1911 Encyclopædia Britannica/River Engineering
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 freshwater 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 inland, have to traverse vast stretches of valleys and plains before reaching 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 extenton 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 800 m., 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 m. 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 inyin the year to over 80 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 Saône 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 100 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. Sometimes 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. 2). 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 banks in flood-time and inundates the adjacent low-lying lands to an extent depending upon the level of the ground and the
Flood Diagrams, Seine basin, 1903–1904.
:Fig. 1.—Little Seine at Nogent.
:Fig. 2.—Marne at St Dizier.
:Fig. 3.—Seine at Paris.
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 of grass and leaves, so that the amount of detritus carried down into the river is greatly reduced.
Proposals have sometimes been made to reduce the height of floods in rivers and restrict the resulting inundations by impounding some of the flood discharge by the construction of one or more dams across the upper valley of a river, and letting it out when the flood has passed down. This arrangement, however, is open to the objection that in the event of a second flood following rapidly on the first, there might not be time to empty the reservoir for its reception. The cost, moreover, of the formation of such-reservoirs could rarely be justified merely for the purpose of reducing the flood-level along an ordinary river valley. Nevertheless, when this provision against floods can be combined with the storage -of water supply for a town, it becomes financially practicable. Thus two masonry dams erected across the narrow valley of the river Furens, at torrential tributary of the Loire, form two reservoirs for the supply of the town of St 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 Etiennefrom inundation. Storage reservoirs also, formed solely for water-supply or irrigation, provided adequate compensation water is discharged from them during dry weather, are advantageous, like lakes, in regulating the flow of the river below.
When a river flowing through flat plains has a very small fall, it requires a proportionately large channel to carry away the drainage waters of the valley; and, accordingly, the low-lying lands bordering the river are very subject to inundations if the rainfall over the higher ground is allowed to flow straight down into the bottom of the valley. By intercepting, however, the flow off the high parts of the valley in small channels excavated along the slopes, termed “catch-water drains,” the ample fall available from this higher elevation can be utilized for conveying the flow farther down the valley; and the congested river is thereby relieved for a certain part of its length from the rainfall over the higher ground.
Methods of increasing the Discharging Efficiency of River Channels.—The discharging efficiency of a river within the limits of its bed depends on the fall and the cross-section of the channel. The only way of increasing the fall is to reduce the length of the channel by substituting straight cuts for a winding course. This involves some loss of capacity in the channel as a whole, and in the case' of a large river with a considerable flow it is very difficult to maintain a straight cut, owing to the tendency of the current to erode the banks and form again a sinuous channel. Even if the cut is preserved by protecting the banks, it is liable to produce changes, shoals and a raising of the flood-level in the channel just below its termination. Nevertheless, where the available fall is exceptionally small, as in lands originally reclaimed from the sea, such as the English fen districts, and where, in consequence, the drainage is in a great measure artificial, straight channels have been formed for the rivers; and on account of the importance of preserving these fertile, low-lying lands from inundation, /additional straight channels have been provided for the discharge of the rainfall, known as drains in the fens. Except where a town is exposed to inundations, a considerable modification of the course of a river and an enlargement of its channel do not produce a reduction in the damage from its floods commensurate with the expenditure involved.
The removal of obstructions, whether natural or artificial, from the bed of a river furnishes a simple and efficient means of increasing the discharging capacity of its channel, and, consequently, of lowering the height of floods; for every impediment to the flow, in proportion to its extent, raises the level of the river above it so as to produce the additional artificial fall necessary to convey the flow through the restricted channel, thereby reducing the total available fall. Accidental obstructions, brought down by floods, such as trunks of trees, boulders and accumulations of gravel, require to be periodically removed. In the absence of legal enactments for the 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 any 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 vim pending 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 shutoff from the floods of the Po by embankments along each side of the river for a distance of about 265 m., extending 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 Gulf of Mexico, and has a length of 600 m. in a straight line with a width ranging between 20 and 80 m., and an area of 29,790 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 512 ft. above the lowest stage of the river, and tend to increase in eight owing to the improved drainage following on the extension o 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 over topped 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 considerable 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 undermining 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 state; 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 débris 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 concentration 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 thelow 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 10 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 ob iterated by the currents in flood-time, and have to be dredged out again afresh every year on the abatement of the floods.
