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RIVER AND CANAL ENGINEERING RIVER AND CANAL ENGINEERING THE CHARACTERISTICS OF OPEN FLOWING STREAMS, AND THE PRINCIPLES AND METHODS TO BE FOLLOWED IN DEALING WITH THEM BY E. S. BELLASIS, M.Inst.C.E. RECENTLY SUPERINTENDING ENGINEER IN THE IRRIGATION BRANCH OF THE PUBLIC WORKS DEPARTMENT OF INDIA 72 ILLUSTRATIONS London E. & F. N. SPON, Ltd., 57 HAYMARKET, S.W. New York SPON & CHAMBERLAIN, 123 LIBERTY STREET 1913 PREFACE RIVER AND CANAL ENGINEERING CHAPTER I INTRODUCTION
1. Preliminary Remarks.—River and Canal Engineering is that branch of engineering science which deals with the characteristics of streams flowing in open channels, and with the principles and methods which should be followed in dealing with, altering, and controlling them. It is not necessary to make a general distinction between natural and artificial streams; some irrigation canals or other artificial channels are as large as rivers and have many of the same characteristics. Any special remarks applicable to either class will be given as occasion requires. 2. Résumé of the Subject.—Chap. II. of this book deals with the collection of information concerning streams, a procedure which is necessary before any considerable work in connection with a stream can be undertaken, and often before it can even be decided whether or not it is to be undertaken. Chap. III. deals with rainfall, and describes how rainfall figures and statistics can be utilised by the engineer in dealing with streams. 3. Design and Execution of Works.—After obtaining full information concerning the stream to be dealt with, careful calculations are, in the case of any large and important work, made as to the effects which will be produced by it. These effects cannot always be exactly foreseen. Sometimes matters can be arranged so that the work can be stopped short at some stage without destroying the utility of the portion done, or so that the completed work can be altered to some extent. 4. The Hydraulics of Open Streams.—When any reach of a stream is altered, say by widening, narrowing, or deepening, so that the water-level is changed, there will also be a change in the water-level, a gradually diminishing change, for some distance upstream of the reach. Also in the lowest portion of the reach the change will gradually diminish and it will vanish at the extreme downstream end of the reach. In the next lowest reach there is no change. Thus if it is desired that the change in the water-level shall take full effect throughout the whole of a reach, the change in the channel must be carried further down. If a weir is built there is no change in the water-level downstream of it except such as may be due to loss of water in the reach upstream of it. The above points are mentioned here because, although they are really questions of hydraulics, they are of much importance and of very general application.
CHAPTER II RAINFALL
1. Rainfall Statistics.—The mean annual rainfall varies very greatly according to the locality. In England it varies from about 20 inches at Hunstanton in Cambridgeshire, to about 200 inches at Seathwaite in Cumberland; in India, from 2 or 3 inches in parts of Scinde, to 450 inches or more at Cherrapunji in the Eastern Himalayas. 2. Available Rainfall.—The area drained by a stream is called its “catchment area” or “basin.” The available rainfall in a catchment area is the total fall less the quantity which is evaporated or absorbed by vegetation. The evaporation does not chiefly take place directly from the surface. Rain sinks a short distance into the ground, and is subsequently evaporated. The available rainfall does not all flow directly into the streams. Some sinks deep into the ground and forms springs, and these many months later augment the flow of the stream and maintain it in dry seasons. The available rainfall of a given catchment area is known as the “yield” of that area. 3. Measurement of Rainfall.—A rain-gauge should be in open ground and not sheltered by objects of any kind. The ordinary rain-gauge is a short cylinder. This is often connected by a tapering piece to a longer cylinder of smaller diameter. In this the rain is stored safely and is measured by a graduated rod. The measurement can be made more accurately than if the diameter was throughout the same as at the top. In other cases the water is poured out of the cylinder into a measuring vessel. If the rain-gauge was sunk so that the top was level with the ground, rain falling outside the gauge would splash into it and vitiate the readings unless the gauge was surrounded by a trench. Ordinarily the top of the gauge is from 1 to 3 feet above the ground. When it is 1 metre above the ground the rain registered is said to be on the average about 6 per cent. less than it should be, owing to the fact that wind causes eddies and currents and carries away drops which should have fallen into the gauge. The velocity of the wind increases with the height above the ground, and so does the error of the rain-gauge. Devices for getting rid of the eddies have been invented by Boernstein and Nipher (Ency. Brit., Tenth Edition, vol. xviii.), but they have not yet come into general use. The Boernstein device is being used experimentally at Eskdalemuir. It would appear that much splashing cannot take place when the ground is covered with grass, and that in such a case the top of the gauge could be 1 foot above the ground, thus making the error very small. 4. Influence of Forests and Vegetation.—When the ground is covered with vegetation, and especially forests, the humus or mould formed from leaves, etc., absorbs and retains moisture. It acts like a reservoir, so that the run-off takes place slowly and the denudation and erosion of the soil is checked. The roots of the trees or other vegetation also bind the soil together. Vegetation and forests thus mitigate the severity of floods and reduce the quantity of silt brought into the streams. They also shield the ground from the direct rays of the sun and so reduce evaporation, and thus, on the whole, augment the available rainfall. Forests render the climate more equable and tend to reduce the temperature, and they thus, at least on hills, increase the actual rainfall to some extent. 5. Heavy Falls in Short Periods.—When rain water, instead of being stored or utilised, has to be got rid of, it is of primary importance to estimate roughly—exact estimates are impossible—the greatest probable fall in a short time. This bears a rough ratio to the mean annual fall. The maximum observed falls in twenty-four hours range, in the United Kingdom, generally from ·05 to ·10 of the mean annual fall—but on one occasion the figure has been ·20,—and in the tropics from ·10 to ·25. Actual figures for particular places can be extracted from the rain registers, but the probability of their being exceeded must be taken into account. The greatest fall observed in twenty-four hours in the United Kingdom is 7 inches, and in India 30 inches in the Eastern Himalayas.
