Lakes and Reservoirs

The natural fresh-water lakes scattered over the face of the earth benefit man in many different ways. Large lakes are used for transport; others provide electric power; the stillness of their waters allows river sediment to sink to the bottom, leaving the water clearer for water supplies; with their large storage capacity they reduce the flood intensity of the out-flowing river and increase its flow in drought; many lakes provide fish for recreation and sometimes for a fish industry; they may provide water for irrigation, or become a dump for municipal sewage, like Lake Geneva and some of the Great Lakes of America.

Lakes are born of a variety of natural occurrences. Some of the largest are formed by earth movements. Two examples are Lake Tanganyika, one of the rift valley lakes of Africa, which averages 1650 feet in depth, and Lake Baikal in Russia, which averages 5200 feet in depth. Many deep lakes in the Azores and Iceland are the water-filled craters of extinct volcanoes. Lakes may also form where a glacier blocks a river valley, or where glaciers melt and shed their load of rock as a terminal moraine, as occurred at Windermere in England, and Loch Lomond in Scotland. Landslides, which are a common feature of alpine regions, can form a natural dam across a river valley. In 1840 an earthquake toppled part of the mountain Nanga Parbat into the river Indus, and produced a lake 40 miles long and 1000 feet deep. Months later the dam gave way, releasing the pent-up waters within 24 hours, with disastrous results. In Japan, as in the Auvergne district of France, lava flows have blocked rivers; Lake Tiberias in Israel was formed in the same way. Deltaic lakes, such as those of the Rhone and Nile deltas, are produced by the building-up of banks of sand and gravel by off-shore currents.

Another way in which lakes are made is by erosion. Over long periods, glaciers have eroded U-shaped valleys in the Lake District, Scotland, Norway, and the Himalayas, producing finger lakes. Drift lakes, formed at the end of the ice age after the withdrawal of the ice-sheet, are shallow and of irregular outline, and abound in Finland and Canada. Icebergs left by the ice-sheet have sunk into soft earth to produce small lakes in Britain and also the Baltic lakes of Denmark and Germany. Other lakes were formed when water dissolved away soluble rocks, as in parts of the Swiss Dolomites.

The composition and concentration of salts and organic matter in lakes and reservoirs are determined by those of their river inflows. The make-up of river water in turn depends on the type of soil over which the water has drained. This drainage area, called the catchment area, is separated from neighbouring areas by a ridge usually known as a watershed . Many alpine lakes contain very little organic matter, because soil and vegetation are sparse on their catchment areas; they also contain a very low concentration of salts because they are surrounded by hard insoluble rock. In contrast, limestone and dolomite areas give water a pronounced hardness, while sulphur and other compounds occur in volcanic lakes.

In temperate climates, lakes rarely dry up by evaporation. But in the tropics and subtropics, where the rate of evaporation may exceed the rate of the river inflow, or where there is no outlet, a fresh-water lake may disappear; or it may become very saline, like the Dead Sea. In the tropics, evaporation may exceed 100 inches a year, and if runoff is also low, lake levels can fluctuate by as much as 40 feet.

Drought can also have serious effects outside the tropics. For example, a critical situation arose in the Great Lakes of America at the beginning of the navigation season of the St. Lawrence Seaway in 1964. Water-levels were at their lowest point since records began, and so serious was the position in Montreal harbour that ships had to reduce their draught by loading 9000 tons of grain instead of 14,000.

Until recently it was thought that planting forests {afforestation on catchment areas helped to conserve water; a few even claimed that trees actually increased rainfall. Admittedly, afforestation often reduces soil erosion, especially on steep slopes, and so prevents lakes and reservoirs from silting up; forests also reduce the intensity of peak floods by moderating runoff. Today there are second thoughts as to whether afforestation is the right policy in all cases. Trees, especially conifers, intercept on their leaves a large proportion of the rainfall, which then evaporates instead of reaching the ground. Runoff, and the replenishment of aquifers by the downward infiltration of water through the soil, may therefore actually be reduced by afforestation. But the effects of forests on runoff and infiltration, the interception of rain by leaves, and evaporation, are very complex factors, and each catchment area must be considered individually before deciding whether more is to be gained by planting or by cutting down various types of plant cover.