Figs. 4 and 5.—River Rhone.
Fig. 6.—River Rhine.
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 the bed 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 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 1 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 323 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 10 ft. from the frontier to Cologne, 812 ft. from Cologne to Kaub, and 612 ft. through the rocky defile up to Bingen, beyond which the same depth is maintained up to Philippsburg, 2212 m. above Mannheim. Works, moreover, are in progress by which it is anticipated that the minimum depth of 612 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 shallow and shifting reaches, with the object of obtaining a minimum 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 iles 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 brus wood 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 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 de th 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 their ordinary summer level has to be raised by impounding the flow with weirs at intervals across the channel (see Weir), while a lock (see Canal and Dock) 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.
Fig. 7.—Canalized River Main.
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 1012 ft. from its tidal limit up to Paris, a distance of 135 m., and a depth of 612 ft. up to Montereau, 62 m. higher up. Regulation works for improving the river Main, from its confluence with the Rhine opposite Main; up to 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 612 ft. (figs. 7 and 8).
Fig. 8.—Locks, Weir and Haven near Frankfort.
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 723 ft. just below the lowest lock, and 745 to 813 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 1012 ft. has been obtained on the Lower Seine by weirs placed on the average 1312 m. apart, on the Upper Seine weirs are required at intervals of only about 423 m. to attain a depth of 612 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 up 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 atera cana.
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 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 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 2212 ft.; the approach channels to the river Pernau (fig. 9) and other Russian rivers flowing of the outlet across the foreshore, 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. 10).
Fig. 9.—Jetty Outlet into Baltic: River Pernau.
Fig. 10.—Shifting Outlet, fixed by Jetties: River Yare.
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. 11 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 comparatively shallow diverging channels, almost devoid of fall, have to force their way in order to convey the fresh-water discharge into the sea (fig. 13).
Figs. 11 and 12.—Jetty Outlet into North Sea: River Maas.
Fig. 13.—Mississippi Delta.
These deltaic channels deposit their burden of sediment in front of their outlets, forming bars which advance with the delta and whose rate of progress seawards and distance in front of each outlet are proportionate to the discharge of the several channels. A channel simply dredged on the bar in front of one of the outlets of a deltaic river is only maintained for a moderate period on account of the large volume of deposit continually, accumulating at the outlet. Thus the channel in front of the outlet of the south-west pass of the Mississippi delta, when deepened from 13 ft. to 18 ft. over its bar by dredging many years ago, was soon silted up again on the discontinuance of the dredging; Whilst the depth of the outlet channel of one of the branches of the Volga delta, which was increased from 4 ft. to 8 ft., could only be maintained by regular yearly dredging.
Parallel Jellies at Delta Outlets.—In order to procure and maintain for some time an adequate deepening across the bar in front of the outlets of delta channels, recourse has been had to the scour of the issuing current concentrated and extended out to the bar by parallel jetties, forming prolongations seawards of the banks of the channel. he requisite conditions for the success of this system of improvement are a good depth in the sea beyond the bar, allowing of a considerable deposit of alluvium before the increased depth is interfered with, and a littoral current carrying pa portion of the alluvium away from the outlet, both of which retard the progression of the delta in front of the outlet and the inevitable eventual formation of a new bar farther out. The rate of advance of a delta depends also on the proportion of solid matter contained. in the river water and on the specific gravity and size of the particles of alluvium discharged into the sea; for the heavier and coarser materials, and especially those which are rolled along the bed of the channels, come first to rest. Moreover, as the larger channels of a delta bring down a larger volume of alluvium on account of their larger discharge, and as their bars form farther seawards from their outlets owing to the issuing current being less rapidly arrested in proportion to the volume discharged, the rate of advance of the delta in front of an outlet is proportionate to the size of the channel, and the length of the jetties required for lowering the bar by scour in front of any channel is proportionate to the discharge of the channel. Consequently, the conditions are more unfavourable for the improvement of the outlets of the larger delta channels than of the smaller ones; though, on the other hand, the larger channels crossing the delta are generally more suitable for navigation on account of their size, and the natural depth over their bars is greater owing to the larger discharge.
