Laboratory investigation of erosion in the downstream of relaxation ponds for the outlet of pipes

To receive a master's degree

Civil Engineering - Water

Abstract:

Relaxation ponds should be designed in such a way that they can depreciate the potential energy stored in the reservoir water of dams and dams in such a way as to transfer it to the natural bed so as not to cause the loss of the bed and in critical conditions to destroy the pond and dam itself. In various sources, depending on the flow conditions and the shape of the flow outlet, based on the results of numerous laboratory studies, methods for the design of relaxation ponds are presented, which in some cases are extremely uneconomical and lead to excessive length and excessive consumption of materials. In this research, several of the best models presented for relaxation ponds at the outlet of pipes were modeled and the performance of the models were compared through the introduction of the dimensionless parameter of the scour depth ratio to the distance of the maximum scour location from the beginning of the erodible bed. In order to properly compare the models, in all experiments, by creating the same hydraulic conditions upstream and downstream of the pond, or in other words, placing all the ponds between two equal energies and modeling the same erodible bed at the bottom of all models, the performance of the ponds was checked by comparing the parameter of the scour depth ratio to the distance of the maximum scour from the beginning of the sediment pond and the profile of the middle of the scour hole. Finally, by making changes in the proposed ponds, including the wall angle introduced in USBR type six pond [1] and creating a reverse slope in the pond floor, we managed to reduce the length of the pond by 20%.

The research results show that there is an optimal angle for the wall inside the pond to improve the performance of the pond independent of the inflow descent, and the reverse slope of the pond floor improves the pond's performance. Although the relaxation pond introduced in this research has a shorter length than the best models introduced, it creates less erosion in the same hydraulic conditions and shows a more independent function compared to the groundwater level.

Key words: base water, erosion, relaxation pond, Froude number, normal depth

Chapter 1

Introduction, generalities and definitions

Introduction

The word erosion, which is pronounced in English as erogen and in French as erosion, comes from the Latin root erodri [1], which means abrasion and is the wearing away of the earth's surface. In general, soil erosion [2] is the movement and separation of soil particles from their original substrate and its transfer to another place due to the transfer agent (Ellison [3], 1947). If the agent that separates particles from the bed and transports them is water, it is called water erosion[4]. When water moves from one level to a lower level, its potential energy is converted into kinetic energy. If the high kinetic energy of the flow is not restrained in the long term, it can cause damage and erosion in the downstream of the flow. In order to prevent damage caused by the super energy of water at super critical speeds and also in order to eliminate the excess kinetic energy in such water, special structures called energy receivers[5] are generally used, which are built downstream. In addition to destroying water energy, such structures are also considered as a means to control and restrain hydraulic jump and create the conditions for its occurrence in a specific location. In other words, when a hydraulic jump occurs on a smooth surface, the slightest change in the depth of the upstream and downstream flow can cause the location of the jump to change to one of the two sides, so it is logical to use structures in the ponds to reduce sensitivity, including base water[6].

In general, there are two methods to reduce water energy, the first method is to use local transformations or other components that cause serious disturbances in the flow, the second method is based on throwing water at a point. It is far away that in the meantime the flow turns into small particles and loses a lot of its energy when it collides with the air and returns to the ground (Abrishami 1380).

Although the length of the hydraulic jump and the drop created is one of the sensitive parameters in the design, but in general it cannot be calculated through mathematical analysis and it is necessary to use experimental and laboratory results in each case (Abrishami 1380).

One of the places where the flow with speed It is prone to cause damage in the downstream of the flow, the output of the pipes is under pressure, the high energy of this flow must be efficiently taken to avoid damage and erosion in the downstream.Water jets, culverts, and water transfer tunnels are examples of structures that require energy-absorbing structures (USBR, 1974).

In the past 60 years, many studies have been conducted on relaxation ponds at the outlet of pipes. Different ponds are provided in different forms and lengths based on the descent of the output stream, many of them are relatively long in addition to low efficiency, and their construction requires spending a lot of money. A suitable criterion for checking the performance of ponds and their efficiency in consuming water energy is the amount of erosion created in the aquifer under similar hydraulic conditions. In other words, the better the pond's performance, the more turbulence it causes, and as a result, the output water has less energy and its erosion power is reduced.

In this research, the amount of scouring and the form of the scouring hole are considered as criteria for comparing the performance of ponds.

1-2 Importance and Necessity of Conducting Research

To express the importance of conducting this research, as mentioned in the introduction, there are several models for the design of relaxation ponds at the outlet of pipes. have been introduced, all of which are based on laboratory modeling. In the first part of this research, the researcher considers the need for a comprehensive research and the use of a suitable criterion to compare the proposed models necessary by examining the available sources, so that by examining and introducing the weaknesses and strengths of the introduced models, the least expensive and most effective model presented by others can be introduced. In this research, the first part is focused on the review and recognition of the introduced models. The second basic point about the relaxation pond at the outlet of the pipes is to investigate the possibility of improving the performance of the pond based on the weaknesses and strengths of other models. Based on the complexity of the solution field in the relaxation ponds, for analytical or numerical solution to investigate the amount of loss created in different types of relaxation ponds, the majority of studies have been conducted in a laboratory manner, accordingly, the basis of the comparisons in this research is also laboratory modeling. Due to the large number of variables affecting the performance of relaxation ponds at the outlet of the pipes, it is not possible to examine all the parameters at the same time in one research, and in the second part of this research, due to the large length of previous models, by examining some parameters that have not been examined in previous researches, the possibility of reducing the length of the pond in a way that does not reduce the performance of the pond has been examined. The reasonable assumptions of the performance of the ponds should be investigated and compared with the previous models according to the new changes at different depths.

1-3 main research questions

Which model performs better among the introduced models?

Is it possible to reduce the length of the relaxation pond?

Given that the reverse slope causes the weight component to be stuck in the momentum equations, can it improve the performance of the relaxation pond?

Is there an optimal angle for the wall introduced in the standard of the United States Department of Public Works [1] that improves the performance of the pond by creating more turbulence? The effect of gravity is investigated in the form of a dimensionless dynamic parameter called the Froude number. This number in each section of the flow is defined as follows:

In relation (1-1), V is the average speed, g is the acceleration of gravity and L is the characteristic length of the flow, which in closed channels is the characteristic length of the pipe diameter (Abrishami, 1380). their efficiency becomes (Di [9], 2006).

1-4-2-1 types of scouring

Scouring is divided from different aspects as follows:

- scouring in terms of the cause of its occurrence, which includes general scouring, scouring caused by the narrowing of the section and local scouring.

- scouring in terms of sediment transport, which includes scouring In the case of clear water, scouring in the state of water contains sediment (Publication 318, 1389).

1-4-2-1-1 General scouring

This type of scouring occurs when the flow in a section of the river is able to move and carry the bed particles along the way, which causes a decrease in the level of the river bed in the same section.