CHAPTER III COLLECTION OF INFORMATION CONCERNING STREAMS
1. Preliminary Remarks.—The information which is required concerning streams depends on the character of the stream and on the nature of the work which is to be done. For the present let it be supposed that the stream is large and perennial. Other kinds of streams will be dealt with in Arts. 6 and 7. In dealing with a large perennial stream it is nearly always necessary to know the approximate highest and lowest water-levels, and these can generally be ascertained by local inquiry, combined with observations of water marks; but a higher level than the highest known and a lower level than the lowest known are always liable to occur, and must to some extent be allowed for. If navigation exists or is to be arranged for, the highest and lowest levels consistent with navigation must be ascertained. The highest such level depends chiefly on the heights of bridges. A plan to a fairly large scale is also necessary in most cases. 2. Stream Gauges.—Unless the stream being dealt with is an artificial one, it is unlikely that the flow in the reach with which the work is concerned will be uniform. The rise and fall of the water at one place cannot therefore be correctly inferred from those at another. It will be desirable to have two gauges, either read daily or else automatically, recording the water-level, one near each end of the reach concerned, with intermediate gauges if the reach is very long. If, in or near the reach, there is already a gauge which has been regularly read, it may be sufficient to set up only one new gauge, and to read it only for such a period of time as will give a good range of water-level, and to compare the readings with those of the old gauge. The readings of the new gauge for water-levels outside the range of those observed can then be inferred, but if the stream is very irregular this may involve some trouble (Art. 4). 3. Plan and Sections.—Making a survey and plan, and laying down on it the lines for longitudinal and cross-sections, and taking levels for the sections, are ordinary operations of surveying. If any land is liable to be flooded, its boundaries should be shown on the plan and on some of the cross-sections. Unless the water is shallow, it is necessary to obtain the bed levels from the water-level by soundings, the level of a peg at the water-level having been obtained by levelling. All the sections should show the water-level as it was at some particular time, but the water-level will probably have altered while the survey was in progress, and allowance must be made for this. The pegs at all the cross-sections and on both banks of the stream—for the water-levels at opposite banks may not be exactly the same—may, for instance, be driven down to the water-level when it is steady, and thereafter any changes in it noted and the soundings corrected accordingly. 4. Discharge Observations.—For a large stream it is necessary to observe the discharges by taking cross-sections and measuring the velocity. If there is a sufficient range of water-levels, it will be possible to make actual observations of a sufficient number of discharges. If soundings cannot, owing to the depth or velocity, be taken at high water, they must be inferred from those previously taken, but this does not allow for changes in the channel, which are sometimes considerable and rapid. If there is not a sufficient range of water-level, the discharges for some water-levels must be calculated from those at other water-levels. In this case observations of the surface slope will be required, and the discharge site should be so selected that no abrupt changes in the channel will come within the length over which the observations are to extend. This length should be such that the fall in the water surface will be great enough to admit of accurate observation. If the cross-section of the stream is nearly uniform throughout the whole of this length, or if it varies in a regular manner, being greatest at one end of the length and least at the other end. the differences in the areas of the two end sections being not more than 10 or 12 per cent., then the velocity and cross-section of the stream can be observed in the usual manner at the centre of the length; but otherwise they should be observed at intervals over the whole length, or at least in two places, one where the section is small and one where it is great, and the mean taken. Or the velocity can be observed at only one cross-section and calculated for the others by simple proportion and the mean taken. The coefficient C can then be found from the formula C = V/√(RS). To find the discharge for a higher or lower water-level, the change in the value of C corresponding to the change in R can be estimated by looking out the values of C in tables, and the discharge calculated by using the new values of C and R and the new sectional area, S remaining unaltered. But if the channel is such that, with the new water-level, a change in S is likely to have occurred, this change must be allowed for. Any such change will be due to the changed relative effects of irregularities, either in the length over which the observations extended or downstream of that length. The effect of irregularities in the bed is greatest at low water. The effect of lateral narrowings is greatest at high water. Since a change of 10 per cent. in S causes a change of only 5 per cent. in V, it will usually suffice to draw on the longitudinal section the actual water surface observed and to sketch the probable surface for the new water-level. If the whole channel is fairly regular for a long distance downstream of the discharge site, no slope observations need be made nor need several sections be taken in order to find V. The changed value of C should, however, be estimated in the manner above indicated. For this purpose any probable value of S will suffice. 5. Discharge Curves and Tables.—Ordinarily it will be possible, by plotting the observed discharges as ordinates, the gauge readings being the abscissæ, to draw a discharge curve and from it construct a discharge table. Unless the channel is of firm material and not liable to change, there are likely to be discrepancies among the observed discharges, so that a regular discharge curve will not pass through all the plotted points. If the discrepancies are not serious, they can be disregarded and the curve drawn so as to pass as near as possible to all the points, but otherwise trouble and uncertainty may arise. The soundings should be compared in order to see whether changes have occurred in the channel. If such changes do not account for the discrepancies, the cause must be sought for in some of the recorded velocities. If no sources of error in these can be found, such as wind, it is possible that the velocity has been affected by a change in the surface slope owing to some change in the channel downstream of the length. Failing this explanation, the discrepancies must be set down to unknown causes. With an unstable channel and where accuracy is required, the sectional areas and velocities should be regularly tabulated or plotted so that changes may be watched and investigated. To do this it may be necessary to take surface slope observations, or to set up extra gauges which will show any changes in the slope. 6. Small Streams.—Small streams will now be considered, those, for instance, which are too small to be navigable and which occasionally run dry or nearly dry. If the water of the stream is to be stored for water supply, power or irrigation purposes, full information as to discharges and silt carried will be required. If the stream is small enough the discharges can be ascertained by means of a weir of planks. The discharge is then known from the gauge readings. Cross-sections and large scale plans will not be required unless the stream is to be altered or embanked. If the water, instead of being stored, is to be got rid of, as in drainage work, the only information required as to discharges is the maximum discharge. Large scale plans, sections, or information as to silt or water-levels (except as a means of estimating the discharge) will not be required unless the stream is to be altered or embanked. 7. Intermittent Streams.—In the case of large streams whose flow is intermittent, the information required will, as before, depend upon the circumstances. Such streams occur in many countries. The difficulty in obtaining information is often very great. To obtain figures of daily discharge a gauge must be set up in the stream and a register kept. The chief difficulty in an out-of-the-way place is likely to be the obtaining correct information as to the maximum discharge. Information, derived from reports or from supposed flood marks, as to the highest water-level, may be inaccurate, and information based on rainfall figures may be extremely doubtful owing to the large size of the catchment area, the absence of rain gauges, and the difficulty, especially if the rain is not heavy, in estimating the available fall. All sources of information must be utilised and, whenever possible, observations should be made over a long period of time. 8. Remarks.—Very much remains to be done in collecting and publishing information concerning the ratio of the discharges to the rainfall. By observing a fall of rain and the discharge of a stream before and after the fall, it is possible to ascertain the figures for that occasion, but they will not hold good for all occasions. Continuous observations are required. The chief obstacle is the expense. Not only have measuring weirs and apparatus for automatically recording the water-level to be provided, but the weirs would often cause flooding of land involving payment of compensation. The most suitable places for making observations are those where reservoirs for water-works exist or are about to be made.