The discharge of the main branch of the Rhone, which formerly flowed into the Mediterranean and the Gulf of Foz through six mouths, was in 1852–57 concentrated in the direct eastern channel by embankments along sides, which closed all the lateral channels. The entire flow of the river, being thus discharged through the eastern outlets, increased for a time the depth over its bar Rhone. from 412 ft. to 934 ft.; but as the great volume of alluvium brought down, including an unusually large proportion of sand rolled along the bed of the river, was also all discharged through the. one outlet, the bar soon formed again farther out, and naturally advanced with the delta in front of the outlet more rapidly than formerly when the deposit was distributed through six divergent mouths. Accordingly, the very moderate deepening produced by the embankments was not long maintained, and the average depth over the bar has not exceeded 612 ft. for many years past; the St Louis Canal was constructed to provide a deeper outlet for the navigation. This want of success was due to the selection of an outlet opening on a sheltered, somewhat shallow bay, instead of a southern outlet discharging into deep water in the Mediterranean and having a deep littoral current flowing across it, and also resulted from the closing of all the-other outlets, whereby the whole of the deposit, as well as all the discharge, was concentrated in front of the badly situated eastern outlet. The southern Roustan branch was reopened in 1893 to prevent the silting-up of the outlet of the St Louis Canal. The Danube traverses its delta in three branches, the northern one of which, though conveying nearly two-thirds of the discharge Danube of the river, is unsuitable for improvement owing to its splitting up alon portions of its course into several channels, and eventually flowing into the sea through twelve mouths of a small independent delta advancing about 250 ft. annually across a shallow foreshore. The central Sulina branch was selected for improvement in 1858 in preference to the southern St George’s branch, which had a more favourably situated outlet and a better channel through the delta, on account of the much smaller expenditure required for carrying out jetties to the bar in front of the Sulina outlet, which was only half. the distance from the shore of the bar of the St George’s outlet, owing to the much smaller discharge of the Sulina branch. The jetties, begun provisionally in 1858 and subsequently consolidated and somewhat extended, were finally completed in 1877. They increased the depth over the bar from an average of about 9 ft. previously to 1858 up to 2012 ft. in 1873, which was maintained for many years. In 1893, however, the increasing draught of vessels rendered a greater depth necessary; the wide inshore portion of the jetty channel was therefore narrowed by inner parallel jetties, and a powerful dredger was set to work in the jetty channel and outside, whereby the depth was increased to 24 ft. in 1897, and was fairly maintained up to 1907, when a second dredger became necessary to cope with the shoaling. The somewhat small ratio of sediment to discharge in the Danube, the fineness of the greater portion of this sediment, its comparatively moderate amount owing to the small proportion of the discharge lii owing through the Sulina branch, and its partial dispersion by the southerly littoral current and wave action, have prevented the rapid formation of a shoal in front of the Sulina outlet. Nevertheless, the lines of soundings are gradually advancing seawards in the line of the outlet channel, and there are signs of the formation of a new bar farther out, whilst the deposit to the south by the current and waves has deflected the deepest channel northwards. <Accordingly, a prolongation of the jetties will eventually be necessary, notwithstanding the removal of a portion of the deposit from the outlet channel by dredging.
