Page:EB1911 - Volume 18.djvu/633

 not, as had been supposed, in Louisiana, serve as reservoirs to diminish the maximum wave in great flood seasons. Furthermore, the report argued that no diversion of tributaries was possible; that no reservoirs artificially constructed could keep back the spring freshets which caused the floods; that the making of cut-offs, which had sometimes been advocated as a measure of relief, was in the highest degree injurious; that outlets were impracticable from the lack of suitable sites; and, finally, that levees properly constructed and judiciously placed would afford protection to the entire alluvial region.

During the Civil War (1861–65) the artificial embankments were neglected; but after its close large sums were expended by the states directly interested in repairing them. The work was done without concert upon defective plans, and a great flood early in 1874 inundated the country, causing terrible suffering and loss. Congress, then in session, passed an act creating a commission of five engineers to determine and report on the best system for the permanent reclamation of the entire alluvial region. Their report, rendered in 1875, endorsed the conclusions of that of 1861, and advocated a general levee system on each bank. This system comprised: (1) a main embankment raised to specified heights sufficient to restrain the floods; and (2) where reasonable security against caving required considerable areas near the river to be thrown out, exterior levees of such a height as to exclude ordinary high waters, but to allow free passage to great floods, which as a rule occur only at intervals of five or six years. An engineering organization was proposed for constructing and maintaining these levees, and a detailed topographical survey was recommended to determine their precise location. Congress promptly approved and ordered the survey; but strong opposition on constitutional grounds was raised to the construction of the levees by the government.

In the meantime complaints began to be heard respecting the low-water navigation of the river below the mouth of the Ohio. A board of five army engineers, appointed in 1878 to consider a plan of relief, reported that a depth of 10 ft. could probably be secured by narrowing the wide places to about 3500 ft. with hurdle work, brush ropes or brush dykes designed to cause a deposit of sediment, and by protecting caving banks by light and cheap mattresses. Experiments in these methods were soon begun and they proved to be effective.

The bars at the efflux of the passes at the mouth of the Mississippi were also serious impediments to commerce. The river naturally discharges through three principal branches, the south-west pass, the south pass and the north-east pass, the latter through two channels, the more northern of which is called Pass à l’Outre. In the natural condition the greatest depth did not exceed 12 or 13 ft. After appropriations by Congress in 1837, 1852 and 1856, a depth of 18 ft. was finally secured by dredging and scraping. The report of 1861 discussed the subject of bar formation at length, and the stirring up of the bottom by scrapers during the flood stages of the river (six months annually) was recommended by it. After the war this recommendation was carried into effect for several years, but experience showed that not much more than 18 ft. could be steadily maintained. This depth soon became insufficient, and in 1873 the subject was discussed by a board of army engineers, the majority approving a ship canal. In 1874 Congress constituted a special board which, after visiting Europe and examining similar works of improvement there, reported in favour of constructing jetties at the south pass, substantially upon the plan used by Pieter Caland (b. 1826) at the mouth of the Meuse; and in 1875 Captain James B. Eads (1820–1887) and his associates were authorized by Congress to open by contract a deep channel through the south pass upon the general plan proposed by this board. As modified in 1878 and 1879 the contract called for the maintenance for twenty years of a channel through the pass and over the bar not less than 26 ft. in depth throughout, a width of not less than 200 ft. and with a middle depth of 30 ft. The work was begun on the 2nd of June 1875. The required depth was obtained in 1879, and with few interruptions has been maintained. In 1902 Congress authorized preparations for the construction of a deeper (35 ft.) and a wider channel through the south-west pass; the work was begun in 1903 and virtually completed in 1909.