CHAPTER IV THE SILTING AND SCOURING ACTION OF STREAMS
1. Preliminary Remarks.—When flowing water carries solid substances in suspension, they are known as “silt.” Material is also moved by being rolled along the bed of the stream. The difference between silt and rolled material is one of degree and not of kind. Material of one kind may be rolled and carried alternately. The quantity of silt present in each cubic foot of water is called the “charge” of silt. Silt consists chiefly of mud and fine sand; rolled material of sand, gravel, shingle, and boulders. When a stream erodes its channel, it is said to “scour.” When it deposits material in its channel, it is said to “silt.” Both terms are used irrespective of whether the material is silt or rolled material. A stream of given velocity and depth can carry only a certain charge of silt. When it is carrying this it is said to be “fully charged.” 2. Rolled Material.—If a number of bodies have similar shapes, and if D is the diameter of one of them and V the velocity of the water relatively to it, the rolling force is theoretically as V2 D2, and the resisting force or weight as D3. If these are just balanced, D varies as V2, or the diameters of similarly shaped bodies which can just be rolled are as V2 and their weights as V6. From practical observations, it seems that the diameters do not vary quite so rapidly as they would by the above law, the weights being more nearly as V5. 3. Materials carried in Suspension.—It has long been known that the scouring and transporting power of a stream increases with its velocity. Observations made by Kennedy have shown that its power to carry silt decreases as the depth of water increases (Min. Proc. Inst. C.E., vol. cxix.). The power is probably derived from the eddies which are produced at the bed. Every suspended particle tends to sink, if its specific gravity is greater than unity. It is prevented from sinking by the upward components of the eddies. If V is the velocity of the stream and D its depth, the force exerted by the eddies generated on 1 square foot of the bed is greater as the velocity is greater, and is probably as V2 or thereabouts. But, given the charge of silt, the weight of silt in a vertical column of water whose base is 1 square foot is as D. Therefore the power of a stream to support silt is as V2 and inversely as D. The silt charge which a stream of depth D can carry is as V½. V is called the “critical velocity” for that depth, and is designated as V0. 4. Methods of Investigation.—The quantity of silt in water is found by taking specimens of the water and evaporating it or, if the silt is present in great quantity, leaving it to settle for twelve hours—an ounce of alum can be added for every 10 cubic feet of water to accelerate settlement—drawing off the water by a syphon, and heating the deposit to dry it. The deposit is then measured or weighed. It is best to weigh it. If clay is filled into a measure, the volume depends greatly on the manner in which it is filled in. When silt deposits in large quantities in a channel, or when heavy scour occurs, the volume deposited or scoured is ascertained by taking careful sections of the channel. 5. Quantity and Distribution of Silt.—The quantity of silt present in water varies enormously. Fine mud, even though sufficient to discolour the water, may be so small in volume that it only deposits when the water is still, and even then deposits slowly. In the river Tay, near Perth, the silt was found to be ordinarily 1/10,000 of the volume of water, and at low water only 1/28,000. In the river Sutlej at Rupar, near where it issues from the Himalayas, the silt in the flood season is extremely heavy. Out of 360 observations, made at various depths, during the flood seasons of four successive years, in water whose depth ranged up to 12 feet, the silt was once found to be 2·1 per cent. by weight of that of the water. It was more than 1·2 per cent. on four occasions, and more than 0·3 per cent. (or 3 in 1000) on sixty-four occasions. Generally about one-half of the silt was clay and sand of classes finer than (·10), about one-third was sand of class ·1/·2, and the residue was sand of class ·2/·3. The sand of the river Chenab is generally coarser than that of the Sutlej. There are very great differences in the degree of coarseness of river sand. The sand in any river becomes finer and finer as the gradient flattens in approaching the sea. Sea sand has been found to be of class (·20). In the Sirhind Canal, which takes out from the Sutlej at Rupar, the maximum quantity of suspended silt observed in the four flood seasons was 0·7 per cent., on one occasion out of 270, and it exceeded 0·3 per cent. on twenty-five occasions. About 80 per cent. of the silt was clay. 6. Practical Formulæ and Figures.—A stream which carries silt generally rolls materials along its bed. The proportion between the quantities of material rolled and carried is never known, and this makes it impossible to frame an exact formula applicable to such cases, but Kennedy, from his observations on canals fully charged with the heavy silt and fine sand usually found in Indian rivers near the hills, arrived at the empirical formula for critical velocities V = ·84 D·64 The observations were made on the Bari Doab Canal and its branches, the widths of the channels varying from 8 feet to 91 feet, and the depths of water from 2·3 feet to 7·3 feet. The beds of these channels have, in the course of years, adjusted themselves by silting or scouring, so that there is a state of permanent régime, each stream carrying its full charge of silt, and the charges in all being about equal. From further observations referred to above (Art. 3, par. 2) it appears that this kind of silt forms about 1/3300 of the volume of the water, and that on the Sirhind Canal, sand coarser than the (·10) class, formed 1/35,000 of the volume of water. 7. Action on the Sides of a Channel.—It has been seen that the laws of silting and scour on the bed of a channel depend on the ratio of the depth to the velocity. The same laws probably hold good in the case of a gently shelving bank, so that here again V ought to vary as D·64. The velocity near the angle where the slope meets the water surface seems to decrease faster than D·64. At all events, silt tends to deposit in the angle and the slope to become steep. 8. Action at Bends.—At a bend, owing to the action of centrifugal force and to cross-currents caused thereby, there is a deposit near the convex bank and a corresponding deepening—unless the bed is too hard to be scoured—near the concave bank. The water-level at the concave bank is slightly higher than at the convex bank. The greatest velocity instead of being in mid-stream is nearer the concave bank. 9. General Tendencies of Streams.—Since the velocity is greater as the area of the cross-section is less, a stream always tends to scour where narrow or shallow, and to silt where wide or deep. The cross-section thus tends to become uniform in size. Suppose two cross-sections to be equal in size but different in shape. The velocities of the two sections will be equal. The tendency of the bed to silt will (Art. 6) be greater at the deeper section and, when silting has occurred on the bed, the section will be reduced and there will be a tendency to scour at the sides. Thus the cross-sections tend to become also uniform in shape. If a bank of silt has formed in a stream, the tendency is for scour to occur. There is also a tendency for silt to deposit just below the point where the bank ends. Hence a silt bank often moves downstream.
