Online Encyclopedia

RIVER ENGINEERING

Online Encyclopedia
Originally appearing in Volume V23, Page 380 of the 1911 Encyclopedia Britannica.
Spread the word: del.icio.us del.icio.us it!
RIVER ENGINEERING. Before undertaking works for the improvement of rivers, either with the object of mitigating the effects of their inundations, or for increasing and extending their capabilities for navigation, it is most important that their physical characteristics should be investigated in each case, for these vary greatly in different rivers, being dependent upon the general configuration of the land, the nature of the surface strata and the climate of the country which the rivers traverse. Physical Characteristics of Rivers The size of rivers above any tidal limit and their average fresh-water discharge are proportionate to the extent of their basins, and the amount of rain which, falling over these basins, reaches the river channels in the bottom of the valleys, by which it is conveyed to the sea. River Basins.—The basin of a river is the expanse of country, bounded by a winding ridge of high ground, over which the rainfall flows down towards the river traversing the lowest part of the valley; whereas the rain falling on the 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 coast and flowing straight down into the sea, up to immense tracts of great continents, when rivers, rising on the slopes of 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 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 Britain, with its very limited area, cannot possess large river basins, its largest being that of the Thames with an area of 5244 sq. m. Even on the mainland of Europe, river basins augment in extent on proceeding eastwards with the increasing width of the continent; in France the largest basin is that of the Loire with an area of 45,000 sq. m., while the Rhine has a basin of 86,000 sq. m. with a length of 80o in., the Danube a basin of 312,000 sq. M. with a length of 1700 m., and the Volga a basin of 563,000 sq. m. with a length of 2000 M. The more extensive continents of Asia, Africa and North and South America possess still larger river basins, the Obi in Siberia having a basin of about 1,300'000 sq. m. and a length of 3200 m., the Nile a basin of 1,500,000 sq. m. with a length of over 4000 m., and the 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 Amazon of 2,250,000 sq. m. is due to the chain of the Andes almost bordering the Pacific coast-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 Atlantic Ocean. Available Rainfall.—The rainfall varies considerably in different localities, both in its total yearly amount and in its distribution throughout the year; also its volume fluctuates from year to year. Even in small river basins the variations in rainfall may be considerable according to differences in elevation or distance from the sea, ranging, for instance, in the 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 season of the year. The available rainfall has, indeed, been found to vary from 75% of the actual rainfall on impermeable, bare, sloping, rocky strata, down to about 15% on flat, very permeable soils. Fall of Rivers.—The 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 friction against its bed and 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 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 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 regular discharge. Variations in the Discharge of Rivers.—The irregular flow of rivers throughout their course forms one of the 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 flood during the rainy season and have hardly any flow during the 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 weather than in the winter months, so that the rivers fall to their low 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 cold season, extending from May to October and from November to April respectively; the rivers are low and moderate floods are of rare occurrence during the first 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 sources amongst mountains clad with perpetual snow, and are fed by glaciers; their floods occur in the summer from the melting of the snows and ice, as' exemplified by the Rhone above the 'Lake of 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 Lyons has a more uniform discharge than most rivers, as the summer floods of the Arve are counteracted to a great extent by the low stage of the 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-time, derived mainly from the disintegration of the surface-layers of the hills and slopes in the upper parts of the valleys by glaciers, frost and rain. The 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 attrition in their onward course into shingle, gravel, sand and silt, simultaneously with the gradual reduction in fall, and, consequently, in the transporting force of the current. Accordingly, under 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 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 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, 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 mass of the sea, especially in the absence of a 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. Influence of Lakes on Rivers.—Sometimes a peculiar depression along part of a valley, with a rocky barrier at its 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 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, 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 long maintained, owing to the confluence a 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 burden of detritus and the regulation of their flow. Thus the Neva, conveying the outflow from Lake Ladoga to the Baltic, is relieved by the lake from the detritus brought down by the rivers flowing into the lake; and the Swine outlet channel of the Oder into the Baltic is freed from sediment by the river having to pass through the Stettiner Haff before reaching its mouth. The St Lawrence, again, deriving most of its 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 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 constant erosive action to maintain a fairly regular channel; though rivers having soft beds and carrying down sediment erode their beds during floods and deposit 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 depth inversely with