Vol-7, Special Issue3-April, 2016, pp1755-1760 http://www.bipublication.com
Review Article
Study of the Environmental Impact of Constructing
Dams on the Environment
Somayeh Babazadeh
Msc. of Hydrology, Department of geology,
Science and research branch, Islamic Azad University, Tehran, Iran
Email: solmaz_bzadeh@yahoo.com
ABSTRACT
Undoubtedly, water is considered as one of the pivotal and fundamental bases for the development of communities in different countries. The development of water resources by constructing big and small dams in accordance with their situation and circumstances has been caused enormous environmental changes. Mutual impacts of dams on the environment can be assessed at two aspects of positive and negative effects. The dam as a barrier on the river creates different physical and chemical features in the place, shoal and mirage of the dam. despite various advantages which this function has had on the limitation of exploitation, such as decreasing the abundance and intensity of floodwaters and the accessibility to water in arid or drought years, but constructing such structures can strike the balance of natural systems and it would leave many short term and long term environmental impacts on that region or other regions which are affected by constructing dams. These impacts may have some influence on the condition and behavior of aerology, biology and hygienic, culture, monuments etc. and seriously will cause to change it . As a result, regarding the importance of positive effects of constructing dams, in order to stable development their negative environmental impacts should be minimized. In the present paper, the environmental impact of constructing dams on the environment has been assessed from different aspects .
Key words: Dam, Environmental Effects, Environment
1. INTRODUCTION
According to the vital role of water in all period of the human life and ever increasing population, the water shortage crisis has been always predictable and it has persuaded experts to decrease water loss by providing methods and plans of water restrain (Sait Tahmicioglu at. al., 2007). For many years constructing dams as a barrier against the water flow and reserving it in huge reservoirs, controlling the floodwater, generating energy and so on, has been considered as a fundamental approach. For many authors, leaders, engineers, staff, nationalists and revolutionists in the past six decades, huge dams have been considered as a symbol of national pride and the dominance of human genius on the nature, electricity, water and
food supplier, controlling floodwater, desert regenerative and the national independence guarantor. (Salehi Hafashjani at. al., 2013). Therefore, constructing big dams which in the first half of twenty century they were not over 5000, in the first decade of twenty first century and during less than sixty years exceeded 52000 big dams. The great dam-builder in the world –china- has over 19000 big dams that except eight of them others have been constructed after 1949 revolution. After china, is the United States of America with 5500 big dams and then Russia and India .
disadvantages as well, that in some cases they are reparable and though impacts of water on the human life and the expansion of civilization has been known all over the world but it has been claimed that concerned economical advantages have not been achieved from projects which were designed for exploitation of water resources, as well as necessary predictions for decreasing the environmental, economical and social disadvantages were not accomplished properly (Salehi Hafashjani at. al., 2013).
Even some international organizations, in order to stooping water supply projects in developing countries have accomplished some studies. Therefore, in the water resources management, the cultural, social and economical development has been considered and environmental impacts as a result of these studied is of the utmost importance. Nowadays, in designing studies which are accomplished based on new findings, in addition to technical and economical issues, environmental, social and cultural values should be considered as well. In this paper environmental influences of constructing dams on a majority of environmental aspects have been examined (Brandt, 2000).
2. Environmental influences of constructing
dams
2.1.Influence on rivers
Rivers have different biological features in terms of having rapid, slow or calm flowing waters in which any industrial or economical activity have great influences on the river life. Rivers according to the kind of their water flow and the geographical location and altitude have different biological communities. These aquatic environments as a running water ecosystem have different vegetation. Different macrophytes of flowering plants (lentic) generally form the vegetation of fresh water rivers so that these terrestrial plants through some mechanisms adapt themselves to aquatic condition .
Algae are considered as one the important plants in running waters that generally are centralized in
some parts with high speed of water flow. These kinds of algae are sticky .
In contrast, speed of water flow decreases in downstream of the river, thus the background is provided for formation of Phytoplankton algae and Diatoms .One of the animal species in rivers are fishes that their diffusion pattern is related to dissolved oxygen in water (Hanai. at. al., 2008). Usually at the time of dam exploitation, the speed of the water flow at that part of the river will alter and these changes leave their influence up to kilometers in the upstream and downstream of the dam. For example, in the upstream of a dam, where the reservoir is formed, the speed of water flow decreases up to a large extent, and it will lead to accumulation of sludge and slit in the reservoir and it can influence on the life of algae and cause to perish them. By perishing one ring of the ecosystem, destruction of other rings that form the ecosystem chain will be accelerated (Brandt, 2000).
