banks in flood-time and inundates the adjacent low-lying lands to an extent depending upon the level of the ground and the volume and height of the flood.
| Flood Diagrams, Seine basin, 1903–1904. Fig. 1.—Little Seine at Nogent. Fig. 2.—Marne at St Dizier. Fig. 3.—Seine at Paris. |
An enlargement of the bed of the river, principally by deepening it, in order to increase its discharging capacity sufficiently to prevent inundations, is precluded by the cost, and also, in rivers bringing down sediment, by the large deposit that would take place in the enlarged channel from the reduction in the velocity of the current when the flood begins to subside. Where, however, the depth of a tidal river has been considerably increased by dredging for the extension of its sea-going trade, the enlargement of its channel and the lowering of its low-water line have greatly facilitated the passage of land floods from the river above for some distance up, and consequently reduced their height; for instance, the Glasgow quays along the deepened Clyde are no longer subject to inundation, and the lands and quays bordering the Tyne have been relieved from flooding for nearly 10 m. above Newcastle by the deepening of the river from Elswick to the sea (fig. 18).
Sometimes works are carried out in a river valley for diminishing the height of floods by delaying the discharge of part of the rainfall into the main river; whilst others are designed to increase the discharging efficiency of the river channels. In certain cases, moreover, it is very important to restrict or to prevent the inundation of some riparian districts by embankments; and occasionally low-lying lands are so unfavourably situated that pumping has to be resorted to for the removal of their drainage waters.
Works in River Valleys for diminishing Floods.—Rain falling on bare, impervious, hilly slopes rapidly flows into the nearest water-course, carrying with it any loose soil or disintegrated materials met with in its rush down the ravines, thereby intensifying the torrential character of the river, increasing the height of its floods and adding to the sediment obstructing its course to the sea. By encouraging the growth of vegetation and restricting its use for pasturage, and by planting trees on the mountain slopes, which have often been denuded of their natural covering by the reckless clearing of forests, the flow of the rain off the slopes is retarded; the soil, moreover, is bound together by the roots of the plants, and the surface strata are protected from disintegration by the covering of grass and leaves, so that the amount of detritus carried down into the river is greatly reduced.
Proposals have sometimes been made to reduce the height of floods in rivers and restrict the resulting inundations by impounding some of the flood discharge by the construction of one or more dams across the upper valley of a river, and letting it out when the flood has passed down. This arrangement, however, is open to the objection that in the event of a second flood following rapidly on the first, there might not be time to empty the reservoir for its reception. The cost, moreover, of the formation of such-reservoirs could rarely be justified merely for the purpose of reducing the flood-level along an ordinary river valley. Nevertheless, when this provision against floods can be combined with the storage of water supply for a town, it becomes financially practicable. Thus two masonry dams erected across the narrow valley of the river Furens, at torrential tributary of the Loire, form two reservoirs for the supply of the town of St Étienne, in which the water is kept down several feet below the full level in order to provide for the reception of the surplus flood-waters, and thereby protect St Étienne from inundation. Storage reservoirs also, formed solely for water-supply or irrigation, provided adequate compensation water is discharged from them during dry weather, are advantageous, like lakes, in regulating the flow of the river below.
When a river flowing through flat plains has a very small fall, it requires a proportionately large channel to carry away the drainage waters of the valley; and, accordingly, the low-lying lands bordering the river are very subject to inundations if the rainfall over the higher ground is allowed to flow straight down into the bottom of the valley. By intercepting, however, the flow off the high parts of the valley in small channels excavated along the slopes, termed “catch-water drains,” the ample fall available from this higher elevation can be utilized for conveying the flow farther down the valley; and the congested river is thereby relieved for a certain part of its length from the rainfall over the higher ground.
Methods of increasing the Discharging Efficiency of River Channels.—The discharging efficiency of a river within the limits of its bed depends on the fall and the cross-section of the channel. The only way of increasing the fall is to reduce the length of the channel by substituting straight cuts for a winding course. This involves some loss of capacity in the channel as a whole, and in the case of a large river with a considerable flow it is very difficult to maintain a straight cut, owing to the tendency of the current to erode the banks and form again a sinuous channel. Even if the cut is preserved by protecting the banks, it is liable to produce changes, shoals and a raising of the flood-level in the channel just below its termination. Nevertheless, where the available fall is exceptionally small, as in lands originally reclaimed from the sea, such as the English fen districts, and where, in consequence, the drainage is in a great measure artificial, straight channels have been formed for the rivers; and on account of the importance of preserving these fertile, low-lying lands from inundation, additional straight channels have been provided for the discharge of the rainfall, known as drains in the fens. Except where a town is exposed to inundations, a considerable modification of the course of a river and an enlargement of its channel do not produce a reduction in the damage from its floods commensurate with the expenditure involved.
The removal of obstructions, whether natural or artificial, from the bed of a river furnishes a simple and efficient means of increasing the discharging capacity of its channel, and, consequently, of lowering the height of floods; for every impediment to the flow, in proportion to its extent, raises the level of the river above it so as to produce the additional artificial fall necessary to convey the flow through the restricted channel, thereby reducing the total available fall. Accidental obstructions, brought down by floods, such as trunks of trees, boulders and accumulations of gravel, require to be periodically removed. In the absence of legal enactments for the
