On the effect of green nonstructural materials on scour reduction downstream of grid dissipators

Experimental research shows that screens positioned perpendicular to a supercritical flow path can dissipate flow energy (Rajaratnam & Hurtig 2000; Bozkus et al. 2007; Sadeghfam et al. 2014). Supercritical flow is created through a sluice gate; energy dissipation and hydraulic jump occur in the gap between the gate and the screen, which is called the stilling basin. After leaving the stilling basin, the flow will enter an unprotected area; one aim of this study is to reduce the length and depth of the scour hole in this area. The importance of scour investigation is revealed when the scour depth is significant. This is so that this scour reaches the foundation of river structures or moves or endangers the stability of these structures. On the other hand, construction materials such as concrete used to stabilize the bed can also have harmful environmental effects. One of the challenges that the current research brings with it is the process of dispersing nanomaterials. Due to the use of nano in the sedimentary bed, we cannot use it directly in the bed. For this purpose, we have to use dispersing materials to separate nanoparticles. The challenge that arises here is that the process of dispersing nanomaterials is a long and time-consuming process that requires high precision. But the advantage that this challenge brings is the drastic reduction of the scouring phenomenon. However, the advantages of using nanomaterials are more than the difficulties and time. Among other challenges, we can point out that nanomaterials should not be allowed to agglomerate after dispersing. Because in this case, the obtained results will not bring a positive result. On the other hand, these nanomaterials are environmentally-friendly and do not harm the environment.

Several studies in the field of downstream scouring of hydraulic structures have been conducted by researchers such as Chabert & Engeldinger (1956), Lee et al. (1961), Chiew (1992), Zarrati et al. (2004), Sanoussi & Habib (2008), Karimaee & Zarrati (2011), Singh & Maiti (2012), Elsebaie (2013), and Nasr-Allah et al. (2016). Tuna & Amiroglu (2011, 2013), Abdelhaleem (2003), and Elnikhely (2016) studied the downstream scouring of stepped and other spillways. They also investigated the effect of water depth and hydraulic conditions of the flow on the downstream scouring rate. Goel (2010) investigated scouring around bridge piers and geometrical changes in scouring (length, depth, and volume). The results indicated an increase in scouring geometrical parameters with an increasing flow rate. Lu et al. (2021) evaluated the hydrochemical evolution of pore water in the sedimentary zone of the riverbed during the infiltration of its bank. The results of their research showed that in the process of infiltration of river water, a series of redox reactions occur in the sedimentation area of the bed, and there are differences in different infiltration depths where strong respiration and denitrification occur.

Sun et al. (2021) used the combined model of support vector regression and the fruit fly optimization algorithm to predict scouring caused by ski-jump and reached the conclusion that the proposed method of support vector regression significantly improved the results of the support vector machine. Li et al. (2021) investigated sediment avoidance diversion and the coordinated dispatch of water and sediment at an injection-water supply project on a sediment-laden river. The results showed that during a period with high sediment content, large-scale silting occurs in a diversion channel. Also, to reduce sediment in a diversion channel, strategies have been presented to reduce mud. Jia et al. (2022) numerically evaluated the effect of obstruction on eddy flow in partly obstructed channels and showed that two vortex evolution patterns including sequent sole along-canopy vortices (SCVs) and alternating hybrid along-canopy and along-wall paired vortices (HCWVs) correspond to small and large blocking ratios, respectively.

Widyastuti et al. (2022) investigated the energy of dam failure with a porous structure in order to cope with scouring around bridge foundations. The results of laboratory studies showed that the percentage of friction speed in the energy containment area before and after the porous structures decreased by 31.42% and increased by 9.27%, respectively. Qasim et al. (2022) investigated the effect of channel bed roughness on the hydraulic parameters of a weir gate. The research showed that there are certain results between the Reynolds number and the downstream Froude number, the flow rate and the flow velocity in the downstream of the flow area passing through the gate and the downstream Froude number.

The methods that researchers have used to prevent scouring are placing structures in and near the place of scouring or changing the destructive flow pattern. The proposed method of this research is a new one that can be used along with other methods that will not disturb the river environment. In this research, a suitable solution to reduce the aforementioned problems by using materials compatible with the nature and ecology of the river and with the aim of reducing the downstream scouring depth of the hydraulic structures is provided. On the other hand, the length and depth of downstream scouring of recently used screens have not been fully evaluated so far, and the necessity of studying it has been felt by the authors of this study.