Fishes are among the most abundant aquatic organisms in rivers that some of them swim against the water to reach themselves to the upstream for spawning. After constructing a dam on the river the motion path is cut and it leads to vanishing rare species of fishes and the animal life in the river will be destroy. Perishing fishes will cause to gradual disappear of organisms which feed from fishes, but what is the solution ?
To solve the problem of fish emigration in rivers that a dam has been constructed on them, a kind of lifts are used to carry fishes from the dam to the river upstream that the above problem is solved to some extent only on the condition that dam designers be care about the river ecosystem. To carry fishes from the place of dam toward the river upstream some lifts are used which to some extent can solve the above problem only under the condition that dam designers devote attention for the ecosystem of river.
•A dam acts as a barrier against the movement of floating things across the rivers, such as trees, ice cubes, fishes, ships, and so on.
•The effect of sediment in the reservoir for blocking hatches and tippers.
•The surface evaporation increases due to the growth of water level .
The salinity of farmlands is due to an increase in the level of underground water and drainage of farmlands flow from the surface irrigation (Brandt, 2000).
When the surface of underground water is close to the earth, the capillary phenomenon takes the available salt to the earth. Even when the water usage is under control, due to low quality of underground water, probably the secondary salinity will arise. The Salinity of land causes to decrease the productivity of salt sensitive products and thus the land becomes unproductive.
2.2. Changes of the river outflow due to
constructing a dam in its shoal
Dams despite their real advantages have influences on the bed and boarders of the river way especially in their shoal. The impact of dam construction is related to the hydrological feature of the shoal with the morphometery of basins, dams’ volume, geometric features of the river beds and the amount of released waters from reservoirs during the time. A majority of dams at least release a little water toward downstream at the beginning of exploitation and until filling the reservoir. Decreasing the outflow causes main problems in the downstream beds that the amount of these changes depends on the quantity and magnitude of constructed dams across a river. In addition to effects of constructing dams on released water as well as their impact on decreasing floodwaters, also they create some changes in the daily and annually pattern of the river flow. It should be mentioned that daily changes occurs when the water power is used (Hanai. at. al., 2008). Since producing and consuming electricity during the night is more than day time, releasing water in night is more than day. Also, seasonal changes occur due to
changes in the precipitation pattern that changes will appear in downstream in the form of delay in reflection of outflow changes.
What the important is in terms of bed changes and in the view point of geomorphology is changes in the dominant flow or in the amount of flow that fills the river path. The dominant outflow plays the primary role in adjusting the main bed. According to the relation between geo-morphological changes of the river bed with the amount of outflow and its changes during the time, the amount of changes before and after constructing a dam should be considered in a way that analysis of geo-morphological changes become possible .
Some researchers believe that for studying the amount of flow, percentage of changes in the average of daily flow before and after of constructing a dam should be considered that this would be possible by using the following equation (Salehi Hafashjani at. al., 2013):
PC = 100 ((X2/X1) / (Y2/Y1))
That in this regard:
PC: the Percentage of daily average changes of flow in a specific time
X2 / X: The average flow of the river during the
specified time, in an under studied place, for the periods before and after constructing the dam.
Y2 / Y: Daily average of daily flow at a station that
has not been affected by dam construction.
2.3. The impact of constructing dams on the
river sediment load
dams; of course the amount of accumulative sediment behind the dam depends on some factors such as regional and lithological features of basins. In the zonal or semi-arid regions, the surface of hillsides is not completely kept by vegetation, as well as their material is removed by running waters of showery precipitations. Therefore, in these regions the amount of river sediment load and also the amount of sediments behind the dam are higher. In addition to changes which occur after constructing dams in the amount of sediment load due to changes in the performance of the process, some changes in the increase of sediment load are related to operations of constructing dams and the hillsides turmoil of it.
Of course, it should be mentioned that, other factors are able to maximize or minimize impacts of constructing dams in terms of changing the amount of river sediment load in the fluvial systems. For example, the amount of sediment transfer through the dam is affected by the quality of accumulated water behind it and its changes during the time. Also, thermal changes in the basin and the amount of water have influence on the sediment carriage capacity (Sait Tahmicioglu at. al., 2007).
2.4. The influence of constructing dams on
earthquakes and activity of faults
Based on the seismology, when tectonic tensions are accumulated across a fault to reach a critic point due to sudden evacuation of them, the earthquake will happen (Wang at. al., 2012). Always there are some arguments about what causes to start an earthquake? Is it conceivable that the trigger of the earthquake is squeezed by human ?
