Treatise on the Improvement of the Navigation of Rivers. With a new theory on the cause of the existence of bars

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TREATISE ON THE IMPROVEMENT OF THE NAVIGATION or RIVE R S: M [1 II a NEW THEORY ON THE CAUSE OF THE EXISTENCE OF BARS. BY WILLIAM ALEXANDER BROOKS, M. Inst. C.E. " No* BTTCMB mnt, en cettc tuatiert, dW toute aulre consequence que dan* les objOU degoftt, de l.ixc. ou d'apement; parceque toujours il en resnlte, ou un dmnmage "reel, ou 1« pffU' de qmlque (wtoUg* preneux."— Dt; Bcat. LONDON: PUBLISHED BY JOHN WEALE, ENGINEERING AND ARCHITECTURAL EOOKSKI.I.ER, .'3, Hir,H HOtiBOfcS^ L841. LONDON : OILRERT & IUVINGTON, fRINTERS, st. John's square. CONTENTS. PAGE

. Report on the Improvements in the river Clyde, by W. A. Brooks, C. Engineer, made by order of the Tees Navigation Company, in 1833 A TREATISE, fye.

Chapter I. On The Bars Of Rivers

The subject of this chapter is one of the very highest importance, whether it be considered in relation to national objects, or to science. This has been so much felt in past ages, as well as in our own times, that many works of great mag nitude have been undertaken for the purpose of ob viating or removing the impediments to navigation, arising from the presence of bars. Many theories have been advanced to account for the formation of bars, amongst which I shall first notice that propounded by Major Rennel, in a note to his translation of Herodotus, which, moreover, describes in clear and general terms, the appearance usually presented by them. " All rivers," says Major Rennel, " preserve to a certain extent of space, which is proportioned to the *<■ velocity of their streams, a current of water into the sea, beyond the points of land that form their embouchures ; when by the continued resistance of the sea they at last lose their motion.

The mud and sand suspended in these waters during their motion, are deposited when that motion ceases, or rather, they are gradually deposited as the current slackens, according to the gravity of the substances that are suspended. This deposition then will form a bank or shallow in the sea, and which will be of a fan-like shape, consistently with the form in which the water of the river disperses itself. This bank is of very considerable breadth, and is, of course, constantly on the increase in height as well as extension, and the additions constantly made to the breadth will be on the side towards the sea, until the bank rises up nearly to the surface of the river water, which is constantly poured into the sea, and escapes freely over it ; but when the bank has risen so high, as to inclose the water in a kind of lake, it is then compelled to force its way through the bank, although the passage will be both narrow and shallow while the bank remains under water.

This passage is technically termed a BAR ; for such it is in respect of the channel of the river, although it be the deepest part of the entrance to it." The Vice President of the Geological Society, Mr. De la Beche, in his Geological Manual, advances another theory to account for the formation of bars. in these words : " The action of waves on coasts is not only exhibited in piling up detritus in the direc tion of their greatest force on the shore, by which embouchures of rivers are turned on one side, but also by heaping up bars, as they are termed, even at their mouths, rendering their navigation dangerous, and in many instances preventing it altogether, though behind these barriers the rivers may have considerable depth and breadth. " In some situations these bars are partially dry at low water, at others they are never uncovered, though rendered visible by the breaking of a furious surf."

A third theory attributes the existence of bars to the casual circumstance of the strength of the current of the flood tide not running in the same chan nel with that of the ebb ; or to the embouchure of the river not being directly in the line of, or freely open to the course of the tidal current. A fourth, assigns their formation to the occasional circumstance of the lower reach, or embouchure of the river, discharging its waters at right angles, or nearly so, into the sea, in lieu of entering it obliquely ; or, in the words of one who has devoted much time in promulgating this theory : — " The cause of the existence of bars is the con flicting action of effluent currents, or tides, passing into the ocean at right angles to the shore. " This is the sole cause of bars at the mouths of b2 ■I ON THE BARS OF RIVERS. harbours ; and wherever there is a backwater, or what is termed a scouring power, in operation, there a bar exists ; and it is an invariable fact, that whenever attempts have been made to remove bars by increasing the velocity of the natural currents into the ocean, there the bars have also increased in proportion.

