ABSTRACT
A rostral drop structure is a specialized type of vertical drop designed as a new and optimized hydraulic structure. This structure is designed to enhance energy dissipation, reduce erosion, and achieve better flow control. Considering that the natural riverbed materials are a combination of sand and clay and aiming to use bed materials that are close to the natural state of the river, clay has been used in the movable bed. The results showed that using gabion rostral drop and clay in the bed has significantly reduced scour. On the other hand, the maximum scouring depth has decreased by 40 and 16.5%, respectively, in the lowest and highest discharge compared to the case without clay. Also, the value of the relative energy dissipation parameter in the gabion rostral drop was 32.15 and 25.5%, respectively, compared to the upstream and downstream, more than the simple rostral drop. On the other hand, the comparison of simple rostral and gabion models with simple vertical drop showed that gabion and simple vertical drop models had increased the relative depth of the downstream by 67.97 and 60.46%, respectively, compared to the simple vertical drop model.
HIGHLIGHTS
The use of clay in the stabilization of the downstream mobile bed of the rostral drop for the first time.
Investigating the effect of gabion baskets on reducing scouring rate.
Investigating the simultaneous effect of gabion baskets and clay in stabilizing the downstream of the mobile bed.
SYMBOLS
Parameters | Symbol | Unit | Dimension |
Discharge | Q | m3/s | [L−3T−1] |
Channel width | B | m | [L] |
Scouring depth | ds | m | [L] |
Upstream depth | yu | m | [L] |
Critical depth | ycr | m | [L] |
Drop height | H | m | [L] |
Scouring length | Ls | m | [L] |
Drop edge angle | θ | Deg. | [-] |
Porosity | n | - | [-] |
Channel slope | S0 | Deg. | [-] |
Sediment density | ρs | kg/m3 | [ML−3] |
Gravity acceleration | g | m/s2 | [LT−2] |
Water density | ρ | kg/m3 | [ML−3] |
Dynamic viscosity | μ | Pa.s | [ML−2T−1] |
Median sediment particle size | D50 | m | [L] |
Maximum scouring depth | Dmax | m | [L] |
Upstream flow velocity | Vu | m/s | [LT−1] |
Downstream flow velocity | Vd | m/s | [LT−1] |
Time | t | S | [T] |
Equilibrium time | te | S | [T] |
Standard deviation | σ | - | [-] |
Upstream flow energy | Eu | m | [L] |
Downstream flow energy | Ed | m | [L] |
Shear velocity | u* | m/s | [LT−1] |
Parameters | Symbol | Unit | Dimension |
Discharge | Q | m3/s | [L−3T−1] |
Channel width | B | m | [L] |
Scouring depth | ds | m | [L] |
Upstream depth | yu | m | [L] |
Critical depth | ycr | m | [L] |
Drop height | H | m | [L] |
Scouring length | Ls | m | [L] |
Drop edge angle | θ | Deg. | [-] |
Porosity | n | - | [-] |
Channel slope | S0 | Deg. | [-] |
Sediment density | ρs | kg/m3 | [ML−3] |
Gravity acceleration | g | m/s2 | [LT−2] |
Water density | ρ | kg/m3 | [ML−3] |
Dynamic viscosity | μ | Pa.s | [ML−2T−1] |
Median sediment particle size | D50 | m | [L] |
Maximum scouring depth | Dmax | m | [L] |
Upstream flow velocity | Vu | m/s | [LT−1] |
Downstream flow velocity | Vd | m/s | [LT−1] |
Time | t | S | [T] |
Equilibrium time | te | S | [T] |
Standard deviation | σ | - | [-] |
Upstream flow energy | Eu | m | [L] |
Downstream flow energy | Ed | m | [L] |
Shear velocity | u* | m/s | [LT−1] |
INTRODUCTION
Drop structures are used to convert the natural slope of the land into a designed slope, with a high kinetic energy of water downstream that typically causes local scour. Combining gabion structures with drop structures can reduce energy flow and, in the presence of a movable bed, reduce scour depth. The scouring process occurs when falling jets from drop structures hit the bed and wash away materials, leading to instability. Therefore, studying gabion structures and their scour characteristics is essential. Stabilizing sediment using non-structural materials that naturally exist in the riverbed is important due to their compatibility with natural conditions, ease of implementation, and lower costs. Improving the resistance of the sediment bed by adding clay, due to its environmental compatibility, relatively high resistance, and adhesive properties, is one method of reducing scour depth. Thus, the background of the present study is presented in two sections: gabion structures and scouring investigation.
