A vast engineering project involving the expenditure of more than £75,000,000 has been devised to regulate the flow of the Tennessee River and to combat erosion of the surrounding land. The task of the Tennessee Valley Authority is to regulate the Tennessee River, which flows for 800 miles before it joins the Ohio
ONE OF THE LARGEST NAVIGATION LOCKS in the world under construction at Pickwick Landing Dam, Tennessee. The building of the dam has caused the formation of a lake 53 miles long.
OF the many schemes fostered by the United States Government to promote economic recovery, that known as the Tennessee Valley Authority, popularly alluded to as T.V.A., is one of the most remarkable. It is a vast engineering project designed to solve the major economic problems of a region 40,600 square miles in extent — about four-fifths the area of England. The scheme is achieving complete control of the Tennessee River and its tributaries and is using this control to effect navigation, flood control, agricultural and industrial development. The T.V.A. was created by special Act of Congress in May 1933. In the following month operations in the field were begun. Two colossal dams had been built by 1937 and another four were at that time under construction, in addition to other major engineering undertakings. In all, eleven large dams were projected and scheduled for completion by 1940. The estimated cost was £37,000,000.
The Tennessee River, whose farthest affluents rise in the mountains of Virginia, is the largest tributary of the Ohio. In its course of 800 miles it flows through several States. The river and its tributaries water the State of Tennessee and parts of Virginia, North Carolina, Alabama, Mississippi and Kentucky. Two million people dwell in the Tennessee Valley and another four million in the mountains flanking it on either side. The principal agricultural products of the region are maize, wheat, cotton and tobacco. Dairying sheep farming are carried on. Coal, ore and copper are found. Industries include iron and steel, flour, lumber, cottons and cotton-seed oil. All these are directly affected by the T.V.A. Schemes.
The Tennessee Valley includes ground ranging from 325 feet to 6,700 feet above sea level, with a mean annual rainfall varying from below 40 in. to over 80 in., and with equally marked temperature contrasts. Originally the entire region was forested, but large tracts of the valley and lower slopes have long since been cleared for farmland. The higher ground is still timbered and there are important undeveloped mineral resources. But the population is largely agricultural, living standards have remained low, and the trend of the last few decades has been down rather than up. This tendency is explained partly by the fact that the southerners in general are easy-going; partly by the serious impoverishment of the land, which has lost much of its topsoil by erosion; and partly by a variety of historical and economic causes which have combined to retard the development of the region.
The task of the T.V.A. is to rehabilitate this backward region by regulating and using its water resources for power and river navigation; by the introduction of cheap electricity on a regional scale for use in homes, farms, and factories; by the development of the mineral resources of the area; by checking erosion with soil-conservation measures; and by improving crop yields with artificial fertilization, thus leading to radical changes in variety, quality and yield of agricultural produce. Such an extensive scheme could not be carried out unless the T.V.A. obtained control of the entire Tennessee River system.
There have been abortive attempts in the past by the Government as well as by private agencies to make the river navigable and to control floods. In a state of Nature the Tennessee River is one of extremes. Violent floods may swamp its banks, menace towns and cities, and increase the destructive high-water crests of the Ohio and Mississippi Rivers. Then there are periods of drought, when the waters run low, navigation is impossible and power development is largely nullified. In times of drought water in certain stretches of the river falls as low as three feet, or even as low as one foot in some places.
Congress called for a river with a minimum depth of nine feet, which is sufficient for the light-draught boats used on the internal waterways of the United States. A channel of this depth could have been obtained by a series of low dams. This, however, would have proved too costly for the benefits derived and low dams would not have solved the problem of flood control.
After exhaustive surveys it was decided to build a number of high dams with ample storage capacity, forming a. series of gigantic steps with the river represented by locks. Between Paducah, Kentucky, where the Tennessee joins the Ohio, and Knoxville, Tennessee, 650 miles upstream, ten of these dams were planned. Of these, three were already in existence — the Government’s Dam No. 1 below Muscle Shoals, Alabama, the Wilson Dam at Muscle Shoals and the privately owned dam at Hales Bar, Tennessee. Between Knoxville and Paducah the river falls 475 feet, an average of less than 9 in, a mile. The fall is far from being evenly distributed, however, so that dams of uniform height and spaced at equal intervals were out of the question. Apart from this, the geological setting of the various sites involved such a diversity of problems, calling for special treatment, that entirely different specifications were needed in each instance.
