Document Type : Original Article
Authors
1 Department of water engineering, Lorestan University, Khorramabad, Iran
2 Assistant Professor, Department of Civil Engineering, Islamic Azad University, Buin Zahra Branch, Buin Zahra, Iran.
3 Lorestan University, Water Engineering Department
Abstract
Extended Abstract
Introduction
Compound channels are hydraulic sections consisting of the main channel and flood plains. In natural rivers, the formation of a compound cross-section is common because, during floods, a part of the river discharge is carried by the flood plains. In nature, flood plains are usually covered with vegetation that affects the flow transfer capacity in the flood plain and the main channel. Knowing the hydraulic flow conditions and the interaction of the main channel and the floodplain is necessary to protect human lives and facilities. Many experimental and numerical studies have been carried out in compound channels with vegetation (Myers et al., 2001., Hosseini, 2004., Kang and Choi, 2006., Huai et al., 2009, Proust et al., 2013., Hamidifar et al., 2013., Yonesi et al., 2014., Theoharris and Panagiotis, 2016., Hamidifar et al., 2016., Kumar et al., 2016., Shankar and Kumar, 2018., Samadi Rahim et al., 2020., Dovlati and Rezaei, 2021., Shokri and Mehdipour, 2021., Zang et al., 2022., and Samadi Rahim et al., 2023). According to the review of previous studies, most of them have been done in straight compound channels, but the numerical study of asymmetric compound channels with divergent floodplains covered by vegetation has rarely been of interest. Despite the high accuracy of laboratory studies in investigating hydraulic phenomena, high costs, limited laboratory space, and scale effects have also inclined researchers to use numerical methods. Numerical methods can be very efficient for investigating the effects of different parameters on a phenomenon by spending less time and money, provided that the numerical model results have been validated. The current research aim is to study the flow mechanism numerically in a diverging compound channel with vegetation and to investigate the relative depth change in the flow mechanism.
Methodology
For the purpose of this research, the 3D flow field in a compound channel with a divergent floodplain in two cases, with and without vegetation, at three relative depths (the ratio of the flow depth in the floodplain to the flow depth in the main channel) has been simulated. Flow3D was used and validated. Then, velocity contours, velocity profiles, flow depth, and discharge in the flood plain and the main channel for the two mentioned states were compared and analyzed.
Results and Discussion
The results showed that in both cases, by the increase in relative depth, the maximum velocity in the main channel decreased. In floodplains without vegetation, at the beginning of the divergence, the depth-averaged velocity is about 80% of the main channel one. And as it progresses towards the end of the divergence, this ratio has gradually increased. The vegetation and its resistance to the flow caused a decrease in this ratio of 30 to 60 percent. By increasing depth, vegetation creates more resistance to the flow, and the resistance vegetation effect dominated by the effect of relative depth. The amount of depth-averaged velocity reduction in floodplain due to vegetation compared to the condition without vegetation for the relative depths of 0.15, 0.25, and 0.35 was 85, 82 and 84%, respectively. Accordingly, the depth-averaged velocity increase in the main channel was 12, 25, and 30%. The investigation of the changes in the flow depth from the beginning to the end of the channel showed that for Dr=0.15, floodplain without vegetation have led to an increase of 3.7% in the flow depth in the main channel from the beginning to the end of the divergence area. The vegetation has prevented the rapid increase in flow depth, and an increase of about 0.7% occurred gradually up to a distance of about two times the length of the divergence area, and then there is a 1.5% increase in depth. At Dr=0.35, about 4.5% increase in flow depth has occurred in the main channel, but despite vegetation, the flow depth has gradually increased by 1.7%. Investigating the effect of vegetation and relative depth on the amount of flow passing through the main channel and floodplain showed that the flow in the main channel is always higher than the flow in the floodplain, and the highest amount of flow passes through the main channel in the case of vegetation. With the increase in relative depth, the amount of flow passing through the floodplain has increased while the amount of flow passing through the main channel has decreased. About 80% of the discharge has passed through the main channel and 20% through the floodplain. With the increase in relative depth, the flow through the floodplain has reached 38%. The vegetation has increased the flow through the main channel between 4 and 9 percent.
Conclusions
In this research, the flow field is simulated in an asymmetric compound channel with a diverging floodplain. The simulation is carried out at different relative depths for two states, with and without vegetation, using Flow3D. The high-velocity core of the flow occurred in the center line of the main channel and below the free surface. The quantitative value of the depth-averaged velocity difference in the main channel and the flood plain in the relative depths of 0.15, 0.25, and 0.35 in the case of no vegetation is 36, 22, and 20%, respectively, and with vegetation is 91, 89, and 90%. That is, with the increase in relative depth, the difference in depth-averaged velocity in the floodplain and the main channel has decreased, but in the case of vegetation, relative depth changing has not led to depth-averaged velocity difference, and the difference is due to the presence of vegetation. By increasing the cross-section’s width and decreasing the flow velocity, increasing the depth can be justified to keep the specific energy constant. This phenomenon continued to occur with increasing relative depth. The vegetation and preventing the flow from entering the floodplain has led to a gradual increase in flow depth at divergence region. The discharge in floodplain is always lower than in the main channel.
Keywords
Main Subjects
)