments were carried out with incoming water of the same hydraulic properties: average discharge of 0.122 m3/s, streamwise velocity of 0.066 m/s, water depth of 1 m, Reynolds number of 124732 (>4000, turbulent flow) and Froude number ranging of 0.027(<1), subcritical flow. The results showed that Salix Purpurea, Pennisetum macrourum and Sesbania sesban are most effective species to reduce water velocities near the bank. Almost all vegetations showed power in reducing lateral shear stresses responsible for bank erosion except Saccharum officinarum and Syzygium guineense less effective because of their big diameters compared to the remaining species. It was also seen that the main reason that makes species more effective is its leave density. Less leafy species do not increase roughness that impact on velocity. In all treatments, Pennisetum macrourum were tested with high stem density of about 256 stems/m2. Due to time constraints, it was not easy to play with variation of stem density, diameters, plantation patterns and water levels to get much insight in the matter. This is what future research can work on for better understanding.