the width. A river channel, accordingly, often presents considerable irregularities in section, forming shallow rapids when the river flows over a rocky barrier with a considerable fall, and consisting of a succession of pools and shoals when the bed varies in compactness and there are differences in width, or when the river flows round a succession of bends along opposite banks alternately. A river flowing through a flat alluvial plain has its current very readily deflected by any 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 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 Lombardy plains between Casale and the confluence of the 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 Marne, draining an impermeable part of the Upper Seine basin, is subject to these sudden torrential floods in the cold season, as illustrated by a diagram of the variations in height of the river at St Dizier from November to March 1903–4 (fig. z), On the contrary, rain falling on permeable strata takes longer in reaching the rivers; and the floods of these rivers rise more gradually, are less high, continue longer and subside more slowly than in rivers draining impervious strata, as indicated by the diagram of the Little Seine at Nogent during the same period, which has a permeable basin (fig. 1). A main river fed by several tributaries, some from impermeable and others from permeable strata, experiences floods of a mixed character, as shown by the diagram of the same floods in 1903–4 of the Seine at Paris, below the confluence of the torrential Marne and Yonne, where the floods of the gently flowing Upper Seine and other tributaries with permeable basins also contribute to the rise of the river (fig. 3). High floods are caused by a heavy rainfall on land already sodden by recent rains at a period of the year when evaporation is inactive, and especially by rain falling on melting snow. A fairly simultaneous rainfall over the greater part of a moderate-sized river basin is a tolerably common occurrence; and under such conditions, the floods coming from the torrential tributaries reach their maximum height and begin to subside before the floods from the gently flowing tributaries attain their greatest rise. Exceptional floods, accordingly, only occur in a main river when a heavy rainfall takes 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 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 DECEMBER . 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. 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 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 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 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, a torrential tributary of the Loire, form two reservoirs for the supply of the town of St 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 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-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 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 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 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 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 mining refuse, sluice-gates for mills, fish-traps, unduly wide piers for 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 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 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 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 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. 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 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 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 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 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 m., ex-tending from Cornale, 89 m. below Turin, to its outlet. The system has been developed on a very extensive scale along the alluvial valley of the Mississippi, which is below the high flood-level of the' river from Cape Girardeau, 45 M. above Cairo, to the Gulfof Mexico, and has a length of 600 m. in a straight line with a width ranging between 20 and 8o m., and an area of 29,990 sq. m. These embankments, having been begun by the French settlers in 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 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 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 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 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 Orleans on the Mississippi and Szegedin on the Theiss in Hungary, the channel of the river should be improved to facilitate the passage of floods past the town. The town also must be enclosed within very solid embankments, raised above the highest possible flood-level, to obviate the contingency of an exceptional flood, or a gradually raised flood-level, overtopping the protecting bank at a low part, leading to an inevitable breach and a catastrophe such as overwhelmed the greater part of Szegedin in March 1879. Effect of Embankments in raising the River Bed.—A most serious objection to the formation of continuous, high embankments along rivers bringing down considerable quantities of detritus, especially near a part where their fall has been abruptly reduced by descending from mountain slopes on to alluvial plains, is the danger of their bed being raised by deposit, producing a rise in the flood-level, and necessitating a raising of the embankments if inundations are to be prevented. Longitudinal sections of the Po taken in 1874 and 1901 show that its bed was materially raised in this period from the confluence of the Ticino to below Caranella, in spite of the clearance of sediment effected by the rush through breaches; and therefore the completion of the embankments, together with their raising, would only eventually aggravate the injuries of inundations they have been designed to prevent, as the escape of floods from the raised river must sooner or later occur. The periodical devastating floods of the Hwang Ho or Yellow River in China are due to the raising of the bed of its embanked' channel by detritus brought down from the hills, followed by the raising of the banks, whereby the river is forced to flow above the level of the plains. When the river was first embanked, a consider-able space was left between it and its banks on each side, which allowed for deviations in the channel, and also afforded a fair area for the deposit of detritus away from its bed, and a good width for the discharge of floods. Later, however, in order to appropriate and bring under regular cultivation the riparian land thus prudently left within the embankments and exposed to every flood, lines of inner embankments were formed close to the river, thereby greatly confining the flood-waters, and, consequently, raising the flood-level and the river-bed, besides exposing these embankments to under-mining by merely a moderate change in position of the river channel. 