Investigating the impact of clay percentage on scouring downstream of labyrinth weirs

Using labyrinth weirs instead of linear weirs is a solution to increase the hydraulic efficiency of weirs. Since weirs are exposed to regular flooding, it is vital to ensure the stability of the structures against downstream scouring. Many studies have examined the scour depth downstream of hydraulic structures (e.g., Novak 1961; Bormann & Julien 1991; Hoffmans & Pilarczyk 1995; Dargahi 2003; D'Agostino & Ferro 2004; Guan et al. 2013; Amin 2015; Elnikhely 2018 and Dah-mardeh et al. 2023). Additionally, investigations have been carried out on the scouring that occurs downstream of labyrinth weirs. Romero & Brañez (2009) studied scour processes downstream of four 2-cycle labyrinth weirs. The results showed that a labyrinth weir with an opening angle of 35° produced less scoured volumes and eroded areas, minimum scoured depths, and shorter erosion lengths compared to the other studied structures. Emiroglu et al. (2017) studied the effects of antivortex structures on discharge capacity and scouring at trapezoidal labyrinth side weirs. The findings demonstrated that antivortex structures decrease scour depth around the water intake region. Tunç & Emiroğlu (2018) studied local scour depths downstream of triangular labyrinth side weirs in a live bed. The results showed that the highest scour depth generally occurred at the downstream end of the triangular labyrinth side weir. Ikinciogullari et al. (2022) investigated scour properties downstream of labyrinth weirs. They employed the finite-volume approach to solve the k–ɛ turbulence model using Flow-3D. The results showed reasonable agreement between numerical and physical model results. The findings indicated that the local scour downstream of labyrinth weirs was lower than that of the linear weirs. Elnikhely & Fathy (2020) predicted scour downstream of triangular labyrinth weirs under four different apex angles. They concluded that the best weir apex angle is 60°, which produced minimum values of scour parameters downstream of the triangular labyrinth weir. Rajaei et al. (2020) investigated local scours downstream of grade control structures with a labyrinth planform. The results of their study demonstrated that the shape of the labyrinth weir significantly reduces the scour depth. Moreover, experimental observations and comparison of results showed that trapezoidal and rectangular labyrinth weirs reduced scouring by an average of 19 and 10% in comparison with linear weirs, respectively. Tunç & Emiroğlu (2020) examined the scour depth around the labyrinth side. The findings indicated that as the flow intensity increased, the equilibrium scour depth also increased. Conversely, increasing the median grain size of the bed material resulted in a decrease in the equilibrium scour depth. Ghaderi et al. (2020) investigated scouring parameters downstream of stepped spillways. Their findings showed that tailwater depth is an essential parameter for decreasing maximum scouring depth. Tunc et al. (2022) studied the local scour at triangular labyrinth side weirs located on an alluvial channel. It was determined that scour depth is dependent on the approach flow intensity, dimensionless side weir crest height, dimensionless median grain size, dimensionless side weir opening, and dimensionless volumetric amount of upstream sediment feed under live-bed scour conditions. They also developed empirical equations for dimensionless scour depth. Daneshfaraz et al. (2023) investigated the effect of sedimentary materials with a combination of clay and montmorillonite nanoclay on scouring. The experimental results showed that by adding clay and montmorillonite nanoclay mixtures, the scour depth was reduced by up to 39 and 46%, respectively. Abdi Chooplou et al. (2024) investigated the local scour downstream of piano key weirs (PKWs) with various shapes in plan, including rectangular, trapezoidal, and triangular. The result showed that the scour parameters downstream followed similar patterns, albeit with variations in the maximum values of scour depth, scour hole length, scour hole area, and scour hole volume. Yasi & Azizpour (2024) studied the downstream scour of labyrinth weirs with triangular, trapezoidal, and curved apexes, where the curved planform proved more efficient. In low flow conditions, the scour downstream of the trapezoidal planform was 19 and 28% greater than that of the triangular and curved planforms, respectively. Corresponding values were 22 and 7% for mean flows, and 11 and 5% for high flows. Since the type of materials used downstream of the weirs determines the scour depth, it is necessary to evaluate their effects. Therefore, this research investigated the effect of sediment non-uniformity, compaction rate, and clay percentage on scouring downstream of labyrinth weirs.

A total of 6 experiments without clay and 12 experiments with clay were performed (Table 1).

According to the figure, the maximum scour occurred on the sides of the channel by up to 48, 119, and 162 mm for discharge rates of 5, 10, and 15 l/s, respectively. Moreover, the results of Elnikhely & Fathy (2020) showed that the maximum scouring occurs near the sides of the triangular labyrinth weir, which corresponds to the results of the present experiments. The apex angle of the investigated labyrinth weir was 60°. Similarly, Elnikhely & Fathy (2020) concluded that the lowest scouring rate was recorded for the apex angle of 60°.

The maximum scour by changing the discharge rate from 5 to 10 l/s resulted in a 145% increase and from 10 to 15 l/s showed a 36% increase in scouring. The second maximum depths of scour occurred near the apex of the weir at 28, 95, and 105 mm for the discharge rates of 5, 10, and 15 l/s, respectively.

