Flow structures and sedimentation characteristics around clump-type vegetation

Models comprising 7, 19, 37, or 61 equally spaced circular cylinders in a staggered hexagon-shape arrangement were mounted on a water-flume bed, and the characteristics of flow structures in uniform flow around the models were investigated to elucidate the effects of number, density, and the emergent or submerged condition of the cylinders as a model of clump-type roughness. Drag force was measured with changing both the relative optical gap (G/D) and relative height (h/H), where G is the optical gap between neighboring cylinders in cross-section, D is the cylinder diameter, h is the model height, and H is the water depth, and the drag coefficient Cd was calculated. The flow structure around the clump-type roughness models and the drag coefficients changed greatly with changing G/D. The number of cylinders in the clump-type model greatly affected the value of Cd when G/D was large, because the sheltering effect is changed with the number. Flow structures changed around G/D > 1. The Cd in the emergent condition became slightly larger than that in the submerged condition but was not much changed in comparison with the G/D. Sedimentation around and behind vegetation was investigated in the field after a flood event. When the vegetation density was high (G/D < 0.5–1), such as with willow (Salix subfragilis) or dense grass (Eragrostis curvula, a clump-type vegetation), sedimentation occurred behind the vegetation region. When the vegetation density was low (G/D > 1), such as with Robinia pseudoacacia or Phragmites japonica, it occured inside the vegetation region. The flume experiment and field study indicate that the flow pattern changes depending on the G/D with a wide range of Reynolds numbers and that it affects the sedimentation pattern at an actual flood event.