The selection of the outlet of the south pass of the Mississippi delta for improvement by parallel jetties in 1876–79, in spite of the south-west pass possessing a larger channel and a better depth over its bar, was due, as at the Danube, to motives of economy, as the bar of the south-west pass was twice as far off from the shore as that of the south pass (fig. 13). There fascine Mississippi. mattress jetties, weighted with limestone, and with large concrete blocks at their exposed ends (see Jetty), 214 and 112 m. long, and curved slightly southwards at their outer ends to direct the sediment-bearing current more directly at right angles to the westerly littoral current, increased the depth of 8 ft. over the bar in 1875 up to 31 ft. between the jetties and out to deep water (fig. 14). The prolonged current of the river produced by the jetties has; as at the Sulina outlet, carried the main portion of the heavier sediment into fairly deep water, so that the greatest advance of the foreshore in front of the south pass has occurred in the 70-ft. line of soundings, though the shallower soundings have also advanced.
Fig. 14.—Deltaic Jetty Outlet, South Pass, Mississippi.
The shoaling, however, in the jetty channel necessitated its reduction in width by mattresses and spurs from 1000 ft. to 600 ft., and also dred ing to maintain the stipulated central depth of 30 ft., and 26 ft. depth for a width of 200 ft., out to deep water; whilst the outer channel was deflected to the east and narrowed by the alluvium carried. westwards by the littoral current and also deposited in front of the jetty outlet. Accordingly, dredging has been increasingly needed to straighten the channel outside and maintain its depth and width; and since the United States engineers took in hand its maintenance in 1901, the available depth of the outlet channel has been increased from 26 ft. up to 28 ft. by extensive suction dredging.
In order to provide for the increasing requirements of sea-going vessels, the dredging of a channel 35 ft. deep and 1000 ft. wide, cut from the large south-west pass outlet to deep water in the gulf, was begun at the end of 1903; and jetties of fascine mattresses weighted with stone and concrete blocks have been carried out about 4 and 3 m. respectively from the shore on each side of the outlet for maintaining the dredged channel (fig. 15). These works differ
Fig. 15.—Deltaic jetty Outlet, South-West Pass, Mississippi.
from the prior improvement of the south pass in the adoption mainly of suction dredging for the formation of the channel in place of scour alone, so that it will be unnecessary to restrict the width of the jetty channel to secure the desired depth; whilst as the discharge through the south-west pass is rather more than three times the discharge through the south pass, and the bar is double the distance seawards o the outlet, the slightly converging jetties, in continuation of the south-west pass, are placed about 3400 ft apart at their outer ends; and have been given about twice the length of the south pass jetties. As soon as the dredging of the channel has been completed (which depends on the appropriations granted by Congress) the south pass will be abandoned, and the south-west pass will form the navigable approach. Dredging will be required for preserving the depth of the outlet of the south-west pass; and when the large volume of sand and other alluvium discharged by the pass accumulates in front sufficiently to begin forming a bar farther out, an extension of the jetties will be necessary to maintain the elongated channel free from drift, and extend the scour, especially in flood-time.
Improvement of Tidal Rivers for Navigation.
Whereas the size of tideless rivers depends wholly on their fresh-water discharge, the condition of tidal rivers is due to the configuration of their outlet, the rise of tide at their mouth, the distance the tide can penetrate inland, and the space available for its reception. Accordingly, tidal rivers sometimes, even when possessing a comparatively small fresh-water discharge, develop. under favourable conditions into large rivers in their lower tidal portion, hav1ng a much better natural navigable channel at high tide than the largest deltaic rivers, as shown by a comparison of the Thames, the Humber and the Elbe with the Danube, the Nile and the Mississippi. Tidal water is, indeed, unlimited, in volume; but, unlike the drainage waters which must be discharged into the sea, it only flows up rivers where there is a channel and space available for its reception. Consequently, it is possible to exclude the tide by injudicious works, such as the sluices which were erected long ago across the fen rivers to secure the low-lying lands from the inroads of the sea; the tidal influx is also liable to be reduced by accretion in an estuary resulting from training works. The great aim, on the contrary, of all tidal river improvement should be to facilitate to the utmost the flow of the flood-tide up a river, to remove all obstructions from the channel so as to render the scouring efficiency of the flood and ebb tides as great as possible, and by making the tidal flow extend as far up the river as possible to reduce to a minimum the period of slack tide when deposit takes place.
