The effect of nano-clay on critical shear stress of cohesive materials in water conveyance canals

Study on the movement and deposition of cohesive sediments due to electromagnetic and chemical nature of the particles could be difficult. Estimation of suspended sediment load would also be difficult in the rivers that these particles are present in their sediment load. Cohesive sediments include fine-grained sediments such as silt and clay. These materials have large surfaces that can absorb each other and even other small materials and lead to the formation of aggregate and flocculate structures in low shear stress. Deal with the problem of sedimentation of canals, including issues that should be included in the design and planning of irrigation networks. Considering the recent advances in the use of nano-scale materials, the study of the performance of these materials and their surrounding materials is essential in reducing the bed erosion of the earthen water conveyance canals. For this purpose, nano-clay has been used in this research. In this study, using the experimental data, the effect of nano-clay on the critical shear stress of cohesive materials of bed earthen canals was investigated.
The used flume was a rectangular canal that its framework was made of iron and its walls were made of glass and plexiglas. Width, length and height of the flume were 50, 1000 and 70 cm, respectively. Discharge of flume was obtained according to the weir discharge-height data. The depth of water inside the flume was measured by the depth gauge. Canal slope was adjustable by an electromotor which was installed below the flume. The flume was placed in different slopes of 0.0005, 0.002, 0.003, 0.006 and 0.008, respectively. The threshold of cohesive bed materials in the earth canal C25 (embranchment of Ganj Afrooz diversion dam located in Alborz Project) was determined using hydraulic experiments; also, the nano-clay effects on soil erosion were evaluated by a hydraulic test. In order to find the threshold of cohesive sediment, erosion was measured by measuring the bulk density of sediments and eroded sediment volume after each experiment. The experiments were carried out by increasing the slope until observing the erosion with high volume. This process was repeated 5 times.
The relationship between flow shear stress and sediment erodibility was determined in both conditions, before and after adding nano-clay to the soil. With increasing shear stress, the erosion rate increased, However, this increase in the amount of erosion before and after the threshold of sediment was a small amount, but after the moment of sediment threshold, the amount of erosion increased also the gradient of erosion increased several times, that the reason for this was the conversion of erosion from single-clay particles to mass erosion of sediments. The sediment segments which were separated from the bed moved massively at first and after a while, they were crushed into smaller pieces, and the small particles were suspended at the end. By adding nano-clay to the soil, the bulk density of the soil decreased by 10.33% that this was attributed to microstructures with large cavities and to the re-structure of the mass of soils that are combined with nano-particles which was due to the high porosity of micro-particles and nano-particles, inside the particles. Nano-particles, due to their specific surface area and surface load, cause interactions between the particles and lock and fix the particles of the soil, resulting in the formation of relatively thick and massive masses in the soil mass. Due to the continuity of these masses, the critical shear stress and critical velocity sediments after mixing with nano-clay were increased 50 and 12/71%, respectively. After the threshold, due to the integrity of the soil particles, erosion occurred as mass, and this resulted in an increase in the mass and volume of erosion of the sediments after the combination of soil with the nano-clay. As the slope more increased, the erosion increasing was more evident.
In order to design water canals in areas where the cohesive and fine-grained sediments are high in the combination of sediments entering the canal, instead of using the noncohesive sediment transport criterion, it is better to use the criteria for cohesive sediments such as critical shear stress. As a result, the critical shear stress indicates greater particle stability against erosion so with the help of nano-clay can overcome the problems caused by erosion and sedimentation in rivers or canals.