CHAPTER V METHODS OF INCREASING OR REDUCING SILTING OR SCOUR
1. Preliminary Remarks.—Most important works which affect the régime of a stream have some effect on its silting or scouring action, but this is not generally their chief object. Such works will be dealt with in due course, and the effects which they are likely to produce on silting or scouring will be mentioned. In the present chapter only those works and measures will be considered whose chief object is to cause a stream to alter its silting or scouring action. It does not matter, so far as this discussion is concerned, whether the object is direct, i.e. concerned only with the particular place where the effect is to be produced, or indirect, as, for instance, where a stream is made to scour in order that it may deposit material further down the stream. The protection of banks from scour is considered in Chap. VI. Dredging is dealt with in Chap. VIII. 2. Production of Scour or Reduction of Silting.—Sometimes the silt on the bed of a stream is artificially stirred up by simple measures, as, for instance, by scrapers or harrows attached to boats which are allowed to drift with the stream, or by means of a cylinder which has claw-like teeth projecting from its circumference and is rolled along the bed, or by fitting up boats with shutters which are let down close to the bed and so cause a rush of water under them, or by anchoring a steamer and working its screw propeller. It is thus possible to cause a great deal of local scour, but the silt tends to deposit again quickly, and it is not easy to keep any considerable length of channel permanently scoured. The system is suitable in a case in which a local shallow or sandbank is to be got rid of and deposit of silt a little further down is not objectionable. It may be suitable in a case in which the bed is to be scoured while a deposit of silt at the sides of the channel is required, especially if some arrangement to encourage silt deposit at the sides is used (Art. 3, par. 4; also Chap. VI., Art. 3). 3. Production of Silt Deposit.—Works or measures for causing silt deposit may be undertaken in order to cause silt deposit in specific places where it will be useful, or in order to free the water from silt. Sometimes both objects are combined. 4. Arrangements at Bifurcations.—At a bifurcation, as where a branch takes off from a canal, it is possible to reduce the quantity of rolled material entering the canal by raising its bed or constructing a weir or “sill” in its head. This arrangement may have great effect in excluding boulders, shingle, or gravel. As regards rolled sand, it has much less effect than might be expected (Chap. IV., Art. 2). If the canal is reduced in width (fig. 5) there will be eddies below the bed level of the branch. They will stir up the sand and some of it will enter the branch. If the canal is not reduced in width, eddies will be produced in the surface water, and they will affect the bed. 5. A Canal with Headworks in a River.—In the case of a canal taking off from a river and provided with complete headworks, it is possible to do a great deal more. The case of the Sirhind Canal, already referred to (Chap. IV., Arts. 5 and 6), is a notable example. The canal (fig. 6) is more than 200 feet wide, the full depth of water 10 feet, and the full discharge about 7000 cubic feet per second. In 1893 when the irrigation had developed, and it became necessary to run high supplies in the summer—July, August, and part of September—the increase in the silt deposit threatened to stop the working of the canal. In the autumn and winter, say from 25th September to 15th March, the water entering the canal is clear and much of the deposit was picked up by it, but not all. In the five years 1893 to 1897 inclusive, the following remedial measures were adopted. Increased use was made of the escape at the twelfth mile. This did some good, but there was seldom water to spare. In 1893 to 1894 the sill of the regulator was raised to 7 feet above the canal bed, and it was possible to raise it 3 feet more by means of shutters. This had little effect. The coarsest class of sand was ·4, and the velocity of the water, even of that part of it which came up from the river bed and passed over the sill, was over 2 feet per second, so that all sand was carried over. In 1894 to 1895 the divide wall, which had been only 59 feet long, was lengthened to 710 feet, so as to make a pond between the divide wall and the regulator,8 but probably the leakage through the under-sluices was often as much as the canal supply, and the water in the pond was thus kept in rapid movement and full of silt. The canal was closed in heavy floods. This did some good, but probably the canal was often closed needlessly when the water looked muddy but contained no excessive quantity of sand. The above comments on the measures taken were made by Mr Kennedy when chief engineer. The above measures did not reduce the silt deposits, but the scour in the clear water season improved, probably because higher supplies were run owing to increased irrigation. The deposit in the upper reaches of the canal, when at its maximum about the end of August of each year, was generally more than twenty million cubic feet. From the year 1900 a better system of regulation was enforced, the under-sluices being kept closed as much as possible, so that there was much less movement in the pond and much less silt in its water. By 1904 the deposit in the canal had been reduced to three million cubic feet, and no further trouble occurred. 6. Protection of the Bed.—It is possible to afford direct protection from scour to the bed of a stream by constructing walls across it, but unless the walls are near together the protection will not be effective. An arrangement used in some streams in Switzerland consists of tree trunks secured by short piles and resting on brushwood. But as long as the walls are not raised above the bed they cannot entirely stop scour, unless extremely close together. If raised above the bed they form a series of weirs.
CHAPTER VI WORKS FOR THE PROTECTION OF BANKS
1. Preliminary Remarks.—The protection of a length of bank from scour may be effected by spurs, which are works projecting into the stream at intervals, or by a continuous lining of the bank. A spur forms an obstruction to the stream (Chap. IV., Art. 1), and when constructed, or even partly constructed, the scour near its end may be very severe, even though there may be little contraction of the stream as a whole. If the bed is soft a hole is scoured out. Into this hole the spur keeps subsiding, and its construction, or even its maintenance, may be a matter of the greatest difficulty. A high flood may destroy it. If it does not do so, it may be because the stream has, for some reason, ceased to attack the bank at that place. A continuous lining of the bank is not open to any objection, and is generally the best method of protection. Spurs made of large numbers of rather small trees, weighted with nets filled with stones, have been used on the great shifting rivers of the Punjab which swallowed up enormous quantities of materials. The use of spurs on such rivers has now, in most cases, been given up. If L is the length of a spur measured at right angles to the bank, the length of bank which it protects is about 7 L—3 L upstream and 4 L downstream,—but the spur has to be strongly built, and its cost is, in many cases, not much less than the expense of protecting the whole bank with a continuous lining. 2. Spurs.—A spur may be made of— 3. Continuous Lining of the Bank.—The lining or protection of a bank may be of stone or brick pitching (figs. 12 and 13), loose stone (fig. 14), fascines (fig. 15), turfing, plantations, brushwood, or of other materials laid on the slopes. Before protecting a bank it is best to remove irregularities and bring it to a regular line. This can generally be done most easily by filling in hollows, but sometimes it is done by cutting off projections. It is also necessary to make the side slope uniform. Where the slope is as shown by the dotted lines in figs. 12 to 14, filling in can be effected, but cutting away the upper part of the slope is also feasible. Such cutting away has been proposed as a remedy in itself in cases where the steep upper part of the slope was falling in, but it is not much of a remedy. 4. Heavy Stone Pitching with Apron.—On the great shifting rivers of India a system of bank protection is adopted, consisting of a pitched slope with an apron (fig. 22). The system is used chiefly in connection with railway bridges or weirs, but it has been used in one instance, that of Dera Ghazi Khan, for the protection of the bank near a town. When, as is usual, the flood-level is higher than the river bank, an artificial bank is made. In any case the bank is properly aligned. The pitching has a slope of 2 to 1, and consists of quarried blocks of stone loosely laid, the largest blocks weighing perhaps 120 lbs. The apron is laid at the time of low water on the sandbank or bed of the stream. If necessary, the ground is specially levelled for it. It is intended to slip when scour occurs. The following dimensions of the apron are given by Spring (Government of India Technical Paper, No. 153, “River Training and Control on the Guide Bank System,” 1904). The probable maximum depth of scour can be calculated as explained in Chap. XI., Art. 3. If this depth, measured from the toe of the slope pitching is D, and if T is the thickness considered necessary for the slope pitching, then the width of the apron should be 1·5 D, and its thickness 1·25 T next the slope and 2·8 T next the river. It will then be able to cover the scoured slope to a thickness of 1·25 T. This thickness is made greater than T because the stone is not likely to slip quite regularly. The thickness T should, according to Spring, be 16 inches to 52 inches, being least with a slow current and a channel of coarse sand, and greatest with a more rapid current and fine sand; but since the sand is generally finer as the current is slower, it would appear that a thickness of about 3 feet would generally be suitable. Under the rough stone there should be smaller pieces or bricks. Along the top of the bank there is generally a line of rails so that stone from reserve stacks, which are placed at intervals along the bank, can be quickly brought to the spot in case the river anywhere damages the pitched slope.
CHAPTER VII DIVERSIONS AND CLOSURES OF STREAMS
1. Diversions.—When a stream is permanently diverted the new course is generally shorter than the old one, and the diversion is then often called a cut-off. The first result of a cut-off is a lowering of the water-level upstream and a tendency to scour there, and to silt downstream of the cut-off. Fig. 23 shows the longitudinal section of a stream after a cut-off A B has been made. The bed tends to assume the position shown by the dotted line. If both the diversion and the old channel are to remain open, the water-level at the bifurcation will be lowered still more, and the tendency to scour in the diversion will be reduced. 2. Closure of a Flowing Stream.—The closure of a flowing stream by means of a dam is usually attended with some difficulty and sometimes with enormous difficulty. There may be little trouble in running out dams from both banks for a certain distance, but as soon as the gap between the dams becomes much less than the original width of the stream, the water on the upstream side is headed up and there is a rush of water through the gap, which tends to deeply scour the bed and to undermine the dams. The smaller the gap becomes the greater is the rush and scour. 3. Instances of Closures of Streams.—In 1904 the Colorado River broke into the Salton Sink—a valley covering 4000 square miles. Unsuccessful attempts were made to close the stream by two rows of piles with willows and sandbags between them, by a gate 200 feet long, supported on 500 piles, and by twelve gates each 12 feet wide. A “rock-fill” dam was then constructed on a mattress 100 feet wide and 1·5 feet thick. The river, which was 600 feet wide, broke through, but was stopped by the construction of three parallel rock-fill dams in the gap (Min. Proc. Inst. C.E., vol. clxxi.).
