Effect of biological mulches on the shear resistance of soil channels under soil erosion and rainfall simulation

Introduction
Mechanical properties of most soils alter upon the increase of moisture and saturation. In some soils, certain phenomena appear due to increased moisture. Some of these phenomena lead to major damage in development projects. These soils are called Sensitive soils to water”. The most significant types of these soils are swelled soils, dispersive soils, and collapsible soils. Dispersive soils refer to clay soils that are easily washed up in waters with a low concentration of salt. Dispersion is a progressive phenomenon that starts from one point and is gradually extended. The start point of the dispersive phenomenon may refer to cracks resulting from shrinkage, soil deposition, or cracks made due to the roots of plants. This phenomenon is of great importance in such plans as soil dams and water supply channels where there is a water concentration inside the soil too. One of the essential reasons for soil erosion and sediment production in irrigation and drainage networks is the instability of canal soil, so it is necessary to use erosion control methods in parts of the canal route, especially at the intersection with other structures. To this end, the solution investigated and tested in this article is the use of biological mulches to reduce the erosion of the side slopes of earthen channels.
 
Materials and Methods
This research has been conducted on soil with a Loam sandy texture of irrigation and drainage channels network of the Arayez Plain of Khuzestan that lies on the west side of the Karkheh River. Using a simulated system, two-nozzle rainfall of the K18 feature was performed. To determine the effect of mulches on parameters of side wall erosive resistance of the soil structure, after the soil of the region passes through a sieve of 4.76 mm was put in the basin designed for 1 x 0.33 x 0.1 m for about 25 kg. After filtration, leveling, and pressing the soil to the edge of the Flume basin proportionate to the physical special weight of the soil, the stabilizing materials, and different bio mulches were sprayed in their different concentrations. Then, the basins inside the Flume with a side slope of 80% were put into a depth of 10 cm. In this research, with the objectives of determining the type and most appropriate level of biological mulches and their effect on the soil erosion resistance parameters of the lateral wall of the drainage channels of the Araiz plain of Khuzestan, the necessary experiments were conducted in the rain simulator laboratory of the Soil Conservation and Watershed Research Institute. So that using four types of water-based biological mulch, which were named with numbers 1 to 4 and at three levels of minimum (C1), medium (C2), and maximum (C3), experiments were conducted on the soil sample of the researched area. Each of the experimental treatments on a flume with a side slope of 1 to 1.25, similar to the side slope of canals in nature, was repeated and simulated three times in two rainfall intensities of 30 and 80 mm h-1. In each experiment, primary and secondary soil moisture, runoff volume, sediment weight, the intensity of water penetration in the treatments, and the shear strength of the treatments were measured. The experiments were conducted in a randomized complete block design in the form of split plots and the data were analyzed using SAS software and the averages were compared using the Student–Newman–Keuls (SNK) multi-domain test. Finally, by using mathematical models with optimization and minimization of sediment amounts and economic costs, the most suitable treatment for stabilizing the lateral walls of drains was determined. SPSS software was used for the statistical analysis of data and ANOVA and the Duncan test were used for the statistical comparison of data. Considering the tables obtained from data variance analysis and considering the statistic F and significance level, it can be said that all treatments are different in a significance level of 5% concerning the extent of sediment and there is a significant difference in the sediment amount.
 
Results and Discussion
The tests conducted on the soil of drainage channels of Arayez plain with a loamy – sand texture indicated that the presence of mulch coverage results in reduced sediment arising from rainfall in the manner that by the increase of density in any of the mulches, sedimentation reduces accordingly. The results obtained from the statistical analysis of this research confirmed that there is a significant difference in a level of 5% between sediment amounts of test control and mulch treatments for the sample of the soil under study. Therefore, bio mulches have an effective role in erosion control and the decrease of sediment from the walls of soil channels. The results indicated that only two types of biological mulches used in this study had a positive effect on shear resistance. General Linear Model (GLM) results showed that the interaction of two parameters of rainfall intensity with the type of treatment on shear strength is significant, so the interaction of rainfall intensity parameters with the level of materials used was not significant at the 5% level. In addition, the interaction of the type of treatment with the level of the materials used was also significant. According to the results of the analysis of variance in investigating the role of rainfall intensity on biological mulch treatments, it was found that rainfall intensity affected shear strength.
 
Conclusion
According to the above photos, all mulches compared to the control treatment have had a remarkable effect on the decrease of sediment. Moreover, it was found that the increase in the density of mulches used in all densities has had a remarkable effect on the decrease of outlet sediment. Furthermore, it was realized that the rainfall factor affects the increase of sediment. Thus, this effect in mulch 2 is the least effective in such a manner that upon increased rainfall in mulch 2m we see the minimum increase in the amount of sediment. The results after conducting system engineering and analyses related to the optimization of sediment amounts and economic costs indicated that mulch 2 at the C3 level has the best efficiency in increasing shear resistance. This conclusion could help the decision makers to allocate their soil conservation budget for the best performance activities.