Investigation of scouring and downstream protection methods of river hydraulic structures (case study: the second bridge of Minab river)

Thesis of master's degree: Civil Engineering (M.S.C)

Thesis: Hydraulic structures

Academic year 1391-1392

Abstract

Minab river is the most important fresh water river in Hormozgan province. This river is the drainage of the surface waters of the Minab catchment area, and two bridges have been built on it, the first one is the connecting road to Minab city and the second one is 1500 meters after the first bridge on the Minab-Jask ring road. The subject of this research is to investigate scouring phenomenon on the foundations of the second bridge of Minab. The effect of scour on water structures including bridges is one of the important topics in civil engineering and river engineering, bridges are one of the oldest engineering structures. One of the most important factors in the destruction of bridges is the problem of local erosion around its foundations. This type of erosion can be identified by the hole that forms around the structure. If this hole expands in depth, it can cause damage and eventually collapse of the bridge. In order to accurately investigate the flow and predict the local erosion around the bridge foundations, it is necessary to understand the flow pattern around the foundations. By fully understanding the flow, the flow field can be completely modeled by solving the governing equations, and along with solving the sediment transport equations and relying on the significant advances in Computational Fluid Dynamics (CFD) [1], an accurate method can be obtained to estimate local erosion around these structures. In the upcoming research, simulation has been done using Flow-3d software, which is capable of simulating flow turbulence in three dimensions, for this purpose, using the Shallow water model, the flow characteristics including water level, speed, pressure, etc., due to the flow of discharge with a return period of 100 years (2723) were calculated in the range from the beginning of the first bridge to 100 meters downstream of the second bridge of the Minab river. Sediment scour model was investigated in 3D form around the foundations of the second bridge of the Minab river. The results show that the most scouring occurs in the third, fourth and fifth foundations. Keywords: local scour, computational fluid dynamics, Flow-3D model.

1

Research overview

One of the structures whose destruction has irreparable consequences is bridge foundations. In recent years, the local erosion of bridge foundations during flood flows has exceeded Other factors have caused the destruction of bridges, and for this reason, it has been the subject of many researches conducted in the last hundred years, and this issue has occupied the minds of researchers for years. Today, the scientific principles of structural design of bridges are well known. But until now, there is no single theory to estimate the local erosion around the bridge foundations, which is due to the complexity of the erosion problem. According to a complex process, the flow hitting the base turns into a horseshoe vortex and causes the flow to separate from the base and create a rising vortex. Design, calculation and construction of bridge foundations are one of the most important and sensitive stages of a bridge construction project, especially when this bridge is located at the crossing of a river. At this time, the designer must analyze the hydrological and hydraulic information of the area to choose the length and number of openings and the minimum depth of the foundations. One of the most important things that can be mentioned in this regard is the information about the level of erosion of the bed, which if not taken into account, one should witness dangerous consequences, including threatening the stability of the bridge and eventually destroying it. (Breusers et al., 1991)

1-1-Necessity and Research Objectives

Every year many bridges are destroyed all over the world, the destruction of these bridges is caused by not considering the role of hydraulic factors in the design of bridges. Most of the problems that we face after the construction of the foundations are due to the lack of full and sufficient attention to issues such as predicting the rise of water upstream of the foundations or estimating erosion around the foundations and methods of reducing it.In order to determine the scour depth in the vicinity of foundations, it is necessary to have sufficient knowledge of this phenomenon and the flow pattern around it, in order to determine a suitable method for estimating the scour depth. Normally, three practical methods are used to determine and predict scour depth. These methods include:

Physical models

Using special and equipped equipment, in order to measure the behavior of scouring created at the foundation site

Mathematical and computer models

  The first and second methods are completely experimental methods and are based on experiment and observation. The second method is a precise method that is more suitable for built foundations, and with the help of it, the existing problems are identified and the design is protected and strengthened against scouring threats. The main problem in this regard is that the equipment with scrubbing behavior measurement capabilities used in this method are very expensive and costly. Therefore, most of the studies and researches carried out are in the form of physical models and the equations related to determining scour depth are also presented based on these models. By using the first method, it is possible to check the scouring behavior and extent for foundations under construction and foundations built, but unfortunately, the results obtained from physical models are not reasonable and acceptable in most cases, because in this research, many of the complexities of the flow range around the foundations are considered very simple and most of the parameters are considered economical (Qiping Yang, 2005)

  Physical scales, flow characteristics and boundary conditions in small-scale models (laboratory conditions) should be extracted from the prototype [2] or real conditions and according to the laws of hydraulic similarity. Geometric similarity is usually necessary for all physical models. On the other hand, the Reynolds similarity for the models in which the flow around solid bodies is examined and the Froude similarity for the models including the flow with a free surface are relevant and should be considered. The mentioned simplification and the fact that many parameters are considered in the physical models is due to the fact that in the physical model of scouring around the foundations, it is not possible and practical to consider all the mentioned similarities. This method is basically based on theories and mathematical relationships, so that at first, a mathematical model is prepared by using the relationships related to erosion and the theories presented in relation to hydraulics and erosion of foundations. After this step and according to the prepared mathematical model, a computer model that can be adapted to the different conditions and states of this phenomenon is made. (Richardson et al., 1998)

In our beloved country of Iran, every year many bridges are destroyed due to scouring phenomenon and the damages are They bring a lot of life and money and sometimes irreparable. The statistics of table (1-1) show that despite the improvement and promotion of technical knowledge about the structural design of bridges and the advancement of construction technology, the amount of destruction of bridges has increased. Therefore, it seems necessary to investigate this phenomenon as much as possible in a correct and principled way in order to achieve accurate and acceptable results. Especially, carrying out quantitative research using specialized software and computer models can increase civil engineers' knowledge of this phenomenon and fill the gap in this field. Obviously, the realization of this important will reduce the cost of design, implementation and maintenance of river water structures in similar cases. While providing a suitable platform for similar and other researches.

The purpose of conducting this research can be divided into two main and sub-parts:

A- Main objectives: The main purpose of this research is to numerically model the scope of the second bridge of the Minab River and investigate the extent of scouring process development in the foundations of this bridge and provide appropriate solutions to control, reduce and prevent erosion caused by scouring in this scope.

B- Objectives Sub: After modeling the scope of the second bridge of the Minab river, parameters such as flow level profile, velocity distribution, around the foundations and the river bed will be investigated and evaluated.