In the year in which Captain Eads opened the south pass of deep-water navigation Congress created a commission of seven members to mature plans for correcting and deepening the channel of the river, for protecting its banks and for preventing floods, and since then large expenditures for improvement between the head of the passes and the mouth of the Ohio have been under the control of this commission. In protecting the banks, mattresses of brush or small trees, woven like basket-work, were sunk on the portion of the bank at the time under water, by throwing rubble stone upon them, an excess of stone being used. A common size of mattress was 800 ft. long, counted along the bank, by 250 ft. wide. Sometimes a width of 300 ft. was used, and lengths have reached 2000 ft. The depth of water was often from 60 to 100 ft. At first these mats were light structures, but the loss of large quantities of bank protection by the caving of the bank behind them, or by scour at their channel edges, forced the commission steadily to increase the thickness and strength of the mattress, so that the cost of the linear foot of bank protection, measured along the bank, rose from $8 or $10 to $30 in the later work. The contraction works adopted were systems of spurs or pile dykes, running out from the shore nearly to the line of the proposed channel. Each dyke consisted of from one to four parallel rows of piles, the interval between rows being about 20 ft. and between piles in a row 8 or 10 ft. The piles and rows were strongly braced and tied together, and in many cases brush was woven into the upper row, forming a hurdle, in order further to diminish the velocity of the water below the spur. By 1893 it was evident that the cost, which had been estimated at $33,000,000 in 1881, would really be several times that amount, and that the works would require heavy expense for their maintenance and many years for their execution. Navigation interests demanded more speedy relief. The commission then began experimenting with hydraulic dredges, and in 1896 it adopted a project for maintaining a channel from the mouth of the Ohio to the passes that should be at least 9 ft. deep and 250 ft. wide throughout the year. Centrifugal pumps are used, the suction pipes being at the bow and the discharge at the stern through a line of pipes about 1000 ft. long, supported on pontoons. Water jets or cutters stir up the material to be dredged before it enters the suction pipes. The later dredges have a capacity of about 1000 cub. yds. of sand per hour, the velocity in the 32- to 34-in. discharge pipes being from 10 to 15 ft. per second. They cost from $86,000 to $120,000, and their working during a low-water season costs about $20,000. These dredges begin work on a bar where trouble is feared before the river reaches its lowest stage, and make a cut through it. A common cut is 2000 ft. long by 250 ft. wide, and 3 or 4 ft. deep. Since 1903 a channel of the proposed depth or more has been maintained.

In 1882 occurred one of the greatest floods known on the Mississippi, and extensive measurements of it were made. A maximum flood of 1,900,000 cub. ft. per second crossed the latitude of Cairo. Much of it escaped into the bottom lands, which are below the level of the great floods, and flowed through them to rejoin the river below. The flow in the river proper at Lake Providence, 542 m. below Cairo, was thus reduced to about 1,000,000 cub. ft. per second, while if the river had been confined by levees the flow between them would have been double, or about 2,000,000 cub. ft. per second. The volume of the levees in 1882 was about 33,000,000 cub. yds., and by the 30th of June 1908 had been increased to 219,621,594 cub. yds., of which the United States had built about one-half, and has expended on them $22,562,544. The length of the levees is about 1486 m., and they are continuous save where interrupted by tributaries or by high lands, from New Madrid, or 80 m. below Cairo, to Fort Jackson, 1039 m. below Cairo. The width of the interval between levees on the opposite banks of the river varies greatly; in many places the levees are built much nearer the normal margin of the river than is consistent with keeping the flood heights as low as possible. This has arisen from two causes: firstly, to give protection to lands already cultivated, which lie usually near the bank of the river; secondly, to avoid the lower ground, which, owing to the peculiar formation, is found as one goes back from the river. Another bad result of this nearness of the levees to the bank of the river is the loss of levees by caving, which was nearly 5,000,000 cub. yds. in 1904–1905, and can only be prevented by bank protection, costing $150,000 per mile, to protect a levee perhaps 16 ft. high costing about $30,000 per mile. The levees have top widths of 8 ft., side slopes of one-third, and banquettes when their heights exceed about 10 ft. The grades of the levees are usually 3 ft. above the highest water, and have to be raised from year to year as greater confinement of water gives greater flood heights. When this system is completed there will probably be hundreds of miles of levee with heights exceeding 14 ft. In 1899, after about $28,000,000 had been spent on levees by the United States and by the local authorities, the commission submitted an estimate for additional work on levees, amounting to 124,000,000 cub. yds. and costing $22,000,000. The effect of the levees has been to increase flood heights. Though the