Previous studies on gabion structures have been conducted by researchers such as Kohji et al. (2007), Leu et al. (2008), and Mohamed (2010). Salmasi et al. (2011) studied the energy dissipation rate in the gabion step spillway by considering three porosities and two slopes of 1:1 and 1:2 and showed that with increasing porosity and decreasing the slope of the table, energy dissipation has increased. Tavakoi-Sadrabadi et al. (2018) in their study, calculated by considering three pores at the broad-crested gabion weirs with side slopes; they concluded that the increase in the diameter of the materials causes the increase of the coefficient of discharge and energy loss and causes the decrease of water behind the weirs. Ghaderi et al. (2019) reported that the creation of vertical constriction on the vertical drop has a favorable effect on energy dissipation and causes a reduction in the relative depth of the pool, the relative downstream water depth, and the remaining energy.
Torkaman Sarabi & Rajaei (2020) investigated the effect of stone aggregate with three scales (4, 6, and 8) cm and the number of steps (2 and 3) on the energy loss of a gabion stepped spillway. Their results showed that the increase of energy loss in the diameter of the materials is 4 cm and the number of steps is more than three. Salmasi et al. (2021) set sills in triangular and rectangular shapes at the gabion stepped weirs with the number of three steps and three slopes downstream and with three roughnesses (10, 25, and 40) mm. They stated that the performance of the rectangular sill on the energy dissipation table was better compared to the triangular sill, and in the gabion with a diameter of 10 mm with a downward slope of 1:2, the greatest effect on energy dissipation was found. Mirzaee et al. (2021) investigated the effect of the horizontal serrated edge in vertical drop on energy dissipation using a numerical method. The results show that serrating the edge causes an increase in energy dissipation compared to the simple case, and also reduces the number of edges, and increasing its dimension has a good effect on the energy dissipation and reduces the downstream Froude number. As a result, the dimensions of the stilling basin are reduced, and it becomes more economical.
Majedi Asl et al. (2021) created a gabion basket downstream of the ogee spillway, the materials of which contain stones with diameters of 1.5, 2.2, and 3 cm. Four repetitions (10, 15, 20, and 30) in the width of the base water, they reported that the increase in the height of the base water causes an increase in the table of relative energy dissipation, and the highest energy dissipation is in materials with a diameter of 1.5 cm. The face was taken, but the increase in the opening in the width of the sill caused a reduction in the relative energy dissipation. Naseri & Kashefipour (2022) in their laboratory survey, considering three porosities (35, 40, and 45) at gabion stepped weirs, showed that the most loss related to porosity was 40%. The gabion stepped weirs reduce the jump length, submergence length, and conjugate depths. Mobayen et al. (2023) estimated the energy loss in gabion spillways using evolutionary polynomial regression (EPR) and multivariate adaptive regression spline (MARS) models. Their results showed that the MARS model has higher accuracy compared to the EPR model.
Many studies have been conducted on the scour phenomenon and the presentation of empirical relationships, which have been addressed by creating breakwaters to reduce scour depth (Momeni et al. 2008; Mehraein et al. 2011; Khosravi Nia et al. 2014). Lodhi et al. (2016) investigated the effect of the sticky sediment repose on the amount of scour holes around the submerged groyne. They reported that the scour hole impressed the amount of percentage of mixed clay in the mixture of sand and clay. Aminpour et al. (2018) studied the time scale of local scour downstream of stepped spillways; they concluded that the dimensions of the stepped spillways are significantly reduced. Also, the increase in the slope of this spillway is also related to the decrease in the dimensions of the scour hole.