Fig. 16.—Simultaneous Tidal Lines: River Hugli.
Tidal Flow in a River.—The progress of the flood-tide up a river and the corresponding ebb are very clearly shown by a diagram giving a series of simultaneous tidal lines obtained from simultaneous observations of the height of the river Hugli during a high spring tide in the dry season, taken at intervals at several stations along the river, and exhibiting on a very distorted scale the actual water level of the river at these periods (fig. 16). The steep form assumed by the foremost part of the flood-tide lines from the entrance to beyond Chinsura, attaining a maximum in the neighbourhood of Konnagar and Chinsura, indicates the existence of a bore, caused by the sand-banks in the channel obstructing the advance of the flood-tide, till it has risen sufficiently in height to rush up the river as a steep, breaking wave, overcoming all obstacles and producing a sudden reversal of the flow and abrupt rise of the water-level, as observed on the Severn, the Seine, the Amazon and other rivers. A bore indicates defects in the tidal condition and the navigable channel, which can only be reduced by lowering the obstructions and by the regulation of the river. No tidal river of even moderate length is ever completely filled by tidal water; for the tide begins to fall at its mouth before the flood-tide has produced high water at the tidal limit, as most clearly shown in the case of a long tidal river by the Hugli tidal diagram. Every improvement of the channel, however, expedites and increases the filling of the river, whilst the volume of water admitted at each tide is further augmented by the additional capacity provided by the greater efflux of the ebb, as indicated by the lowering of the low-water line.
Deepening Tidal Rivers by Dredging.—The improvement of tidal rivers mainly by dredging is specially applicable to small rivers which possess a sufficient navigable width, like the Clyde and the Tyne; for such rivers can be considerably deepened by an amount of dredging which would be quite inadequate for producing a similar increase in depth in a large, wide river, with shifting channels. Both the Clyde below Glasgow and the Tyne below Newcastle were originally insignificant rivers, almost dry in places at low water of spring-tides; and the earliest works on both rivers consisted mainly in regulating their flow and increasing their scour by jetties and training works. They have, however, been brought to their present excellent navigable condition almost wholly, since 1840 on the Clyde and 1861 on the Tyne, by continuous systematic dredging, rendered financially practicable by the growing importance of their sea-going traffic. The Clyde has been given a minimum depth of about 22 ft. at low water of spring-tides up to Glasgow, and can admit vessels of 27 to 28 ft. draught. In the Tyne (figs. 17 and 18), it was decided in 1902 to provide a minimum dredging depth in the river channel at low water of 25 ft. from the sea to the docks, of 20 ft. thence to Newcastle and of 18 ft. up to Scotswood, the rise of spring-tides increasing these depths by 15 ft. In 1906 it was determined to make the channel 30 ft. deep at low water of spring tides from the sea to the docks, and in 1908 to deepen it between the docks and Newcastle swing bridge from 20 to 25 ft., and also between the swing bridge and Derwenthaugh from 18 to 25 ft. The natural scour of these rivers has been so much reduced by such an exceptional enlargement of their channels that a considerable amount of dredging will always be required to preserve the depth attained.
Figs. 17 and 18.—Improvement of Tidal River by dredging:
Regulation and Dredging of Tidal Rivers.—Considerable improvements in the navigable condition of tidal rivers above their outlet or estuary can often be effected by regulation works aided by dredging, which ease sharp bends, straighten their course and render their channel, depth and flow more uniform. Examples are the Nervion between Bilbao and its mouth (figs. 19 and 20), and the Weser from Bremen to Bremerhaven at the head of its estuary (figs. 21 and 22). These works resemble in principle the regulation works on large rivers with only a fresh-water discharge, previously described; but on tidal rivers the main low-water channel should alone be trained with an enlarging width seawards to facilitate the tidal influx, and the tidal capacity of the river above low water should be maintained unimpaired.
