Dissertation for obtaining a master's degree
in the field of civil engineering of hydraulic structures
October 2013
This study is to quantitatively evaluate the effects of watershed operations and with the aim of evaluating the efficiency of watershed projects and using the results It is done for future decisions. For this purpose, in the studied basin, the amount of sediment behind the short dams created in the area is estimated and also Changes in the slope of the waterway as a result of the construction of the mentioned short dams are investigated and evaluated with the amount of sediment and discharge recorded in the hydrometric station in the time period before the construction of the short dams and after their construction. Results It shows that the input peak discharge from 1.3 m3/s during the peak 0.3hr to the exit peak discharge 1.165m3/s has been reduced in 0.4hr time and based on the estimates made in all the studied structures about 117073 cubic meters of sediment has been extracted. This means that as a result of the construction and existence of the mentioned structures, the reduction of the volume of Nahand Dam has been prevented and finally, despite these structures, the useful life of the dam has increased. First chapter
Introduction and Generalities
Introduction
The growing population growth in the country and the need for development in various economic fields, including agriculture and animal husbandry and the development of rural and urban settlements, have made the acquisition of natural areas inevitable, and on the other hand, the frequency of destructive floods along with the removal of about half of 413 The country's billion cubic meters of atmospheric precipitation, the destruction of forests and pastures, the reduction of land fertility and other issues and problems are among the problems that show the need for comprehensive special attention to watersheds.
A look at the past shows that the increasing erosion probably started at the same time as the beginning of agriculture and productivity of the land, and as a result, the chaos imposed by humans on the environment and the fertile and green lands turned into deserts. It has become bare, waterless and grassy, ??and people's habitats have been abandoned due to floods and the destruction of fertile lands. After the occurrence of these events, people think of fighting against erosion and forceful factors of nature, which affects the way of planting and harvesting and the use of wide terraces. can be Based on this, in the last century until now, various studies have been carried out in the field of combating erosion and optimal use of agriculture, water and soil (Agha Majidi et al. 2013).
Soil is gradually eroded under the influence of erosive factors and is transported by water flow from high places to low areas. Erosion refers to the movement of soil from its original location by water or wind. The phenomenon of erosion takes place in three stages as follows:
- the separation of soil particles
- the transfer of soil particles
- the settling of soil particles
These three stages will not be harmful if they occur naturally. but if the natural balance of the soil is disturbed due to the change of vegetation in different forms, the erosion will be intensified and harmful (Ziaei 2010). Currently, soil erosion is considered a serious threat to human life. Erosion has reduced the fertility of the soil and abandoned the fields, the resulting sediment settles in canals, waterways and reservoirs of dams and reduces their water intake capacity. Therefore, it is necessary to protect the soil and fight against erosion. Water and soil conservation operations have been common among human societies for centuries. In those years, this operation was performed initially. The progress and expansion of water and soil protection operations has grown significantly in the last fifty years, but it has not been able to curb soil erosion. Paying attention to water control operations and combating erosion and sedimentation in agriculture, watershed management, dam construction, irrigation, road construction and rural and regional development projects is of great importance. One of the main factors of comprehensive watershed management plans, in reviving and preventing the wasting of the national wealth of a watershed, is to recognize the erosion of the watershed. Investigating the amount of erosion and production of basin sediment and its spatial distribution is very important.This research was conducted in order to understand the erosion mechanism of the watershed surface and to estimate the amount of produced sediment through the evaluation of all the effective factors in causing erosion (Refahi 2018). Today, evaluation is one of the basic elements of plans and projects, and in advanced societies it is used as a tool to measure the effectiveness of plans and programs according to the intended goals. It is necessary to evaluate watershed management plans according to the goals and the environmental effects of the plan on the watershed and outside it (Ghorti 1383).
According to the statistics prepared by the United Nations, among natural disasters, floods and storms have caused the most casualties and damage to human societies (Saenei et al., 1384). The trend of floods in recent years indicates that most regions of the country are exposed to the invasion of destructive floods, and the dimensions of the damages and casualties of floods are increasing (Khosroshahi and Thaqfian, 2014b). Since flood and its dangers have always been considered as one of the natural disasters and with its destructive effects and widespread human and financial losses, especially in recent years, it has been one of the main preoccupations of the country's leaders, and it has seriously reduced the level of safety and security of the people and residents along the rivers and flood plains (Beynam, 2007), but unfortunately, the issue of managing and reducing its losses has not been given serious attention in the country, and only for a while. that a destructive flood will flow and a disaster will occur, drawing the attention of officials and experts to it. It can be done (Saneyi et al., 2004). 1-2 1-2 Statement of the problem
In the semi-arid country of Iran, the implementation of watershed management and water and soil projects is necessary to maintain national interests and it causes the correct use of water and soil resources, prevents soil erosion and improves water productivity. Watershed plans are the starting point for preserving natural resources and their continuation for better exploitation and preventing the wastage of national resources, and by evaluating them quantitatively, a clear view can be obtained about the effectiveness of such measures in watersheds.
The solution to the problems of water challenges on the one hand, the increase in population and the need for more people for healthy food, the lack of water needed by the agricultural sector, the drop in the level of underground aquifers, the lack of atmospheric precipitation and the decrease in agricultural production, on the other hand, the need to implement plans It has made watershed more than ever. The important point in this regard is the level of effectiveness and profitability of watershed management projects, which requires the review and evaluation of watershed management operations so that, based on the results, while determining the efficiency of watershed management operations in the above basin, it also provides decision-making regarding the implementation of similar plans in other watersheds. Evaluation of watershed management plans in order to check the level of achievement of goals, improve methods, review macro and micro policies and Inventing new methods is an inevitable necessity. If the performance of these plans is evaluated in appropriate ways and from various technical and engineering aspects, the results can lead to finding the reasons for the failure of the proposed operation and providing suitable solutions to solve them (Eskandari et al. 2013).
At the global level, the World Food and Agriculture Organization[1] has paid special attention to the issue of evaluating watershed management operations, in such a way that it has published various publications and views in this field, and its working method is based on the principle that before the implementation of the project, , the initial conditions should be studied, then the impact of each watershed management operation in reducing runoff and sediment should be evaluated (Timori et al. 2010).
More than a decade has passed since the new period of watershed management in the country, which has been accompanied by a fundamental change in its structure and importance. A development that resulted in the improvement of the qualitative and quantitative level of programs at different levels and departments of management, implementation, studies and coordination with other relevant departments in the fields of nature. The view of the planners and agents of the watershed department is to implement a series of limited implementation projects, with the aim of controlling sediment upstream of large dams, now spread throughout the vast country of Iran, with goals such as water and soil protection, dealing with drought, flood control, improving the conditions of green land cover, paying attention to agricultural infrastructure resources, improving the economic conditions of watershed residents, etc. It has changed and evolved.