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RIVER ENGINEERING
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See also:RIVER See also:ENGINEERING . Before undertaking See also:works for the improvement of See also:rivers, either with the See also:object of mitigating the effects of their inundations, or for increasing and extending their capabilities for See also:navigation, it is most important that their See also:physical characteristics should be investigated in each See also:case, for these vary greatly in different rivers, being dependent upon the See also:general configuration of the See also:land, the nature of the See also:surface strata and the See also:climate of the See also:country which the rivers See also:traverse. Physical Characteristics of Rivers The See also:size of rivers above any tidal limit and their See also:average fresh-See also:water See also:discharge are proportionate to the extent of their basins, and the amount of See also:rain which, falling over these basins, reaches the river channels in the bottom of the valleys, by which it is conveyed to the See also:sea. River Basins.—The See also:basin of a river is the expanse of country, bounded by a winding See also:ridge of high ground, over which the rainfall flows down towards the river traversing the lowest See also:part of the valley; whereas the rain falling on the See also:outer slope of the encircling ridge flows away to another river draining an adjacent basin. River basins vary in extent according to the configuration of the country, ranging from the insignificant drainage-areas of streams rising on high ground very near the See also:coast and flowing straight down into the sea, up to immense tracts of See also:great continents, when rivers, rising on the slopes of See also:mountain ranges far huand, have to traverse vast stretches of valleys and plains beforereaching the ocean. The size of the largest river basin of any country depends on the extent of the See also:continent in which it is situated, its position in relation to the hilly regions in which rivers generally rise and the sea into which they flow, and the distance between the source and the outlet of the river draining it. Great See also:Britain, with its very limited See also:area, cannot possess large river basins, its largest being that of the See also:Thames with an area of 5244 sq. m. Even on the mainland of See also:Europe, river basins See also:augment in extent on proceeding eastwards with the increasing width of the continent; in See also:France the largest basin is that of the See also:Loire with an area of 45,000 sq. m., while the See also:Rhine has a basin of 86,000 sq. m. with a length of 80o in., the See also:Danube a basin of 312,000 sq. M. with a length of 1700 m., and the See also:Volga a basin of 563,000 sq. m. with a length of 2000 M. The more extensive continents of See also:Asia, See also:Africa and See also:North and See also:South See also:America possess still larger river basins, the Obi in See also:Siberia having a basin of about 1,300'000 sq. m. and a length of 3200 m., the See also:Nile a basin of 1,500,000 sq. m. with a length of over 4000 m., and the See also:Mississippi, flowing from north to south, having a basin of 1,244,000 sq. m. with a length of 4200 in. The vast basin of the See also:Amazon of 2,250,000 sq. m. is due to the See also:chain of the See also:Andes almost bordering the Pacific coast-See also:line, so that the river rising on its eastern slopes has to traverse nearly the whole width of South America, at its broadest part before reaching the See also:Atlantic Ocean. Available Rainfall.—The rainfall varies considerably in different localities, both in its See also:total yearly amount and in its See also:distribution throughout the See also:year; also its See also:volume fluctuates from year to year. Even in small river basins the See also:variations in rainfall may be considerable according to See also:differences in See also:elevation or distance from the sea, ranging, for instance, in the See also:Severn basin, with an area of only 4350 sq. m., from an average of under 30 in. in the year to over 8o in. The proportion; moreover, of the rain falling on a river basin which actually reaches the river, or the available rainfall in respect to its flow, depends very largely on the nature of the surface strata, the slope of the ground and the extent to which it is covered with vegetation, and varies greatly with the See also:season of the year. The available rainfall has, indeed, been found to vary from 75% of the actual rainfall on impermeable, See also:bare, sloping, rocky strata, down to about 15% on See also:flat, very permeable soils. Fall of Rivers.—The See also:rate of flow of rivers depends mainly upon their fall, though where two rivers of different sizes have the same fall, the larger river has the quicker flow, as its retardation by See also:friction against its See also:bed and See also:banks is less in proportion to its volume than that of the smaller river. The fall of a river corresponds approximately to the slope of the country it traverses; and as rivers rise See also:close to the highest part of their basins, generally in hilly regions, their 'fall is rapid near their source and gradually diminishes, with occasional irregularities, till, in traversing plains along the latter part of their course, their fall usually becomes quite See also:gentle. Accordingly, in large basins; rivers in most cases begin as torrents with a very variable flow, and end as gently flowing rivers with a comparatively See also:regular discharge. Variations in the Discharge of Rivers.