khosrow hoseini

Soil is one of the most important natural resources of any country, which plays a key role in food security, self-sufficiency in food production, national economy, and sustainable agriculture. Soil erosion is one of the most obvious factors of soil loss, and rain erosion is one of the most important forms of erosion. Therefore, the knowledge of the processes governing soil erosion and sediment transport is very important in water and soil resources management, as well as, the development of soil erosion models to achieve sustainable development is of great importance. Previous research has shown that rainfall patterns are one of the factors influencing rain erosion. Vegetation also reduces soil erosion by protecting the soil against the effects of raindrops and runoff. Rain erosion is especially important in arid and semi-arid areas due to the lack of vegetation and low initial soil moisture. This research was conducted, regarding the effect of rainfall patterns on rain erosion, by investigating the rainfall pattern and vegetation changes over 25 years in Ebrahim Abad and Royan watersheds situated in Semnan City. 

First, the physical characteristics of the watersheds were obtained; using ArcGIS software, and the precipitation information was extracted from the rain gauge sheets with an accuracy of 10 minutes. To compare the rainfall for different amounts of precipitation, the dimensionless cumulative rainfall curve of each event was obtained. The time of each rainfall was divided into 10 parts and the percentage of rainfall was determined for each part. The rainfall curve was divided into 4 quartiles (1st, 2nd, 3rd, and 4th quartiles) depending on the occurrence of the maximum precipitation. According to the information on the sediment layers in check dams located at the outlet of each watershed and the precipitation data, the storm-related to each sediment layer was determined and the effect of the storm pattern on the sediment pattern was investigated. To check the similarity of precipitation and sedimentation patterns in check dams, the average difference in precipitation and sedimentation in each time step and standard deviation changes were used. Considering the dynamic changes of vegetation compared to other characteristics of the watershed, remote sensing data were used to investigate the changes in vegetation and its area. Due to the effective performance and high accuracy of NDVI index and landsat satellite images in dry areas, Google Earth Engine system was used to estimate vegetation cover, manage and recall the satellite images. Then, the influence of watershed characteristics such as slope, area, soil type, shape factor, and vegetation cover on watershed sedimentation was investigated. 

The average similarities in precipitation and sediment pattern in Ebrahim Abad and Royan watersheds were 48.2 and 46.1%, respectively. Also, the percentage of coarse-grained sediments augments by increasing the precipitation quarter number, during each storm event, which shows the important role of the rainfall pattern on the sedimentation pattern in each watershed. The average monthly vegetation cover (obtained from Landsat images) in Ebrahim Abad and Royan watersheds during the mentioned period was 5.15 and 4.99%, respectively, which is less estimated than reported by previous research. In this research, a threshold limit of 0.1 has been used for the NDVI index, in which very weak vegetation has been omitted.

In both watersheds, in more than 51% of cases, by increasing vegetation cover in each storm event, the thickness of the corresponding sediment layer augments, which shows the effect of vegetation cover on the erosion and sedimentation of the watersheds.

In order to understand the mechanism of flow patterns in vegetated channels, the flow located in a rectangular channel was numerically investigated by using OpenFOAM software. Firstly, two solvers of that software (i.e. icoFoam and pimpleFoam) were used to calculate the velocity profiles in both longitudinal and cross-sectional directions for four selected sections in a rectangular channel with a square cylinder. By comparing the simulation results with the available data, the icoFoam solver with a better performance (six Percent Error) was selected for the next developed model. A new model was then created with two tandem square cylinders with spacing ratios of two and a half and five. Flow patterns, velocity distribution and pressure characteristics in the channel with different inlet flow velocities were investigated for two cases. It was observed that a flow field disturbance occurred in all simulations and the current changed from steady state to unsteady one at a critical velocity. This instability occurred in a distance between the cylinders for the spacing ratio of five at an average Reynolds number of eight, while for the ratio of two and a half it is occurred at an average Reynolds number of 32. The maximum values ​​of longitudinal and transverse velocity timelines in a period of 200 seconds for four states (including two Reynolds numbers and two different spacing ratios) were plotted in two spatial ranges and fully investigated. According to the results, it can be said that the overlap has an important role on the flow characteristics in tandem arrangements and by increasing the distance ratio between the cylinders by 55 percent, the critical velocity value decreases by 74 percent.

One of the most important parameters in the design of bridge abutments and piers is to calculate the depth of scouring. The previous relationships for measuring the scour depth were based on steady flow, and it is not expected that the amount of scour depth computed from these relationships is accommodated to the actual value during a flood wave. Inasmuch as in this condition, the flood hydrograph occurs in an unsteady state in which the discharge is variable between base and peak values. In this study, experiments were conducted to measure the clear-water scour depth of a rectangular abutment under steady and unsteady flow conditions by approaching flood waves with triangular hydrographs and equivalent stepped hydrographs in a compound channel. Then, by using the measured scour depths values in steady flows, a relationship is calculated to estimate the maximum scour depth in terms of dimensionless time parameter and flow intensity. By changing the peak discharge time parameter, the effect of slope at ascending and descending part of the triangular hydrograph on the scouring was investigated. A comparison of the scouring of two stepped hydrographs with the same time duration showed that the difference between the scouring values was 1.5%. Moreover, the results of the scour depth measurements of triangular and stepped hydrographs were compared.

Landslides are complex phenomena that commonly occur in mountains, oceans, bays, and reservoirs. Considering their economic and life consequences in recent years, it is necessary to find effective methods for prediction of landslides and their destructive effects. Landslide is a complicated phenomenon (especially under water), which involves a highly-reformative multiphase flow of granular materials. Considering the restrictions of the traditional mesh-based numerical models in the modeling of large deformations and the discontinuities, meshless Lagrangian (particle) methods can be the effective alternatives for simulation of landslides. The meshless Lagrangian methods such as Smoothed Particle Hydrodynamics (SPH) and Moving Particle Semi-implicit Method (MPS) provide the opportunity to model the deformations and fragmentation in sediment flow. In this research, an MPS meshless Lagrangian model has been developed for modeling of rigid and deformable landslides. In this model, the solid phase is considered as a non- Newtonian visco-plastic fluid, whose behavior is predicted using the µ (I) rheological model. The model has been validated and evaluated in comparison with the available experimental and numerical data. The water surface and the sediment profile obtained from numerical model shows a good compatibility with the experiments. The RMSE value in predicting the numerical water surface and sediment profiles of current study found to be lower compared to the other numerical methods.