Mosalman Yazdi et al. (2020) in an experimental laboratory investigated and compared the characteristics of the scour hole in the overflows of the rectangular and trapezoidal piano key weirs with sand materials. The results of their investigation showed that at lower discharge, the depth of the scour hole in the rectangular model was greater than that of the trapezoidal-shaped model, and with the increase of the discharge and the water head on the tabletop, the difference in shape decreases. Also, in the three-tails water depth and discharge, the length of the scour hole in the trapezoidal-shaped model is less than in a rectangular model. Ghodsian et al. (2021) by examining the scouring downstream of triangular and trapezoidal piano key weirs in three tailwater depths with sandy materials, showed that, at lower Froude numbers, the maximum scouring depth was at the triangular piano key and a distance close to the weirs, but at higher Froude numbers, this process changed. In both models, with the increase of the Froude number, the scouring depth was found to increase.
Mirzaee et al. (2023), in their laboratory study, investigated the scouring caused by a symmetrical crossing jet on a coherent sediment bed with three percent moisture (13, 16, and 19), at three jet impact angles (45, 75, and 105). Their results showed that the increase in humidity percentage and also the increase in the angle of incidence of the jet decrease the scouring depth. Akbari Dadamahalleh et al. (2022) in the experimental study, the effect of the tailwater depth and the location of the floating objects in the form of free and floating on the scouring of the bridge pier, the results of their studies showed that with the increase of the tailwater depth, the scouring increases, also the accumulation of floating objects caused the increase of scouring, and its effect was more in the free state than in the buried condition.
Daneshfaraz et al. (2023) showed that the addition of montmorillonite nanoclay and clay reduced the scour depth by 39 and 46%, respectively, by combining clay and montmorillonite nanoclay with the materials in the channel bed. Daneshfaraz et al. (2024), by investigating the change of geometry of vertical drop edges equipped with a screen on the amount of energy dissipation, showed that increasing the angle of the vertical drop is edge equipped with a grid plate significantly increases energy dissipation. But without a screen, the highest energy dissipation of the drop is at a lower angle of the drop.
A review of previous research shows that while studies have been conducted on erosion depth downstream of hydraulic structures and sediment bed stabilization, these factors can be influenced by various modifications. Additionally, stabilizing the bed using different materials can help reduce sediment erosion. For this reason, the present study, for the first time, investigates the effect of rigid and gabion rostral drops on controlling and reducing mobile bed erosion. Furthermore, to enhance stability and minimize bed erosion, the study examines the impact of both the gabion model and the addition of clay under free-flow and submerged-flow conditions.
MATERIALS AND METHODS
Experimental set-up
The depth of the scour hole downstream of the rostral drop has been measured at specific time intervals throughout the test. Due to the extreme changes in scouring depth concerning time in the early stages of the experiment, the samples were measured at short intervals of 5 min (1 measurement), 10 min (2 measurements), 15 min (3 measurements), and finally 30 min (2 measurements). Ultimately, through trial and error, a total of 130 min was chosen as the equilibrium time. At each time interval, the water flow was stopped, and data collection was conducted until the scour hole reached a state of equilibrium. To ensure the accuracy of the obtained results, the experiments have been repeated for the number of samples mentioned and for the three times as well. After collecting the data, we used the average of the calculated data as the final data. The total length of the mobile bed was chosen to be 120 cm. This value was obtained based on trial and error in the initial experiments and at the maximum discharge so that the scouring amount would reach zero at the end. Then, the mobile bed was divided into 5 cm longitudinal intervals to provide a longitudinal scouring profile with high accuracy. To measure the length and depth of scouring within the mobile bed, after completing each experiment at different times, the scouring depth was measured at 5 cm longitudinal intervals from the beginning to the end of the mobile bed. To increase the accuracy of data collection, the scouring depth in the channel cross-section was also measured at 5 points, and finally, their average was considered as the scouring depth. The complete bed profile was prepared using plot digitizer software. In this way, high-quality images of the flow passing through the gabion and simple rostral drops and entering the mobile bed at specific times were recorded by digital cameras. At the end of each experiment, plot digitizer software was used to determine the longitudinal and depth expansion of the scour hole in the control section by taking a photo of the scour section with a camera. Also, to calibrate the points in the plot digitizer software, a very small computer millimeter label installed on the channel wall was used.