—The irregular flow of rivers throughout their course forms one of the See also:main difficulties in devising works, either for mitigating inundations or for increasing the navigable capabilities of rivers. In tropical countries, subject to periodical rains, the rivers are in See also:flood during the See also:rainy season and have hardly any flow during the See also:rest of the year; whilst in temperate regions, where the rainfall is more evenly distributed throughout the year, evaporation causes the available rainfall to be much less in hot summer See also:weather than in the See also:winter months, so that the rivers fall to their See also:low See also:stage in the summer and are very liable to be in flood in the winter. In fact, with a temperate climate, the year may be divided into a warm and a See also:cold season, extending from May to See also:October and from See also:November to See also:April respectively; the rivers are low and moderate floods are of rare occurrence during the first See also:period, and the rivers are high and subject to occasional heavy floods after a considerable rainfall during the second period in most years. The only exceptions are rivers which have their See also:sources amongst mountains clad with perpetual See also:snow, and are fed by glaciers; their floods occur in the summer from the melting of the snows and See also:ice, as' exemplified by the See also:Rhone above the 'See also:Lake of See also:Geneva, and the Arve which joins it below. But even these rivers are liable to have their flow modified by the influx of tributaries subject to different conditions, so that the Rhone below See also:Lyons has a more See also:uniform discharge than most rivers, as the summer floods of the Arve are counteracted to a great extent by the low stage of the See also:Saone flowing into the Rhone at Lyons, which has its floods in the winter when the Arve on the contrary is low. Transportation of Materials by Rivers.—Another serious obstacle encountered in the improvement of rivers consists in the large quantity of detritus brought down by them in flood-See also:time, derived mainly from the disintegration of the surface-layers of the hills and slopes in the upper parts of the valleys by glaciers, See also:frost and rain. The See also:power of a current to transport materials varies with its velocity, so that torrents with a rapid fall near the sources of rivers can carry down rocks, boulders and large stones, which are by degrees ground by See also:attrition in their onward course into See also:shingle, See also:gravel, See also:sand and silt, simultaneously with the See also:gradual reduction in fall, and, consequently, in the transporting force of the current. Accordingly, under See also:ordinary conditions, most of the materials brought down from the high lands by the torrential water-courses are carried forward by the main river to the sea, or partially strewn over flat alluvial plains during floods; and the size of the materials forming the bed of the river or See also:borne along by the stream is gradually reduced on proceeding seawards, so that in the Po, for instance, pebbles and gravel are found for about 140 M. below See also:Turin, sand along the next 10o m., and silt and mud in the last 110 m. When, however, the fall is largely and abruptly reduced, as in the case of rivers emerging straight from mountainous slopes upon flat plains, See also:deposit necessarily occurs, from the materials being either too large or too great in volume to be borne along by the enfeebled current; and if the impeded river is unable to spread this detritus over the plains, its bed becomes raised by deposit, causing the river in flood-time to rise to a higher level. The materials, moreover, which are carried in suspension or rolled along the bed of the river to the sea, tend to deposit when the flow of the river slackens and is finally brought to rest on encountering the great inert See also:mass of the sea, especially in the See also:absence of a See also:tide and any littoral current, and this is the cause of the formation of deltas with their shallow outlets, barring the approach to many large rivers. See also:Influence of Lakes on Rivers.