Study of Velocity over Submerged Rigid Vegetation with random Non-Uniform Height in Open Channels

The roughness of the open channels bed and the walls have a great impact on the hydraulic characteristics of the flow, and to perform the best management and control tasks, the change in the behaviour and characteristics of the flow in the presence of these roughness should be known well. Vegetation is one of the most important and common roughness, which have various types and characteristics. One of the most important characteristics is their degree of flexibility, which are usually divided into two general categories: rigid and flexible. From the past until now, researchers are always trying to predict the effect of the presence of vegetation on the flow path and to provide more accurate relationships for predicting different characteristics of the flow, including the flow velocity. The vegetation studied in this research is rigid and inflexible, and to bring the vegetation conditions close to their natural state, its height distribution is considered non-uniform and random.

Experiments were carried out using a rectangular flume with a length of 10 m, a width of 0.5 m and a depth of 0.6 m. At the downstream end of the flume, a rectangular weir with an adjustable angle was installed to adjust the flow height inside the channel and the vegetation submergence ratio. Vegetation models used in the experiments include stainless steel cylinders at three heights 3.5, 5 and 7 cm, which are used to simulate hard plant stems. During the experiments, the submergence ratio of the cylinders has been considered variable. The diameter of the cylinders is 5 mm. The distribution of the cylinders is staggered, where the longitudinal and lateral distances are the same, but the height distribution is completely random, and Mathematica software was used to generate random numbers to determine the location of each cylinder. The distance between the cylinders and, as a result, the vegetation density per unit area is variable and in total three different densities were considered for stems. The flow discharge varied between 5 and 60 lit/s and an ultrasonic flowmeter was used to measure it. Velocity measurement started from 4 m from the channel entrance. A 3D ultrasonic velocimeter (ADV) with a frequency of 25 Hz was used to measure the point velocity. The flow is assumed to be uniform and steady for all experiments.

The distance between the cylinders and, as a result, the vegetation density per unit area is variable and in total three different densities were considered for stems. The flow discharge varied between 5 and 60 lit/s and an ultrasonic flow meter was used to measure it. Velocity measurement started from 4 m from the channel entrance. A 3D ultrasonic velocimeter (ADV) with a frequency of 25 Hz was used to measure the point velocity. The flow is assumed to be uniform and steady for all experiments.Analytically using the momentum equation in the layers with vegetation and the logarithmic law in the layers without vegetation. The velocity profile is presented in this research, unlike the uniform distribution, has two inflection points whose position depends on the vegetation density. To determine the CD, a special plant Reynolds number (Re*) has been introduced, and as this Reynolds number increases, the drag coefficient decreases. An empirical relationship between CD and the effective parameters is presented. The effect of different dimensionless parameters that were extracted through dimensional analysis, was investigated on the flow velocity and the results show that with the increase in the flow Reynolds and the Froude numbers, the velocity increases in all the flow layers, but the change of these numbers has no effect on the position of the inflection points of the velocity profile. With the increase of the vegetation density, the flow velocity in the vegetation layers and the upper layer decreases and increases, respectively. By increasing the submergence ratio of the cylinders for a constant flow rate, the flow velocity decreases in all three layers, but the effect of the submergence increase on the flow velocity is different for the flow layers, so for example, with a 30% increase in the submergence ratio, the average velocity decrease in the vegetation layer is 10%, however, shows a decrease of 25% in the upper layer. As the results showed, with the change of vegetation conditions, many hydraulic properties of the flow changed, so this field still needs to be discussed. The present study was conducted on hard vegetation that has a random height distribution with regular intervals. In future studies, researchers can study natural vegetation with random intervals and bring their laboratory conditions closer to the conditions of the vegetation distribution in natural riverbeds.