In terms of geological time, lakes are a very transient feature of the earth’s surface. During their brief existence, they are constantly changing: inflowing streams deposit sediment in deltaic fashion into the calm, shallow upstream end of the lake; simultaneously, the outlet stream gradually cuts deeper into the natural barrier holding back the water, and the lake level slowly falls until, choked by weed, silt, and vegetation, the lake becomes marshland. Biologically, it has reached its richest state on the borderline between the two great systems of land and water. To the naturalist, this aged specimen has a new value as a wetland teeming with life . But the lake is dead to the engineer, the economist, and the planner.

The physical, chemical, and biological changes that take place in lakes and reservoirs are so important that a whole science has grown up around them, called limnology. These changes depend very much on the size of a lake and on the climate, but the basic principles are best shown by considering a large, deep lake—such as Lake Michigan—where there is a very cold winter and a hot summer. Suppose we start in the autumn, when the lake is cooling rapidly. As the surface water cools, its density increases, the water sinks, and is replaced by warmer water from below. This process continues until the whole lake has cooled to 4°c—the temperature at which water’s density is greatest . Because the whole lake has the same density, it then circulates freely under the influence of the autumn winds.

When the air temperature falls to below 4°c, the upper layer of water decreases in density and so remains at the surface. With a hard frost, a layer of ice up to two feet thick may form at the surface, and it floats on the water beneath, which has a temperature above freezing point. This curious relation between water’s temperature and density, is one of the happiest accidents in nature, for without it the lake could freeze from the bottom upward, destroying all life in the process.

During the winter, water immediately beneath the ice cools down to near freezing point, but water in the deeper parts remains at 4°c. When, in the spring, the days become longer and warmer, the ice melts and the surface water soon warms up to 4°c. The whole lake is again almost uniform in density, and a slight wind is enough to circulate the water. As spring proceeds, the surface water warms up, becomes less dense than the water beneath, and so forms a distinct layer, or epilimnion, which may be as deep as 30 feet. Below, there is a zone a few feet thick where the temperature rapidly changes; this zone is called the thermocline, and the cool, static water trapped beneath is called the hypolimnion.

During the winter, as we saw, the lake is stratified, with ice and the coldest water at the surface. In summer, the lake is again stratified, but the other way round, with the warmest water at the top. During summer, water in. the epilimnion is circulated by winds, and warm inflowing streams add to its depth. The hypolimnion is unaffected by the sun or winds, and remains cold.

In spring, increased light and heat encourage the growth of dormant algae in the epilimnion. These multiply very quickly, and the dead algae are heavy enough to penetrate the thermocline and sink into the hypolimnion. By midsummer, the epilimnion still has plenty of oxygen, but has run short of nutrients; the growth of algae then slows down considerably, and many species die. In the hypolimnion the dissolved oxygen is soon consumed in breaking down the dead algae. The process of decay is then taken over by anaerobic organisms, which flourish in the absence of oxygen, and produce obnoxious compounds such as hydrogen sulphide, soluble iron salts, and methane. When this happens, only the epilimnion is fit to be used by water undertakings. The stratified condition persists until autumn, when the cooling of the surface water causes the whole lake to turn over, and the upper layers are again charged with nutrients to provide for algal growth the following year. If there has been much anaerobic decay during the summer, the autumn mixing means that water undertakings are unable to use water from any level of the lake until the products of decay are oxidized.

Such is the broad outline of the seasonal behaviour of a lake, but there are many variations. In temperate climates, seasonal changes differ from the situation in Lake Michigan only in so far as a lake may not reach 4°c throughout. But there is still an autumn mixing as soon as the epilimnion cools to the same temperature as the hypolimnion. In the tropics, many lakes are stratified all the year round, with an epilimnion permanently deficient in nutrients and with a hypolimnion highly charged with nutrients. Such lakes are poor producers of plankton and fish, and the only time when the upper layer contains nutrients is after a high wind has driven the epilimnion to one end, exposing the hypolimnion at the other. Then surges of water between gusts cause some mixing of the two layers at the windward end of the lake.

Thermal stratification in upland lakes and reservoirs is usually unimportant, because inflowing streams contain little organic and inorganic matter to support algae. Lowland water, on the other hand, is usually polluted with minerals and organic matter from industrial and domestic effluents and so provides an ideal breeding ground for algae, which may later clog filter beds and make the water taste and smell.

