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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.

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  • 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.

energy dissipation, gabion, rostral drop, scour
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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] 
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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.

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Experimental set-up

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Experiments were applied in a rectangular channel with a length, width, and height of 5, 0.33, and 0.5 m, respectively, in the hydraulic laboratory at the University of Maragheh. To have a smooth surface with minimal roughness, the walls of the channel are made of Plexiglass. The channel flow is provided by two pumps, each with a maximum flow rate of 7.5 L/s. These pumps are connected to a small reservoir at the channel entrance. The flow rate is measured with two taps connected to two rotameters installed at the pump outlet. Upstream of the channel, a simple gabionic rostral drop with a vertex angle of 120° and a height of 15 cm has been used. The percentage of porosity of the gabionic drop was equal to 35%. The length and thickness of the sedimentary bed were determined experimentally according to the flow characteristics after the screens. In addition, the scouring value was obtained from the initial test. The initial test showed that the scour hole length increases to 1 m at high flow rates. Also, the hole depth advances to floor level. The bed thickness was 12 cm, accounting for the appropriate depth to reduce scouring due to flow. To create a sedimentary bed with a specific thickness, two smooth polyethylene sheets were used. The length of the first floor is 1 m from the toe of the drop (the stilling basin area), and the length of the second floor is 40 cm, which is installed at a distance of 3.5 m after the rostral drop, as shown in Figure 1.
Figure 1
Schematic view of laboratory channel and rostral drop used.
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Schematic view of laboratory channel and rostral drop used.

Figure 1
Schematic view of laboratory channel and rostral drop used.
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Schematic view of laboratory channel and rostral drop used.

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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

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Equations presented by Yalin & Yalin (1971) were used in order to determine the diameter of sediment particles. According to the research of Raudkivi & Ettema (1983), to prevent the formation of ripples in the alluvial bed of the channel and also to eliminate the cohesion of sediment particles, the average diameter of the particles should be larger than 0.7 mm. If the standard deviation of sediments obtained from the equation is less than 1.3, the material is uniform, and if it is greater than 1.3, the material is non-uniform. Scouring depth in non-uniform sediments is lower than scouring depth in uniform sediments, and scouring depth decreases as the standard deviation of sediments increases (Melville 1997). To achieve uniform granulation in the sediments used in the experiments, the granulation of particles is usually expressed in the form of the weight percentage of the sample passed through standard sieves. In this study, sand with an average diameter of 1.8 mm and a standard deviation of 1.28 was selected. In this case, both maximum scouring is obtained and ripples are prevented. The granulation curve of bed sediments is presented in Figure 2 and the mechanical characteristics of bed sediments are presented in Table 1. For the second and third tests, about 90% of the sediment particles of the sand bed left from sieve #16 with the granularity specified in Figure 2, and the rest of the fine particles mixed with clay with the granularity specified in Figure 3 are presented.
Table 1

Mechanical characteristics of bed sediments

D10 (mm)D30 (mm)D60 (mm)σ gCuCc
Sedimentary bed particles 1.3 1.7 2.1 1.28 1.61 0.69 
D10 (mm)D30 (mm)D60 (mm)σ gCuCc
Sedimentary bed particles 1.3 1.7 2.1 1.28 1.61 0.69 
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Figure 2
Granulation curve of bed sediments (coarse grain).
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Granulation curve of bed sediments (coarse grain).

Figure 2
Granulation curve of bed sediments (coarse grain).
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Granulation curve of bed sediments (coarse grain).

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Figure 3
Granulation curve of bed sediments (fine grain).
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Granulation curve of bed sediments (fine grain).

Figure 3
Granulation curve of bed sediments (fine grain).
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Granulation curve of bed sediments (fine grain).

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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%.

Table 2

Clay specifications added to the bed

GroupPlasticity indexPLLLSpecific weight
CH 23.3% 26.9% 50.2% GS = 2.75 
GroupPlasticity indexPLLLSpecific weight
CH 23.3% 26.9% 50.2% GS = 2.75 
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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

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Dimensional analysis was done by using the Π-Buckingham method and the effective parameters in the present research are presented in a dimensionless form based on Figure 4 and the following equation:
(1)
Figure 4
Effective parameters on scour downstream of rigid and gabion rostral drop.
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Effective parameters on scour downstream of rigid and gabion rostral drop.