To these consequences no exception is to be found in the whole globe, and in many cases, harbours have, from the causes alluded to, been blocked up and lost ! But if the current or tide be by arti ficial means conducted into the ocean so as to join the sea tide at an acute angle, no conflicting action can arise, and then no bar will accumulate, but what ever silt or other matter be in that case held in sus pension by the tides or currents will be carried away from the harbour's mouth in the direction of the tidal water of the ocean '." A fifth assigned cause is an imagined insufficiency of backwater. Commencing with the theory given by the talented translator of Herodotus, I venture to submit that it is insufficient to account for the formation of bars, because the operations described as producing the latter, take place in all rivers in a greater or less de gree, and in those which, although their waters are most abundantly loaded with sand or mud, are never theless free from the presence of bars ; and here it Nautical Magazine, p. 487. may be observed, that deposits on either side of the channel of a river do not come under the denomina tion of bars ; such for example as the sands at the mouth of the river Thames, lying between the Nore and the sunk lights.

The theory that bars are caused by the action of waves is also insufficient, because those impediments to navigation are not only found in the most sheltered situations, or where they are comparatively little exposed to the action of waves ; but many rivers whose waters are loaded with silt, and whose embouchures are open to the most furious seas, are nevertheless free from any deposit forming bars. The action of waves in piling up detritus can only take place where local causes exist, such as a shelter from, or a check to the onward impulse to the sand or shingle produced by the action of the prevailing gales ; the tendency of which latter is, as stated by Professor Playfair, to carry all detritus seaward; whether that detritus be produced by the action of the elements upon the cliffs, or brought down by the river waters ; and this because of the preponderating effect arising from the slope, or declination of the shore towards the depths of the sea.

That the casual direction of the lower reach, or position of the mouth of a river, cannot be truly as signed as the cause of the existence of bars, is easily proved by observations on rivers subject to great variation at their entrance, the bar being always found to exist quite independent of the direction of the discharge into the sea. This fact at once refutes the third and fourth theories which have been noticed above. The fifth theory, a supposed insufficiency of back water, can offer no cause for the formation of a bar ; but solely for a general deficiency of sectional area, or limitation to the dimensions of the channel. The mighty waters of the Orinoco, and Mississippi, are obstructed by bars, having on them less depth of water than is found at the mouths of rivers utterly insignificant in comparison to the former, as regards the volume of water they discharge.

The consideration of the cause of the formation of bars has engrossed much of the attention of the Italian and French engineers, and the subject has been fully entered into by Colonel Emy of the Engineers, ex-director of the fortifications of Ro chelle and Bayonne, in his interesting treatise on the " Movement of Waves, and of Hydraulic Works ;" in which the Colonel has started an entirely new and ingenious theory to account for the existence of bars, which cannot without injustice to him be referred to, except by large extracts descriptive of his views. In section 94, Colonel Emy thus introduces the subject. " A bar is a bank of sand which obstructs the embouchure of a river. The cause to which has been attributed the formation of bars is that a stream, flowing on a horizontal bed, on its approach to the sea, spreads its surface and diminishes in depth, so that on its meeting with the current of the flood, or even solely with the mass of the water of the sea, a stagnation is caused, by which the sands are depo sited, whether brought by the river or by the flood tide.

" It is easy to comprehend that this explanation is not satisfactory, for the repose supposed to take place at the meeting of the currents cannot exist. In the Mediterranean, the current of the flood is never opposed to any stream. In the ocean, one of the two currents overcomes alternately the other. " If the assumed repose resulted from the equili brium of the two currents, or solely from the resistance of the sea to the current of the river, all rivers would have bars, and they would certainly be at a greater distance than where we find them, since the current of the river always extends far into the sea. " The action of the ground waves, "slots de fond," is the true cause of the phenomenon of the bar. Those bottom waves which have traversed the shore current, and which present themselves directly to the mouth of a river, enter, or are stopped, according to the manner in which the bed of the river is con nected with that of the sea.

If the bed of the river be a continuation of the bottom of the sea, or if it be united with it by an extremely easy inclined plane, or by a gentle curvilineal figure, the ground waves pene trate into the river. In tempests they carry with them sand, which is deposited in banks of little height, because they are able to spread, and all that is raised above a certain level, is scoured out again by the action of the current of the river during calm wea ther. In great tides, the ground waves meeting with nothing opposed to their progress, even on the banks of sand which have not steep sides, produce, more or less, before them in the river the phenomenon of the bore. It is thus that bauks of sand are formed at the mouth of the Gironde, and that the bore shows itself higher up in the Dordogne. But if an irregularity of the bottom, such as an acclivity, or a steep slope, exist at the mouth, the ground waves rise in encountering the obstacle, and being no longer able to continue their course under the tidal stream, because their movement ceases to be in harmony with the undulation, they are met and dispersed by the current of the river ; and during the moment of repose that their waters find in mixing with those of the river, the sand which they held is pre cipitated, and quickly thrown above the declivity, by the succeeding ground waves, which repeat the operation.

