Lessons from Aswan Dam The Aswan Dam in Egypt was once the pride of the Egyptian people and government, but soon after its completion, its adverse impact on the environment became increasingly serious, gradually changing people's evaluation of it. 10 years ago, Egyptian President Mubaramaru told scientists from all over the world at a scientific conference: "Brothers and sisters, from now until the year 2000, Egypt will have to face some major challenges. You must help us win. These challenges, that is, the serious problems we must face now and in the future, require serious scientific research from all angles, the most prominent of which is the impact of the Aswan Dam. " 1 Assumption and Construction of Aswan Dam As early as the beginning of the 20th century, some experts suggested that Egypt could build a high dam on the upper reaches of the Nile, thus regulating the river flow and expanding the irrigation area. In 1950s, the then Egyptian (Arab) government made an ambitious economic development plan. Because the population of Egypt is growing rapidly at the rate of 3% per year, the available natural resources are very limited. Therefore, the Egyptian government hopes to develop new resources and promote economic development by building the Nile High Dam. At that time, the Egyptian government and water conservancy experts thought. It is a wise move to build the Nile high dam. First of all, high dams can not only control the flood of river water, but also store river water for irrigation and other purposes in dry season. The arable land in Egypt is mainly located on the banks of the Nile River and the flood plain in the Nile Delta. After the completion of the high dam, the area of irrigated farmland can be greatly expanded to adapt to the rapidly growing population. Secondly, after the completion of the dam, it can generate huge power generation capacity and provide rich and cheap energy for industrialization. Third, building a giant reservoir formed by a dam and adjusting the downstream water level can develop freshwater aquaculture and inland navigation. With the financial and technical assistance of the Soviet Union, the Egyptian government completed the design of the Aswan Dam in 1959, broke ground in 1 960, closed the dam five years later, and officially completed the Aswan Dam in 1967. This dam was the largest high dam project in the world at that time. It is 1 12m high and 5km long, which cuts off the Nile and forms a huge reservoir-Lake Nasser with a length of 650km and a width of 25km in the upper reaches of the high dam. By 1970, all 12 hydro-generator sets installed on the dam have been put into operation. The huge storage capacity of Daba Reservoir not only regulates the downstream flow and prevents flooding, but also uses accumulated water to expand the irrigation area. Therefore, nearly 6,543,800 hectares of desert can be reclaimed as cultivated land. At the same time, the dam power station generates 8 billion kilowatt hours a year, which solves the energy shortage problem in Egypt. It can be said that the expected goals of the Egyptian government to build the Aswan Dam at that time were all realized. However, due to the limitations of people's understanding at that time, the seriousness of siltation in the reservoir area was underestimated, so the possible benefits of the dam project were too optimistic. The huge Nasser Lake formed when the dam was built, due to the natural deposition of sediment, the effective storage capacity of the reservoir gradually decreased, resulting in a decrease in the storage capacity of the reservoir. The designer of the dam project failed to accurately estimate the speed and process of sediment deposition in the reservoir area. According to the design of Aswan Dam water conservancy project, 26% of the reservoir capacity is dead capacity, and the sediment brought into the reservoir from the upstream is about 6000 ~18000 t every year. According to the sediment concentration of Nile water, the designer thinks that it will take 500 years for sediment to fill the dead storage capacity, and that the problem of sediment deposition has little effect on the benefit of the reservoir. However, after the completion of the dam, the actual situation is that the sediment is not uniformly deposited in the dead storage area of the reservoir, but quickly deposited in the slow water flow upstream of the reservoir; In this way, a large amount of sediment deposited in the upstream of the reservoir forms a triangular state at the entrance of the reservoir; In this way, shortly after the completion of the reservoir, its effective storage capacity decreased significantly, and the benefits of water conservancy projects were greatly reduced. In addition, the water surface evaporation of the vast reservoirs is very large, and the annual evaporation loss is equivalent to 1 1% of the storage capacity, which also reduces the expected benefits of water conservancy projects. What's more, when building such a magnificent dam, the Egyptian government and engineering designers ignored the impact of the dam on the ecology and environment, neither made a serious assessment of it, nor carefully considered the countermeasures after the ecology and environment were destroyed. The destruction of Aswan