Figure 4
Effective parameters on scour downstream of rigid and gabion rostral drop.
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Effective parameters on scour downstream of rigid and gabion rostral drop.

Close modal

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.

H, n, θ, S0, B, and have a fixed value, so it is ignored. Considering the parameters yu, g, and ρ as repeated parameters and also making significant and dividing some parameters by each other, the dimensional analysis is presented as follows.
(2)

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.

After making some parameters meaningful, we will have:
(3)
In a similar method for parameters of ΔE/Eu, ΔE/Ed, and yd/H the dimensional analysis is in the form of Equation (4).
(4)
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Figure 5 shows the longitudinal profiles of scour in different models of the current research, which include the scour of the mobile bed downstream of simple and gabion drops in the presence and absence of clay at a flow rate of 5 L/s for all the times taken. Carefully in Figures 5(a)–5(d), it can be seen that in the simple rostral drop model with a movable bed without clay, the highest amount of scour occurred during the equilibrium time, which was about 3 cm, but this is while adding 10. The weight percentage of clay to scour bed materials has decreased by about 1 cm during its equilibrium. On the other hand, the results have been different in gabion rostral drop. The use of the gabion model has been more effective in reducing scour compared to the simple rostral drop. In the gabion model without clay, the amount of scour is lower than in the simple drop model with similar characteristics, and on the other hand, with the addition of clay in the gabion model, we can see a great decrease in scouring depth at all times, as well as scouring equilibrium time. With more reviews of longitudinal profiles, conclusions can be drawn that in the models of simple soffit breakers, after creating a scour pit, the harvested materials are piled up like a hill and create a chute. However, this phenomenon does not appear in gabion models due to the decrease in speed and dissipation of flow energy inside the gabion basket, which is clearly shown in the diagrams.
Figure 5
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.
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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.

Figure 5
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.
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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.

Close modal
Figure 6 shows the changes in the maximum relative scour depth against the relative scour equilibrium time in all investigated discharges for the simple and gabion escarpment drop models. It is carefully deduced in Figures 6(a)–6(d) that in all the studied discharges, the amount of scour has reached the equilibrium value at the end times and the scour depth has been assigned a constant value. Figures 6(a) and 6(b) show, respectively, the changes related to the relative scour in the model of a simple drop without clay and a combination of bed materials and clay. By further examining the charts, it can be seen that the model in which a percentage of clay is combined with the substrate material has a lower scour depth than the case without clay. In the model containing clay, the maximum relative scouring depth has decreased by 40% and 16.5%, respectively, in the lowest and highest discharge compared to the case of bed materials without clay. Figures 6(c) and 6(d) also show the changes related to the relative scour in the gabion rostral drop model for the state without clay and the bed materials containing clay, respectively. Carefully in these figures, the gabion effect of the rostral drop can be observed. The use of gabion baskets and the creation of internal currents have increased the energy loss of the flow, and as a result, the decrease in the flow speed has reduced the relative scour depth downstream. Meanwhile, the addition of clay to the downstream materials in this type of drop has greatly reduced the scour depth. So the decrease in the lowest and highest discharge for models containing clay was 60 and 23%, respectively, compared to the case without clay.
Figure 6
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.
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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.

Figure 6
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.
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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.

Close modal
Figure 7 shows experimental images of scour change in different models of the study at a discharge rate of 8.33 liters per second. Figures 7(a) and 7(b), respectively, display, from top to bottom, the changes in scour in the movable bed from the initial time to equilibrium for the simple and gabion rostral drops. According to Figure 7, the scour depth in the gabion models and clay bed models is significantly less than in other models, as shown in these images. The reasons for this behavior are explained in detail in subsequent sections of the present study.
Figure 7
(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.
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(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.

Figure 7
(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.
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(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.

Close modal
Figure 8 shows the column diagram of the maximum depth of relative scour at the time of relative balance t/te = 1. It is clear in this chart that the depth of the scour has decreased relatively due to the use of gabion drops. On the other hand, in all studied models, the use of clay has caused the adhesion of sedimentary materials and increased shear strength, which has reduced the depth of scour downstream of the drop structure.
Figure 8
Column chart of changes in maximum relative scouring depth t/te = 1.
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Column chart of changes in maximum relative scouring depth t/te = 1.

Figure 8
Column chart of changes in maximum relative scouring depth t/te = 1.
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Column chart of changes in maximum relative scouring depth t/te = 1.