—Sometimes a See also:peculiar depression along part of a valley, with a rocky barrier at its See also:lower end, causes the formation of a lake in the course of the river flowing down the valley. The intervention of a lake makes the river, on entering at the upper end, deposit all the materials with which it is charged in the still See also:waters of the lake; and it issues at the lower end as a perfectly clear stream, which has also a very regular discharge, as its floods, in flowing into the lake, are spread over a large surface, and so produce only a very slight raising of the level. This effect is illustrated by the river Rhone, which enters the Lake of Geneva as a very turbid, torrential, See also:glacier stream, and emerges at Geneva as a sparkling, limpid river with a very uniform flow, though in this particular case the improvement is not See also:long maintained, owing to the confluence a See also:short distance below Geneva of the large, rapid, glacial river, the Arve. The influence, of lakes on rivers is, indeed, wholly beneficial, in consequence of the removal of their See also:burden of detritus and the regulation of their flow. Thus the See also:Neva, conveying the outflow from Lake See also:Ladoga to the Baltic, is relieved by the lake from the detritus brought down by the rivers flowing into the lake; and the See also:Swine outlet channel of the See also:Oder into the Baltic is freed from sediment by the river having to pass through the Stettiner Haff before reaching its mouth. The St See also:Lawrence, again, deriving most of its See also:supply from the chain of Great Lakes of North America, possesses a very uniform flow. River Channels.—The discharge of the rainfall erodes the beds of rivers along the lowest parts of the valleys; but floods occur too intermittently to See also:form and maintain a channel large enough to contain the flow. A river channel, indeed, generally suffices approximately to carry off the average flow of the river, which, whilst comprising considerable fluctuations in volume, furnishes a sufficiently See also:constant erosive See also:action to maintain a fairly regular channel; though rivers having soft beds and carrying down sediment See also:erode their beds during floods and deposit See also:alluvium in dry weather. As the velocity of a stream increases with its fall, the size of a channel conveying a definite average flow varies inversely with the fall, and the See also:depth inversely with the width. A river channel, accordingly, often presents considerable irregularities in See also:section, forming shallow rapids when the river flows over a rocky barrier with a considerable fall, and consisting of a See also:succession of pools and shoals when the bed varies in compactness and there are differences in width, or when the river flows See also:round a succession of bends along opposite banks alternately.
A river flowing through a flat alluvial See also:plain has its current very readily deflected by any See also:chance obstruction or by any difference in hardness of the banks, and generally follows a winding course, which tends to be intensified by the erosion of the See also:concave banks in the bends from the current impinging against them in altering its direction round the curves. Some-times also a large river, bringing down a considerable amount of detritus, shifts its course from time to time, owing to the obstruction produced by banks of deposit, as exemplified by the Po in traversing the portion of the See also:Lombardy plains between Casale and the confluence of the See also:Ticino.
Floods of Rivers.—The rise of rivers in flood-time depends not merely on the amount of the rainfall, but also on its distribution and the nature of the strata on which it falls. The upper hilly part of a river basin consists generally of impermeable strata, sometimes almost bare of vegetation; and the rain flowing quickly down the impervious, sloping ground into the water-courses and tributaries feeding the main river produces rapidly rising and high floods in these streams, which soon pass down on the cessation of the rain. The river See also:Marne, draining an impermeable part of the Upper See also:Seine basin, is subject to these sudden torrential floods in the cold season, as illustrated by a See also:diagram of the variations in height of the river at St Dizier from November to See also: Exceptional floods, accordingly, only occur in a main river when a heavy rainfall takes See also:place at such periods over different parts of the basin that the floods of the various tributaries coincide approximately in attaining their maximum at certain points in the main river. Mitigation of Floods and See also:Protection from Inundations.—As the size of the channel of a river is generally quite inadequate to carry down the discharge of floods, the river overflows its 376 banks in flood-time and inundates the adjacent low-lying lands to an extent depending upon the level of the ground and the yNOe See also:DECEMBER . See also:JANUARY. ~Ni1 25 - • /O /520 J6 S /0 /5 22 2- 3/. 6 %O %5~2 2S 6 /0 13 20 X22 NOV" DECEMBER. JANUARY. See also:FEBRUARY. MARCH. ~/ '3,1021 30.3 10 /S 20 5 3/. 5 /0 /3 20 23 3L S O /3 4529. $ /O /S 2 3h/SFS Flood Diagrams, Seine basin, 1903-1904. FIG. 1.-Little Seine at Nogent. FIG. 2.