" This sand forms the bar, which increases rapidly in stormy weather, because at that time the ground waves bring to it an abundance of materials, and deposit them as on a flat shore. " The bar increases quickly, and would even rise above the level of high water, if the current of the backwater and that of the ebb united had not sufficient force to repulse a portion. " The sea side slope of the bar, being kept up rather steep by the action of the ground waves, pro duces in regard to them the same effect as the sides of sand banks, or elevations of the bed of the ocean ; it causes them to break on the surface of the water. " A great number of rivers have bars which render their entrances difficult and dangerous ; one of the most remarkable, and which has engaged the atten tion of many engineers, is that of the Adour.

" It appears to be principally formed from the siliceous debris of the shores of Spain and Portugal, brought along by shore currents, which the ground waves drive afterwards to the shore of the Landes, where they furnish by their trituration the materials of the sand hills. " A portion of these sands forms the bar of the Adour, the height of which varies between the level of high and low water, the overplus being repulsed by the current of the backwater, and by that of the ebb during calms, when there cannot exist but very weak ground waves, which have not the power to throw up as much sand as these currents are able to drive out to sea. We observe in fact, that the extent of the bar is diminished in long calms, and that it augments rapidly during storms. " At high water, and when there are waves, the position and figure of the bar is discerned by the form which the ground waves impress upon those of the surface, which become short, and sometimes broken, in passing over.

" In storms, the tumultuous foam from the breaking of the waves marks out the bar ; but in calms, as there are then no surface waves, and consequently no sensible ground waves, nothing indicates at high water the place of danger." The reader will perceive from the foregoing extracts, that Colonel Emy attributes the formation of bars, to what he calls " ground waves," " les slots de fond." A phenomenon believed by him to exist, but of which a description in his own words is requisite, in order to place the gallant officer's theory fairly before the public. " Plots defond. Behold me arrived at the principal object of my researches, to the description of ground waves, a submarine phenomena which has caused the ruin of a great number of maritime works, which has given to the sea the means to ravage its borders and to destroy cities, which in other situa tions has augmented portions of continents, has filled up ports, has surrounded them with sands and deposits of clay, and has left in the midst of land, towns whose walls were once lashed by the waves.

All these effects have hitherto been attributed to currents of the sea, and to the simple action of waves, but the phenomenon of ground waves is about to re sume its rights as a formidable agent of the power of the seas on the globe." Without the necessity of any horizontal sections to illustrate Colonel Emy's theory, it is sufficient to state, that this assigned theory for the supposed exist ence of ground waves, independent of those surface waves which are apparent to all, is based upon the belief, that wherever the bed of the sea presents an irregularity of surface within the limits of the vibra tions of the upper waves, the previous undula tory motion of the lower filaments of the wave is changed to that of a horizontal, or purely progressive one, independent of the undulatory motion of the upper division of the wave, which continues as before.

Thus, supposing the bed of the sea, within the limits of the orbits described by the lower filaments, or strata, of the waves, happens to rise in successive lifts or platforms, such as frequently occur in the stratification of the rocks forming the bed of the sea ; the result, according to the theory of Colonel Emy, is, that on the lower portion of the wave striking against the abrupt face of the submarine cliff, which is presented to it, the body of water, in lieu of rising to the surface, and forming breakers, according to my observations, merely surmounts the obstruction, taking a subsequent horizontal, or level course along the surface of the platform, or elevated bed of the sea, and below the superincumbent waves, which continue their undulations towards the shore. On this point he says : " Although the ground waves glide on the bed of the sea, that bed does not occasion any sensible reaction on them ; or rather they guarantee the superior undulations from that reaction ; so that however great may be the breadth of the platform or elevated bed, provided the ground waves meet with no greater accident, they are only animated with a horizontal movement ; they continue their course under the mass of undulating water, and not only are they not thrown from below npivards, but they assist in the completion of the undulatory movement of that mass of water, since by their translation they make up for the want of the undulation of that part of the volume of the wave, which is obstructed by the elevation of the bed of the sea.