The need for more water and the rising cost of land both encourage the construction of deeper storage reservoirs, at teast 100 feet deep. In the past, two objections were levelled against deep reservoirs: first, the hypolimnion would be deoxygenated and therefore useless just when the summer demand reached its peak; and second, the reservoir would be unusable during the autumn overturn, when the polluted hypolimnion would contaminate the epilimnion.

Today, these objections no longer apply, because there are now ways of preventing stratification, such as introducing water into reservoirs at high velocity to cause turbulence and mixing. Another method is to use a vertical air-lift pump, which continuously lifts water from the hypolimnion into the epilimnion. The ‘aero-hydraulic’ gun, for instance, consists of a plastic tube that discharges a compressed-air bubble that entraps water as it rises. It is a cheap and effective device with negligible maintenance costs; it also requires very little power because the transference of water beneath the surface requires very little energy.

The flow of most rivers fluctuates widely from season to season; in North Africa many dry up completely every summer. The maximum amount that can be abstracted for water supplies throughout the year is determined by the minimum flow, called the dry-weather flow; surplus water during wet periods is wasted unless it is stored in reservoirs. A reservoir is simply an artificial lake, or ‘water bank.’ It may be a natural lake whose outlet has been dammed so that the water-level can be adjusted. Alternatively a completely new lake may be formed by damming a river valley. In the few places where the river flow is uniform, as when it is fep! by an ample spring, reservoirs are unnecessary; nor are they needed in areas, such as a village on the Amazon, where water requirements are only a small fraction- of the dry-weather flow of the river. But as more and more water is taken from a river, so the risk of it drying up in drought increases and the need for a reservoir grows.

The traditional type of reservoir—which also produces the cleanest water—is made by building a dam across a relatively uninhabited upland valley. Another type occurs at Plover Cove, Hong Kong, for example, where a sea inlet is being converted to hold fresh water running down from the hills by building a dam across the inlet; similar schemes are being considered in England. Another type of reservoir is formed on flat land by an encircling embankment to receive water pumped from a nearby river. This type of reservoir is particularly useful when upland sites are distant, as in the case of London. One of the most efficient types of reservoir is that used for river regulation, where a reservoir on the head waters both supplements the flow of the river in drought and acts as a retention basin during winter floods.

The economics of reservoir design are governed by two extremes: the dry-weather flow of the river, and the intensity of the anticipated peak floods. These extremes are worked out from meteorological records covering a period of at least 20 years, and are best recorded on an automatic gauge across the river. From these readings it is possible to estimate statistically the likely intensity of a drought or flood at intervals of, say, 20, 50, 100, or 1000 years. Much more difficult to decide is what intensity of extremes the reservoir and dam should be designed to meet. Obviously the once in a hundred years drought is far more severe than the once in ten years, and far more costly to allow for. In some parts of the world it would be impracticable to allow for every eventuality. In Mauritius, for example, the prospect always exists of the top layer of water being blown right out of the reservoir by a hurricane at the beginning of the dry season. Another problem that faces water engineers is the amount of room that should be allowed in a reservoir for silting. The life-span of a reservoir may vary from 20 years in some parts of Bolivia or India to 800 years in the few silt-free drainage areas of Western Europe. With all these variables to consider, any estimate of the safe yield of a reservoir can only be approximate, and it is not surprising that some reservoirs periodically go dry, or that dams occasionally give way under exceptional floods.

The creation of reservoirs in bare river valleys involves no actual construction work on the valley basin itself; all that has to be done is to clear away vegetation and re-house the valley population. The really difficult and costly part of any reservoir scheme is building the dam and its foundations. Men have been building dams for over 5000 years. At first these were laid across the beds of small streams, sometimes to be swept away with the next flood, at other times to last for many centuries. Most of these early dams were built to store water for irrigation and for drinking, though some were made to trap fish, otters, beavers, and other animals. The oldest dam in existence stands in the eastern desert of Egypt across the Wadi Gerrawi . Made of rough masonry 3000 years ago, it once stored the brief winter rains to supply workers in the nearby alabaster quarries. Around 1300 B.C., the Lake of Horns in Syria, 20 square miles in area, was formed by the Orontes Dam, whose sluices still supply the neighbouring irrigation canals. Many ancient dams still exist in Arabia, the most notable near Marib in the Yemen, of fine masonry keyed together with copper fastenings. The Romans built many large masonry dams in Italy and North Africa from which water flowed to cities along aqueducts. In the Middle East, cement-lined tanks and stone cisterns abound; one at Aden has a capacity of over 30MG. The Israel coast is still dotted with Roman cisterns that were constructed to collect some of the expected annual 56 days of rain.