Of the dams needed to provide the nine-feet channel the T.V.A. engineers had in 1937 completed the Wheeler Dam, and work was well advanced on dams at Chickamauga, near Chattanooga, Tennessee; at Pickwick Landing, in the same State, and at Guntersville, Alabama. The next dams in the scheme are at Coulter Shoals (below Knoxville), Watts Bar (Tennessee) and Gilbertsville (Kentucky).
The dams will create a series of lakes varying with the gradient of the stretch involved. The length of the lake created by Gilbertsville Dam will be 184 miles, that by Pickwick Landing Dam 53 miles, that by Wilson Dam 15 miles, that by Wheeler Dam 86 miles, and those by the dams at Guntersville 81 miles, at Hales Bar 35 miles, at Chickamauga 59 miles, at Watts Bar 74 miles, and at Coulter Shoals 44 miles respectively. It is estimated that by 1950 river traffic will have increased from below 3,000,000 tons to over 17,000,000 tons a year. Because of the extremely variable flow of the Tennessee, it is necessary, apart from the dams, with their locks on the main river, to control its tributaries. Upon three important tributaries — the Clinch, Hiwassee and Little Tennessee Rivers — storage dams with no lock system are being built. That on the Clinch, known as Norris Dam, was completed by 1936. The main object of these tributary dams is to control flood waters and to ensure a steady flow for hydroelectric power.
Storage of Flood Water
As the largest tributary of the Ohio, the Tennessee River is an important factor in the Mississippi floods. At the peak of the great flood of April 1927, which inundated 28,000 square miles along the Mississippi, the Tennessee was responsible for 285,000 cubic feet a second, or 14 per cent of the total discharge. By storing the heavy seasonal rains of March 1937 Norris Dam reduced the subsequent flow crest in the Tennessee River at Chattanooga by about four feet, and so saved 1,000 acres of urban property from being flooded. This represented a saving of £200,000 in property damage alone. Under the present plan, dams on the Tennessee and its tributaries will have a storage capacity of 7,000,000 acre feet, which may ultimately be raised to 10,000,000 acre feet by further damming of the minor tributaries.
Norris Dam, lying in a wooded valley 800 feet above sea level, on the Clinch River, six miles downstream from its confluence with the Powell River and twenty miles north-west of Knoxville, is typical of the engineering work which the T.V.A. is carrying out. The masonry section of the dam, 1,570 feet long, is in addition to a rolled earth embankment 302 feet long, with a reinforced concrete core wall connecting the concrete dam with the east hillside. The base thickness of the dam in the spillway section is 204 feet, and the height 265 feet from the lowest point in the power-house foundation to the roadway across the top. Along the crest of the spillway, separated by 12-feet concrete piers, are three hydraulically operated steel-drum gates, each 100 feet long, designed for dealing with flood waters. A concrete-encased steel girder bridge for the roadway spans the three spillway openings. There are eight outlet conduits through the dam, grouped in pairs.
BUILDING THE NAVIGATION LOCK at Chickamauga Dam, near Chattanooga, Tennessee. This dam was designed to create an artificial lake 59 miles in length, the formation of which necessitated the rearrangement of hundreds of miles of roads and railways and the building of many bridges.
he power house, 96 ft. 6 in. wide and 203 ft. 8 in. long, is situated on the east side of the river immediately downstream from the dam and adjacent to the spillway. There are two turbines, each generating 66,000 horse-power. The current generated is carried by conductors through an underground tunnel and ducts to step-up transformers in an outdoor switch-yard, from which it is carried to various distribution points.
