1841.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



189 



REVIEVJ^S. 



Treatise on the Tmproremertt of the Navigation of Rivers:, mith a New 

 Theory of the Cause of the Existence of Bars. By William Alex- 

 ander Brooks, M. Inst. C.E. London: Weale, 1841. 



This work is the result of much reading and much research, deriv- 

 ing its materials not only from the engineering literature of England, 

 but also from the best and latest continental writers. Although the 

 volume is small, the labour and attention which have been bestowed 

 are considerable, and none can peruse it without recognizing the spirit 

 of inquiry which animates the writer. One only remark we have to 

 make, which is, that our author does not seem to have done full justice 

 to the contributers to this Journal, and other English writers, in omit- 

 ting to mention the names of many of the parties, to whose theories lie 

 alludes. 



In his introductory chapter Mr. Brooks detines several of the theories, 

 proposed for explaining the formation of bars. First, Major Rennel's, 

 which is that they are caused by the current losing its strength at a 

 certain distance in the sea, and so depositing the substances carried 

 with it. — 2. Mr. Delabeche attributes it to the ocean piling up detritus 

 on the shore — 3. Mr. Rookc attributes bars to the strength of the current 

 of the flood tide not running in the same channel with that of the ebb; 

 or to the embouchure of the river not being freely open to the course 

 of the tidal current. — 4. Mr. Barrett's theory, as our readers know, is, 

 that they are caused by the conflicting action of effluent currents pass- 

 ing into the ocean at right angles to the shore. — 5. Another, and the 

 most favourite theory is an imagined insufficiency of backwater. — Ij. 

 An opinion entertained abroad is that bars arise from the streams in 

 their approach to the sea spreading in surface and diminishing in 

 depth, so as to deposit the sands. — 7. Colonel Emy, an eminent French 

 engineer quoted by our author, attributes these obstacles to the ground 

 waves, or fots de fond. — 8. We now come to the theory put forward 

 by Mr. Brooks, which we shall let him give in his own words. 



An accurate examination of the state of a bar river will exhibit a great 

 irregularity of its surface at low water ; in lieu of the river presenting at that 

 period a longitudinal section of a succession of inclined planes, described in 

 tlie preceding description of rivers free from bars, as becoming more anil 

 more gentle in proportion to their proximity to the ocean, it will be often 

 found that the declination or slope of some of the upper reaches is less than 

 those nearer the ocean ; and the fall at low water in the lower reaches of the 

 river is always so great, as to produce a striking difference in the vertical rise 

 of tide, even at a short distance from the sea ; and attendant upon this de- 

 fective state of the section presented by the surface of tlie river at low water, 

 is a great extension of the duration of the ebb, beyond that of the upward 

 current of the flood tide. 



The river being in this irregular state, the process by which the Ijar is 

 formed may be thus described. 



During the period of the first quarter flood, the cuirent, in lieu of being 

 able to take its natural upward course, as in rivers where no bar exists, is 

 opposed, or effectually checked, by the effluent backwater; the declination 

 of the stream in the lower division of the river presenting a head which in- 

 sures a strong downward current, long after the tide would have been able 

 to maintain an upward course, provided the backwater had had a free dis- 

 charge. At tills period the flood tide, by reason of its greater specific gravity, 

 occupies the lower stratum of the tide-way, and like a wedge endeavours to 

 force its course up the channel, which it is unable to effect, but merely ele- 

 vates the lighter effluent water, the lower strata of which, being checked by 

 the opposition of the tid.l water, yields to the latter the sand or other ma- 

 terials, which it was capable of holding in suspension, previously to its en- 

 countering the conflicting action of the flood-tide ; and where this takes place 

 the bar is formed. 



To the theory of Majoi Rennel (No. 1,) Mr. Brooks objects that it is 

 insufficient because the operations described as producing bars take 

 place in all rivers, even in such as having their waters most abundantly 

 laden with sand or mud, are yet free from bars. On Mr. Delabeche's 

 (No. 2), Mr. Brooks says that the action of the waves cannot be the 

 cause, as bars are found in the most sheltered situations, while other 

 rivers abuunding with silt are nevertheless free from deposits in the 

 most furious seas. To Mr. Rooke and Mr. Barrett (Nos. 3 and 4), the 

 objection of our author is that in rivers subject to great variation at 

 their entrance, the bar is always found to exist independently of the 

 direction of the discharge into the sea. The backwater theory (No. 