Now scientifically designed, modern dams approach heights of 800 feet. It is perhaps difficult to appreciate that large dams are among the most costly of all engineering projects. Large dams contain an enormous amount of material. The Hoover Dam, which stores the entire flow of the Colorado River for two years, is as high as a 60-storey skyscraper, while the Grand Coulee Dam, with a waterfall over its spillway more than twice as high as the Niagara Falls, contains enough concrete to build a highway across the U.S.A. and back.

Water-supply dams impound water for domestic and industrial use for cities like New York, . ..»¦¦: ?

Los Angeles, and Manchester . Flood control dams are essential in some parts of the world to save lives and property, such as the five dams of the Miami River valley, which protect Dayton, Ohio. Navigation dams, usually consisting of a stair-like series of dams and locks, are built to maintain a minimum depth of water for ships.

Irrigation dams are a common feature in arid areas such as India and Pakistan. Irrigation is also the primary function of the Hoover Dam in the U.S.A. Before it was built, the Colorado River flooded the Imperial valley in Arizona when the mountain snows melted, and became a sluggish stream in summer. Now the reservoir saves the floodwater and has greatly increased the irrigated area. The Grand Coulee Dam on the Columbia River was built partly because the river had cut its bed too deep. From the reservoir created by the dam, large pumps lift water to another reservoir on the plateau, to supply farms through a canal system.

A very important irrigation dam is the Aswan High Dam, started in Egypt in 1960 near to the old Aswan Dam built in 1902. Half Egypt’s population are farmers who rely entirely on irrigation. In the past they produced only one crop a year, by the old system of basin irrigation. After the new dam is built, there will be sufficient water for several crops a year, and the irrigated land will be increased by 1.7 million acres. It will also produce 2100 MW of electric power. Virtually a rainless country, Egypt depends for its existence entirely on the Nile, the second longest river in the world. In summer, the Nile’s flow, increased by monsoon runoff from the Abyssinian highlands, floods large areas of the adjacent land. Formerly, most of this summer floodwater drained into the sea, and about once every decade, disastrous floods destroyed agricultural land and canals. The Aswan High Dam will create a giant lake, 315 miles long and an average 5 miles in width, to store this surplus water from one year to the next. One quarter of its capacity will be reserved to accommodate the freak floods, while a small fraction will accommodate the silt carried by the Nile.

Hydro-electric power dams use the force of falling water to move turbine blades connected with an electric generator. The turbines and generators are housed in a power station in or below the dam, or in a diversion tunnel in the valley side. The amount of electricity produced depends on both the head and the volume of the water. One of the most important African hydroelectric schemes is the Volta Dam in Ghana, completed in 1965. Ghana had relied on cocoa and gold for most of its income, but in 1953 President Nkrumah decided to expand the economy by creating industries. Since industries require power, hitherto obtained from expensive imported diesel oil, it was decided to use the flow of the Volta River to produce 512,000 KW of electricity. Of this, 60 per cent would be used for a new aluminium smelter, and the remainder for domestic use and for future industries.

Since the flow of the Volta River during the dry season is 300 times less than during flood, it was necessary to create an enormous dam, to form in 1965 the largest man-made lake in the world—3275 square miles in area and 250 miles long. Apart from its main function of providing power, the Volta scheme also serves other purposes. It provides a cheap means of transport, opening up new opportunities for developing the natural iron deposits and other industries to the north. Because 600 square miles of the lake shore will flood seasonally, new farming communities will be established to grow rice and other crops. Other communities around the lake margin will be able to catch an estimated 10,000 tons of fish a year. The whole project, though dramatic, is based on sound principles, but whether the country can afford to repay the borrowed £61 million, only time will show.

Strange as it may seem, water provides the only economical means of ‘storing’ electrical energy for use at times of peak demand. In pumped storage schemes, surplus electricity from the grid is used to pump water into higher reservoirs, from which it is released during the day to produce hydro-electric power and augment existing power supplies.