Dam on ecology and environment has a certain positive impact on ecology and environment. For example. Before the dam was built, with the alternation of thousands of rainy seasons every year, the vegetation on both sides of the river showed periodic withering and glory; After the completion of the reservoir, a desert 5300 ~ 7800km away from the reservoir appeared along the lake, which not only attracted many wild animals, but also helped to stabilize the lake shore and maintain water and soil, and played a certain role in protecting the reservoir surrounded by desert. However, after the dam was built for more than 20 years, the negative effects of the project gradually appeared, and with the passage of time, the damage of the dam to the ecology and environment became increasingly serious. These unforeseeable consequences not only worsen the ecology and environment of coastal basins, but also have a negative impact on the economic and social development of the whole country. 1) The dam project has caused a continuous decline in the soil fertility of cultivated land along the river basin. Before the dam was built, agriculture in the lower Nile benefited from the seasonal changes of Shihe River. Every year when the rainy season comes, the flooded river covers a lot of fertile sediment on the cultivated land, which periodically supplements the soil with fertility and water. However, after the completion of the dam, although water diversion irrigation can ensure that crops are not threatened by the previous drought. However, due to sediment blocking in the upstream of the reservoir area, the land in the downstream irrigation area can not be nourished. As a result, the fertility of the land is declining. 2) After the completion of the dam, soil salinization appeared on both banks of the Nile. Because the river is no longer flooded, there will be no more salt in the soil in the rainy season, and continuous irrigation will raise the groundwater level and bring the salt in the deep soil to the surface. Coupled with the high salt content in irrigation water and many chemical residues, the soil will be salinized. 3) The water quality of the Nile River in the reservoir area and downstream of the reservoir has deteriorated, and the health of residents who use the river as their living water source has been endangered. After the completion of the dam, the water quality and physical properties of the reservoir are obviously worse than the original Nile water. Water evaporation in reservoir area is an important reason for water quality change. Another reason is that the decline of land fertility forces farmers to use a large number of chemical fertilizers, and the residual part of chemical fertilizers returns to the Nile with irrigation water, which increases the nitrogen and phosphorus content of the river and leads to eutrophication. The average density of phytoplankton in the downstream river increases from 160mg/ L to 250 mg/L. In addition, soil salinization leads to a large increase of salt and chemical residues in the soil, which not only pollutes groundwater, but also increases the salt content of Nile water. These changes not only have a significant impact on the survival of organisms in the river and the irrigation of cultivated land in the basin, but also poison the drinking water of residents in the lower reaches of the Nile. 4) The change of river water nature makes aquatic plants and algae spread everywhere, which not only evaporates a lot of river water, but also blocks the river irrigation channels and so on. Due to the regulation of river flow, the turbidity of river water decreases and the water quality changes, which leads to the proliferation of aquatic plants. These aquatic plants not only spread all over irrigation channels, but also invaded major rivers. They hinder the effective operation of irrigation channels and need to be cleaned by mechanical or chemical methods frequently. This way. This also increases the maintenance cost of the irrigation system. At the same time, aquatic plants also transpiration a lot of water. According to the estimation of the Ministry of Irrigation in Egypt, the annual water loss due to transpiration of aquatic weeds reaches 40% of the irrigation water. 5) The riverbed of the lower Nile River is seriously eroded, and the coastline of the Nile estuary recedes. After the completion of the dam, the sediment concentration in the lower Nile decreased sharply, the suspended solids in the water decreased from 1 600ppm to 50ppm, and the turbidity decreased from 30 ~ 30~300mg to 15 ~ 40 mg/ 1. The decrease of sediment in the river has led to the erosion of the riverbed in the lower Nile. During the 12 years after the completion of the dam, the riverbed was eroded by an average of 2 cm per year from Aswan to Cairo. It is expected that the Nile will continue to change. It will take more than a century to form a new and stable river course. The decrease of sediment content in the lower reaches of the river, coupled with the Mediterranean circulation washing away sediment deposited in the estuary, led to the continuous retreat of the coastline of the Nile Delta. A former Egyptian soldier said that the lighthouse where he once stood has now fallen into the sea, far from the current coast.