The research that has been done in the previous studies is related to the structural methods to reduce the amount of scour in the mobile bed or around the bridge piers. Meanwhile, in the present study, an attempt has been made to reduce the scouring rate of the mobile bed by using nonstructural and environmentally friendly methods. For this reason, in this research, the effect of adding clay and the combination of clay and montmorillonite nanoclay to sedimentary materials downstream of screens has been investigated in order to reduce the depth and length of scouring. To import clay or a mixture of clay and montmorillonite nanoclay, the grouting method is used at the maximum depth equivalent to the scouring of the no-clay layer or a mixture of clay and nanoclay.

The effect of adding clay and the combination of clay and montmorillonite nanoclay to the sedimentary material downstream of the screens has been investigated in order to reduce the scour profile. The experimental conditions of Table 1 were used. It was determined with the preliminary tests that the sand with an average diameter of 1.8 mm and a standard deviation of 1.28 mm is suitable for use.

Table 1

Experimental conditions

Type of tests .	Range of flow depth (vena contracta), cm .	Range of Fr no. (vena contracta) .	Q (L/s) .	Number of tests .
Control test (only sedimentary materials)			3, 5, 6.25	18
Mixing clay into the sedimentary bed
Combination of clay and montmorillonite nanoclay

Type of tests .	Range of flow depth (vena contracta), cm .	Range of Fr no. (vena contracta) .	Q (L/s) .	Number of tests .
Control test (only sedimentary materials)			3, 5, 6.25	18
Mixing clay into the sedimentary bed
Combination of clay and montmorillonite nanoclay

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To perform the experiments, a channel with a rectangular cross-section with a length, width, and height of 5, 0.3, and 0.45 m, respectively, was constructed in the hydraulic laboratory of the University of Maragheh. In order to have a smooth surface with minimal roughness, the channel floor and walls 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. To create the supercritical flow, a sluice gate was used installed 1 m from the entrance to the channel. The distance between the sluice gate and the screens was also 1.5 m and its opening was 1 cm. This was kept constant during the experiments. 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 present research is designed for high Froude numbers. For this reason, a vertical sluice gate with an opening of 1 cm has been used to create high Froude numbers. To create a sedimentary bed, river materials with the mentioned specifications have been used.

To use nanomaterials, firstly, nanomaterials are dispersed, and after several steps, they are injected into the sedimentary bed. The remarkable thing is that the dispersed nanomaterials have been injected without any change in the sediment bed by injecting clay and montmorillonite nanoclay at regular intervals, which result in minimizing scouring. On the other hand, in addition to dispersing nanomaterials, it should also be noted that the uniform distribution of nanoclay in the sediment bed was done without agglomeration. Because if the distribution of nanoclay is done with agglomeration, positive results will not be obtained from these materials in order to reduce scouring.

In Figure 1, section A is the starting point of the supercritical flow (vena contracta), section B is the starting point of the hydraulic jump, section C is the flow depth at the end of the hydraulic jump, section D is the beginning of the flow and the initial depth of the hydraulic jump after the screen, section E is the end of the hydraulic jump after the screen, and section F shows the downstream depth. The depth of the scour hole downstream of the screen has been measured at specific time intervals throughout the test. Due to the extreme changes in scouring depth with respect to time in the early stages of the experiment, the samples were measured at short intervals of 30 s (10 measurements), 5 min (4 measurements), 10 min (2 measurements), and finally for 30 min. In order 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 use the average of the calculated data as the final data. To determine the longitudinal and depth expansion of the scour hole downstream of the screen, at the end of each test, a point gauge and a digital camera were used and the entire bed profile was prepared using plot digitizer software. In this way, high-quality images of the flow passing through the screen and entering the mobile bed at specific times were recorded. 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, in order to calibrate the points in the plot digitizer software, a very small computer millimeter label installed on the channel wall was used.

It should be mentioned that since the calculated particle uniformity coefficient (C_u = 1.61) is less than 2, the materials can be considered as uniform particles (Lambe & 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 3. The fine-grain granulation curve is presented in Figure 4.

The nanoclay added to the channel bed is montmorillonite nanoclay. Montmorillonite nanoclays are a group of mineral clays whose structure consists of gibbsite surrounded by silica sheets on top and bottom and with van der Waals bonds. The length and width of these particles are in the range of a few tenths to 1.5 μm, and their third dimension is 1 nm with a significant difference in length and width (Uddin 2008). This product, after being added to fine-grained soils in a wet or dry method, improves the physical parameters of the soil by creating a chemical and physical reaction through cation and anion exchanges and by bonding and connecting nanoparticles and other soil particles. Research has shown that the addition of montmorillonite nanoclay up to 1% using the dry method and 0.5% using the wet method significantly improves shear resistance and reduces its permeability by 300–1,000 times (Zhang 2007). The physical and chemical characteristics of montmorillonite nanoclay are presented in Table 4.