—Marne at St Dizier. volume and height of the flood. An enlargement of the bed of the river, principally by deepening it, in See also: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 See also:extension of its sea-going See also: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 See also:Glasgow quays along the deepened See also:Clyde are no longer subject to inundation, and the lands and quays bordering the See also:Tyne have been relieved from flooding for nearly 10 m. above See also:Newcastle by the deepening of the river from See also: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 See also:soil or disintegrated materials met with in its See also: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 See also:bound together by the roots of the See also:plants, and the surface strata are protected from disintegration by the covering ofgrass 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 sonle 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 See also: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 See also:provision against floods can be combined with the storage .of water-supply for a See also:town, it becomes financially practicable. Thus two See also:masonry dams erected across the narrow valley of the river Furens, a torrential tributary of the Loire, form two reservoirs for the supply of the town of St See also:Etienne, 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 Etienne from inundation. Storage reservoirs also, formed solely for water-supply or See also:irrigation, provided adequate See also: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-lowlying 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, -how-ever, 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 See also: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 See also: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 See also: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 See also:fens. Except where it 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 See also:expenditure involved. The removal of obstructions, whether natural or artificial, from the -bed of a river furnishes a See also: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 periodic-ally removed. In the absence of legal enactments for the 0 /WI DECEMBER. JANUARY. FEBRUARY. MARCH. p.,,540 2S 30. S 10 /3 90 25 J/ 5 /O /,t 20 2S /0 If BO 222 5 /0 /5 20 2S J//Jrx 10 conservancy of rivers, numerous obstructions have in many cases been placed in their channel, such as See also:mining refuse, sluice-See also:gates for See also:mills, See also:fish-traps, unduly wide piers for See also:bridges and solid weirs, which impede the flow and raise the flood-level. Stringent prohibitions with regard to refuse, the enlargement of sluice-ways and the compulsory raising of their gates for the passage of floods, the removal of fish-traps which are frequently blocked up by leaves and floating rubbish, a reduction in the number and width of the piers of bridges when rebuilt, and the substitution of movable weirs for solid weirs, greatly facilitate the discharge of a river, and consequently lower its flood-level. Prediction of Floods in Rivers.—By erecting gauges in a fairly large river and its tributaries at suitable points, and keeping continuous. records for some time of the heights of the water at the various stations, the rise of the floods in the different tributaries, the periods they take in passing down to definite stations on the main river, and the influence they severally exercise on the height of the floods at these places, are ascertained. With the- help of these records, by observing the times and heights of the maximum rise of a particular flood at the stations on the, various tributaries, the. time of arrival and height of the See also:top of the flood at ally station on the main river can be predicted with remarkable accuracy two or more days beforehand. By telegraphing these particulars about a high flood to places on the lower river, the See also:weir-keepers are enabled to open fully beforehand the movable weirs for the passage of the flood, and the riparian inhabitants receive timely warning of the impending inundation.
Embankments along Rivers to prevent Inundations.—Where portions of a See also:riverside town are situated below the maximum flood-level, or when it is important to protect land adjoining a river from inundations, the overflow of the river must be confined within continuous. embankments on both sides. By placing these embankments somewhat 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, whilst leaving the natural channel unaltered for the ordinary flow. Low embankments may be sufficient where only exceptional summer floods have to be excluded from meadows. Occasionally the embankments are raised high enough to retain the floods during most years, whilst provision is made, for the See also:escape of the rare exceptionally high floods at See also: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. In this manner, the increased cost of embankments. raised above the highest flood-level of rare occurrence is saved, and the danger of breaches in the banks from an unusually high flood-rise and rapid flow, with their disastrous effects, is avoided. Both the above methods afford the See also: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 See also: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 confinement ,of the flood within the embankments, instead of spreading over the low-lying land; and they have to be made perfectly watertight and strong enough to resist the water-pressure and