Considerable excavation work in the bed of the river was necessary to obtain a solid foundation for the dam, and much of this work was carried out in the dry season to escape risk of floods. For this foundation work cofferdams, huge timber structures strengthened and protected by interlocking sheet piling, were used.
There were three of these structures. One, extending from the east bank of the river, enclosed the power-house area, a large portion of the tailrace, and the dam and spillway as far as midstream, the river being diverted round the western end of the structure. A second cofferdam was built out from the opposite bank and, finally, a third in midstream enclosed the midstream portion of the dam and the spillway. This third cofferdam was not built until preparations had been made for diverting the river through low blocks in the partly completed barrier on either side of the river.
What engineers have to guard against in the building of these great reservoirs is seepage. However solidly built a barrier may be, if the rocky bed on which it stands is faulty, water is liable to find its way under the dam. This may prove a serious matter, particularly if a large volume of water is being held up. The enormous pressure of the impounded water would be liable in time to tear away the rock and force a large opening beneath the dam wall. To avoid such a catastrophe high-pressure grouting is resorted to, and this was done on a fairly extensive scale on the site of Norris Dam.
Precautions Against Seepage
The grouting is effected through core holes spaced about 10 feet apart. The depth of the holes varies from 70 feet to 200 feet, according to the character of the foundation rock below. In all there were approximately 18,400 feet of core borings. In Chickamauga Dam, 51 miles of core borings were drilled, and some of them had a diameter of 36 in., large enough for men to be lowered into them. Into the boreholes liquid cement is forced under high pressure. The excavated material taken out for the foundations of Norris Dam totalled 156,205 cubic yards of earth, and 218,165 cubic yards of rock.
To speed up the cement-casting after the foundations had been obtained, it was necessary to deliver semi-liquid concrete to different sections of the dam in turn, allowing each pouring to set before more was cast on top. To avoid resort to cooling by artificial refrigeration, individual pourings of concrete were limited to 5-feet depths and an interval three days was allowed before the next pouring on the same block, so that the heat generated by the setting cement might dissipate naturally. Batches of concrete from giant mixers high on the valley walls had therefore to be deliverable without delay at any point over an area of 30,000 square feet and perhaps as high as 265 feet above the bedrock.
WHEELER DAM, at the head of Wilson Lake, Alabama. The length of this concrete dam is 6,343 feet, and the water impounded has formed a lake 86 miles long. Although Wheeler Dam was built primarily to assist navigation, provision was made also for the generation of hydroelectric power. In the foreground of the illustration is the power house, which is equipped with two 48,003 horse-power turbines.
The concrete was delivered from the mixing sheds to the men working upon the dam by an overhead electric cableway which spanned the site, and was carried on two tall steel towers. The engineer in the control room on the north bank had full control of the moving buckets and could bring a charge of concrete exactly over any desired spot without interfering with work in progress 30 feet away. The greatest amount poured at Norris Dam in any one day was 4,108 cubic yards. The concrete was cast in blocks, 56 feet long, which were keyed into one another, each block having embedded in it sensitive electrical and mechanical apparatus to detect and record the least changes of temperature or pressure due to yielding under unequal strains, to leakage among block-joints, or to chemical changes occurring in the composition of the material.
Ordinarily this great retaining wall will hold up the waters of the Clinch River for a distance of 72 miles and those of its tributary, the Powell, for 56 miles, thus forming a lake 37,400 acres in extent, or nearly 59 square miles. This is the maximum storage of the reservoir for power purposes, representing 1,710,000 acre feet of water. During floods the water level can be raised another 32 feet. The surface area of the lake is then increased to 52,000 acres (81 square miles), and the quantity of water impounded to 3,090,000 acre feet. The clearance of this area was an arduous task. Hundreds of farms and buildings had to be removed, thousands of trees were uprooted and tons of debris had to be destroyed.