Close modal
Figure 9(a) shows the changes in relative energy dissipation with respect to relative critical depth in the rigid and gabion rostral drop and compares it with the results of Daneshfaraz et al. (2022). Careful examination of the figure reveals that the energy dissipation in the rigid and gabion rostral drop is higher compared to the study by Daneshfaraz et al. (2022) on the vertical drop. This indicates the effect of the rostral drop edge on the relative energy dissipation. Additionally, the comparison of the results for the vertical drop (control condition) shows good agreement with the findings of those researchers. Figures 9(a) and 9(b) show the changes in flow energy dissipation relative to upstream and downstream, respectively. It can be seen carefully in these figures that in all the current research models, with the increase of the relative critical depth, the relative energy dissipation of the flow has taken a downward trend. This result is that in the gabion rostral drop, the relative energy loss increased by 32.15 and 25.5%, respectively, compared to the upstream and downstream, more than the simple rostral drop. On the other hand, comparing the results obtained from the present study with the study of Daneshfaraz et al. (2022), it was observed that the relative energy loss of the flow increased by 31.25% compared to upstream. The reason for this is that when the current passes through the gabion basket, some of the current is dissipated in this way, and another amount of energy is dissipated due to the formation of a hydraulic jump. On the other hand, by comparing the simple rostral drop 120° angle with the vertical drop, it can be seen that the rostral drop dissipates 50.33% more flow energy than the vertical drop. The reason for this is that in the rostral drop, the flow falls from the three directions of the sides and the top of the drop to the downstream side, and this is a factor for further increasing the energy loss of the flow.
Figure 9
Variation of flow energy dissipation vs. relative critical depth: (a) upstream relative energy dissipation, (b) downstream relative energy dissipation.
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Variation of flow energy dissipation vs. relative critical depth: (a) upstream relative energy dissipation, (b) downstream relative energy dissipation.

Figure 9
Variation of flow energy dissipation vs. relative critical depth: (a) upstream relative energy dissipation, (b) downstream relative energy dissipation.
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Variation of flow energy dissipation vs. relative critical depth: (a) upstream relative energy dissipation, (b) downstream relative energy dissipation.

Close modal
Figure 10 shows the changes in the downstream relative depth against the relative critical depth. With the increase in the flow discharge, the relative depth of the downstream has increased. In the gabion rostral drop model, the downstream relative depth is a little higher than the flow depth in the simple rostral drop model. So the relative depth of the downstream in the rostral gabion model is 33.33% higher than the simple rostral drop. On the other hand, the comparison of the simple rostral and gabion models with a vertex angle of 120° with the vertical drop shows that the gabion and simple rostral drop models have increased the relative depth of the downstream by 67.97 and 60.46%, respectively, compared to the vertical drop model.
Figure 10
Variation of relative downstream depth vs. relative critical depth.
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Variation of relative downstream depth vs. relative critical depth.

Figure 10
Variation of relative downstream depth vs. relative critical depth.
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Variation of relative downstream depth vs. relative critical depth.

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Figures 11(a) and 11(b) show the variation of the Froude number over the relative critical depth on the simple and gabion drops, respectively. According to these figures, it can be seen that per-flow inlet discharge, the flow regime was in the form of subcritical, so the table of the Froude number was in the range of 0.034 to 0.054 m. Also, it can be confirmed in the figures that by adding clay to the materials available at the bottom of both models of the simple and gabion drops, the value of the Froude number has been reduced. Based on the above information, the reason for this can be stated as that in the model with the edge of the gabion reinforced with the stones in the down discharges, it was inbound flow. And with the increased discharge of the flow, its essence is that it flows in and out. As a result of the loss of water energy in more discharge, it increases and the Froude number decreases. Also, the use of clay in the bed materials causes water absorption and coherence of the materials, and the speed of the flow has been reduced and the return flow has been reduced. Therefore, the Froude number is downward and decreasing.
Figure 11
Variation of Froude number vs. relative critical depth: (a) simple drop, (b) gabion drop.
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Variation of Froude number vs. relative critical depth: (a) simple drop, (b) gabion drop.

Figure 11
Variation of Froude number vs. relative critical depth: (a) simple drop, (b) gabion drop.
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Variation of Froude number vs. relative critical depth: (a) simple drop, (b) gabion drop.

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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.

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No funding was received to assist with the preparation of this manuscript.

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The data that support the findings of this study are available on request from the corresponding author.

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The authors declare there is no conflict.

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