current of the highest floods. The See also:system has been very extensively adopted where large tracts of fertile alluvial land. below flood-level stretch for long distances away from the river. Thus the fens of See also:Lincolnshire, See also:Cambridgeshire and See also:Norfolk are protected from inundations by embankments along their rivers and drains; a great portion of See also: These embankments, having been begun by the See also:French settlers in See also:Louisiana, are called levees, and have a total length of 1490 M. They, however, do not afford complete protection from inundations, as they are not quite continuous and are not always strong enough to withstand the water-pressure of high floods, which have at See also:Vicksburg a maximum rise of 59 ft. above the lowest stage of the river, and tend to increase in height owing to the improved drainage following on the extension of cultivation. Breaches, or crevasses as they are termed in the See also:United States, resulting from a deficiency in the strength or consistency of the banks, or from their being overtopped or eroded by the current, produce a sudden rush of the flood-waters through the opening, which is much more damaging to the land in the neighbourhood of the See also:breach than a gradual inundation. Moreover, the velocity of the outflowing water is intensified by the sloping down of the land on these alluvial plains for some distance away from the river, owing to the raising of the ground nearest the river by the gradual deposit of layers of sediment from the flood-waters when they begin to overflow the river banks. The levees on the Mississippi are breached in weak places every year during the See also:spring floods, and are liable to be destroyed along considerable lengths by the rapid erosion resulting from their being overtopped by exceptional floods at intervals of about ten years; and in places they are undermined and overthrown by changes in the course of the river from the caving-in of concave banks at bends, necessitating reconstruction some distance back from the river at points thus threatened. When towns have been established below the flood-level of an adjoining river, like New See also: This reckless policy of securing additional land regardless of-consequences has greatly contributed to the more frequent occurrence of the very widespread inundations resulting from the bursting of the vast volume of pent-up flood-waters through breaches in the banks, which descend with torrential violence upon the plains below, causing great destruction of See also:life and See also:property. The restriction of the floods on the lower Mississippi by the levees, placed about See also:double the width apart of the ordinary channel, has caused the river to enlarge its very soft alluvial bed, resulting in a lowering of the water-line at the low stage; and it is, therefore, anticipated that the further scour by floods when the levees have been made continuous will, in this instance, prevent any material raising of the flood-level by the levees. Protection of Vessels during Floods.—On large open rivers, where vessels during high floods are exposed to injury from large floating debris and ice floes, shelter can be provided for them in See also:refuge ports, formed in a See also:recess at the side under the protection of a solid See also:jetty or See also:embankment constructed in the river parallel to the bank, these ports being closed against floods at their upper end and having their entrance at the lower end facing down-stream. Many such ports have been provided on several See also:German and North See also:American rivers; where the See also:port, being near a town, is lined with See also:quay walls, it can also be used for river See also:traffic, a See also:plan adopted at the refuge port on the Main just below See also:Frankfort (fig. 8). Regulation of Rivers for Navigation. As rivers flow onward towards the sea, they experience a considerable diminution in their fall, and a progressive increase in the basin which they drain, owing to the successive influx of their various tributaries. Thus • gradually their current becomes more gentle and their discharge larger in volume and less subject to abrupt variations; and, consequently, they become more suitable for navigation. Eventually, large rivers, under favourable conditions, often furnish important natural highways for inland navigation in the lower portion of their course, as, for instance, the Rhine, the Danube and the Mississippi; and works are only required for preventing changes in the course of the stream, for regulating its depth, and especially for fixing the low-water channel and concentrating the flow in it, so as to increase as far as practicable the navigable depth at the lowest stage of the water-level. Regulation works for increasing the navigable capabilities of rivers can only be advantageously undertaken in large rivers with a moderate fall and a fair discharge at their lowest stage; for with a large fall the current presents a great impediment to up-stream navigation, and there are generally great variations in water-level, and when the discharge becomes very small in the dry season it is impossible to maintain a sufficient depth of water in the low-water channel. Removal of Shoals.