The greatest benefit is invariably obtained from those water schemes that serve several purposes. Such multipurpose reservoirs may provide any combination of the following services: hydro-electric power, water supply, irrigation, flood control, navigation, fishing, and recreation. One might expect that these different objectives would at times conflict, but in practice this seldom happens. ‘ – –

The first multipurpose scheme, started during the great American depression of the 1930s, was created by the Tennessee Valley Authority to develop the poverty-stricken Tennessee basin. Most of the population were farmers who had to contend with bad soil fertility and erosion, and a river too shallow for navigation in summer and liable to violent flooding in winter.

Today there are 31 large and 12 small dams along 650 miles of the river and its tributaries. The lake levels are kept low just before the winter floods, during which as much water as possible is allowed to discharge past the dams. After the flood season ends, the reservoirs fill up with the spring rains. During summer and autumn this water is released to maintain an adequate river flow, leaving the lake levels low again for the start of the next winter floods. The TVA scheme produced a notable rise in the standard of living and agriculture. Flood control alone saves about £185 million annually. The scheme also provides large amounts of hydroelectric power for industrial and domestic use. The dams and locks have turned the river into a navigable stairway of lakes—a great advantage to the many new industries. There are also 50 times more fish than there were before, providing new opportunities for recreation.

We now explain the principles involved in building dams, which will be more easily grasped with the help of the diagrams. A dam must be strong enough to withstand the tremendous force of water pushing against its walls. Contrary to what one might expect, this for,ce has no relation to the length of the reservoir; for the same depth of water and area of dam exposed, the water pressure is exactly the same in a reservoir 10 miles long as it is in one of 100 miles. The horizontal thrust on a dam depends solely on the depth of water, increasing by 624 pounds per square foot for every ten-foot increase in depth. The force is therefore greatest at the base of the dam; in a reservoir 100 feet deep this will be 2.8 tons per square foot, and a dam 100 feet high and half a mile long will have to sustain a total water pressure of 352,000 tons. There is thus a tendency not only for the whole structure to be pushed downstream, but also for it to be tipped over. Such tendencies are usually counteracted by making the dam broadest at the base, and sufficiently heavy. Excessive pressure on the dam during floods is prevented by building a spillway, which may take the form of a diversion tunnel through or around the dam, or may simply be a chute over the crest.

If there is leakage under the dam, another force comes into play that tends to lift the dam off its foundations. Such seepage is prevented by building a cutoff wall from the dam down to impermeable bedrock; if the dam already rests on fir-m rock, any rock cracks are filled up by injecting liquid cement—a process called grouting. It is also important to prevent leakage through the dam itself, not so much to prevent the escape of water as to prevent the stability of the structure from being undermined. Waterproofing is achieved by having an impervious centre or an impervious upstream face.

Several factors have to be considered before deciding what type of dam to build, where it should be sited, and how much margin of safety there should be. Wrong decisions have led to disasters in the past, so today great care is taken to make a proper preliminary investigation. This starts with a geological survey to determine whether the rock foundations are strong and stable enough to take the weight of the dam without shifting later. The choice of dam also depends on the nature of the available materials. As the movement of material, sometimes involving shifting 10 million tons, forms a large proportion of the total cost, materials are usually used from the immediate neighbourhood.

The most permanent dam and the one that requires least maintenance is the solid gravity concrete dam. So long as the underlying rock can withstand the enormous weight of these massive structures, this kind of dam can be built almost anywhere and can be made much higher than those of earth or rock. The spillway usually takes the form of a chute over the crest. If the dam is used for hydro-electric power, water passes to the power house either through the dam or through a diversion tunnel at the side. In other cases, large pipes called penstocks convey water from the dam to the power house.

In the few sites where there is a narrow gorge with strong sides, a curved concrete wall can be built so that the force of water is transferred to the cliff sides. Such an arch dam, unlike a solid gravity concrete dam, relies for stability on its shape, rather than on its weight. It also contains much less concrete and so is cheaper to construct. The greatest stability is achieved when the gorge is narrow and deep, and the thinner the dam the more it depends on the strength of the gorge sides. For this reason, thin arch dams need to be very carefully designed.

In a wide valley a concrete multiple arch, or buttress, dam may be built. This type is really a modification of the arch dam, and consists of a series of short arches supported by buttresses. The arches slope at an angle of about 45° on the upstream face, so that water bears down on the dam and helps to give it stability. Sometimes flat slabs of reinforced concrete are used instead of arches.