Seismologists call these kinds of earthquakes as inductive. The inductive earthquake is applied to those which occur due to construction of dams, filling and realizing water. The main factors in occurrence of such earthquakes are a follow :
• The condition of reservoir and quality of water supply
• The proceeding of rise in water height , specially a rapid increase after a long period of relative stagnation in its changes
• Achieving the stored water level to the maximal
• A time span in which water of the reservoir is kept in the maximal
• Geological, tectonic and seismological features
• Existence of gaps and holes in the beneath layers of the reservoir
Regenerator factors and the mechanism of earthquakes which are attributed to dams has not been know completely like natural earthquakes yet; but based on carried studies, when the height of stored water exceed from 100 meters, a powerful earthquake could occur (Salehi Hafashjani at. al., 2013).
When a dam is constructed near active faults, water penetration in the separation levels of faults will make them slippery and thus, relocation of top and low plates towards each other will be easily. Besides, water penetration to lower beds will cause to increase the pressure between gaps of available rocks in the depth. Accordingly, the hydraulic pressure of artificial lakes in the centralized and dense layers and beds will help an earthquake to happen .
Therefore, in the case of specific geological and tectonic conditions, constructing big dams and creation of deep reservoirs will cause to earthquake or some changes in the seismic activity of nearby regions will happen .
Even in regions that have already known stable and calm in terms of seismicity, in some cases creating artificial lakes may cause to motivate the mechanisms of orogenic movements, accumulation of tension, and releasing accumulated tensions and sometimes it appears in the form of an earthquake.
2.5. The impact of constructing dam on health
and hygiene of the river inhabitants
and hygiene of their inhabitants, that among them it can be mentioned to the outbreak of diseases which their carriers use stagnant waters, including Malaria (Salehi Hafashjani at. al., 2013).
The influences of reservoirs on biological collections can be divided into two categories; the first category is created changes by the structure of the reservoir and operation of constructing dams in the surroundings and in terms of productivity it will affect on other activities. The second category depends on the management and quality of directing the dam and reservoir. Sometime, the biological process in the dam is against the main purpose of dam construction (supplying power or drinking and agricultural water). The rapid growth of forage in the water may prevent turbines from moving or the growth of alga makes the water lake useless for domestic or industrial uses (Sait Tahmicioglu at. al., 2007). Another issue related to rivers that a dam is constructed on them is about the discharge of domestic and industrial sewage and agricultural waste. Since the river normally has the self-purification property therefore it can gradually decrease its pollution while it is running along by the river by absorbing oxygen. Dams decrease the speed of river to reach it to a steady state behind dams and it will cause to reduce the self-purification property of the river and during the time due to accumulation of pollutants behind dams living organisms in the lake and lands around the dam become infected (Wang at. al., 2012).
Furthermore, since the reservoir water is used for various consumptions, it can transmit contamination from water to human and other animals that live around dams. This affair itself causes an outbreak of diseases and will transmit it through animals to other ecosystems .
With the arrival of devices and machinery for constructing dams, another environmental effect of it is that the noise level increases in the area. The generated sound by related machinery as well as explosions that occur during the operations of constructing dam will strike the peace of the area
and have undesirable impacts on its wildlife. Leaving the nest, emigration to adjacent areas, the possibility of abortion in mammals and birds are among effects and consequences of noise pollution caused by activities of the dam construction (Sait Tahmicioglu at. al., 2007). 2.6. Economical and social effects of constructing dams
Among destructive social and economical influences of constructing dams on the environment around the river, its impact on the adjacent villages can be mentioned. Villages and communities that are submerged in the reservoir will lose their farmlands and it causes a growth in emigration to cities and creation of false jobs (Fearnside, 2016).
Also, by constructing dams, roads and power transmission lines due to passing from within the reservoir will destroy and therefore the accessibility to some region of dams, old watercourses and dikes would be impossible(Sait Tahmicioglu at. al., 2007).
Another problem sometimes is engaging foreign workers for constructing dams and developing the tourism industry during its exploitation that to some extent it can strike the security of cities and villages and in the long time it is able to change the cultural and traditional structure of the city or village adjacent to the river (Hanai. at. al., 2008). Sometime, monuments, historical places and some places with beautiful and specific topography that are found rarely would disappear under water.
3. CONCLUSION
exploitation, striking the balance of biological systems and immigration of different species of human and animals and finally spreading different disease and hygienic issues because of decreasing the quality of surface and underground waters and creation of marshes. It may be better that to avoid such destructive effects alternative methods are thought to control water and floodwater or in designation and construction of such great and costly installation, at least the environmental results in their affected regions to be considered.
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