Impacts of vegetation over bedforms on flow characteristics in gravel-bed rivers

Abstract

In the present study, among six gravel-bed river reaches, two natural gravel-bed river reaches with approximately similar bedforms and flow conditions have been found, one river reach with vegetation on bedforms and the other without vegetation on bedforms. Based on field measurements, the impacts of vegetation over bedforms on flow characteristics have been investigated. Flow velocity, Reynolds stress and turbulence intensity distributions over bedforms with vegetation have been compared with those over gravel bedforms without vegetation. By using the quadrant analysis, the dominant events of turbulent flow structures have been assessed for flows over gravel bedforms with vegetation and without vegetation. Results show that the effect of vegetation enhances the anisotropy in turbulence and generates strong secondary currents, which lead to the occurrence of dip phenomenon at the stoss section and the crest of vegetated bedforms. The change of bed roughness from gravel bed to vegetated bed causes a development of a new boundary layer. This resulted in the different shape of velocity distributions over bedforms with vegetation from those over bedforms without vegetation, which can affect the lives of aquatic animals. Reynolds shear stress at downstream section of gravel bedform without vegetation is much higher than that of vegetated bedform, which shows that the erosion takes place from downstream of vegetated bedforms, while in the bare beds; the deposition is at downstream of bedforms. It is found that the presence of vegetation causes a chaotic distribution pattern of Reynolds shear stress, while it has more orderly pattern at the downstream section of gravel bedform without vegetation. Besides, the distribution of turbulence intensity in the lee side of vegetated bedform has a concave shape and in the stoss section, turbulence is generated in ejections and sweeps near the vegetation cover. These findings are significant to the practice of river ecological restoration.