CHAPTER VIII THE TRAINING AND CANALISATION OF RIVERS
1. Preliminary Remarks.—When a stream is trained or regularised it is generally made narrower, but sometimes narrow places have to be widened. Deepening has also very frequently to be effected. The object of training is generally the improvement of navigation, but it may be the prevention of silt deposit. Some natural arms of rivers which form the head reaches of canals in the Punjab are wide and tortuous, and they are sometimes trained. Training often includes straightening or the cutting-off of bends, as to which reference may be made to Chap. VII. 2. Dredging and Excavating.—When a flowing stream is to be deepened, the work is usually done by dredgers. Dredgers can remove mud, sand, clay, boulders, or broken pieces of rock. The “bucket ladder” dredger is the commonest type. The “dipper” dredger is another. Both these can work in depths of water ranging up to 35 feet. The “grab bucket” dredger can work up to any depth and in a confined space. The “suction dredger” drawls up mud or sand mixed with water. A dredger may be fitted with a hopper or movable bottom, by means of which it can discharge the dredged material—this, however, involves cessation of work while the dredger makes a journey to the place where the material is to be deposited—or it can discharge into hopper barges or directly on to the shore by means of long shoots. For small works in comparatively shallow water the “bag and spoon” dredger, worked by two men, can be used. 3. Reduction of Width.—If a channel which is to be narrowed is not a wide one, the reduction in width can be effected by any of the processes described under bank protection (Chap. VI.). But in a wide channel, reduction of the width by any direct process is generally impracticable. The expense would generally be prohibitive. Earth, if filled in, is liable to be washed away unless protected all along. Reduction in the width of a large channel is nearly always effected either by groynes (fig. 26) or by training walls (fig. 27). Spurs or short groynes for bank protection have been already described (Chap. VI., Art. 2). Groynes for narrowing streams are made in the same way and of the same materials, but are longer. They are at right angles to the stream or nearly so. Groynes in the river Sutlej have been mentioned in Chap. V., Art. 5, and are shown in fig. 6, p. 55. Whether groynes or training walls are used, the object is to confine the stream to a definite zone and to silt up the spaces at the sides. These spaces when partly silted can be planted with osiers or with anything which will grow when partly submerged, and this will assist in completing the silting. 4. Alteration of Depth or Water-Level.—When the width of a stream is altered, the depth of water—the gradient being supposed to be unchanged—must alter in the opposite manner. A narrowing of the channel by training necessitates an increase in the depth of water, and the same remark applies if an arm of the stream is closed. The increase in depth may be effected either by raising the water-level or by lowering the bed—as may be convenient—or both. If the bed is to be lowered and is of hard clay, it may be necessary to dredge it and, when this has been done, training may be unnecessary. If the bed is of soft mud, a dredged channel is likely to fill up again, and training alone will be the method to adopt. If the bed is moderately hard, say compact sand, it may be suitable to train the channel first and then to dredge if necessary. In any case, shoals of hard material may have to be dredged or rocks, whether these form shoals or lateral obstructions, to be blasted or otherwise broken up (Art. 2). In cases where it is desired to raise the water-level without any lowering of the bed, training is of course necessary. In any case in which the bed is likely to scour to a lower level than is desired, or if the bed is to be raised, the measures described in Chap. V., Art. 6, may be adopted, but they are hardly likely to be suitable and satisfactory in all cases. 5. Training and Canalising.—The steps so far described, together with any of those described in Chaps. V. and VI., exhaust the list of what can be done so long as only the cross-section of a stream is dealt with. This is often called the “regulation” of a stream, though “training” is a more satisfactory term.10
CHAPTER IX CANALS AND CONDUITS
1. Banks.—All banks which have to hold up water should be carefully made. The earth should be deposited in layers and all clods broken up. In high banks the layers should be moistened and rammed. The dotted lines in fig. 29 show two possible courses of percolation water. The vertical height—from the water-level to the ground outside the bank,—divided by the length of the line of percolation is the hydraulic gradient, as in the case of a pipe, and this gradient is more or less a measure of the tendency to leakage. A bank which has water constantly against it nearly always becomes almost water-tight in time. The time is less or greater according as the soil is better, and according to the amount of care with which the bank is made. 2. Navigation Canals.—A navigation canal is sometimes all on one level, but generally different reaches are at different levels, the change being made by means of locks. A “lateral” canal—the most common kind—runs along a river valley more or less parallel to the river. It is frequently cheaper to construct such a canal than to canalise the river. A “summit” canal crosses over a ridge and connects two valleys. A navigation canal requires a supply of water to make good the losses which occur by lockage, leakage, or absorption and evaporation. A canal may be of any size, according to the size of the boats which are to be used. There is always room, except in short reaches where the expense of construction has to be kept down, for two boats to pass one another. 3. Locks.—An ordinary lock is shown in fig. 29A. The space above the head gates is called the “head bay,” and that below the tail gates the “tail bay.” The floor of the lock is often an inverted arch. Sometimes the floor is of cast-iron. The “lift wall” is generally a horizontal arch. The gates when closed press at their lower ends against the “mitre sills”; and the vertical “mitre posts” at the edges of the gates meet and are pressed together. The gate, in opening and closing, revolves above the cylindrical “heel post”—which stands in the “hollow quoin” of the lock wall—and when fully open is contained in the “gate recess.” 4. Other Artificial Channels.—The method of calculating the discharges of channels in which water is to flow is a question of hydraulics. The principles and rules to be followed, in the design of earthen channels, have been stated in Chap. IV., Art. 6, and in Chap. VIII., Art. 5. The design of banks has been dealt with in Art. 1 of this Chapter. For conveying water for the supply of towns, or for other purposes, masonry conduits are often used. A usual form is shown in fig. 30. The curving of the profile of the cross-section gives an increased sectional area and hydraulic radius, and hence an increased discharge.