Figs. 19 and 20.—Training Tidal River and protection of Outlet:
To secure a good and fairly uniform depth on a tidal river, it is essential that the flood and ebb tides should follow the same course, in order to combine their scouring efficiency, and form a single, continuous deep channel. In wide, winding reaches, however, the flood tide in ascending a river follows as direct a course as practicable; and on reaching a bend, the main flood-tide current, in being deflected from its straight course, hugs the concave bank, and, keeping close alongside the same bank beyond the bend, cuts into the shoal projecting from the convex bend of the bank higher up, forming a blind shoaling channel, as clearly indicated near the Moyapur Magazine in fig. 23, and a little below Shipgunj Point in fig. 24. This effect is due to the flood-tide losing its guidance, and consequently its concentration, at the change of curvature beyond the termination of the concave bank, where it spreads out and passes gradually over, in its direct course, to the next concave bend above along the opposite bank. The ebb tide, on the contrary, descending the river, follows the general course of the fresh-water discharge in all rivers, its main current in the Moyapur reach keeping close along the concave bank between Ulabaria and Hiragunj Point, and crossing over opposite the point to the next concave bank below (fig. 23); whilst in the James and Mary reach the main ebb-tide current runs alongside the concave bank in front of Ninan and Nurpur, and crosses over near Hugli Point to the opposite concave. bank below Gewankhali (fig. 24). The main currents, accordingly, of the flood and ebb tides in such reaches act quite independently between the bends, forming channels on opposite sides of the river and leaving a central intervening shoal. The surveys of the two reaches of the Hugli, represented in figs. 23 and 24, having been taken in the dry season, exhibit the flood-tide channels at their deepest phase, and the ebb-tide channels in their worst and least continuous condition.In tidal rivers the main ebb-tide current, being reinforced by
Figs. 21 and 22.—Training Tidal River at Estuary: River Weser.
the fresh-water discharge, generally forms the navigable channel, which is scoured out during floods. Narrowing the river between the bends to bring the two channels to ether would unduly restrict the tidal flow; and in a river like the Hugli dependent on the tidal influx for the maintenance of its depth for two-thirds of the year, and with channels changing with the wet and dry seasons, »so that deepening by dredging in tie turbid river could not be permanent, training works below low water to bring the ebb-tide current into the flood-tide channel, which latter must not be obstructed aided by dredging, the best prospects of improve at all, offer,
Fig. 23.—Moyapur Reach, River Hugli, Jan. 1896.
- Fig. 24.—James and Mary Reach, River Hugli, April 1890.
The average rate of enlargement adopted for the trained channel of the Nervion, in proportion to its length, is 1 in 75 between Bilbao and its mouth, and) 1 in 71 for the Weser from Bremen to Bremerhaven; and these ratios correspond very nearly to the enlargement cf the regulated channel of the Clyde from Glasgow to Dumbarton of 1 in 83, and of the Tyne from Newcastle to its mouth of 1 in 75. Accordingly, a rate of enlargement comprised between 1 in 70 and 1 in 80 for the regulated or trained channel of the lower portion of a tidal river with a fairly level.bed may be expected to give satisfactory results.
Works at the Outlet of Tidal Rivers.—Tidal rivers flowing straight into the sea, without expanding into an estuary, are subject to the obstruction of a bar formed by the heaping-up action of the waves and drift along the coast, especially when the fresh-water discharge is small; and the scour of the' currents is generally concentrated and extended across the beach by parallel jetties for lowering the bar, as at the outlets of the Maas (figs, II and 12) and of the Nervion (figs. 19 and 20). In the latter case, however, the trained outlet was still liable to be obstructed by drift during north-westerly storms in the Bay of Biscay; and, except in the case of large rivers, the jetties have to be placed too close together, if the scour is to be adequate, to form an easily accessible entrance on an exposed coast. Accordingly, a harbour has been formed in the small bay into which the Nervion flows by two converging breakwaters, which provides a sheltered approach to the river and protects the outlet from drift (fig. 19), and a similar provision has been made at Sunderland for the mouth of the Wear; whilst the Tynemouth piers formed part of the original design for the improvement of the Tyne, under shelter of which the bar has been removed by dredging (fig. 17).
