Experimental Study of Flow Through Rigid Vegetation in Open Channel

Abstract

The present study investigates the various hydraulic characteristics of vegetated open channel under both emergent and submerged flow conditions with various discharges and flow depths. Cylindrical rigid iron rods of height 10 cm and diameter 6.5 mm planted in staggered pattern in a tilting hydraulic flume with a vegetal density of 76 per unit bed area of 1 m2 are used to simulate the effect of hydraulic characteristics on vegetation in open channel. Measurement of flow velocity inside the stem/vegetation layer under emergent condition indicates that the velocity profile is one layer with almost a uniform constant velocity at all points along the path of longitudinal direction of flow. However, under submerged flow condition, velocity of flow in the surface layer above the top of vegetation is very high and follows logarithmic law. The various hydraulic resistances like vegetal drag coefficient, CD, Manning’s roughness coefficient n, and Darcy-Weisbach friction factor, f are found to be more for vegetated open channel than open channel without any vegetation. These resistance factors are found to vary with flow depths and submergence ratios whereas channel without vegetation bears a constant roughness coefficient. Under submerged flow conditions, when the depth of flow increases, values of n and f are found to decrease but values of CD are found to increase. However, under emergent flow conditions, reverse trends are noticed. Values of CD, n and f are found to increase with increasing depth of flow. Equations have been shown to predict the stem/vegetation layer velocity under both emergent and submerged flow cases. The calculated velocities are found to be close to the observed velocities indicating that the developed equation can be used to predict the flow for both emergent and submerged conditions. Further, regression based multi-linear models are developed relating to drag coefficient and various hydraulic and vegetative parameters for both emergent and submerged flow conditions and the models are found to work satisfactorily after validation with the present experimental data and data of other investigators