The nanoclay used in this research was mixed with the required amount of water using a mixer with a high rotation speed and a suspension containing 1% of montmorillonite nanoclay that was uniformly added to the soil. 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% (Mohammadi & Niazian 2013; Majeed et al. 2014). 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. The assumption of uniform distribution of nanoclay in the sedimentary bed comes from the injection of nanoclay in networks with fixed and regular intervals.

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.

Also, with the addition of clay and the combination of clay and nano montmorillonite, there is an average decrease of 39 and 46% in scouring depth, respectively. This improvement and the positive effect of clay and especially its combination with montmorillonite nanoclay on increasing the cohesion strength of sediment particles and increasing the shear strength of particles are more evident at high-flow rates.

In other words, it can be seen that the combination of clay and montmorillonite nanoclay can be used in flood discharges and downstream hydraulic structures, especially in areas where there is a high possibility of heavy rains and floods. In those places, it can be useful in further reducing erosion and scouring downstream structures.

The addition of nanoclay increases the shear strength of clay and sandy-clay soils. This increase in shear strength effectively prevents the increase of abrasion. On the other hand, by carefully looking at the results and especially Figures 5–9, it can be concluded that the use of nonstructural methods can be a very important and effective factor in the scouring of the bed, which includes the scouring depth and length. Some of these nonstructural methods compared to structural methods, in addition to reducing related costs, prevent damage to the environment and water resources. According to the materials used in this research, it can be concluded that due to the compatibility of montmorillonite nanoclay with the environment, in addition to the drastic reduction of the two parameters mentioned above, there is no environmental damage to the river, aquatic life, ecology of the river, and even the existing vegetation does not reach it. As mentioned above, using clay and the combination of montmorillonite clay and nanoclay, the scouring depth has decreased by 35 and 49%, respectively, compared to the case without additives. On the other hand, due to the high rate of leaching in nature, this material can be easily obtained from nature. These results have been obtained while the supercritical flow regime with high Froude numbers has occurred with a hydraulic jump on the sedimentary bed and high energy dissipation without disintegration of the sedimentary bed with the performance of nanomaterials. On the other hand, if we look at the issue from a geotechnical point of view, we will reach the conclusion that the existing bed gets its major shear strength from intergranular interlocking due to angular particles. Adding clay to the sedimentary bed creates proper cementation and increases the shear strength of the soil. In fact, adding clay improves the structure of cementation and increases the shear strength of clay and the substrate.

In this research, the effect of adding clay and its combination with montmorillonite nanoclay to the building materials downstream of the energy-dissipating structure (screen) was investigated as a means to reduce the length and depth of scour.

The experimental results indicate the positive effect of clay and its combination with nanoclay on improving and reducing scouring depth and length, especially in the early section of data measurement downstream of the screen. Also, with the increase in discharge, the greatest reduction in scouring depth and length increases. In other words, at the maximum flow rate of 6.25 L/s and at the end of the measurement time (30 min), the maximum scouring depth was about 6.91 cm in the first test. With the addition of clay and the combination of clay with montmorillonite, nanoclay has decreased by about 35 and 49%, respectively. The positive effect of clay and its combination with montmorillonite nanoclay is also related to the reduction of scouring spread, so that the initial scouring length (36 cm) is decreased by 19.5 and 33%, respectively, for the claim and the combination of clay/montmorillonite nanoclay.

These results indicate the remarkable success of clay and nanoclay in controlling the scouring bed erosion downstream of hydraulic structures, especially in flood discharges. Considering that clay and nanoclay are compatible with the environment and are economical and accessible, it has a good compatibility with river systems and their ecologies, its use in the screen application system is recommended to researchers, operators, and designers. It is also expected that by increasing the concentration of nanoclay materials in the channel bed, it is possible to witness a better performance of these materials in reducing the maximum depth of scouring at the location of hydraulic structures, especially downstream of the screens and during floods.

Considering some laboratory limitations, including access to nanomaterials and other laboratory limitations, the following suggestions can be put forward as suggested topics in future research, including the practical effect of adding nanoclay amounts of less than 1% and different granulations of the sedimentary bed, the effect of the diameter of the grid dissipators, and also the changes in the gate opening rate.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.