—The possibility of securing uniformity of depth in a river by the lowering of the shoals obstructing the channel depends upon the nature of the shoals. A soft shoal in the bed of a river is due to deposit from a diminution in velocity of flow, produced by a reduction in fall and by a widening of the channel, or to a loss in copcentration of the scour of the main current in passing over from one concave bank to the next on the opposite side. The lowering of such a shoal by dredging merely effects a temporary deepening, for it soon forms again from the causes which produced it. The removal, moreover, of the rocky obstructions at rapids, though increasing the depth and equalizing the flow at these places, produces a lowering of the river above the rapids by facilitating the efflux, which may result in the See also:appearance of fresh shoals at the low stage of the river. Where, however, narrow rocky reefs or other hard shoals stretch across the bottom of a river and See also:present obstacles to the erosion by the current of the soft materials forming the bed of the river above and below, their removal may prove a permanent improvement by enabling the river to deepen its bed by natural scour. The deepening of the bed of a non-tidal river along a considerable length by dredging merely lowers the water-level of the river during the low stage; and though this deepening facilitates the passage of floods in the first instance, it does not constitute a permanent improvement even in this respect, for the deposit of the detritus brought down by the river as the floods abate soon restores the river to its See also:original See also:condition. Nevertheless, where sand-banks obstruct and divert the low-See also:state channel of a river at its low stage, as in parts of the Mississippi below Cairo, it has been found possible before the river has fallen to its lowest level to form a channel through these sand-banks, with a depth of 9 or Io ft. and 250 ft. wide, by suction dredgers, aided by revolving cutters or water-jets (see DREDGING), which discharge the sand through floating tubes into a part of the river away from the channel; and the navigation can thus be maintained throughout the low stage at a reasonable cost. Though, however, these channels across the shoals, connecting the deeper parts of the river, can be easily kept open on the Mississippi till the return of the floods, they are obliterated by the currents in flood-time, and have to be dredged out again afresh every year on the See also:abatement of the floods. Regulation of the Low-Water Channel.—The capability of a river to provide a waterway for navigation during the summer or throughout the dry season depends upon the depth that can be secured in the channel at the lowest stage. Owing to the small discharge and deficiency in scour during this period, it is important to restrict the width of the low-water channel, and concentrate the flow in it, and also to See also:fix its position so that, forming the deepest part of thebed along the line of the strongest current, it may be scoured out every year by the floods, instead of remaining an undefined and shifting channel. This is effected by closing subsidiary low-water channels with dikes across them, and narrowing the channel at the low stage by low-dipping cross dikes extended from the river banks down the slope, and pointing slightly up-stream so as to See also:direct the water flowing over them into a central channel (See also:figs. 4 and 5). The contraction also of the channel is often still more effectually accomplished at some parts, though at a greater cost, by low Regulation Works. FIGS. 4 and 5.—River Rhone. FIG. 6.—River Rhine. longitudinal dikes placed along either side of the low-water channel, some distance forward from the banks but connected with them generally at intervals by cross dikes at the back to prevent the current from scouring out a channel behind them during floods (figs. 4 and 6). By raising these dikes only slightly above the surface of the bed of the river, except where it is expedient to produce See also:accretion for closing an old disused channel or rectifying the course of the river, the capacity of the channel for discharging floods is not affected; for the slight obstruction to the flow produced by the dikes at the sides is fully compensated for by the deepening of the low-water channel in the central course of the river. This system of obtaining a moderate increase in depth during the low stage of a river, whilst leaving the river quite open for navigation, has been adopted with satisfactory results on several large rivers, of which the Rhone, the Rhine and the Mississippi furnish notable examples. Regulation works were preferred on the Rhone to canalization from Lyons nearly to its outlet, in spite of its large fall, which reaches in some places I in 250, on account of the considerable quantities of shingle and gravel carried down by the river; the See also:comparative regularity of the discharge, owing to the flow being derived from tributaries having their floods at different times