Characteristics of bed sediment particles and clay
Mechanical characteristics of bed sediments
. | D10 (mm) . | D30 (mm) . | D60 (mm) . | σ g . | Cu . | Cc . |
---|---|---|---|---|---|---|
Sedimentary bed particles | 1.3 | 1.7 | 2.1 | 1.28 | 1.61 | 0.69 |
. | D10 (mm) . | D30 (mm) . | D60 (mm) . | σ g . | Cu . | Cc . |
---|---|---|---|---|---|---|
Sedimentary bed particles | 1.3 | 1.7 | 2.1 | 1.28 | 1.61 | 0.69 |
It should be mentioned that since the calculated particle uniformity coefficient (Cu = 1.61) is less than 2, the materials can be considered uniform particles (Lamhe & Whithman 1969).
The fine-grained soil in the coarse-grained bed, which makes up nearly 10% of its total, was classified based on the tests to determine the liquid limit (LL) and the plastic limit (PL) using the standard method ASTM D4318-87 and the hydrometric test of standard ASTM D421-58 based on Table 2. The fine-grain granulation curve is presented in Figure 3. In the present research, adding clay to the bed material to stabilize it was in the form of the weight percentage of the bed material, which in this research was equal to 10%.
Clay specifications added to the bed
Group . | Plasticity index . | PL . | LL . | Specific weight . |
---|---|---|---|---|
CH | 23.3% | 26.9% | 50.2% | GS = 2.75 |
Group . | Plasticity index . | PL . | LL . | Specific weight . |
---|---|---|---|---|
CH | 23.3% | 26.9% | 50.2% | GS = 2.75 |
The research shows that the major part of the increase in soil shear strength obtained from the equation is due to the increase in soil cohesion, and the maximum value of the increase was observed at 1% by weight of clay and other chemical compounds, including Na2O, CaO, Al2O3,(Mohammadi & Niazian 2013). This phenomenon, as well as the decrease in permeability, reduces erosion. The shear strength of soil is a function of the friction angle and cementation between soil particles, and any improvement method, such as compaction or stabilization, focuses on the modification and improvement of one of those parameters. Unlike compaction methods that lead to an increase in ϕ, stabilization methods mainly emphasize improving cementation between soil particles. Weakening of the effective parameters of friction angle and cohesion reduces the shear strength of the soil. Reducing soil permeability causes less contact of water with cement materials around soil particles. While preventing the deterioration of the shear resistance, it leads to the reduction of scour length and depth compared to the permeable state.
Dimensional analysis
Effective parameters on scour downstream of rigid and gabion rostral drop.
Parameters are introduced in the symbol section along with their units and dimensions.
Dmax represents the maximum scour depth created in each run of the experiment. Each experiment along the mobile bed has different values for scour depth (ds), where the maximum scour depth is created at a section of the mobile bed length, for which parameter Dmax is used.
In the above relation, ,
, and Re represent the upstream Froude number, downstream Froude number, and Reynolds number, respectively. In order to simplify and eliminate some parameters from the dimensional analysis, considering that the flow created in the laboratory system of the present study was turbulent and the Reynolds number, which represents the type of flow regime, was in the range of 48,686 to 110,160, due to the turbulent nature of the flow in all the flow rates studied and also by referring to research (Daneshfaraz et al. 2020; Kalateh & Aminvash 2023; Aminvash et al. 2024; Kalateh et al. 2024a, b), parameter Re can be omitted from the dimensional analysis parameters because it has a constant regime.
