The role of slope and vegetation in the flood risk of the Zhalaki river basin

Dissertation for Master's Degree

Field: Natural Geography, Orientation: Geomorphology in Environmental Planning

Abstract

Flood is one of the most destructive natural disasters, which is usually associated with many human and financial losses. Floods every year in every corner of the country cause a lot of damage to economic resources. The occurrence of floods and the resulting damages in Iran has been increasing in recent years; However, not much attention has been paid to it. Every year, some of the buildings and facilities located downstream of the country's rivers suffer a lot of damage due to the occurrence of seasonal floods and the release of larger flows than the healthy capacity of the rivers. The occurrence of floods in a catchment area depends on many variables, these variables do not have constant conditions and values ??every year. In this research, an attempt has been made to investigate and compare the influence of geomorphological features, especially slope and vegetation, on flood risk by comparing the influence of the effective variables in the basin. The research method was based on the analytical method, and the work technique was the quantification of each of the effective variables in flooding (vegetation, slope, etc.). By examining the natural conditions, it can be seen that there are very favorable natural conditions for floods to occur. However, due to the high slope and the predominance of other investigated variables, the shear force of the water increases, and as a result, the erosive power of the stream increases, and therefore the flood risk is high in this basin. The obtained results show that the Zhalaki River basin is close to thermal in terms of morphometric features and obviously this basin has the ability to flood. According to the physical characteristics of the basin such as area, high slope, low permeability, low concentration time, along with other effective factors such as indiscriminate cutting of forests, destruction of pasture lands due to excessive grazing and so on. Especially in recent years, it has caused the height of the basin's runoff to be high, and every few years destructive floods cause problems for the sub-river's watershed. Using flood estimation methods, the flood discharge with a return period of more than 10 years produces a larger flood volume.

Key words: watershed, flood, lower river basin, environmental factors, vegetation, slope

Introduction

The importance of water in the life of living beings and the emergence of human civilizations is not hidden from anyone. Water has always been considered as one of the most abundant and at the same time one of the most precious weather elements. Rainfall is one of the most variable climatic parameters in any climate region, whose long-term deficiency can cause drought and severe or long-term rainfall can cause floods. On the other hand, not only the characteristics of rainfall can affect the characteristics of flooding or water flow of a river, but other climatic factors can also affect the characteristics of flooding and water flow of a river. Therefore, examining the changes in the flow of a river over time or examining the changes in the time of flooding can determine the effects of changes or lack of changes in the climatic conditions of a region, especially rainfall.

Throughout history, river valleys have been filled with people, but humans are still facing disasters such as floods and river erosion. Humans use the fertile lands of flood plains of rivers, which are suitable places for agriculture. Rivers provide sufficient and abundant drinking water for human consumption, domestic and even wild animals, land irrigation and other human uses in industry. Most floods are natural calamities, but the changes that humans have made in the river channels and urbanization in the valley floor have increased the fluctuation and intensity of many floods, in other words, these calamities are also caused by human factors. Next to the big rivers, there is enough water for urban and rural, industrial and farm irrigation. Therefore, a large part of the development of developed urban, industrial and agricultural centers located along rivers is caused by the flow of river water. The occurrence of terrible floods in the river is considered a threat to the facilities built near it. Although today, an important part of the floods of important rivers have been controlled, but still, floods cause a lot of collateral and financial damage to human societies and their capitals (Khaldi, 2013, p. 195).

Climate change is one of the current problems of human society and it is considered a threat and disaster for the planet earth (Baratian and Rahimzadeh, 2017). This phenomenon has a wide temporal and spatial scope. (Eshghi, 1378). In addition to the role of natural factors in climate change, humans also play an effective role in the intensity and extent of climate change, so that nowadays, when climate change is discussed, most of the opinions are focused on the role of humans and the activities that cause these changes or widen their temporal and spatial scope. This is mainly due to the increase in the consumption of fossil fuels, urbanization, deforestation, desertification, etc.

Floods are one of the most important factors that threaten the water and soil resources of the country. Conservation of water, soil and natural resources is one of the important strategies and goals of water conservation. Protecting the soil and preventing erosion is necessary for the preservation and restoration of renewable natural resources and comprehensive development of water resources. Damages and risks caused by erosion and movement of soil masses in watersheds are the most important issues and problems that have comprehensively affected the restoration of watersheds. The intensity of erosion and displacement of soil masses is a function of environmental, natural and geological factors, and causes the separation of particles in different ways and their displacement towards low and low-lying areas (Shariat Jafari, 1376, p. 124).

The characteristics of this phenomenon are also known in Iran. In other words, during the last half century, there has been an increasing trend in terms of number, volume and damage, and during the last forty years, more than 3700 important flood events have been recorded in the country, 52% of which are related to the last 10 years. The number of floods in the decade of 1370-80 was about 5 times that of the decade of 1330-1340 (Sharifi and Nowrozi, 1381).

Gilan is one of the parts of the country that has experienced the effects of droughts and sudden floods in recent years. It prevents severe erosion and movement of particles, especially soil, and this shows the importance of maintaining vegetation on sloping surfaces.

Obviously, the strength of resistance to erosion, natural conditions including the amount of rainfall and its penetration, slope and vegetation, type of rocks, weather and season, etc. . It also depends on human activity and often the largest amount of sediment is obtained from the initial rainfall after the low rainy season, and especially heavy and sudden rainfalls play a major role in the increase of solid discharge. In hydrology, it is important to determine the volume and time of the biggest flood, and this issue is also important from the point of view of geomorphology, because when a flood occurs, the characteristics of the waterway undergo changes and transformations (Motamid, 2019, p. 184).   

Various factors are involved in flooding, including: rainfall intensity, basin slope, land permeability, topographical conditions, vegetation characteristics and degree of soil saturation can be mentioned as effective factors in flooding. Of course, nowadays, due to human intervention, the flood risk factor of the basins has changed and mainly reduced its return period. Human activity has increased the possibility of floods in various ways, including building construction in the floodplain of the river and encroaching on the river boundary, which has reduced the natural capacity of the river. In this way, the area of ??the floodplain that goes under water during the flood becomes wider. (Thomas 11968, p. 45)

Urbanization and removal of plants reduce permeability and increase surface runoff. On the one hand, the large volume of runoff increases the size of the flood, and on the other hand, it increases the transfer of sediments, which, when deposited, reduces the capacity of the main river bed.

Obviously, it is possible to predict floods by knowing the flood time of catchment basins and determining the volume of runoff caused by rainfall and the flood levels in specific places along the waterways of the catchment basin, and knowing the factors influencing the occurrence of floods in the study area. It has helped to reduce the destructive effects of floods and its optimal use has reduced these damages to a great extent and has used this angry appearance of nature for the benefit of mankind and by channeling the floods, it has used its sediment and strengthened regional water reserves.