109 



RIVER ENGINEERING. 



RIVER ENGINEERING. 



110 







been raised by the effect of storms to a greater height than those of 

 the syzigies. The disturbing effect of storms is, however, the greatest 

 in seas with a small tidal range ; as upon the French shores of the 

 Mediterranean, where the normal rise of the tides is only from 6 to 12 

 inches, it U thus often increased to 3 feet. The most remarkable cause 



irbance in the range of the tides is. however, to be met with in 

 the oceanic currents, and the manner in which they act varies in almost 

 every case, producing some very singular interferences with the discharge 

 of the upland waters, and with the deposition of the alluvial matters 

 in the embouchures of riven, whether the alluvions be brought from 

 the sea, or from the interior of the country. 



For instance, upon the shores of England we find that in consequence 

 of the set of the tidal currents in the offing, the bars at the mouths of the 



iwtween Land's End and Portland, point from N.W. to S.E., as 

 a rough general rule ; between Portland and Selsea Bill, the in-shore tide 

 runs in a direction opposite to that of the tide in the offing, and the 

 ban of the riven aW> point from N.E. to S.W. On the Atlantic 

 shores of Ireland, the ban point rather S.K. to N.W. ; they have the 

 aame direction on the western coasts of Scotland ; whilst on the eastern 

 eeast of Scotland and of England, the bars upon the open coast point. 

 nearly from S'.W. to S.K. In the mouth of the Thames two tiilal 

 .is meet, which have an origin separated by 12 hours' interval ; 

 so that not only are the day and night tides of that river equal in 

 their amplitudes, but the alluvial matters transported by the tides are 

 thrown down at the point where the velocities of the currents are 

 destroyed, in banks which are constantly shifting. ( hi the . st coast 



land again, there are in the riven many instances of the double 

 the reflection of the great tidal wave from a pro- 



i situated at a ]x>int beyond their embouchures, on the line of 

 advance of that wave ; and the conditions of the deposition of alluvial 

 matters in those riven are thus very different from the conditions ordi- 

 narily prevailing. Nor is this the only technical peculiarity connected 

 with these riven ; because, in fact, the double tide is not so powerful as 

 the normal tide for the purpose of carrying forward a barge navigation ; 

 but in another sense it is more favourable for the operations of com- 

 merce ona Urge scale, by reason of its retaining a great depth of water 

 in docks and harbours, during the whole of the (so called) ttatl-lidr. 



thaujjiton, this slack-tide lasts for nearly two hours ; on the 



; coast the same phenomenon occun at Havre, and in the Ome, 

 during about 1 J hour in each tide. Great oceanic currents, wherever 

 they exist, naturally produce analogous effects to these minor inter- 

 ferences with the progression of the tidal waves ; but of course upon a 

 much larger scale. A study of the history of the deltas of such riven 

 as the ,,'eii, Indus, Irriwaddy, Amasonaa, Mississippi, 4c., 



would be necessary for any one who desires to become acquainted with 

 this important detail connected with hydraulic engineering. The 

 actual records of the changes in the delta of the Rhine add of the 

 Scheldt, would be of gnat interest and value also to the student ; 

 because they hare been more closely and more scientifically observed 

 than the changes which have recently taken place in other riven of 



.. riven exposed to the action of a complicated series of tides 

 and currents, it is far more difficult to maintain a clear navigable 

 channel than it is in riven wherein the disturbing causes are more 

 simple ; and bar riven, although extremely dangerous during stormy 

 weather, are sometimes less inconvenient for the ordinary purposes of 

 commerce than ore those which, like the Thames, the Scheldt, or the 

 Seine, are exposed to the injurious effects of the shifting sands thrown 

 down at their embouchures. It is, moreover, to be observed that in 

 the majority of > aes the oceanic currents bring into the sheltered bays 

 they meet with, a greater quantity of alluvial matters with the flood 

 tides than the ebb tide can remove ; and there is on this account a 

 tendency in all such bays to silt up, a tendency which may distinctly 

 be traced in the three riven just mentioned. The maintenance of a 

 navigable channel in these cases becomes a matter of difficulty, and it 

 can only be effected temporarily by natural means, by securing the 

 greatest possible scouring action of the tidal wave. The efforts of 

 hydraulic engineers for this purpose should therefore be directed 

 rather to concentrate the flow of the water in one well proportioned 

 channel, than to secure merely a large volume of tidal water, without 

 reference to the conditions of velocity it may be possible to impress 

 he Utter. . For this reason it U preferable, as a general rule, in 

 designing works for the improvement of the navigable channels of 

 riven discharging their waters in tidal seas, to limit the width of the 

 water-way in the lower portion, so as to drive the tidal wave as far 

 ' as possible ; and for the aame reason every interference with 



rect transmission of the tide is objectionable. Many illustra- 

 tions of the truth of these propositions might be found on our own 

 bora* ; but the success of the works executed for the improvement of 

 the Clyde, Tyne, Seine, Ac., must suffice to prove that the most im- 

 for the maintenance of a clear tidal navigation is, 

 that the water should always possess a unit' 