CHAPTER X WEIRS AND SLUICES
1. Preliminary Remarks.—Every structure which interferes at all with a stream causes an abrupt change in the stream (Chap. IV., Art. 1). At an abrupt change there are always eddies, and these have a peculiar scouring effect. This effect is greatest where the velocity of the stream is abruptly reduced as where, for instance, after being contracted by an obstruction, it expands again or where it falls over a weir or issues from a sluice opening. In all cases of this kind the protection of the structure from scour is of primary importance. 2. General Design of a Weir.—Unless the bed and sides of the channel are of rock, a weir has side walls and rests on a strong floor or “apron.” These need not extend far upstream, but must extend some way downstream because of the scouring action of the water.11 A common type of weir is shown in fig. 32. The downstream face is made sloping, so that the water may not fall vertically and strike the floor below the weir. The thickness and length of the floor depend on the volume of water to be passed and on the height which it will fall and on the nature of the soil, and are generally matters of judgment, though rules regarding them, applicable to certain special cases, are given in the next article. 3. Weirs on Sandy or Porous Soil.—If the channel is very soft or sandy the weir may be built on one or more lines of wells. The wells are not so much to support the weir as to form a curtain and prevent streams, due to the hydraulic gradient A E (fig. 33), from forming under the structure and gradually removing the soil. It is assumed in the case represented by the figure that the maximum head occurs when the downstream channel is dry. Any removal of soil from under the weir may cause its destruction. The wells should be as close together as possible, and the spaces between them carefully filled up with brickwork or concrete to as great a depth as possible, and below that by piles. Instead of wells, lines of sheet piling—cast-iron or wood—can be used. A good fit should be made, but it is not necessary that the joints should be absolutely water-tight. The object is to flatten the hydraulic gradient by increasing the length travelled by the water from B E to B L G H E. Of course, no flattening occurs at a point where the curtain is not water-tight, but if only small interstices exist, none but small trickles of water can pass, and the interstices will probably soon be choked up, just as the sand in a filter bed becomes clogged and has to be washed. In any case, no important stream could develop otherwise than round the toe of the curtain. It has been stated that when a curtain is water-tight the water follows the line B L M G H K E, but this requires proof. Another plan is to cover the bed and sides of the channel with a continuous sheet of concrete extending upstream of the weir from B to D—thus flattening the hydraulic gradient from A E to F E. Instead of concrete, clay puddle can be used with pitching over it. The choice between the different methods depends largely on questions of cost and facility of construction. It has been said that a certain amount of leakage occurs under structures such as the Okla weir (Art. 4), which nevertheless remains undamaged. There have, however, been cases in which failures of works have occurred, especially when there has been a great difference between the water-levels of the upstream and downstream reaches, from no other apparent cause than the passage of water underneath the works. 4. Various Types of Weirs.—The type of weir shown in fig. 32 may be varied by steepening or flattening the slopes of one or both faces. Flattening increases the cost but gives a greater spread for the foundations. It may, however, be combined with a decrease in the width of the crest. Flattening of the downstream slope reduces the shock of the water on the floor, but the slope itself, especially the lower portion, has to stand a good deal of wear, and the length exposed to this is increased. Flattening the upstream slope facilitates the passage of floods. The same result is obtained by making the crest slope upwards (fig. 34). In a small stream or in an irrigation distributing channel, a weir may be a simple brick wall with both faces vertical and corners rounded. 5. Weirs with Sluices.—The long weirs built across Indian rivers below the heads of irrigation canals generally extend across the greater part of the river bed. In the remaining part—generally the part nearest the canal head—there is, instead of the weir, a set of openings or “under-sluices” (fig. 40) with piers having iron grooves in which gates can slide vertically. The piers may be twenty feet apart and five feet thick. The gates are worked by one or more “travellers,” which run on rails on the arched roadway. The traveller is provided with screw gearing to start a gate which sticks. When once started it is easily lifted by the ordinary gears. The gates descend by their own weight. The gate in each opening is usually in two halves, upper and lower, each in its own grooves, and both can be lifted clear of the floods. In intermediate stages of the river these gates have to be worked a good deal. (See also Chap. V., Art. 5.) Usually the weir has, all along its crest, a set of hinged shutters, which lie flat at all seasons, except that of low water in the river. 6. Falling Shutters.—In Thénard’s system, first used in France, a shutter (fig. 43) is hinged at its lower edge and is held up by a strut. When the lower end of the strut is pushed aside it slides downstream and the shutter falls flat. To enable the shutter to be raised again an upstream shutter, which ordinarily lies flat and is held down by a bolt, is released, and it is then raised by the current to the extent permitted by a chain attached to it. The downstream shutter is then raised. Thénard’s system was not much used in France because the river had to fall to a level somewhat too low for navigation before the shutters could be raised. The sudden jerk on the chain of the upstream shutter is also liable to do damage. The system has been adopted on some of the long weirs which cross Indian rivers downstream of the heads of irrigation canals. To prevent damage by shock, a hydraulic brake was designed by Fouracres. It consists of a piston which travels along a cylinder and drives water out through small holes. The shutters are placed on the top of the fixed weir, where they usually lie flat, except in the low water season, any adjustments of the river discharge being effected by means of the under-sluices. 7. Adjustable Weirs.—Drum weirs, invented by Desfontaines, have been used in France and Germany. Two paddles (fig. 47) are fixed on a horizontal axis and can turn through about 90°, the lower paddle, which should be slightly the larger, working in a “drum,” which is roofed over and can, by means of sluices, be placed in communication with either the upper or lower reach of the stream. According as the upper paddle is to be raised or lowered, water is admitted from the upper reach above or below the lower paddle, the water on its other side being at the same time placed in communication with the lower reach. On the weirs first made on the Marne, the height of the upper paddle was 3 feet 7½ inches, and there were, in a weir, a number of pairs of paddles, each being 4 feet 11 inches wide. By having sluices at both abutments communicating with both reaches, and by opening or closing each of them more or less, the various paddles can be made to take up different positions, and thus perfect control over the discharge is obtained by simply turning a handle to control a sluice gate. A weir has since been made with a single pair of paddles extending right across the opening (33 feet), and the height of the upper paddle is over 9 feet.16 8. Remarks on Sluices.—In all kinds of sluice openings or regulators, the principles of design as regards protection of the bed and sides, splaying and curving of walls and piers, thickness of floor, and prevention of the formation of streams under the structure are the same as laid down for weirs.
CHAPTER XI BRIDGES AND SYPHONS
1. Bridges.—Bridges are of many kinds. In this book only those parts of them are considered which are exposed to the stream. If a bridge has piers, there must be some disturbance of the water. The disturbance will be least when the area of the waterway of the bridge is at least as great as that of the stream, and when its shape is as nearly as possible the same. For small streams, a single span clearing the whole stream may be adopted, especially when the channel is of soft material, but for a large stream the cost of intermediate piers, even with a certain amount of protection for them or with deep foundations, will be more than counterbalanced by the smaller thickness of arch or depth of girder. 2. Syphons and Culverts.—Syphons are used to pass drainage channels or other streams under canals or other lines of communication. In the case of a masonry syphon under a stream which may be dry while the syphon is full, the weight of the arch and its solid load must be not less than the upward pressure of the water passing through the syphon. The channel sometimes has a vertical drop at the upstream side (fig. 51) and a slope at the downstream side. The slope enables any solid materials to be carried through, and facilitates cleaning out and unwatering. The drop at the upstream side does not give rise to any shock on the floor when the syphon is full, but a slope is preferable if there is room for it, and it causes less loss of head. 3. Training Works.—The object of the upstream and downstream protections already described (Chap. X.) is to prevent damage to the structure owing to the disturbance caused by the structure itself. When a river is given to shifting its course (Chap. IV., Art. 9) and cutting away its banks, protection of another kind is required. The stream, if left to itself, may cut away one bank upstream of the structure for a long distance, and eventually damage, or destroy by undermining, the upstream pitching and the abutment itself. This is known as outflanking. If in the neighbourhood of the line A B (fig. 53) there is nothing for the river to damage,—if, for instance, the structure is a weir with a canal, if any, only on the opposite bank of the river,—and if the land is of no particular value, the case could conceivably be met by protecting the abutment on all sides, but even then there might be a chance of the erosion of the bank continuing until the stream had formed a connection at C with the downstream reach. This, of course, in the case of a weir, would render the work useless and might even destroy it.
