The Regulation of Non-Tidal Rivers

Prediction of Floods.

The rise of floods in the tributaries of a river, and the periods they take in passing down to stations on the main river, are ascertained by means of gauges at suitable points. With the help of these records, and full information of the rainfall over the basin area, the time of arrival and height of the top of the flood at any station can be predicted with re markable accuracy. By telegraphing warnings to places on the river, weir-keepers are enabled to open fully beforehand the movable weirs for the passage of the flood, and the riparian inhabitants receive timely warning. On some rivers, as for example the Mississippi, the Danube and the Po, the flood prediction services serve also for the dissemination of informa tion as to conditions of navigation, as for instance on the river Danube.

Embankments.

Where it is important to protect land from inundations, the overflow of the river must be confined within continuous embankments. By placing these embankments some what back from the margin of the river-bed, a wide flood-channel is provided for the discharge of the river directly it overflows its banks. Low embankments may be sufficient where only excep tional summer floods have to be excluded from meadows. Occa sionally the embankments are raised high enough to retain the floods during most years, whilst provision is made for the escape of the rare exceptionally high floods at special places in the embankments, where the scour of the issuing current is guarded against, and the inundation of the neighbouring land is least injurious. Both the above methods afford the advantage of relieving the embanked channel of some of the sediment deposited in it by the confined flood-waters, when the surplus flow passes over the embankments.

When complete protection from inundations is required, the embankments have to be raised well above the highest flood-level, after allowing for the additional rise resulting from the confine ment of the flood within the embankments. The system has been adopted where tracts of fertile alluvial land below flood-level stretch for long distances away from the river. Thus the fens of Lincolnshire, Cambridgeshire and Norfolk are protected from inundations by embankments along their rivers and drains ; a great portion of Holland is similarly protected; and the plains of Lombardy are shut off from the floods of the Po by embankments along each side of the river for a distance of about 265 miles.

When towns like New Orleans on the Mississippi, and Szegedin on the Theiss in Hungary, have been established below the flood level of an adjoining river, the channel of the river should be improved to facilitate the passage of floods past the town. The town also should be enclosed within embankments raised above the highest possible flood-level to obviate the contingency of an exceptional flood, or a gradually raised flood-level leading to a catastrophe such as overwhelmed the greater part of Szegedin in March 1879 and threatened New Orleans in 1927.

The Mississippi.

A system of levee embankments has been extensively developed along the Mississippi river in its alluvial valley extending from Cape Girardeau, Missouri, to the Head of Passes in the delta, a distance of 1,125 miles by river channel. The alluvial valley has a length in latitude of 600 miles and ranges from 20 to 8o miles in width with a total area of 30,000 square miles. It comprises the St. Francis, the Yazoo, the Tensas and the Atchafalaya basins as well as the alluvial lands around Lake Pontchartrain (fig. 1).

These levees, begun by the French settlers in Louisiana, in the early 18th century were in 1735 about 3 ft. high and had been constructed from 3o miles above New Orleans to 12 miles below. From this beginning the system has been extended, until in 1935 the system included 2,13o miles of levees having an average height of 24 feet and a content of about 1 billion cubic yards of earth. The levees above Baton Rouge, La., are roughly parallel to the river but have a width between them of from 1 to 10 miles thereby greatly increasing the cross-section area of the river in times of flood. Below Baton Rouge the levees follow the bends in the river but are set back from the river bank sufficiently to prevent under cutting by the current during floods. In some parts the spacing is much greater. The low-water discharge, measured at New Orleans, 108 m. from the mouth of the South Pass, has been as low as 135,000 cu.ft. per sec. At Vicksburg a discharge of 1,806,00o sec. ft. was recorded in 1927 (estimated 2,278,00o sec. ft. if flood had been confined to leveed river) which compares with the minimum of record of 97,000 sec. ft. at that place. The levees have not been adequate during certain floods to withstand the water pressure which had (before the record flood of 1927) a maximum rise at Vicksburg of 61 ft. above the lowest stage of record at that point. In 1927 the river at Vicksburg rose 3i ft. higher or to 65 ft. above the lowest record stage. The floods tend to increase in height due to the confinement of the river between levees. Breaches, or crevasses as they are termed in the United States, have occurred during extraordinary floods. They produce a sudden rush of the flood waters through the opening, which is damaging to the land in the immediate vicinity of the breach but the general inundation is gradual and benefits the lands with a fertilizing deposit. The velocity of the out-flowing water is rapid only immediately in front of a crevasse and the water creeps over the delta lands generally, aided by the sloping down of the land on the alluvial plains for some distance away from the river.

The great floods of the Mississippi and its tributary rivers in April–June 1927, were the most serious which had occurred since records of these rivers have been kept. They were due to the extraordinary coincidence of flood conditions in all the chief tribu taries of the river. Normally the eastern floods usually culmi Effect of Embankments upon the River Levels and Bed.— The confinement of a river to a flood-channel of restricted width necessarily produces an increase in the elevation of the high-water surface. It has also been frequently asserted that this confine ment by the construction of embankments is accompanied by a progressive and consequential rising of the river bed, but there not annear to he cnifiriPnt evidence to bear out this conten nate between January and April, while the crests of the Missouri river floods usually enter the Mississippi in June. In 1927 the levees were breached in many places and the flood-waters over flowed throughout the alluvial valley. An area of over 18,000 sq.m. was inundated and 700,000 people were driven from their homes. Apprehension was averted at the city of New Orleans by cutting a gap in the levees at Poydras, a few miles below the city, thus permitting a part of the flood-waters to take a short course of about 5 m. to an arm of the sea instead of following the normal course of the river through the delta. (See also Miss's SIPPI RIVER, Engineering.)