For the 15 l/s discharge, the maximum scour was 130 mm on the sides and 62 mm at the apex, which, compared to S1, shows a 20% reduction in scour on the sides and 40% near the apex. Therefore, due to the involvement of sediment grains inside each other in non-uniform gradation, the S2 experienced less erosion than S1. In addition, the increase in the flow rate increased the non-uniformity effect of S2 on the reduction of scouring, increasing the scour difference between S1 and S2.

For the 10 l/s discharge, the maximum scour was 21 mm, showing an 82% reduction compared to the non-clay sediment. Therefore, the addition of clay has greatly reduced the scouring depth. For the 15 l/s discharge, the maximum scour was 26 mm. The scour has reached 14 mm from 26 mm in a 2 cm longitudinal distance. This shows a decrease in the scour depth as well as the scour area. The scour depth showed an 84% decrease compared to the non-clay sediment in the 15 l/s discharge rate. Hence, the maximum scour depth showed a staggering 80% average reduction when adding clay under different discharge rates.

The maximum scour for non-clay S1 was equal to 48 mm, which shows a scour depth reduction of 85% compared to the non-clay sediment. For the 10 l/s discharge, the maximum scour was 10 mm, which is smaller than the 21 mm scour in S1 with 10% clay. Therefore, by increasing the amount of clay from 10 to 15%, there is a 52% reduction in scour depth. For the 15 l/s discharge, the maximum scour was 16 mm. Considering that this amount was 26 mm in S1 with 10% clay, a 38% reduction in scour depth was achieved with a 5% increase in clay.

For S1 and S2 sediments with 10% clay and 5 l/s discharge, the maximum scour was equal to 13 and 12 mm. These amounts were 26 and 25 mm for the 15 l/s discharge, respectively. As can be seen, the scours in S2 and S1 are close together. It seems that the clay has a significant effect on reducing scour depth by filling soil pores and creating adhesion between particles.

For the 10 and 15 l/s discharge rates, the maximum scours for S2 with 10% clay were 18 and 25 mm, showing a 50 and 100% increase compared to the 5 l/s discharge. Furthermore, there was an 85% decrease in scour for S2 with 10% clay compared to non-clay S2 in the 10 and 15 l/s discharge rates.

The scour in S1 with 15% clay and 5 l/s discharge was equal to 7 mm. Therefore, there is no change in scour between S1 and S2 for the 5 l/s discharge with 15% clay, which was also observed for the scenario with 10% clay in the studied sediments.

For the 10 l/s discharge, the maximum scour for S2 with 15% clay was 8 mm. For non-clay S2, this amount was 90 mm, indicating a decrease of about 91% in the scour depth by increasing the amount of clay to 15%. The scour in S1 with 15% clay and 10 l/s discharge was equal to 10 mm. The S2 sediment with similar conditions showed a 20% reduction in scour depth compared to S1. For the 15 l/s discharge, the maximum scour was 11 mm. For non-clay S2, this amount was 130 mm, which shows that a 15% increase in clay causes a 91% decrease in scouring. The S1 with 15% clay and 15 l/s discharge created a scour of 16 mm, whereas S2 with similar conditions had 31% less scouring. Therefore, the sediment non-uniformity proved more effective in reducing scouring.

Downstream erosion poses a significant threat to the longevity of hydraulic structures, potentially leading to catastrophic failure. This research specifically examined how the percentage of clay in the sediment affects scouring downstream of labyrinth weirs. Two types of sediments were used here, one with uniform grain (S1) and one with non-uniform grain (S2) sizes. The maximum scour depth occurred near the junction of the weir and the channel wall. The non-uniform grain sediment (S2) exhibited less erosion than uniform sediment (S1). This is likely due to the interlocking of different particle sizes, which increases resistance to erosive forces. Increasing discharge in non-clay and clay sediments amplified the impact of non-uniformity on scouring, with non-uniform sediments experiencing less erosion. However, the addition of clay made the scouring rates of the sediments to be closer to each other. The maximum amount of scouring at 5 and 15 l/s discharge rates was 73 and 84% less for S1 with 10% clay than for the non-clay scenario. The scour in S2 with 10% clay at 5 and 15 l/s discharge rates was 76 and 80% less than in non-clay S2, respectively. Therefore, the increase of clay causes a great decrease in scouring depth. Clay's cohesive properties not only bind soil particles together but also increase compaction, further resisting erosion in soil structures. Sediments without clay exhibited a maximum of 90% compaction, while compaction reached 100% by adding clay, which ultimately decreased the scour depth. While increasing clay content from 10 to 15% does reduce scour, the effect is less significant. The amount of erosion in S1 with 15% clay at a 5 l/s discharge rate was reduced by 45% compared to S1 with 10% clay. For the 15 l/s discharge, there was a 38% decrease in scouring depth by increasing the amount of clay. At the 5 l/s discharge rate, the scouring in S2 with 15% clay was 42% less than that in S2 with 10% clay. Increasing the test duration is crucial to understand the long-term stability of clay in the sediment. Future studies can determine the optimal clay content for maximum scour reduction.

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

The authors declare there is no conflict.