The earth dam, made of sand, gravel, or clay, is practical only if these materials are near at hand. Since they have a very broad base, earth dams are particularly suitable where the valley floor is too soft to bear the weight of a concrete dam. They are also fairly flexible, and so are suitable in places where gradual earth movements are likely to occur. Earth dams contain a waterproof core reaching from ground to above high-water level, supported on either side with earth. Puddle clay is often used for the core, but in dry countries like South Africa, parts of America, and Australia, the core is made of less flexible but stronger concrete. The core is invariably taken down into the bedrock as a concrete cutoff wall. The downstream slope of the dam is protected from erosion by heavy rain with gravel or turf, and the upstream slope is protected from wave action by rock or concrete slabs. Unlike the tougher concrete dams, the spillway of an earth dam is always separated from the dam itself in a special overflow leading to a tunnel cut into the side of the valley, because water pouring over the crest would destroy the downstream face. This most ancient of all types of dam has become increasingly common during the last 30 years. While concrete dams remain static in price, earth dams have become cheaper and more reliable because of advances in the science of soil mechanics and better earth-moving machinery. Well-designed earth dams also require very little maintenance.

The rock dam is another ancient type that is becoming more popular. It is built in places where suitable rock can be quarried near the site—particularly in canyons, where rock is first dumped into the river. The upstream face is covered with carefully graded rock, and then faced with concrete to resist seepage and wave action. Sometimes an inner waterproof soil core is used, as in the High Aswan and Volta dams. Rock dams cannot withstand earth movements as well as soil dams, and because of their open texture they need a spillway around the dam. clay core

Public safety first and economics second dictate the choice of dam. Every effort must be taken to ensure sound foundations, even settlement, adequate spillways, and good-quality materials. Some dams now approach the 800-foot-high mark, holding back masses of water unheard of years ago. The benefits are great—the potential dangers sometimes greater, if the designer has miscalculated the forces at work. At Frejus in France, the failure of an arch dam in 1959 caused £20 million of damage, and the loss of hundreds of lives. The rock foundations were said to be suitable for an arch dam thick enough to-cover the supporting rock, which was not homogeneous. Andre Coyne, the greatest dam designer the world had ever seen, produced instead a thin arch dam. A few days before the Frejus tragedy he wrote of his complete faith in his arch dam: ‘Each successful experiment is valuable; if it fails its value is greater.’

Reservoirs are so useful that we tend to forget that they can also be a potential menace. Few realize the fury of unleashed water. Water flowing at about 10 miles an hour can roll 10-foot-diameter boulders along a river bed with ease. Imagine therefore the power of the torrent pouring though the gap torn by the ‘Dam Busters’ in the Mohne Dam, Germany, during World War II. Water passing through a large hole in a dam 100 feet below the surface would in theory travel at nearly 60 miles an hour, a terrifying speed, which, with 100 million tons of water behind it, would cause catastrophic destruction in the towns below.

Yet nature has long been an old hand at the game of dam busting. From early times alpine herdsmen have observed a dam produced by a glacier lifting, to release the water stored in the lake behind. In 1934 the Nevado Glacier, blocking the Rio Plonio valley in the Argentine, suddenly floated and let loose 14,000 million gallons. An earthquake in Chile in 1960 caused a landslide that blocked the outlet of Lake Rinihue, raising the water-level 86 feet and the total storage to 800,000 million gallons. Fortunately the perilous situation was relieved by cutting an escape channel before the natural dam burst.

Landslides are common in steep mountainous areas, where some of the best waters and best dam sites are found. In the Piave valley of Italy, on October 9, 1963, there was a night of terror. Within a few seconds 1000 million tons of Mount Toe fell into the newly built Vaiont Reservoir. This pushed a great wave over the new arch dam and swept 2000 villagers to their doom. The dam remained intact under pressures far greater than those provided for.

A landslide had occurred while the dam was being built, so model tests were made to assess the chances of this happening again. Beneath the limestone of Mount Toe were hard layers of clay, which softened by the penetration of the rising waters of the new reservoir so that the overlying rock slipped. Some authorities claim that the tests were inadequate; others say they were disregarded by the engineers. However, such disasters do stress the special responsibility of the dam designer and the necessity of carefully integrating the structure of the dam with the geology of the whole valley.