The second major project carried out by the T.V.A. was the building of Wheeler Dam, on the Tennessee River, at the head of Wilson Lake, some fifteen miles from Muscle Shoals. The task was completed in 1936. The total length of this concrete structure is 6,343 feet and the length of the roadway which crosses it 6,518 feet, its height being 72 feet. The dam is made up successively of a non-overflow section (1771 feet long) upstream from the service bay and control house; eight forebays (613 feet long) in the powerhouse section; another non-overflow section, 717½ feet long; 5,500 feet of spillway; 1,756 feet of non-overflow; 159 feet of lock; and a cut-off wall, 220 feet long, across the location of a possible future lock, tying the dam into the north bank.
Although Wheeler Dam is primarily a navigation dam, it is designed also for hydro-electric power. The power house, a five-storied structure 48 feet wide and 155 feet long, is situated at the extreme southern end of the dam and is equipped with two turbines, each generating 48,000 horse-power. Vessels can pass up and down the river through a lock 60 feet wide, 350 feet long, and having a lift of 50 feet. Space is provided for a larger lock 110 feet by 600 feet, should it be found desirable. The water impounded by this great dam has created a lake about 140 square miles in area, extending up the river some eighty-six miles to Guntersville.
Pickwick Landing Dam, another navigation barrier on the Tennessee River, is designed as an earth embankment across the valley on either side of the river, with concrete structures across the main channel, including navigation lock, spillway and non-overflow section. The total length is 7,710 feet, and the height 110 feet. The dam impounds water with an area of 48,500 acres (nearly 76 square miles), extending upstream 53 miles to Wilson Dam.
Guntersville and Chickamauga Dams are navigation dams on the Tennessee River. The lakes formed by these dams involve the rearrangement of hundreds of miles of roads and railways and the building of many bridges, in addition to elaborate clearance work in removing settlements, farms, buildings and many thousands of trees.
The whole of this work, calling for an immense amount of organization and engineering skill, is carried out by the officers and men of the T.V.A. In building the dams considerable economies in engineering costs were effected by a policy of serial construction. The dams are started in succession, technical staff and equipment being moved from site to site as the work at each place progresses.
The major items of cost in building dams are construction equipment, operating equipment, materials and labour. If the unit of construction is the single dam, construction equipment must often be sold at a heavy loss when the job is finished. Operating equipment must be specially designed, a labour force must be assembled, trained and discharged when, it may well be, it has reached its peak of efficiency.
Building materials, especially cement, must be bought at prices determined by the amount needed for one dam. If a series of dams is being built, however, under co-ordinated direction, equipment may be moved from one dam to another, elements of design, such as gates, may often be duplicated, trained men may be transferred from job to job, and cement and other materials may be bought more cheaply because they are contracted for in large quantities.
The preliminary work undertaken by the engineers and experts of the T.V.A. before any of their dams are started is most thorough. There is a laboratory which determines the bearing power of soil. In this way the proper distribution of the weight of the proposed structures and the effect of this weight upon the underlying soil strata can be ascertained. The continuous studies of stream flow, for instance, make it possible to predict floods, sometimes to within a few minutes of the time of their arrival and to within a few inches of their height. On the basis of these predictions an engineer may decide whether or not it is necessary to remove his machinery from a cofferdam which may be flooded. Such information may be the means of saving many thousands of pounds.
The T.V.A. engineers go so far as to make large-scale models of their structures and to test them in a special laboratory in conditions approximating as closely as possible to those which will be encountered when the full-sized structures are under construction or are completed. In this way it is possible to determine the effects of pressures and currents on and about proposed dams, and also to ascertain the effect which the structure will have on the flow of the river.
Dam-building and the erection of hydro-electric plants by no means exhaust the activities of the T.V.A. Two of its problems have been to fight erosion and to supply the farmer with a cheap but first-class fertilizer. Among the causes for the backwardness of the region have been the depletion of mineral content of the soil through failure to use any form of crop rotation or artificial fertilization, the progressive impoverishment of the ground by the removal of fertile topsoil from tilled land by rain-wash, and the gullying of entire hillsides by run-off. It was recently estimated that, although floods do £2,000,000 worth of damage annually in the Mississippi Valley, erosion in the same area does £45,000,000 worth of harm.