 5j, is confessedly insufficient, the mightiest rivers of the globe pre- 

 senting staggering e.^ceptions. To the titli, it is opposed that in 

 the Mediterranean no current is ever opposed to any stream, and 

 that consequently the repose supposed to take place at the meet- 

 ing of the currents cannot exist. Further, that in the ocean one of the 

 two currents overcomes the other. Mr. Brooks objects to the ground 

 waves or ^ots defend actmg on sudden elevations of the bed of the sea 



in the manner assumed by Colonel Emy, opposing to it the received 

 opinion that breakers are formed immediately on any portion of the 

 wave meeting violently the vertical face of the obstruction. In sup- 

 port of this view an appeal is made to the geological formation of the 

 north coast of Yorkshire, where nothicg is found to corroborate the 

 Colonel's hypothesis. The case of the Adour quoted by Colonel Emy 

 is well shown by Mr. Brooks to be an influence of local causes. 



With regard to rivers being free from bars, Mr. Brooks supports 

 Mr. Rooke's views, giving a good definition that whenever a navigable 

 river approaches to the form of a simple inlet for the reception of the 

 tide so 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. The same, he observes, may be said of those seaports or pier 

 harbours, which though free from bars in their natural state, are well 

 known to become encumbered by them immediately on the introduc- 

 tion of an artificial scouring power. The views' of the previous 

 writers, Mr. Brooks has carried out still further, and we are prepared 

 to concur in much that he says. He remarks that 



Resuming the investigation into the state of a rivf, w!ic?o c:!trance is free 

 from a bar, we shall find that, from its junction with the ocean, a long Une 

 of navigable course exists with an extremely gentle fall, or slope of its sur- 

 face, 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 slightness 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 har- 

 bours, 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 he an equal duration, 

 or nearly so, of the period taken up by the flow of the flood tide, with that 

 of the ebb, iu 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. 



Our author having propounded his theory, goes on to propose his 

 remedies for the cases in which bars exist'. His first remedy is to 

 make the bed of the river of more regular inclination. 



By this natural elongation of the course of rivers by the deposit of alluvial 

 matter, a gradual amelioration of the navigation must take place, inasmuch 

 as that elongation is necessarily attended with a more gradual junction with 

 the waters of the sea, or the diminutiou of the velocity of the current at the 

 point of discharge ; we have therefore only to assist the operations of nature 

 by directing the course of the current, and thereby the position of the deposit 

 of the alluvions, to insure that the latter shall act beneficially and not pre- 

 judicially to the navigation. 



He then goes on to provide for other cases. 



In a tidal i iver, where a bar exists, and the reduction of the declination of 

 the low water surface cannot be effected, by reason of a long length of rocky 

 bed, too costly to remove, the only means available for its improvement is 

 an artificial elongation of its course, by piers or other works, to bring the 

 raouth of the river within the influence of a stronger current. 



Where the declination of a river is great iu its lower reaches, the result of 

 any cut near the embouchure of the river, which is not attended by a simul- 

 taneous reduction of that declination, must be the increase of the bar. It is 

 however to be observed, that the natural attendant effect of the shortening 

 the course of the current, is the more free discharge of the water and abase- 

 ment of the level of the surface of the current ; and wherever this latter cir- 

 cumstance does not take place, it is solely due to the presence of some geo- 

 logical feature, such as rock or marl, which the current, when unassisted by 

 art, is unable to act upon. 



Upon the use of artificial scouring power, where used with the view 

 of increasing the effect produced by the natural backwater of rivers, 

 we find it observed. 



Assuming, therefore, that the volume of the natural backwater is so small 

 as to be inadequate to maintain a sufficient depth in the harbour for the 

 maritime wants of the port, and that the aid of an artificial scouring power 

 be requisite, still the h-.tter should not be made use of, except during that 

 period of the eljb when its effect is to remove seaward the matter held in 

 sus])ension by the effluent water. 



If, therefore, any portion of the artificial backwater be discharged during 

 still water, or during any period of the flood tide, we may anticipate a rapid 

 deposit, or accumulation on the bar. 



In order to secure the utmost useful effect from an artificial scouring power, 

 it is essential that its action be prolonged to a position which is within the 

 range of a strong tidal current, or within the reach of the effect of the pre- 

 vailing onward impulse by the surf, during on-shore gales. 



Where the scouring power terminates negatively, if 1 may use the expres- 

 sion advanced by Major Rennel, or where the effect of the scorning power is 

 unable to extend into a true tidal shore current, it is unreasonable to expect 

 its utmost useful available residt. 



Thus, supposing the bar prodnced by a scouring power be situated in a 