CHAPTER XII DRAINAGE AND FLOODS
1. Preliminary Remarks.—Arts. 2 and 3 of this Chapter deal with the calculation of flood discharges, Art. 2 dealing with small streams, in which the water has to be got rid of, and Art. 3 with large streams. The remaining articles discuss the methods of predicting floods and of preventing them from doing damage. When the discharge figures have been arrived at in any case, the necessary masonry works can be designed in accordance with the principles described in Chaps. X. and XI. For remarks regarding the design of channels and banks, see Chap. IX., Art. 4, and also Art. 6 of the present Chapter. 2. Small Streams.—In dealing with small streams, such as branch drains or natural streams not far from their sources, the engineer is concerned only with their maximum discharges. He has to design culverts, bridges or syphons to pass the streams under roads or other works, or to design channels or waste weirs for them. In a settled country there may be already some works in existence on the same stream, and these may form a guide, or it may be possible to obtain local information as to the height or volume of floods. Even in such a case rainfall figures will be most useful. In districts where there is no settled population, and in any case where the stream is ill-defined, and the flow fitful, the rainfall figures may afford the only, or at least far the best, means of estimating the discharge. 3. Rivers.—It is possible to apply the methods of the preceding article to large catchment areas, but the results would be quite unreliable. If the calculations were made so as to err on the side of safety, the resulting discharges would often be enormous. The following table shows some figures based on actual flood discharges. None of the localities have excessive rainfalls, though most are liable to occasional very heavy falls. In mountainous districts in the North of England and in Scotland the flood discharges per square mile of catchment area have been found to vary from 64 to 320 cubic feet per second. 4. Prediction of Floods.—At any place high up on the course of a stream, the occurrence of a flood can often be predicted when rain storms—often accompanied in the tropics by lightning—can be seen to be occurring towards the sources of the stream. For any station lower down the stream and for precise information in any case, the readings of gauges higher up the stream can be telegraphed. If the station is at a great distance from the gauge and if there is railway communication, the readings can be sent by post. 5. Prevention of Floods.—The extended use of field drains has, in recent years, done much to increase the severity of floods in England and other countries. One method of mitigating or preventing floods is the construction of reservoirs for storing the water. Reservoirs locally known as “washes,” formed by setting back the embankments, exist on the Fen rivers. One wash, on the Nene, below Peterborough, is 12 miles long and half a mile wide and is filled, in floods, to a depth of 7 feet and holds 1 inch of rainfall over the river basin, and this is found to be sufficient. Reservoir construction is, however, in most cases, impracticable owing to the expense. To store the water which is given by 1 inch of rain in the basin of the Thames, a reservoir would be needed 50 feet deep and covering about 7 square miles. It might cost £7,000,000. 6. Lowering the Water-Level.—The water-level of a given length of stream can be lowered by lowering the bed, widening the channel or straightening the channel. The efficiency of these processes is in the order named. As stated in Chap. I., Art. 4, the alteration to the channel must in any case be continued to some point downstream of the reach under consideration. Let the channel be supposed to be of “shallow” section with sloping sides. Let W be the mean width, D the depth, and S the slope. Let it be required to lower the water-level by an amount equal to D/5. This can be effected by lowering the bed by about 25 per cent. of D, or by increasing the width by about 50 per cent., or by increasing the slope by about 100 per cent. If the bed is lowered, V is not affected, and the mean width is reduced. Increase in W reduces D, and therefore reduces the hydraulic radius and the velocity. Hence the large amount of widening necessary. When S is increased the velocity, if R remains the same, is affected only as √S (Hydraulics, Chap. VI., Art. 2), but the depth of water is reduced and R therefore reduced. Dressing the sides of a channel, so as to make it smoother, produces the same effect as a slight widening. 7. Flood Embankments.—A flood embankment may be close to the edge of the river or it may be set back. If set back it need not follow all the windings of the stream. The setting back of an embankment gives an increased waterway to the stream during floods, and therefore a lower flood-level, but the effect of this is trifling in cases where the depth of the water on the flooded land is small, especially if such land is covered with vegetation, or is otherwise much obstructed. Setting back is generally necessary in cases where the stream is liable to erode the banks to any considerable extent. In such a case the embankment should not be so near to the river as to be in much danger from erosion, but the ground, as already stated (Chap. IV., Art. 9), generally falls, in going away from the river, so that when an embankment is set well back it is in lower ground, more expensive and more liable to breach. The most suitable alignment is a matter of judgment, and depends largely on the extent to which the river is likely to shift.
CHAPTER XIII RESERVOIRS AND DAMS
1. Reservoirs.—The object of a reservoir is to store water for town supply or for irrigation or other purposes. Reservoirs for the water supply of towns are divided into “impounding reservoirs” and “service reservoirs,” the latter being of comparatively small size, and their object being to store, near to the town, a supply sufficient for a short period. Instead of one impounding reservoir there may be several, formed by various dams and one discharging into another. When a reservoir is mentioned without qualification, an impounding reservoir is meant. A reservoir is generally made by blocking up a valley by means of a dam of earth or masonry. The site of the dam should be selected at a place where the valley is narrow. The lowest portion or “bottom water” of a reservoir is usually not drawn upon, because it is less pure than the rest, and it has to be left, in dry weather, for the fish. It is not included in calculating the capacity of the reservoir. 2. Capacity of Reservoirs.—A reservoir depends for its supply on the yield of a particular valley or valleys which form its catchment area, and the capacity of the reservoir or reservoirs can be altered by altering the height or number of the dams. The need for a reservoir is entirely owing to the inequality in the distribution of the rainfall. If the rain fell in equal quantities week by week, the daily fluctuations could probably be equalised by the service reservoirs. The impounding reservoir could be quite small. Actually, a reservoir is needed to “equalise” the flow—that is, to give a steady flow for an intermittent one. The smaller the reservoir, the sooner it will go dry in a drought and the sooner it will overflow in wet weather and cause waste of the water. In other words, the larger the reservoir the better it will fulfil its function of equalising the flow and the greater the degree to which the catchment area will be utilised. 3. Earthen Dams.—Before an earthen dam is made, any soft soil on the site should be removed and the ground downstream of the site should be drained. A few trenches, running parallel to the axis of the dam, can be dug so as to give the dam a hold, though there is never any danger of its being moved horizontally by the thrust of the water. If the ground has a side-long slope it should be benched as shown in fig. 59. The front slope of an earthen dam is generally about 3 to 1, and the rear slope about 2 to 1. The top has a width of ⅓ to ½ the greatest depth of water held up, and is 5 to 10 feet above the highest water-level. The borrow pits from which the earth for the dam is got should not be near enough to it to in any way affect its stability. 4. Masonry Dams.—For heights much exceeding 110 or 120 feet a masonry dam may be cheaper than an earthen dam; and in case a flood occurs while work is in progress the masonry might suffer little injury, while earthwork might be swept away completely. Masonry dams are usually built of random rubble masonry with faces of dressed stone. Such masonry weighs about 140 lbs. per cubic foot, and is ordinarily quite safe when subjected to pressures of 20 tons per square foot, but in a masonry dam a high factor of safety is necessary, and 15 tons per square foot may be allowed. In a wall of such masonry with both faces vertical, the pressure, owing to the weight of the wall, will reach the above limit when the wall has attained a height of about 220 feet.
