Floods.— Floods are of two" kinds, according to the nature of the country traversed by the rivers producing them. Torrential rivers, flowing over impermeable strata and having a rapid fall, rise rapidly after a heavy rainfall, and produce a high flood which quickly subsides. Gently flowing rivers on the contrary rise slowly, and do not attain the same height as torrential rivers; but their floods subside slowly, and consequently, though less high, remain longer 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 sometimes 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.

Floods are generally largest in the winter months, for, owing to the absence of evaporation, a much larger proportion 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.

Floods are due to the inadequacy of the river channel 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 discharge. Large floods occur at too distant intervals to scour a sufficient channel for their passage, and consequently 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 flowing through a plain gradually increases its serpentine course, thereby diminishing its fall and reducing its velocity. 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 completely 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.

Prevention of Floods.— The entire prevention of floods would entail a larger expenditure than the results would 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.

There are three methods by which floods may be prevented or mitigated, namely,—(1) improvement of channel; (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 provided 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 embankments 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 pressure 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 deposit their sediment in the bed of the river, instead of spreading it over the adjacent land, and thus gradually raise the river bed and consequently the height of the floods. The remedies for these defects are strong high banks made of the best materials, and a periodical cleansing of the river bed. Neglect of these precautions has led to serious disasters. Large tracts in the Fens have been occasionally flooded by the bursting or overtopping of badly constructed banks. Numerous breaches have occurred in the embankments of the Po, resulting in the devastation of its valley; and the flood level of the Po has been so much raised that it has been decided not to heighten the embankments for fear of occasioning still greater disasters. The gradual silting up of the river Theiss, near Szegedin, produced a rise in its flood level which led to the overtopping of the protecting embankment in 1879, and the formation of a breach; and the water thus set free destroyed a portion of Szegedin, and inundated a tract of 200 square miles. The rise of the bed of some rivers in Japan, from the deposit of silt, has been followed up by the gradual raising of the embankments; and this system has been carried out to such a degree, and the accumulated deposit is so great, that some of these embanked rivers have their beds as much as forty feet above the level of the plains over which they flow. These high embankments necessarily require constant attention; and any failure is attended with serious inundations. They serve as a warning against the extensive raising of embankments to counteract the silting up of a river.

(3) Pumping.— When lands are very flat and low, lying sometimes actually below the general drainage level of the district and the waterlevel of the streams, it is impossible for rivers to perform their ordinary function of draining the land by gravitation. It is necessary in such cases to create an artificial fall by pumping the drainage waters up so as to be discharged into the adjacent streams. This method has the advantage of ensuring the effectual drainage of the lands, provided adequate pumping power is supplied; but it forms an additional tax on the land, as steam has to be applied to do what is under ordinary conditions effected by nature, and the land has also to be surrounded by banks.

This system has been adopted for the drainage of the Haarlem Meer reclamation, and also for the lands reclaimed from Lake Y in the construction of the Amsterdam ship canal. The drainage of the Fens is, in several instances, supplemented by pumping; and a portion of the Witham basin has been secured against floods by this means.

The formation of large reservoirs in river valleys has been proposed for storing the surplus waters till a flood has subsided. A reservoir has indeed been formed, by constructing a high masonry dam across a narrow gorge of the Furens valley, which both supplies the town of St Étienne with water and preserves it from inundation. It is also proposed to prevent the floods of the river Chagres from interfering with the Panama Canal, by impounding its flood waters in an extensive reservoir to be formed by building a high dam across a suitable point of its valley. In these cases, however, the deep valleys with their narrow gorges are peculiarly well adapted for the formation of reservoirs having a considerable capacity, whereas most river valleys are unsuitable for reservoirs, and the construction of the long lengths of the banks that would be required would entail a very large expenditure, so that this system could only have a very restricted application.

Improvements of the Upper Portions of Rivers.— Most rivers are not suitable by nature for navigation in their non-tidal portion, or, in the case of tideless rivers, at a considerable distance from their mouths, as the fall of their bed increases towards their source, and they generally present irregularities in depth and flow, with occasional sharp bends. Even where the depth is adequate, an irregular or rapid flow offers a great obstacle to up-stream traffic. Moreover, the fall of the waterlevel in dry seasons would often make a river too shallow for navigation. Accordingly, whilst improving the worst bends and removing shoals, it is frequently necessary to retain the water, when the flow is small, so as to maintain a sufficient depth. This is accomplished by dividing the river into a series of sections or reaches, and pounding up the water at the end of each reach by means of a dam or weir.

Formerly rivers used to be penned in by a series of stanches near shoal places, which held up the water, and, when several boats were collected in the pool above a stanch, it was suddenly opened, and the sudden rush of water floated the boats over the shallows below. This primitive method of navigation, termed flashing, was formerly practised on the Thames and the Severn, and also on some of the rapidly flowing rivers in France, such as the Yonne. The stanches on the Severn were removed in 1842; but a few still exist across the Thames above Oxford, where the barge navigation follows a lateral canal. These stanches, consisting of beams swung across the river and supporting a series of spars and paddles, were easily removed in flood time or for the passage of boats. The stanches on the Yonne, which were more recently erected, were of a more elaborate description, known as needle weirs, and are still retained as weirs for holding up the water, though the process of flashing has been discontinued.