" When the bed of the sea rises by successive steps of little elevation, the ground waves formed at the lowest stage continue their course; each wave on arriving at the acclivity rises over it by the effect of the pressure of the undulation of the mass on its outer flank, and becomes merged in the ground wave which is formed on that platform ; but the ground wavesthus produced by theeffects of several successive elevations of the bed are of greater volume than those resulting from a single elevation ; although the latter be equal to the sum of all the smaller ones ; by reason that at every obstruction the ground waves, by their rise upon it, lift the surge of the surface wave, and because these latter, thus lifted up, give place in the same time to the formation of ground waves, which have also a greater relief."

In investigating the correctness of this theory, the inquiry becomes narrowed into the consideration, whether the sudden elevations of the bed of the sea have the effect attributed to them by Colonel Emy ? or whether their result is, according to the usually received opinion, that of the formation of breaker's im mediately on any portion of the wave meeting violently the vertical face of the obstructions The north-eastern coast of Yorkshire presents in a remarkable degree the geological features which would produce the ground waves according to Colonel Emy's theory ; but a residence of several years on its shores, and a close observation of the effects produced by the seas striking against the faces of the rocky shore (the bed of the sea consisting of a succession of platforms of rock, or submarine cliffs), enables me to state, that breakers are formed at the edge of every successive platform ; and that those seas which break against the faces of the outermost platforms are much heavier than those which break nearer the shore.

My reason for devoting my attention to this sub ject, was my desire to ascertain, at what depth in that locality, the waves broke in ordinary and extraordinary gales of wind ; and for this purpose the pecu liar configuration of the coast enabled me to take a position in a line with the march of the waves towards the shore ; from which I could with a sextant deter mine the situation of the commencement of the line of broken water; and on laying down the angles thus obtained, I found them always to coincide with one or other of the faces of the rocky platform of the sea, in proportion to the violence of the gale. The surf pro duced by the opposition of the inner lifts of rock, being also always of small elevation. This shore would be a gaining one if Colonel Emy's theory could be substantiated, but the reverse is the case ; and where the indentations of the coast form bays, there is no diminution of the depth within the latter.

Again, any accumulation which has drifted by the wind beyond the line of high wafer in calms, is in variably, wherever it is reached by the surf of an on-shore gale, carried out to sea by the back sweep of the returning surge, or by what mariners call " the undertow." If ground waves were formed, and were capable of producing the accumulation of sand forming bars at the mouth of rivers, which accumulation is accord ing to Colonel Emy's description only reduced by the discharge of the backwater, the same effects ought to be manifested all along the shores on each side of the bar, or embouchure of the river. So far from this accretion taking place, it will be easy to refer to many bar rivers situated upon coasts upon which no accretion occurs. The bars of rivers exist quite independent of the line of coast losing or gaining ground.

The example which has been adduced by Colonel Emy, of the bar of the Adour being found to increase in extent during tempestuous weather, and to diminish during long calms, shows that the deposit is really due to the onward or progressive motion of the sand along the shore, which being checked by the piers or jetties, between which the Adour is conducted, lodges under their shelter, and thus is the bank of sand augmented during gales of wind, until it is reduced by the out pourings of the river. The increase of sand is such as would occur between the piers of any harbour unprovided with backwater. This has been well expressed in the Naidical Mag azine for February 1838, in a letter signed " Inves tigator :" the only error in which is, in the extension of the operation to the formation of bars, which are always permanent obstructions.

" The sand, or shin gle, travels along the shore in the direction of the pre vailing winds and currents. A pier built across the beach, stops this shingle till it has filled up the angle of the pier ; it then goes on round the end of the pier, part entering the harbour, and the other part, though going on past the opening, shoaling the water in its track, till it reaches the shore on the other side of the pier. This shoaling, or track of the passing shingle, is the bar ; and the depth of water upon it is dependent upon the scouring power of the stream which runs over it. The sectional area of the channel at low water over the bar, being necessarily (in such material) proportionate to the stream which every low water makes its passage over it." There is then at the mouth of the Adour a bar which never can be removed while the river is in its present state ; and there is also an occasional lodgment of sand, which is capable of being removed by the backwater during calm or moderate weather.

From my own personal examination of bar rivers, I do not find that any material alteration takes place in the depth of their entrances ; and certainly during a succession of long calms no increased depth is at tained, such as should be produced by Colonel Emy's theory ; indeed, according to his views, during long calms all bars should totally disappear ; but we know that this is not the case. Happily, his theory is not correct, for if it were, we should find so great an alteration during the long continuance of on-shore gales of wind, that the mouths of rivers would be totally unserviceable when most required for refuge. If the pier at the mouth of the Adour, which is next to the side from which the sand or shingle is brought by the surf, had been made convex to the quarter from whence the impulse .is derived, or that windward pier had been made concave to the action of the current ; the latter, seeking always to escape at a tangent to a curve, would prevent the possibility of any sand lodging in the channel alongside its face.