of the year, has aided the effects of the works, which have produced an increase of about 3; ft. in the available navigable depth below Lyons at the lowest water-level. Owing, however, to the unfavourable natural condition of the river, the depth does not exceed 5 ft. at this stage; and the rapid current forms a serious impediment to up-stream navigation. The Rhine is much better adapted for improvement by regulation works than the Rhone, for it has a basin more than double the area of the Rhone basin, and its fall does not exceed 3.1 ft. per mile up at See also:Strassburg and 2.5 ft. per mile through the rocky See also:defile from See also:Bingen to Kaub, and is much less along most of the length below Strassburg. These works systematically carried out in wide shallow reaches between the Dutch frontier and See also:Mainz, aided by dredging where necessary, have secured a navigable depth at the low stage of the river of ro ft. from the frontier to See also:Cologne, 82 ft. from Cologne to Kaub, and 62 ft. through the rocky defile up to Bingen, beyond which the same depth is maintained up to See also:Philippsburg, 221 M. above See also:Mannheim. Works, moreover, are in progress by which it is anticipated that the minimum depth of 6; ft. will be extended up to Strassburg by 1916. The Mississippi also, with its extensive basin and its moderate fall in most parts, is well suited for having its navigable depth increased
by regulation works, which have been carried out below St See also:Paul in ordinary summer level has to be raised by impounding the shallow and shifting reaches, with the object of obtaining a mini- flow with weirs at intervals across the channel (see WEIR), mum navigable depth during the low stage of 6 ft. along the upper
river from St Paul to St See also: Canalization secures a definite available depth for navigation; and the discharge of the river generally is amply sufficient for maintaining the impounded water-level, as well as providing the necessary water for locking. The navigation, however, is liable to be stopped during the descent of high floods, which in many cases rise above the locks (fig. 7); and it is necessarily arrested in cold climates on all rivers by long, severe frosts, and especially on the break-up of the ice. Instances of Canalized Rivers.—Many small rivers, like the Thames above its tidal limit, have been rendered navigable by canalization, and several fairly large rivers. have thereby provided a good depth for vessels for considerable distances inland. Thus the canalized Seine has secured a navigable depth of See also:Io2 ft. from its tidal limit up to Paris, a distance of 135 m., and a depth of 62 ft. up to See also:Montereau, 62 m. higher up. Regulation works for improving the river Main, from Its confluence with the Rhine opposite Mainz up eroded banks are protected along their upper, steeper part by stone pitching or a layer of concrete, and below low-water level by fascine mattresses weighted with stone, extended a short distance out on the bed to prevent erosion at the toe. Dikes, also, projecting into the channel from the banks reduce the curvature of the navigable channel by pushing the main current into a more central course; whilst curved longitudinal dikes placed in the channel in front of concave banks (figs. 4 and 6) are still more effective in keeping the current away from the banks, which is sometimes still further promoted by dipping cross dikes in front (fig. 5). Regulation of Depth.—The regulation works at bends, besides arresting erosion, also reduce the differences in depth at the bends and the crossings, since they diminish the excessive depth round the concave banks and deepen the channel along the crossings, by giving a straighter course to the current and concentrating it by a reduction in width of the channel between the bends (figs. 4 and 5). Where there are deep pools at intervals in a river, shoals are always found above them, owing to the increased fall which occurs in the water line on approaching the See also:pool, to compensate for the very slight inclination of the water-line in See also:crossing the pool, which serves for the discharge of the river through the ample cross-section of this part of the river-bed. These variable depths can be regulated to some extent by rubble dikes or fascine See also:mattress sills deposited across the bed of the pool, so as to reduce its excessive depth, but not raised high enough to interfere at all with the navigable depth. These obstructions in the pool raise the water-line towards its upper end, in order to provide the additional fall needed to effect the discharge through the pool with its diminished cross section; and this raising of the water-line increases the depth over the shoal above the pool, so that the general depth in these irregular parts of a river is rendered more uniform, with benefit to navigation. Canalization of Rivers. Rivers whose discharge is liable to become quite small at their low stage, or which have a somewhat large fall, as is usual in the upper part of rivers, cannot be given an adequate depth for navigation by regulation works alone; and theirto Frankfort, having failed to secure a minimum depth of 3 ft. at the low stage of the river, canalization works were carried out in 1883–86 by means of five weirs in the 22 M. between the Rhine and Frankfort, and provided a minimum depth of 6a ft. (figs. 7 and 8). r3 See also:Mop wo ,oeo i.00 r_ This depth was subsequently increased by dredging the shoaler portion towards the upper end of each reach, due to the rise of the river-bed up-stream, so as to attain a minimum depth of 7; ft. just below the lowest lock, and 7t to 83 ft. in the other reaches; whilst a See also:sixth weir was erected at See also:Offenbach above Frankfort (fig. 7). The Great Kanawha, Ohio, and other rivers, furnish instances of canalization works in the United States. Limits to Canalization.