The silting up of dams by wash is one of the most difficult problems confronting engineers. The only way to solve this problem of erosion, according to the T.V.A. engineers, is to check it before it starts. On moderate slopes it is being combated by using check-dams and mulch (dead vegetable matter) to hold the soil in place until a new protective cover of vegetation has gained a hold. This work is being done mainly by unemployed boys who are taken out of the cities to Civilian Conservation Corps camps, of which there are several located near the larger towns. Another method of combating this evil is by afforestation.
CONSTRUCTIONAL WORK IN PROGRESS at Guntersville Dam, Alabama. This dam was designed to form a pool, 81 miles in length, extending up the Tennessee River to Hales Bar Dam, Tennessee. The series of dams on the Tennessee River was planned to provide a navigation channel, not less than nine feet deep, for the entire length of the river.
In a period of eighteen months over 7,000,000 trees were planted and 86,000 check-dams built. On the more gentle slopes, regrading combined with grass planting has been found to be sufficiently effective. With erosion proceeding at its former rate, the silt deposited along the shore line of Norris Reservoir would have filled it in a century. Under erosion control, the life of the lake will be extended by 600 years.
Improvement of farmland itself is achieved by the use of a cheap first-class phosphate fertilizer. During the war of 1914-18 the demand for explosives led to the building of the Muscle Shoals power plant at Wilson Dam, near Florence, on the Tennessee. The T.V.A. took over the factory, with its two nitrate plants, one designed for the manufacture of ammonium nitrate by the synthetic process and the other by the cyanide process.
These plants were found to be unsuitable for the production of an efficient fertilizer. After lengthy experiments, upon which nearly £1,000,060 have been spent, the factory is now producing an artificial phosphate fertilizer at one-third of the cost of standard commercial phosphate and with a richer food content. The fertilizer is obtained by smelting phosphate rock, obtainable in the adjoining hills, with coke and silica in electric furnaces at a temperature of approximately 2,750 degrees Fahrenheit. The result is super-phosphate containing 43 per cent available plant food, as compared with the 16-20 per cent phosphate fertilizers on the market. Over 25,000 tons of this fertilizer are now being produced annually.
During 1935 two thousand farms were used for demonstration and experimental purposes, the farmers concerned agreeing to carry out instructions and to keep records of crop yields in return for free fertilizer and other assistance.
Drought Forecast from Trees
With the help of agricultural experiment stations a greatly increased range of crops is being introduced and tested in conditions adapted to local needs. It has now been established that by soil enrichment the yield hitherto considered normal for a given acreage can be secured from half that area. With the help of fertilizer the farmer could secure his present crop from half his land. He could let the other half go fallow, and turn it back into pasturage. In this way he could protect it from further erosion and increase his income by raising livestock.
The pay roll of the T.V.A. numbers over 16,000, including 4,875 skilled workers earning over £300 a year. In addition to a host of engineers, experts in their particular branch, there are surveyors, chemists and geologists.
One of the most highly paid specialists is a woman dendrochronologist. Her work is to discover the climate of the past by measuring the rings of trees. Most interesting results have been obtained by this method, especially in the Arizona Desert, and the method is being adopted in the Tennessee Valley to ascertain the maximum of drought to be expected.
There are also three aeroplanes and they are used for four purposes: aerial mapping (especially during times of flood), malaria control by dusting reservoir shore lines with insecticide during breeding seasons, emergency transportation and forest-fire patrol. Seventy-five forest fires were reported by air patrols during a single season, thirty of them in localities out of range of the forest rangers’ lookout towers.
The outlay of work due for completion by 1940, estimated at £37,000,000, does not include the cost of Wilson Dam and other projects previously carried out at public expense by. other agencies than the T.V.A. These cost £10,000,000. For the full realization of the potential benefits of the unified plan, legislation has been planned to extend the work until 1943 at a further cost of £29,000,000.
BUILDING THE NORRIS DAM on the Clinch River near Knoxville, Tennessee. Considerable excavation work in the river was necessary to obtain a solid foundation, and most of the work had to be carried out during the dry season. Three huge cofferdams were built to protect the foundation work.