The Different Impact of a Half-Separated Gravel and Vegetated Bed in Open Channels

Abstract

This paper investigates the different impact of a half-separated gravel and vegetated bed in open channels. This experimental investigation is based on the 2D Particle Image Velocimetry (PIV) method, which allows data acquisition at fine spatial-temporal resolution. For the simulation of the two porous beds : a) a grass-like vegetation and b) gravel bed of 2 cm height were used to represent the permeable bed. Velocity is measured : a) at the central line of 3.75 cm above the interface of vegetated and gravel bed, b) over the vegetated bed and c) over the gravel bed for different discharges and for the same different total heights. In total, 27 experiments were carried out. Results show that the presence of a half-separated gravel and vegetated bed influences significantly the velocity distribution in comparison with a simple permeable bed (vegetated or gravel). This is due to the fact that the presence of gravel bed in the half-separated bed decreases the velocity of the flow in regard to the simple vegetated or gravel bed due to the great roughness which is observed near the surface gravel-vegetated bed. This fact increases the turbulence and reduces the velocity. Also, the presence of half-separated gravel and vegetated bed influences the values of velocity distribution in a different way in comparison with situations over of half-separated impermeable and permeable bed.