Training Works through Sandy Estuaries.—Many tidal rivers flow through bays, estuaries or arms of the sea before reaching the open sea, as, for instance, the Mersey through Liverpool Bay, the Tees through its enclosed bay, the Liffey through Dublin Bay, the Thames, the Ribble, the Dee, the Shannon, the Seine, the Scheldt, the Weser and the Elbe through their respective estuaries, the Yorkshire Ouse and Trent through the Humber estuary, the Garonne and Dordogne through the Gironde estuary, and the Clyde, the Tay, the Severn and the St Lawrence through friths or arms of the sea. These estuaries vary greatly in their tidal range, the distance inland of the ports to which they giye access, and the facilities they offer for navigation. Some possess a very ample depth in their outer portion, though they generally become shallow towards their upper end; but dredging often suffices to remedy their deficiencies and to extend their deep-water channel. Thus the St Lawrence, which possesses an ample depth from the Atlantic up to Quebec, has been rendered accessible for seagoing vessels up to Montreal by a moderate amount of dredging; whilst dredging has been resorted to in parts of the Thames and Humber estuaries, and on the Elbe a little below Hamburg, to provide for the increasing draught of vessels; and the Mersey bar in Liverpool Bay, about 11 m. seawards of the actual mouth of the river, has been lowered by suction dredging from a depth of about 9 ft. down to about 27 below low water of equinoctial spring tides, to admit Atlantic liners at any state of the tide. Some estuaries, however, are so encumbered by sand banks that their rivers can only form shallow, shifting channels through them to the sea; and these channels require to be guided or fixed by longitudinal training walls, consisting of mounds of rubble stone, chalk, slag or fascines, in order to form sufficiently deep stable channels to be available for navigation. The difficulty in such works is to fix the wandering channel adequately, and to deepen it sufficiently by the scour produced between the training walls, without placing these walls so close together and raising them so high as to check the tidal influx and produce accretion behind them, thereby materially reducing the volume of tidal water entering and flowing out of the estuary at each tide. The high training works in the Dee estuary, carried out in the 18th century with the object of land reclamation, unduly narrowed the channel, and led it towards one side of the estuary; and though they effectually fixed the navigation channel, they produced very little increase in its depth, but caused a very large amount of sand to accumulate in the estuary beyond, owing to the great reduction in tidal volume by the reclamations, and diminished considerably the channel through the lower estuary in width and depth without checking its wanderings. The training of the channel of the Ribble through its estuary below Preston, for improving its depth and rendering it stable, was begun in 1839, and has been gradually extended at intervals; but the works have not yet been carried, out to deep water, and a shifting, shallow channel still exists through the sand banks, between the end of the training walls and the open sea. The high training walls adopted along the upper part of the channel enabled the upper end of the estuary on both sides to be
Figs. 25 and 26.—Training Works in Sandy Estuary: River Seine.