CHAPTER XIV TIDAL WATERS AND WORKS
1. Tides.—The tides or “tidal waves” are caused by the attraction of the moon and the sun. The phenomena are complex, and a full discussion of their causes need not be given here. When the tide rises it is said to “flow,” and it is called the flood tide; when it falls it is called the ebb tide. The period between one tide and the next, e.g. from high water to high water, is about twelve hours, twenty-five minutes. At a spring tide the range of the tide is greater than usual; at a neap tide less. Where there are channels, as, for instance, the seas which surround the British Isles, the tidal waves run up them as the tide rises in the neighbouring ocean, and run back as it falls. At some places, as Southampton, the tide comes in from two directions, and there is a double tide. The times and levels of high and low water at various places have been ascertained by observation, and are recorded. The levels are, however, liable to be affected by winds. A wind blowing towards the shore raises the level of both high and low water; a wind blowing off shore lowers both levels. A severe storm in the North Sea has caused a double tide at London Docks, by accelerating the North Sea tidal wave. 2. Tidal Rivers.—Let A B (fig. 66) be the surface of the lower part or mouth of a river, supposed to be of uniform width, and let B be the mean sea-level. As the tide rises to D the water of the river is headed up and assumes the line A D. When the tide falls to F there is a draw, the river surface taking the line A F. If the rise of the tide B H is so great that the discharge of the river cannot keep pace with it, so as to fill up the whole space between A and H to the level of H, there will be a flow of sea water from H to some point M, and of river water from A to M. The point M will be lower than A and H. If the tide now turns and the water-level H begins to fall, there will still be a flow along H M. For a brief period it will be due to momentum, but it will continue until, by the rise of the water-level at M and the fall at H, the surface has assumed the form indicated by the dotted line A N J. While this is happening, the point corresponding to M—where the concave curve of the upland water meets the convex curve of the tidal water—rises higher and shifts seaward. The character of the two curves remains the same, but they become flatter and the surface N J nearly level. 3. Works in Tidal Rivers.—If any works are required in the tidal portion of a river, the principles to be followed in designing them are the same as if the river was non-tidal. All that has been said in Chap. VIII., Arts. 1 to 3, applies to them. The river may be straightened or trained or dredged. Generally training and dredging are combined. Any dredging in the portion of the river nearest the sea will not, of course, alter the water levels near the mouth, but it will alter them further up. The tide will come up in greater volume and will rise higher and extend further up. The ebb will be facilitated, and the low-water level will be lowered. If any narrowing of the channel near its mouth is effected by training walls for the purpose of lowering the bed, the effect on the volume of tidal water entering the river must be taken into consideration. If the narrowing is confined to a reach near the mouth, and if the resulting deepening is not sufficient to counteract the effect of the narrowing, the volume of tidal water reaching the unnarrowed portions of the channel will be reduced, and this may be injurious. Its scouring action may be insufficient. The proper course may be to continue the narrowing upstream. If this is done, then it is obvious that the width of channel in which deep water is to be maintained at high water, or which is to be kept free from deposit, is reduced in about the same proportion as the volume of tidal water is reduced, and no harm is likely to result. 4. Tidal Estuaries.—If, instead of a river of uniform width, there is an estuary whose width increases steadily towards the sea so that it is funnel-shaped, the conditions described in Art. 2 are modified. An estuary is formed first by the waves of the sea, which wear away the angles at the mouth of the river and allow the tide to enter in greater volume, and then by the flow and ebb of the tides. The slope of the bed of the estuary is usually much flatter than that of the river, and the water surface is as shown in fig. 67. The tidal movements extend further upstream than in the case of a river, not only because of the greater difficulty experienced by the upland water in filling up the wide channel of the estuary, but because of the momentum of the tidal water driving its way up the funnel-shaped channel (Art. 1). The capacity of the estuary is of course much greater than is required for the discharge of the upland water alone. If the sea-level remained always at one height and if the upland water contained silt, it would tend to deposit in the estuary and would certainly deposit in it to some extent. The action of the sea water is the same as described in Art. 2, scouring if it is clear when entering, of less account if it is not clear. Owing to the funnel shape of the estuary, the tide rises higher at its upper end than if the estuary were replaced by a river channel, and the tide also extends further up. This may partly or wholly compensate for the greater tendency of silt to deposit in an estuary as compared with a river channel. 5. Works in Tidal Estuaries.—Estuaries, when shallow, offer great facilities for training. It used at one time to be said that any change which reduces the volume of tidal flow must be injurious. It would be injurious to restrict the mouth of the estuary, unless it were exceptionally wide, and leave the rest untouched. If the whole estuary is narrowed, and a suitable funnel shape preserved, the width to be kept open is, relatively to the size of the mouth, no greater than before, and the tide may flow up as far as before, and rise to as high a level. The narrowing, if properly arranged, will improve the shape of the estuary and cause an increased scour. The effect of the upland water is also greater in the narrower channel. Improvements to estuaries are not, however, restricted to training. There is always one or more deep channels, and the best of these can be selected and improved by dredging. The channel should be one along which both the flood tide and the ebb tide will run. The above remarks as to training do not apply to a case in which there is a bar outside the mouth of the estuary. Training might check the scour at the bar. Bars are treated of in Chap. XV.
CHAPTER XV RIVER BARS
1. Deltaic Rivers.—When a river flows into a tideless sea its silt deposits and forms a shoal or bar. This shoal may in time extend and rise up to the water-level. The current of the river makes its way through it in various directions, and in this way a delta is formed and constantly extends seawards. This flattens the slope of the lower portions of the river, and causes raising of the bed in the reaches upstream, and this again may cause the water to break out further upstream and form fresh channels to the sea. The bars at the mouths of deltaic rivers are generally formed with great rapidity, and they are apt to form a complete hindrance to navigation. They are sometimes partly scoured away by floods in the river, but in this case the scoured material may deposit on the outer slopes of the bar. If a river which carries silt has no delta, it is probably because there is a littoral current, which prevents the silt from depositing. On the other hand, if a river brings down very heavy sediment, a delta may be formed even when tidal flow is not wholly absent. This occurs in the case of the Ganges. 2. Other Rivers.—It often happens that the materials—sand, gravel, and shingle—of which a sea beach is composed shift gradually along the shore. This is known as “littoral drift.” It is by some supposed to be due to the action of the tides, and by others to the action of waves, the drift taking place in the direction of the prevailing winds, excluding those which are off shore. The latter cause is the more probable.
APPENDIX A APPENDIX B INDEX
FOOTNOTES:
Transcriber’s Note:
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