This latter arrangement would guarantee that no lodgment should ever take place within the piers ; the river bar would still exist outside the pier ends ; and its removal or amelioration could only be effected by a treatment which will form the subject of suc ceeding pages. In conclusion, with reference to Colonel Emy's work, which contains much valuable information, I will only add a few remarks on his section for a sea wall, which he proposes to be formed curvilineal at bottom; the curve being so arranged, that the bed of the sea, where it meets the base of the wall, shall be tangential to it ; and upon the utility of which he thus reasons : " The ground waves being masses of water which roll on the bottom of the sea conducted by the undulations, the best means to destroy their shocks is to present to them a surface which shall turn them at each instant of their direction, so that from the horizontal, which that direction is at first, it shall by little and little, and without any shock, be forced to become vertical." c Is not this effect of a vertical direction, or the conversion of the waves into breakers, produced still more completely by the waves meeting the abrupt faces of the submarine cliffs, to which Colonel Emy assigns a different result ; that of the formation of ground waves with a horizontal motion ?

CHAPTER [J. ON RIVERS WHICH ARK FREE PROM BARS It will generally be found, that whenever ;i navigable river approximates to the condition of a simple inlet, for the reception of the tide, 80 far as regards the longitudinal section, presented by its surface at low water, it will either have no bar, or be but lightly obstructed by one 5 the same may be said of those sea ports, or pier harbours, which, though free frorn bars in their natural state, are well known to become encumbered by them immediately on the introduction of an artificial scouring power. Resuming the investigation into the state of a river, whose entrance is free from a bar, we shall find, that, from its junction with the ocean, a long 1 line of navigable course exists with an extremely gentle fall, or slope of its surface, at low water ; the river is in this case in a proper train, its longitudinal section presenting a succession of inclined planes, becoming more and more gentle, as they approach the ocean ; and the lower course of the river, from the slightc l J ON RIVERS WHICH ARE FREE FROM BARS. ness of its fall, approximates to the condition of a frith, or deep inlet, of the coast, or to that of one of those large natural or artificial harbours, which, being mere tidal receptacles, wherein the influx and efflux take place in equal times, are necessarily free from bars.

The river, being in this perfect state, as regards the slope of its surface at low water, a consequent attendant upon the latter will be an equal duration, or nearly so, of the period taken up by the flow of the flood tide, with that of the ebb, in the lower reach of the river ; by the term flow being under stood, the direct upward course of the current of the flood tide, immediately after the true time of low water. Assuming that to the possession of a nearly equal duration of the period of the ebb and flow, in the lower reach of a river, accompanied by an ex tremely gentle inclination of its surface at low water, is to be attributed the freedom from the incum brance of a bar, I will now pursue the investigation, into the actual condition of Bar Rivers. which discharge their waters into tideless seas or fresh water lakes, we shall still be able to trace to the same cause the existence of their bars, viz. to the excess of slope, which their longitudinal sections present near their embouchures: but as here the agency of the tide or counter current does not exist, the deposit which takes place will generally be dif ferently affected from that which forms the bar of a tidal river, which latter does not usually experience that great observable elongation of its course, which takes place in rivers which discharge their waters into tideless seas.

The extension of the course of a tidal river only existing where the latter discharges its waters into a shallow sea or protected bay, the observations on this subject, by Major Rennel, are so clearly expressed, that I cannot do better than give them in the words of their distinguished author. " It is a circumstance well known to the generality of readers, that rivers which deposit great quantities of matter do also very often separate into two or more branches, previous to their discharge into the sea; thus forming triangular spaces, which the Greeks very aptly called Deltas, from the resemblance they bore to the form of that letter of their alphabet, and also that these Deltas almost invariably encroach on the sea, beyond the general, and it may be supposed, the original line of coast. However the formation of such Deltas, even by rivers of the first magnitude, is by no means universal ; on the contrary, some of them terminate in deep inlets or estuaries instead of projecting forms ; or, if the expression be allowed, theyterminate negativelyinstead ofpositively; of this class may be reckoned the great rivers of the Amazons, Plata, and the Orinoco; besides many others, which perhaps bring down an equal quantity of the matter of alluvions with the Nile, Ganges, or any other river, that may form the projecting Delta.