Effect of bentonite clay and slope on sediment concentration and some flow hydraulic characteristics on a loess soil in Golestan province

Soil is an essential part of the environment. It is important for the production of food and other crops. Soil erosion and sedimentation are complicated and least well-known environmental problems worldwide (Mahmoodabadi et al, 2014). Recently, the application of compounds that modify and improve soil quality and also reduce soil erodibility has been more thoroughly researched. These compounds are known by the common name of soil amendments with a relatively high variety such as gypsum, basanite, zeolite, chemical amendments, organic additives, a variety of chemical, biological and composite polymers, soiltac, fungi, polyvinyl acetate, vermicompost and cattle manure, biochar, straw mulching and cyanobacteria and bacteria (Behzadfar et al, 2017). The objective of this study was to investigate the influence of bentonite clay and slope gradient on runoff and sediment concentration and some hydraulic Characteristics in the Loess soil using a rainfall simulator.

Materials and Methods :

The experiments were conducted using a rainfall simulator at the Soil Erosion and Conservation Laboratory, Gorgan University of Agricultural Sciences and Natural Resources, Iran. The soil used for the experiments is taken from the surface layer (0-30 cm depth) of loess lands from Golestan province (37° 55ʹ N and 55° 29ʹ E). The soil sample at first was air-dried, thoroughly mixed and then crushed to pass through 10 mm sieve size. Experiments were done as factorial based on completely random design with three replications. The factors were the bentonite clay at four level (0, 2, 5 and 10 % weight) and slope steepness at three level (10, 20 and 30%). In all experiments, each soil sample was put in the flume, then was saturated from the bottom for 24 h. Afterwards, the drainage water was removed out of the tray, and the experiment lasted for 45 min. For each rainfall event, the sediment-laden overland flow was sampled at selected time intervals and volumetrically measured. The sediment-laden overland flow was sampled at several time intervals and the sediment concentration was determined . The sediment in the collected samples was allowed to settle, separated from the water, and dried in an oven at 105 °C for 24 h. The sediment concentration was determined as the ratio of dry sediment mass to sampled runoff volume for each time interval.Different hydraulic parameters including flow depth, shear stress, stream power, and unit stream power were measured.

The result showed that the sediment concentration decreased with increasing levels bentonite at all slopes. At 10 % slope steepness, the mean sediment concentration varied 32.48 in the control treatment to 24.67 kg m-3 at level 3 bentonite treatment. At 30% slope the corresponding value were 474.52 and 224.14 kg m-3. Therefore, with increasing slope steepness the sediment concentration increased. Totally, the use of bentonite at level 10 % weight could decrease 46% of sediment concentration in comparison with control treatment. Defersha and Melesse (2012) found that rain intensity and slope gradient had significant influences on sediment concentration. Slope gradient is important as more soil particles are splashed down-slope than up-slope (Dunne et al. 2010; Grismer 2012). According to Fig. 2, the obtained flow depth was 1.92×10−4, 1.92×10−4, 1.92×10−4 and 1.92×10−4 m for 0, 2, 5, 10% clay treatment, respectively. Also, the depth flow ranged from 3.6×10− 4 to 1.96×10−4 m on 10 to 30 % slopes. Totally, the depth of flow decreased with increasing slope steepness for all treatments. In fact, due to higher flow velocities at steeper slopes, shallower flow depths were achieved. Statistical analysis (Table 2) confirmed a significant positive main effects of application levels of bentonite and slope on shear stress, power flow and unit power flow. The clay treatment showed significant reductions ranging from 2 to 50% compared to the control treatment for all slopes. Unit stream power varied from 0.0084 to 0.0095 ms-1, 0.0176 to 0.0241 ms-1 and 0.030 to 0.057 ms-1 for 10, 20 and 30 % slope, respectively. Totally, with increasing slope steepness, all the hydraulic parameters, except flow depth, increased. While with increasing percentage of bentonite clay, shear stress and depth flow and stream power, decreased. Consequently, the maximum values were observed at the steepest slope (30 %) and control treatment (0%).

Conclusion :

Based on the results obtained during the present study, it can be concluded that the bentonite can be considered as an effective modifier of soil physicochemical properties leading to better performance in soil and water conservation in loess lands.