Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/596

Rh 572 KIVEE ENGINEERING imer ds. on the land than torrential floods. The valleys, moreover, of torrential rivers are steeper and less fertile than the alluvial plains of gently flowing rivers ; and, consequently, the high short floods of the former are less injurious than the long continuing lower floods of the latter. The long duration of a flood is also the more prejudicial, as some- times a flood remains long enough on the land for a second flood to come down before the first has subsided, thereby producing an increased rise. iter Floods are generally largest in the winter months, for, owing to the absence of evaporation, a much larger pro- portion of rainfall finds its way into the rivers at that period ; and the greatest floods occur when rain falls on melting snow. High floods, however, sometimes occur in the summer after an exceptionally heavy rainfall; and they are necessarily far more injurious at that time of the year, devastating the crops on the land which they inundate. ises Floods are due to the inadequacy of the river channel oods. to carry off the water poured into it within a given period. The bed of a river, being formed and maintained by its stream, is merely adequate to carry the ordinary dis- charge. Large floods occur at too distant intervals to scour a sufficient channel for their passage, and conse- quently they overflow the banks and inundate the adjacent districts. Other causes, moreover, tend to aggravate this evil. A river carries down a large amount of solid matter which has been either ground from the mountain rocks by glaciers, washed from the land by the inflowing streams, or thrown into it as refuse from towns and manufactories. This material tends to settle in the channel wherever the current is checked, and consequently raises its bed and impedes the flow of the stream. Moreover, a river flow- ing through a plain gradually increases its serpentine course, thereby diminishing its fall and reducing its velo- city. Accordingly the tendency of rivers is to deteriorate when left to themselves ; and the discharging capacity of their channels becomes less, whilst the extension of subsoil drainage causes the rain to flow more rapidly and com- pletely into the river upon whose basin it falls. The available fall of the river is frequently diminished by the erection of fixed weirs across the channel, at various places, for the purpose of forming a head of water for mills, or of providing still-water navigation. These weirs are generally constructed with high sills, and of inadequate width; and where flood openings closed by draw-doors are adopted the doors are frequently not fully raised till a flood has actually arrived. These weirs consequently not merely reduce the discharging capacity of the channel by diminishing the available fall of the water surface, but also actually restrict the section of the channel. The result is that floods occur more frequently, rise higher, and remain on the land for a longer period, ent Prevention of Floods. The entire prevention of floods lon * justify. In most cases, the prevention of summer floods and the mitigation of winter floods would suffice; for, whilst summer floods are always very injurious, winter floods prove sometimes beneficial in depositing the mud which they bring down, provided they do not remain very long upon the land. Occasionally, however, where large tracts of low-lying country are exposed to inundation, and especially where portions of towns are below the flood level, it is necessary to extend the protection so as to ensure entire immunity from floods. I0n * (2) embankment of channel ; (3) pumping. (1) Improvement of Channel. The discharging capacity of a river may be increased by enlarging the section of its channel ; by the formation of straight cuts, which reduce its length, and consequently increase its fall ; by dredging away shoals, and thus rendering the fall of its bed more uniform ; and by removing obstacles to its flow at weirs. The channels of the English Fen rivers have been enlarged and straightened, and additional straight drains have been excavated for the more effectual drainage of the low-lying Fen country. Catchwater drains have also been formed to collect the rainfall of the higher lands and convey it into the river lower down, thus gaining a better fall than could be obtained if the upland waters were allowed to flow down to the low lands, besides relieving the low lands of this additional discharge. Straight cuts are very useful when the fall is slight and the velocity of flow is consequently small ; but they are not so suitable for more rapid streams, and, besides modifying the flow, and thus tending to produce shoals below, their straight course is liable to be altered by the irregularities of the current, especially if joining a sharp bend above. The banks in such cases need protection against erosion, which adds considerably to the cost of the works. The improvement of the upper Mississippi by cut-offs has not proved satisfactory ; and it has been found preferable to train the river by brushwood mattresses and dykes. Solid weirs across a river form serious impediments to its flow ; and draw-door or movable weirs should be pro- vided with adequate waterways, and sills level with the bed of the river. Old bridges, also, with wide piers and narrow arches retard the discharge of a river, and their rebuilding with wide openings would afford considerable relief. All obstacles in the river bed, such as weeds, fallen trees, and refuse, should be periodically removed; and fish-traps at weirs should be discarded, as they collect floating leaves and rubbish in their meshes and thus soon become entirely blocked up. (2) Embankment of Channel. When it is essential that the lands bordering a river should be absolutely protected from inundation, the enlargement of a river bed to an adequate extent for discharging the greatest floods would be too costly, especially when the fall is small; and it becomes necessary to resort to the expedient of increasing the channel, above the surface of the ground, by forming embankments along each side. By making the banks with material excavated from the channel, the earthwork serves the double purpose of enlarging the river bed and forming a bank. A flood channel of considerable dimensions can be readily obtained by placing the embankments some distance back from the margin of the river, thus greatly enlarging the section when the waters rise above the level of the land, whilst leaving the natural river bed unaltered for the ordinary flow. In some cases merely low embank- ments are constructed, which retain small floods but are submerged when large floods come down. Embankments, however, formed to secure the surrounding country from inundation must be high enough to exceed the highest flood level of the river, strong enough to resist the pres- sure of the water at that level, and perfectly watertight. If water can percolate through the bank, a breach is readily formed ; and if a high bank is overtopped by the river, the rush of the stream over it soon destroys a portion of the embankment and produces a disastrous inundation from the large volume of water suddenly liberated. The Fens of Lincolnshire, a large portion of Holland, the valley of the Po, and large tracts of low-lying land bordering the Mississippi are protected by embankments. The defects in the system of embanking rivers are that weak points in the banks are liable to be breached ; that the banks are liable to be overtopped by unusually high floods ; and that the muddy waters of the river in flood
 * e " would entail a larger expenditure than the results would
 * hods There are three methods by which floods may be pre-
 * e ' vented or mitigated, namely, (1) improvement of channel ;