reclaimed for a length of 4 m.; whilst the half-tide training walls below, placed unduly close together, have led to considerable accretion at the sides of the estuary and some extension of the sand banks seawards. Moreover, by fixing the channel near the northern shore they have enabled the landowners to carry out large reclamations on the southern foreshore. These works, however, besides fixing the navigable channel, have increased, its depth, especially in the upper part, and augmented the tidal scour.along it by lowering the low-water line; and the trained channel is further deepened by dredging. The training works in the Weser estuary have been confined to constructing a single low training wall at the upper end, which forms a trumpet-shaped outlet for the river below Bremerhaven, and to guiding the navigable channel by occasional low dikes at the side and closing minor channels, so as to concentrate the tidal scour and fresh water discharge in it, whilst additional depth is obtained by dredging (fig. 21). A remarkable improvement has been effected in the navigable condition of the upper portion of the Seine estuary by training works, begun in 1848; for in place of a shallow, intricate channel through shifting sand banks, whose dangers were at times intensified by a bore, a stable deep channel has been provided down to about half-way between Berville and St Sauveur, rendering access easy to the river above at high tide (figs. 25 and 26). The channel, however, was made too narrow between Aizier and Berville and was subsequently enlarged, and large tracts of land were reclaimed in the upper estuary. The reduction in tidal capacity by the reclamations, together with the fixing and undue restriction in width of the channel, occasioned very large accretion sat the back of the lower portions of the training walls and at the sides of the estuary beyond them, and an extension of the sand banks seawards. Moreover, the channel has always remained shallow and unstable beyond the ends of the training walls down to deep water near the mouth of the estuary.
Conclusions about Training Works in Estuaries.—Experience has proved that training works through sandy estuaries, by stopping the wanderings of the navigable channel, produce an increase in its depth, and, consequently, in the tidal scour for maintaining it. This scour, however, being concentrated in the trained channel, is withdrawn from the sides of the estuary, which in its natural condition is stirred up periodically by the wandering channel; and, therefore, accretion takes place in the parts of the estuary; from which the tidal scour and fresh-water discharge have been permanently diverted, especially°wh ere an abundance of sand from outside, put in suspension by the action of the prevalent winds blowing into the estuary, is brought in by the flood-tide, as in the cases of the estuaries of the Dee, the Ribble and the Seine. This accretion, reduces the tidal capacity of the estuary, and, producing diminution in the tidal volume passing through the outlet, promotes the extension of the sand banks seawards, as indicated by the difference in the outer portions of the longitudinal section os different dates of the Weser and Seine estuaries (figs. 22 and 26), To prevent as far as possible the reduction in tidal capacity, the training walls should not be raised more above low-water level than absolutely necessary to fix the channel; and the rate of enlargement of their width apart should not be less than 1 in 80 at the upper end, and should increase considerably towards the mouth of the estuary so as to form a trumpet-shaped outlet. The loss of scour in the channel resulting from this enlargement must be compensated for by dredging to attain the requisite depth. Training works partially carried out through an estuary have the advantage of reducing the length of shallow channel to be traversed between deep water and the entrance to the deepened river; but as these works produce no influence on the channel for any distance beyond their termination, a shallow, shifting channel is always found between their the end of the trained channel and deep water. Accordingly, when training works are started at the head of a sandy estuary, provision should always be made in their design for their eventual prolongation to deep water at the mouth of the estuary, to ensure the formation of a stable, continuous, navigable channel. Experiments with a model, moulded to the configuration of the estuary under consideration and reproducing in miniature the tidal ebb and flow and fresh-water discharge over a bed of very fine sand, in which various lines of training walls can be successively inserted, are capable in some cases of garnishing valuable indications of the respective effects and comparative merits of the different schemes proposed for works which have often evoked very conflicting opinions and have sometimes produced most unexpected results. (L. F. V.-H.)
- ↑ L. F. Vernon-Harcourt, Rivers and Canals, 2nd ed. pp. 187-90, plate 5, figs. 1 and 9.
- ↑ Ibid. plate 5, figs. 2, 3, 4 and 10.
- ↑ Report of the Chief of Engineers for 1906, pp. 382 and 1296 and charts.
- ↑ L. F. Vernon-Harcourt, Rivers and Canals, 2nd ed. pp; 289–293. and plate 9, figs 13 and 14.
- ↑ Id. pp. 293–300, and plate 9, figs. 11 and 12.
- ↑ Rivers and Canals, 2nd ed. pp. 327–342, and plate 10.