—On ascending a river it becomes increasingly difficult to obtain a good depth by canalization in the upper part, owing to the progressive inclination of the river-bed ; thus, even on the Seine, with its moderate fall, whereas a depth of Io2 ft. has been obtained on the Lower Seine by weirs placed on the average 132 M. apart, on the Upper Seine weirs are required at intervals of only about 43 M. to attain a depth of 6; ft. Accordingly, the higher parts of rivers are only suitable for floating down trunks of trees felled on the hills, or rough rafts of timber, conveying small loads of produce, which are broken un on reaching their destination. Moreover, sometimes an abrupt fall or rocky shoals make it necessary to abandon a section of the river and to continue the navigation by lateral canal. Small River Outlets exposed to Littoral See also:Drift. Rivers with a small discharge flowing straight into the sea on an exposed coast are more or less obstructed at their outlet *IV. MAIN. by drift of shingle or sand carried along the coast by the waves in the direction of the prevailing winds. When the flow falls very low in dry weather, the outlet of a river is sometimes completely closed by a continuous line of See also:beach, any inland or tidal waters merely trickling through the obstruction; and it is only on the descent of floods that the outlet is opened out. In rivers which always have a fair fresh-water discharge, or a small fresh-water flow combined with a tidal flow and ebb, the channel sometimes has its direct outlet closed, and is deflected parallel to the See also:shore till it reaches a weak place in the line of beach, through which a new outlet is formed; or, where the current is strong enough to keep the outlet open, a See also:bar is formed across the entrance by the littoral drift, reducing the navigable depth. Jetties at River Outlets.—The bar formed by littoral drift across the outlet of a river not charged with sediment and flowing into a tideless sea can be lowered by carrying out solid jetties on each side of the outlet across the See also:foreshore, so as to scour the bar by concentrating the issuing current over it. Thus by means of jetties, aided by dredging, the depth at the entrance to the Swine mouth of the Oder has been increased from 7 ft. to 221 ft.; the approach channels to the river See also:Pernau (fig. 9) and other See also:Russian rivers flowing into the Baltic have been deepened by jetties, and the outlet channels of some of the rivers flowing into the Great Lakes of North America have been improved by See also:crib-See also:work jetties and dredging. Where the littoral drift is powerful enough to divert the outlet of a river, as in the case of the river Yare, which at one time was driven to an outlet 4 M. south of its direct course into the sea at See also:Yarmouth, and the river See also:Adour in France, whose outlet, owing to the violent storms of the See also:Bay of See also:Biscay, was liable to be shifted 18 m. from its proper position, it has proved practicable to fix as well as to deepen the outlet by means of jetties (fig. To). In such cases, however, where the rivers flow into tidal seas, it is important to place the jetties sufficiently apart to avoid any loss of tidal influx, since the tidal flow assists the fresh-water discharge in keeping the outlet open; whereas, with rivers flowing into tideless seas, a moderate restriction of the width between the jetties increases the scour. The tortuous and somewhat shifting outlet channel of the Scheur See also:branch of the river See also:Maas, emerging on to a sandy coast where the rise of tide is small, and obstructed at its mouth by a bar, has been replaced by a straight cut across the See also:Hook of Holland, and by an outlet guided across the foreshore and fixed in position by fascine mattress jetties (see JETTY), the See also:maintenance of the depth at the mouth by the tidal and fresh waters being aided by frequent dredging (figs. I I and 12). Deltaic Outlets of Tideless Rivers. Large rivers heavily charged with sand and silt, when their current is gradually arrested on entering a tideless sea, deposit these materials as a constantly advancing See also:fan-shaped shoal in front of their mouths, through which comparativelyshallow diverging channels, almost devoid of fall, have to force their way in order to convey the fresh-water discharge ,)pF q •MILCs. 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