It's about 1 ~ 2 kilometers away. 3 Comprehensive evaluation of the advantages and disadvantages of the dam When the Aswan Dam was built in the 1960s, people's understanding of the dam was still one-sided. Some of the ecological and environmental problems after the completion of the Aswan Dam were anticipated in the design, but they are inevitable or unsolvable. Others expected it, but underestimated the seriousness of the consequences; There are still some problems that are completely unexpected. Until today, people still think it is quite difficult to accurately predict the impact of dams on ecology and environment. Because it is very unpredictable to judge the consequences before the completion of the dam project, public opinion and scholars and experts in many countries often oppose or be cautious about the construction of super-large water conservancy facilities. It is not easy to comprehensively evaluate the advantages and disadvantages of dams. First of all, it is difficult to comprehensively measure and measure the impact of dams on ecology and environment with a single capital standard; Secondly, at present, human beings are only observing the short-term consequences of the dam, and it is difficult to predict and judge the long-term effects on the dam, because some effects may not be obvious or revealed in the decades after the dam is completed; In addition, how to observe the changes of ecology and environment accurately and reliably is still a difficult problem. For example, data such as river sediment concentration, reservoir fish production and water evaporation rate can be accurately collected, but there are still many data to be observed and analyzed. In addition, what data information is used to evaluate the effect of super-large water conservancy facilities scientifically and fairly, how to weigh the advantages and disadvantages, how big the advantages are, how deep the disadvantages are, and whether the advantages can offset the disadvantages need further discussion. At this point, after the completion of all the super-large water conservancy projects in the world. The above difficulties may be one of the reasons why no one has established a complete and comprehensive ecological environment monitoring system. For example, from how to evaluate the impact of Aswan Dam on the epidemic incidence, it is very difficult to comprehensively evaluate the advantages and disadvantages of the dam, although it is very necessary. At that time, some experts suggested that the completion of the Aswan Dam would lead to a large number of schistosomiasis patients, mainly because parasitic snails would multiply rapidly in slow-flowing water. However, the statistical data after the dam construction show that there is no obvious difference in schistosomiasis epidemic index before and after the dam construction, but the intestinal schistosomiasis has increased after the dam construction. The specific reason is unknown, but it seems that it has nothing to do with the dam. However, the incidence of epidemic diseases is not only affected by environmental factors. It is also influenced by social progress, economic development, population structure, migration of residents, changes in medical and health level and other factors. If these factors are taken into account, it is more complicated to analyze the difference of schistosomiasis epidemic index before and after the completion of the dam. It may be necessary to find an area in Egypt where the economic, social and medical level is still in the state of 30 ~ 50' s, and observe the influence of dam construction on schistosomiasis epidemic index without obvious social and economic progress. In fact, it is not enough to discuss the impact of dams on ecology and environment. Because the dam's damage to ecology and environment will be transformed into the damage to human society and economy. For example, after the completion of the Aswan Dam, the fertility of the land on both sides of the Nile declined, forcing farmers to use a large number of chemical fertilizers, greatly increasing agricultural costs and reducing agricultural income. 1982, a soil scientist estimated that the net agricultural income decreased by 10% due to the decline of soil fertility and the extensive use of chemical fertilizers and pesticides. In this way, although crops have changed from one season to two seasons a year because of the improvement of irrigation conditions, the annual output per unit land area has increased, but the input cost has increased faster, resulting in a decline in farmers' net income. If a dam has been built for many years, how should human beings face all kinds of positive and negative effects it brings? At present, people all over the world have two diametrically opposite views. Some environmentalists advocate the abolition of reservoir dams and power generation facilities. They think that the benefits brought by the dam are insignificant compared with its negative effects. However, others think that the dam should be rebuilt. Now that the dam has been built, it is necessary to gradually control various related problems. From the perspective of protecting the ecological environment, human beings should pay attention to the damage of dams to ecology and environment and take active measures to minimize these negative effects. "It is not too late to mend." However, we should also see that some damages caused by dams to ecology and environment are persistent and uncontrollable. Therefore, when we consider the direct economic benefits of the dam, we must not ignore the negative consequences of the dam. (