Estimation of Manning's roughness coefficient in rivers (case study of Fahlian river)

Dissertation for M.A.

Abstract

This research deals with the determination of the Manning roughness coefficient in the studied area by knowing the conditions governing the Fahlian river using various experimental and semi-experimental methods and tables for use in designs. And it presents the best and most reliable method of determining the roughness coefficient using field studies and combining it with experimental relationships, and at the end, a number of suggested relationships are presented. For this purpose, the studied area was divided into 5 intervals with a length of 23 km, and in each interval, the granulation test of the substrate material was done, and by using the mentioned methods, n corresponding to each interval was obtained. Then, in order to check the results of the n value obtained, it was entered into the HEC-RAS software, and the discharge curves obtained from the software were compared with the actual discharge curves of the river stations. The results of this study show that the experimental and semi-experimental methods are more accurate than other methods due to the sandy nature of the river bed. Also, in order to verify the accuracy, the longitudinal profile of the river was drawn using HEC-RAS software and the coefficient n of the selected method, and the height of the water surface obtained from this profile was compared with the height measured by the upstream and downstream stations, and the error value of 4 cm was observed. At the end, Manning's roughness coefficient value was 0.037 for this river.

Key words: Manning's roughness coefficient, Eschel discharge curve, HEC-RAS software, granulation test, Fahlian

1- General

1-1- Introduction

The importance of the subject of correctly estimating Manning's roughness coefficient , statement of the problem, goals and necessities of the present research and description of the structure of the thesis are the things that will be stated in this chapter.

In order to analyze the conditions governing uniform flows as well as the design of hydraulic structures and river engineering, there are several relationships, among which Manning's relationship is more important and has a lot of generality. In this regard, estimating the appropriate value of Manning's roughness coefficient plays a significant role in estimating the water level and is of particular importance. According to the research conducted so far in this field, it can be concluded that very small differences of a few hundredths in determining the values ??of Manning's roughness coefficient can change the estimated area of ??flooded lands by tens of thousands of square meters and thus have a significant impact on the estimation of the amount of damages. This also affects the management planning and organization of the river. Therefore, such plans and measures that are directly related to the management and organization of rivers, require great accuracy in choosing the method of determining Manning's roughness coefficient. Manning's roughness coefficient contains all the effective factors in the resistance of the channel bed against the flow and also this coefficient shows the intensity of energy loss in a flow. The proper way to estimate Manning's coefficient more correctly is to know the effective factors in this coefficient. These factors include the roughness of the channel bed, the type of channel, the irregularity of the cross-section, vegetation (its type and degree of density), the shape of the path (straight or spiral), the presence of obstacles in the flow path, and even the depth and flow of the flow, which in addition to the effect on the longitudinal drop in the flow path, to some extent include the drops caused by the change in the shape of the flow (local drops).

1-2- Statement of the problem

It is necessary to design the crossing structures of the river such as bridges, diverting dams or catchment openings, determining the hydraulic factors such as the depth and speed of the flow. The most common relationship in determining speed is Manning's relationship, and to use this relationship, it is inevitable to determine Manning's roughness coefficient. To calculate Manning's roughness coefficient, there is no reliable method for all rivers, and for each river, the best method must be determined according to the conditions of the same river. The roughness value is one of the main parameters of energy loss in canals and rivers, whose changes even by a few hundredths cause tens of thousands of square meters of difference in the level of land flooding. Therefore, by accurately estimating its amount, which is affected by various factors such as the type of bed, vegetation, granularity and size of bed particles, the way the river section changes, etc. In addition to the non-structural prevention of floods, with the help of flood risk zoning, which has more economic justification, we can have more confidence in the results of hydraulic calculations.On the other hand, since the accuracy of determining the depth of the flow is effective in determining the technical and legal characteristics such as the boundaries of the river bed and boundaries, the accurate estimation of the hydraulic roughness coefficient is of legal importance in addition to the technical value. Also, knowing the effect of Manning's roughness coefficient on erosion and sediment transfer, with the appropriate selection of this coefficient, one can witness its important and positive effects on soil protection and watershed management.

The Fahlian River is one of the large rivers in the southwest of the country, located in the northwest of Fars province in Mamsani and Verstam cities. It is formed by Shiv, Kati and Khairabad rivers and flows into the Persian Gulf. Many buildings and hydraulic structures have been built on this river, and now the Parsian (Gorak) dam is also being implemented upstream of the river on one of its main branches located in Doshmen Ziyari Mamsani. Flood management as well as flood flow zoning can be important and effective in designing and protecting and organizing river engineering projects and determining the safe boundary of this river. One of the most important parameters in river engineering is the Manning roughness coefficient, and the results of hydraulic models of the river are highly dependent on this parameter. Therefore, this research aims to determine the most suitable relationship for calculating the Manning roughness coefficient in the Fahlian river, due to the fact that it passes through residential areas, agricultural lands, the presence of many facilities around it is of special importance, it will examine the existing relationships in this field, in the river in question, and improve the management planning and organization of the said areas.

1-3- Objectives and necessity of conducting the research

Given the importance and sensitivity of estimating the Manning roughness coefficient in rivers and considering that there have been no previous studies in this regard on the Fahlian river, it was felt necessary to calculate the Manning roughness coefficient in this river using different methods, compare these methods and estimate the amount of error in each relation.

Comparison of the results Different methods and their analysis will help to carry out further studies in the same field in other rivers as well as studies in different hydraulic fields.                           

In consulting companies, the only common method for estimating Manning's coefficient in rivers is Kavan's method. Carrying out this method requires time-consuming and expensive research, and at the same time, in many rivers, including rivers with sandy beds, estimating numbers larger than the actual number in these rivers will cause additional costs. Therefore, by determining the best method of calculating the Manning roughness coefficient for the studied river using the inverse solution method and Hec-Ras software, in this research, it will help to build more accurate hydraulic structures on these rivers, as well as save time and costs for the construction of these structures. In this research, the error in each method is calculated with the help of the root-mean-square sum of errors relationship. 1-4- Thesis structure: All the contents of this thesis will be presented in five chapters, and the key topics of each of these five chapters are briefly presented below: The necessity of addressing this issue was also expressed. Statement of the problem, goals, and necessities of the research is one of the other parts of this chapter. Chapter Two: Background and background of the research. Due to the wide range and variety of methods for estimating the Manning roughness coefficient in rivers, at the beginning of this chapter, it is tried to present the existing categories in this field. In the following, after stating a number of parameters and terms used in this research, the results of the researches of other researchers in this field will be discussed.

Chapter 3: Materials and Methods

In this chapter, the studied river is introduced, and using the collected information, the studied areas of this river are divided into different sub-areas, and the important description methods used in this research will be discussed. In the following, it will be explained how, with the help of Hec-Ras software, and the use of the root mean square error (RMSE) statistical method, the most appropriate Manning roughness coefficient will be calculated. Also, proposed experimental methods to calculate this coefficient in Fahlian river will be presented.