As the demands of navigation increased, these primitive methods proved inadequate, and, moreover, they were quite unsuited for up-stream traffic. Accordingly weirs were substituted for stanches, to hold up the water in each reach; and locks were constructed, in suitable side channels, for enabling vessels to be passed from one reach to the next with little loss of water, and with equal facility either up or down. Rivers have been thus converted into still-water navigations, with level reaches forming a series of steps, having a fall at each lock, in place of the natural inclination of the river bed; so that the up-river traffic is in a great measure relieved from the serious hindrance of an opposing current. In order that the water held back by the weir may be retained within the channel, it is necessary to raise the banks on each side for some distance above the weir; and, as the gradual rise in the river bed towards the upper end of the reach reduces the depth, it is necessary to deepen the river along the upper portion of the reach to secure a uniform draught of water. A river is thus practically converted into a canal, with this sole difference that it has still to discharge the drainage waters of its basin. This primary object of rivers, to which indeed they owe their existence, was in many instances somewhat overlooked when rivers were utilized for navigation; and weirs appear to have been often regarded merely as dams for retaining the water, rather than as regulators of its flow.

Weirs.—Locks have been already considered in the article on Canals (g.vf; so that it will suffice here to describe briefly the different forms of weirs, which are essentially river works.

Weirs have been divided into three classes, namely, overfall weirs, draw-door weirs, and movable weirs.

Overfall Weirs.—An overfall weir is a solid barrier placed across a stream for the purpose of raising the waterlevel (Plate V. fig. 1 ), and only affords an outlet for the discharge of the river when the water rises above its crest. The waterlevel of the river is thus permanently raised, not merely in dry seasons, but also in flood time,

and its fall is correspondingly reduced. Accordingly, the discharging capacity of a river is materially diminished by the erection of overfall weirs; and this is only partially remedied by the deepening of the channel for navigation, especially as deposit more readily accumulates in the lower part of a reach, owing to the reduction in velocity of the current by the conversion of the river into level reaches. Endeavours have been made to alleviate this defect by placing the weir in a wide place on the river, and at an angle to the cross section of the channel, thereby increasing the length of its sill, and consequently the discharge over it for any definite height of the river (fig. 2). The gain in length of an oblique weir is somewhat neutralized by the weir not being at right angles to the direction of the current; and, even if the cross section of the channel above the sill of the weir is as large as the average section of the river bed, the change in shape, and the small hydraulic radius of the section over the weir, check the discharge of the stream.

Draw-door Weirs.—In order to afford a freer flow than is attainable with the best-designed overfall weir, draw-door weirs are sometimes adopted, which serve equally well to retain the water above during dry weather, and provide a large opening for the discharge of the stream in floods. Draw-door weirs consist of a row of doors, or sluice gates, sliding vertically in grooves formed at the sides of frames, piles, or piers, which are shut down when the flow is small, but are raised to admit the passage of flood waters. These weirs generally serve to supplement an ordinary overfall weir; and, whilst the overfall weir regulates the flow in dry weather, the draw-door weir provides for its more rapid discharge in flood time. The relief, however, afforded by draw-door weirs depends entirely on the opening they furnish; they are rarely, if ever, made equal in section to the river channel, and their sills are usually raised some feet above the bed of the river; but, nevertheless, they are much superior to overfall weirs in respect of drainage, especially when the river has a small fall and low banks.

A large oblique overfall and draw-door weir has been erected across the Thames, by Mr Leach, at the limit of its tidal flow at Teddington, having a total length of 480 feet. This weir is divided into four bays, the two side bays being overfalls, whilst the two central bays, 172∣ feet and 69 j feet wide respectively, are closed by large iron draw-doors sliding in grooves at the sides of strong iron frames supporting a foot bridge from which the doors are raised. The frames rest upon piles, and on the top of a rubble mound raised about a foot above low-tide level.

The friction of large draw-doors against the grooves, in being lifted, is considerable when there is a head of water on one side; but this has been much reduced by Mr F. Stoney, in a large weir in Brazil, by making the doors, 20 feet in width, bear on each side against a row of free rollers suspended in the grooves.

Movable Weirs. —Although draw-door weirs afford a much freer discharge for a river than overfall weirs, the vertical frames or piers in which the doors slide offer more or less impediment to the flow. This defect is avoided by movable weirs, which, whilst equally efficient in retaining the water when raised, can be entirely lowered or removed so as to leave the channel quite open in floodtime.

There are three types of movable weirs which have been regularly adopted abroad, whilst other forms have been occasionally tried. The two types most extensively used are the frame or needle weiland...