جستجوی مقالات مرتبط با کلیدواژه "relative depth" در نشریات گروه "آب و خاک"

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.

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.

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.

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.

Vegetation in the flood plains is effective on the turbulent flow of compound channels, especially in flood conditions where the flow depth changes. Numerical simulation is very useful in investigating the effect of vegetation on turbulent flow. In the present study, the numerical simulation of the flow in the diverging floodplain of compound channel in two cases with and without vegetation at different relative depths has been done using the Flow3D numerical model. The results showed that the flow velocity in the main channel is always higher than that of floodplain. The collision of the flow with the vegetation has led to a decrease in the velocity in floodplain compared to the state without vegetation. Investigation of turbulence parameters showed that the highest value of shear velocity occurred at the common border of the main channel and the floodplain, but with the increase in relative depth, the value of shear velocity and shear stress decreased. Also, at a fixed relative depth, the turbulence intensity is the highest near the walls and the lowest in the middle parts of the main channel. The investigation of the downstream of the divergence area showed that in the case of vegetation, the turbulence intensity is the highest in the center line of the floodplain channel and near the free surface, while in the absence of vegetation, the highest turbulence intensity occurred near the bottom of the floodplain.

Rivers are known as the main sources of surface water in the world, which experience seasonal fluctuations in water level. These resources have severe damage to human societies and nature in flood conditions and have irreparable consequences in the drought seasons. Optimal utilization of these resources with maintaining the environmental conditions of the waterway and minimizing flood damage is considered one of the river engineering goals. Since the conventional methods of river management are imposed serious environmental threats on waterways and wetlands, consideration to these water resources requires attention to issues related to plant ecosystems, solving challenges of coastal bed erosion and predict the condition and management of the river in the future (Callow, 2012; Dawson and Haslam, 1983; Fan et al., 2013; Rose et al., 2010; Rowinski et al., 2018). One of the strategies that cause loss of flow energy in the river improves the hydrological system and river ecosystem is the presence of vegetation in the river banks and floodplains. Native vegetation in floodplains and coastal forests plays an important role in conserving waterway ecosystems, flood management, coastal protection in urban lands and agriculture adjacent to the river (Fathi-Moghadam, 1996). Vegetation will also control the width of the river and increase the stability of the shores by absorbing and settling suspended sediments in river banks. The plant species along rivers and waterways are composed of various vegetative components, mainly affected by the environmental conditions of their habitat, including the distance from the waterway bed, hydrological characteristics of the river, climatic and soil conditions. Obviously, the effect of each plant species in the ecosystem cycle varies and for each section of the river, a specific combination of plants will create optimal conditions.

In this research, using FLOW3D software, the flow velocity distribution and shear stress of the bed in meandering compound channels due to the change of floodplain width and relative depth were investigated. For this purpose, four sections of meandering compound channels with floodplain width having 3.3, 4.31, 5.32 and 6.33 meters and three relative depths of 0.26, 0.35 and 0.45 have been used. The results of numerical simulation show that by increasing the floodplain width, the depth averaged velocity and bed shear stress are reduced. So that with a 92% increase in floodplain width, the maximum flow velocity in the main channel decreases by 24% , and the depth averaged velocity at relative depths 0.45 and 0.26 are reduced by 17% and 21%, respectively; and the rate of change of depth averaged velocity is more noticeable due to a change in the width of floodplain for low relative depths. The effect of changing the floodplain width on the bed shear stress has the highest value in the mid-section between two apexes (CS3 section); so that by 92% increase in the floodplain width in the mid-section (CS3), the bed shear stress is reduced by 35%. Also, the amount of boundary shear stress in the inner arch wall in all channels which is higher than the boundary shear stress in the outer arch wall; and the maximum amount of wall shear stress occurs near the bankfull level of the main channel and by increasing the relative depth, the wall shear stress increases.

The rivers, as the main watercourse and natural drainages, always play a significant role in the conveyance of flood flows. During floods, the water crosses the main section of the river and enters the floodplains. In this case the river crossing becomes a compound cross section. In present research work, by studying meandering compound channels, the effect of changing relative depth of flood currents on the hydraulic flow conditions and the rate of discharge are investigated. For this purpose, six channels with different sinusoidal rates at three relative depths with different flood rates were investigated by FLOW3D software. The results of numerical simulation show that by increasing relative depths from 0.26 to 0.45 (73% increase), the depth averaged velocity in all channels increased by 25% and the rate of discharge passing through the main channel decreased by 33%. Also, the results of this study show that the bed shear stress near the inner arch of the main channel is more than the outer arch and by reducing the relative depth in the compound channel, the amount of bed shear stress and flow velocity decreases. And the amount of shear stress of the inner arch wall of the main channel in all channels is higher than that of the outer arch wall, and by increasing the relative depth the amount of shear stress of the wall is increased.

One of the important aspects in describing river behavior is its curvature. The production of successive bends in natural rivers is an inevitable component of the process through which the river changes. Understanding the flow behavior in the interaction between the main channel and the floodplain, especially during floods, is necessary to protect soil and water structures. This study was conducted to better understand the hydrodynamics of the middle and turbulent flow in compound meandering channels. According to the natural conditions of most rivers, due to variation of discharge and depth ratios (flow depth in floodplain to flow depth in the main channel), in this study, a rectangular meander laboratory channel with a constant sinuosity 1.3 for different depth ratios of 0.35 and 0.55 was investigated. The results showed that the size of velocity components (u,v,w) at 0.35 relative depth was greater than 0.55, which  indicating higher vortex strength and interaction intensity between the main channel and floodplain at lower relative depth. Also, by investigating the secondary flows, the existence of clockwise and counterclockwise rotational eddy currents in the main channel and floodplains and the location of these currents were determined.