Sensitivity analysis of total sediment load in Karun river using HEC-RAS model (Malathani-Farsiat interval)

Dissertation for receiving the master's degree "M. Sc."

Field: Agricultural Engineering

Trend: Water Structures

Abstract

Sediment transfer process is a completely complex, variable and non-linear phenomenon. So, the behavior of the river in terms of sediment transfer capacity for a given flow rate is different in different times. Therefore, due to the complexity of the sediment transport mechanism and the number of factors affecting it, estimating and forecasting the sediment carrying capacity and determining the sediment hydrograph are accompanied by uncertainty. By identifying the effective factors on sediment transfer and their impact and by more accurately measuring the effective factors, uncertainty in sediment estimation can be reduced. One of the methods of estimating the uncertainty of an output variable is its sensitivity analysis to input variables. In this research, the hydraulics of flooding and sedimentation in the Karun River, from Mulathani to Farsiat, were simulated using a mathematical model. Then, the sensitivity analysis of the total sediment flow passing through the Ahvaz hydrometric station was performed with respect to the input variables and parameters of the desired model. The results of the sensitivity analysis show that the total sediment discharge is sensitive to the changes in Shields critical stress, Manning's roughness coefficient, water temperature, and the method of falling velocity of suspended sediment particles, respectively. So that with an increase of 30% in Shields critical stress, the average total sediment discharge decreases by 23.68% and with a 30% decrease in Shields critical stress, the average total sediment discharge increases by 40.82%. Also, with a 30% increase in the Manning roughness coefficient, the average total sediment flow rate decreases by 25.18%, and with a 30% decrease in the Manning roughness coefficient, the average total sediment flow rate increases by 38.1%. With a 30% increase in water temperature, the average total sediment discharge decreases by 18.93%, and with a 30% decrease in water temperature, the average total sediment discharge increases by 25.63%. In addition, if suspended sediment particles fall by Toffaleti, Van rijn and Ruby methods, the average flow rate of the total passing sediment will increase by 0.2%, 0.8% and 2.1%, respectively, compared to the method of falling speed resulting from model calibration (Report12 method). The results of the analysis of the sensitivity of total sediment discharge in the Ahvaz cross-section to changes in the granularity of the bed materials show that with increasing and decreasing the granularity of the bed, the average and total total sediment discharge will not change within the specified tolerance. According to the results of the sensitivity analysis, the degree of uncertainty in the average total sediment discharge at the Ahvaz hydrometric station section due to the uncertainty in the input variables and model parameters is equal to 66%. Keywords: sensitivity analysis, uncertainty, sediment transport, Karun River, HEC-RAS. Introduction Today, the importance of water is not hidden from anyone. Most of the past civilizations were founded near water sources. The passage of time has increased the population of the world and this has caused the importance and value of water to be added. Rivers are one of the most important water resources available to humans.

For this reason, throughout history, humans have invested a lot in controlling and exploiting river water. The construction of various dams and dams along the rivers is one of these investments. Since riverbeds are usually made of alluvial materials, they are constantly eroding and changing. This causes the soil particles, which are called sedimentary particles, to be separated from the bed and walls and move as suspended sediments or bed sediments. In general, knowing, analyzing and predicting the amount of changes in the geometric characteristics of rivers is one of the most complex and at the same time most important topics in sediment hydraulics and river engineering. Currently, with the advancement of computers, it has become possible for the theories related to hydraulics and sedimentation to be made into mathematical models and then solved with the help of computer programs.

The damage caused by coastal erosion and sedimentation of river sediments to nature, agriculture and water structures built on or next to rivers is very extensive and harmful. In order to prevent or minimize the damage, three processes of erosion, transfer and sedimentation of sedimentary materials should be studied. These processes have a certain complexity, because there are many effective factors in creating these processes. In the process of erosion, soil particles are separated from their substrate by the impact of raindrops or by the forces created by the movement of water.Then the separated particles are placed on the threshold of movement and if the forces introduced by the water are more than the resisting forces, the particle is transported along with the water flow. The starting conditions of the particle movement threshold and their transfer rate depend on the characteristics of the sediment materials such as particle size, shape and density, as well as on the characteristics of the flow, such as speed, depth, and on the characteristics of the river shape, such as the hydraulic radius, slope, etc. If, for any reason, the forces applied by the water are reduced in the transmission path, the sediment particles will settle. Each of the three processes of erosion, transport and sedimentation can cause problems. For example, erosion causes the destruction of agricultural lands, the destruction of riverside structures, the destruction of bridges and other buildings near the river, and also causes the deepening of river beds (Figure 1-1). Weathering of rocks and rocks has occurred. Weathering is the process by which hard rocks are broken and worn away. The size, mineral composition, density and other factors such as sediment texture depend on the nature of the original rock. When the original rock is disintegrated, the material is transported and accumulated by rivers from one point to another. The amount of transported materials depends on the size of the particles, the flow rate, the slope of the floor and the characteristics of the catchment area. When the flow rate or the slope of the river decreases, the river is not able to carry the materials that it has taken before, and as a result, the excess materials settle and accumulate in the river bed, causing changes in the river bed, such as the formation and creation of flood zones, deltas, islands, etc.

River sediments are transported in two ways. When these materials are immersed in the water flow and move along with the water, they are called suspended sediment materials. The amount of suspended sediment material that passes through a section of a river in a unit of time is called suspended load[1]. On the other hand, sedimentary materials may move near the bed in one of the forms of sliding, rolling, and jumping, which are called bed load [2]. The type of movement in the form of suspended load or bed load depends on the characteristics of sedimentary materials, flow conditions and river characteristics. In rivers with a steep slope under high velocity conditions, sand particles may also move in suspension, while in rivers with a gentle slope and low speed, as in the recent research period, only very fine grains and silt particles move in suspension. Among the issues that sediment particles can cause during transportation, we can mention damages to turbines, pumps, bridge foundations, canal linings, etc. One of the important phenomena in rivers, especially in arid and semi-arid areas such as Iran, is the occurrence of other floods. The flood wave has a great force and by moving a significant volume of water and sediment, it causes various complications and consequences. Based on Henderson's theory [3], the transfer of bed sediment [4] changes with the second power of discharge. According to this theory, an increase in flow rate to ten times the annual average, which is considered normal in the country's rivers, will increase the sediment flow rate by a hundred times. Therefore, a significant part of the annual sediment supply of the rivers and consequently changes in the level of the bed and transverse and longitudinal displacements and in general morphological transformations, especially in areas like Iran, are related to a few important floods that generally happen every few years. Sediments are generally classified as sticky and non-sticky. Non-sticky sediments include separate and separated particles whose dynamic and kinetic properties directly depend on the external forces that act on the particles and the physical properties of the particles depend on their position in relation to other particles. On the other hand, in cohesive materials, the chemical and electrical bonds between particles will be the determining factor.

     Most sediment engineering activities are often focused on non-cohesive particles and in cohesive materials, it should be mentioned that when the cohesive materials are transported by the flow, they behave like non-cohesive materials and the sediment transport mechanism in these materials is similar to non-cohesive sediments. The moment when the particles start to move is called the movement threshold [5].