RESULTS AND DISCUSSION
Longitudinal profile of scouring: (a) simple drop without clay, (b) simple drop with clay, (c) gabion drop without clay, and (d) gabion drop with clay.
Longitudinal profile of scouring: (a) simple drop without clay, (b) simple drop with clay, (c) gabion drop without clay, and (d) gabion drop with clay.
Variations of maximum relative scour depth vs. relative equilibrium time: (a) simple drop without clay, (b) simple drop with clay, (c) gabion drop without clay, and (d) gabion drop with clay.
Variations of maximum relative scour depth vs. relative equilibrium time: (a) simple drop without clay, (b) simple drop with clay, (c) gabion drop without clay, and (d) gabion drop with clay.
(a). Laboratory images of scouring changes in the present research of simple drop-in Q = 8.33 L/s at different times. (b). Laboratory images of scouring changes in the present research of gabion drop-in Q = 8.33 L/s at different times.
(a). Laboratory images of scouring changes in the present research of simple drop-in Q = 8.33 L/s at different times. (b). Laboratory images of scouring changes in the present research of gabion drop-in Q = 8.33 L/s at different times.
Column chart of changes in maximum relative scouring depth t/te = 1.
Variation of flow energy dissipation vs. relative critical depth: (a) upstream relative energy dissipation, (b) downstream relative energy dissipation.
Variation of flow energy dissipation vs. relative critical depth: (a) upstream relative energy dissipation, (b) downstream relative energy dissipation.
Variation of relative downstream depth vs. relative critical depth.
Variation of Froude number vs. relative critical depth: (a) simple drop, (b) gabion drop.
Variation of Froude number vs. relative critical depth: (a) simple drop, (b) gabion drop.
CONCLUSIONS
In the present study, the combined effect of two structural and non-structural methods to reduce scouring and hydraulic flow parameters was investigated in an experiment for the first time. In the present research, it was tried to use clay to stabilize the bed as part of the environmentally friendly material. Also, by changing the geometry of the vertical drop to the rostral drop, hydraulic analysis was done on the models. A simple gabion drop with a height of 15 cm and a top angle of 120° upstream and a movable bed 2.5 m long at a distance of 1 m from the drop were used. The main results obtained from this research can be classified as follows:
1- The use of gabion rostral drop compared to its simple model has shown a significant reduction in the relative scour depth in both the mixing and non-mixing states of clay. The results indicate the effect of the stone materials used on the edge of the gabion.
2- Adding clay to the movable bed caused the maximum relative scour depth to decrease by 40 and 16.5%, respectively, at the lowest and highest flow rates compared to the bed without clay. This reduction can be attributed to the coherence between the clay soil and its high shear resistance against water flow.
3- The relative energy loss in all models of the present study decreases with increasing relative critical depth. However, in the gabion rostral drop model, the flow energy loss has increased by 32.15 and 25.5% relative to upstream and downstream, respectively, compared to the rigid rostral drop model, which is due to the presence of the gabion basket and the flow passing through the porous media.
4- The comparison of the simple rostral drop with a vertex angle of 120° and the vertical drop showed that the rostral model dissipated 50.33% more flow energy than the vertical drop. This can be explained by the increased disturbance of the flow from three directions – sides and top of the drop – leading to greater energy loss.
5- On the other hand, the comparison of the simple rostral and gabion models with a vertex angle of 120° and the vertical drop showed that the gabion and simple rostral models increased the relative depth of the downstream by 67.97 and 60.46%, respectively, compared to the vertical drop model.
FUNDING
No funding was received to assist with the preparation of this manuscript.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available on request from the corresponding author.
CONFLICT OF INTEREST
The authors declare there is no conflict.