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 life and property. The restriction of the floods on the lower Mississippi by the levees, placed about 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 refuge ports, formed in a recess at the side under the protection of a solid jetty or 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 German and North American rivers; where the port, being near a town, is lined with quay walls, it can also be used for river traffic, a plan adopted at the refuge port on the Main just below 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 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 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 original condition. Nevertheless, where sand-banks obstruct and divert the low-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 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 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 direct the water flowing over them into a central channel (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 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 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 Strassburg and 2.5 ft. per mile through the rocky defile from 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 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 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 Philippsburg, 221 M. above 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 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 Louis just below the confluence of the Missouri, and 8 ft. thence to Cairo at the mouth of the Ohio. Various materials are used for the regulation works according to the respective conditions and the materials available in the locality. On the Rhone below Lyons with its rapid current, the dikes have been constructed of rubble-stone, consolidated above low water with concrete. The dikes on the Rhine consist for the most part of earthwork mounds protected by a layer of rubble-stone or pitching on the face, with a rubble mound forming the toe exposed to the current; but occasionally fascines are employed in conjunction with stone or simple rubble mounds. The dams closing subsidiary channels on the Mississippi are almost always constructed of fascine mattresses weighted with stone; but whereas the regulating dikes on the upper river are usually similar in construction, a common form for dikes in the United States consists of two parallel rows of piles filled in between with brushwood or other materials not affected by water, and protected at the sides from scour by an apron of fascines and stone. Other forms of dikes sometimes used are timber cribs filled with stone, single rows of sheet piling, permeable dikes composed of piles supporting thin curtains of brushwood for promoting silting at the sides, and occasionally rubble-stone in places needing special protection. Protecting and Easing Bends.—Unless the concave banks of a river winding through wide, alluvial plains are protected from the scour of the current, the increasing curvature presents serious impediments to navigation, sometimes eventually becoming so intensified that the river at last makes a short cut for itself across the narrow strip of land at the base of the loop it has formed. This, however, produces considerable changes in the channel below, and disturbances in the navigable depth. Protection, accordingly, of concave banks is necessary to prevent excessive curvature of the channel and changes in the course of a river. On the Mississippi the very easily while a lock (see CANAL and Docx) has to be provided alongside the weir, or in a side channel, to provide for the passage of vessels (fig. 8). A river is thereby converted into a succession of fairly level reaches rising in steps up-stream, providing a comparatively still-water navigation like a canal; but it differs from a canal in the introduction of weirs for keeping up the water-level, in the provision for the regular discharge of the river at the weirs, and in the two sills of the locks being laid at the same level instead of the upper sill being raised above the lower one to the extent of the rise at the lock, as usual on canals. 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 Io2 ft. from its tidal limit up to Paris, a distance of 135 m., and a depth of 62 ft. up to 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 pool, to compensate for the very slight inclination of the water-line in 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 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 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 sixth weir was erected at 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 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 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 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 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 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 Pernau (fig. 9) and other 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 crib-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 Yarmouth, and the river Adour in France, whose outlet, owing to the violent storms of the Bay of 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 branch of the river 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 Hook of Holland, and by an outlet guided across the foreshore and fixed in position by fascine mattress jetties (see JETTY), the 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 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. • 7 i 4 4 1 9 O
End of Article: RIVER ENGINEERING
[back]
RIVER CLYDE
[next]
RIVERINA

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click and select "copy." Paste it into a website, email, or other HTML document.