.itiidinil an. I its i: in M i-f sections. In oases like the one 



occurs at the mouth of thcTji he passage through 



^^^B the enters U contracted by the long bar extend- 



ing from one shore nearly the whole width of the river ; and where 

 nee admitted, can suddenly ex|nd itself over a wide 

 bay, it will be almost impossible to prevent the formation of ollu- 



vial deposits both from the sea and from the land; and if a second 

 contraction of the water-way should take place abruptly above 

 the inner bay, the peculiar phenomenon known by the name of the 

 eagre or bore may be observed. The bore, in fact, is caused by the 

 sudden influx, into a narrow channel, of a large volume of water flow- 

 ing with considerable velocity, which rises in the form of a sudden 

 wave, and constitutes a serious source of danger to the small craft 

 navigating the rivers where it exists. On the Hoogley and on the 

 Amazonas the bore sometimes rises almost literally like a wall of 12 

 feet in height ; on the Severn and the Seine it is often 6 feet high, and 

 advances inland at the rate of about 6 miles per hour ; on the Trent 

 the eagre often rises as much as 8 feet, at Gainsborough, and there are 

 even rudiments of the bore to be detected in the Thames. The bore 

 of the Seine has been diminished by the works lately executed for the' 

 regularisation of the bed of the river above its embouchure, in the bay 

 of Havre. [Bout.] 



The practical rules for the improvement of a river must evidently, 

 from what has been said above, be susceptible of endless varieties, 

 according to the nature of the stream, and the purposes to which it 

 is intended to bo applied. Even if no direct commercial benefit is 

 anticipated from such operations, it must always be desirable that the 

 flow of water should be kept as regular as possible, and the bed of the 

 stream should be made to render the greatest amount of service as an 

 outfall for the surface drainage. There are, however, few rivuletw 

 which might not be made to render service for irrigation purpos, 

 directly the flow of water becomes sufficient to allow the establishment 

 of water-wheels, working occasionally or continuously, the stivams 

 become of great economical value. It ia difficult to discover how the 

 right of property in running waters has been established ; and it is a 

 matter of serious doubt even whether the existence of such rights be 

 not in itself frequently a source of public inconvenience ; but as this 

 state of things has existed from time immemorial it must be dealt with 

 in its actual form, if it were desired to improve any particular tivam. 

 The precise objects aimed at in such operations are mainly to retain 

 the waters in their beds, and to regularise the conditions of the How 

 of water in the channels; to prevent inundations, in fact, and to keep 

 the stream constantly in a state which should enable it to perform the 

 services it is applied to. 



As to the prevention of inundations, that object may at times be 

 effected by establishing, artificially, a system of reservoirs in the upper 

 parts of a river's course, which should perform the same function 

 which the Swiss lakes perform for the rivers coming from the Alps, 

 that is to say, should store the storm waters during the winter months 

 for the purpose of distribution in the dry season. Works of this 

 description have been executed in the East, and they have been 

 recommended for France ; they might be very advantageously intro- 

 duced in our own colonies, and especially in South Africa and Australia. 

 In REHUBVOIIIH the modes of executing this particular class of works 

 has been alluded to ; and the reader may be referred, for further 

 information on the subject of their influence upon inundations, to 

 Frisi's work upon ' Torrents,' to Valles, Savigny, Hiizell, Hun, Ribbe, 

 Babinet, 4e., who have written at length on the inundations of France. 

 For the present it may suffice to add, that when the rivers become of 

 sufficient volume to cause dangerous inundations, there seems to bo 

 less danger when the banks of the river are dressed off at the height 

 of the ordinary winter flood linos (so as to allow the waters when they 

 exceed that level at once to flow over the country), than when tho 

 waters are confined within narrow banks raised to a considerable height 

 above the surrounding lands. In some places on the banks of tho 

 Po, a river subject to violent floods, the banks ore made of two 

 descriptions : " in froldi," when they are raised to the necessary 

 extreme height at once, and on the margins of the stream ; ami " in 

 golene " when they are raised at a great distance from th> n. 

 so as to interfere as little as may be with the spread of the v. 

 The former course is adopted when the valley of the river is narrow 

 and the foundations are good ; the latter, when the valley widens out, 

 and there is any danger of the undermining of the banks. A double 

 system of dykes, such as the one adopted in 1'ii dnmnt, mi'jbt perhaps 

 save the dwellings upon the bunks of the Hhine nnd of th<- M 1 1 1, fiom 

 the periodical returns of their disastrous inundations ; at any rate it 

 would materially increase the security of the banks themselves when 

 ice floats down in large quantities, as the piling up of tin- hummock 

 ice is more injurious to tho light earthen or fascined embankments of 

 those rivers than even the floods themselves. 



The embankments thrown up for the purpose of protecting the 

 submenable districts on the side of a river must be made of a great 

 thickness, and with every imaginable precaution to ensure their 

 stability and their water tightness. The surface exposed to the wash 

 of the current must be protected if the embankment should b, 

 posed of easily transportable materials, by stone pitching, planking, 

 concrete, wattling, cribwork, or by fascines, tunnages, or clayonnages, 

 in the manner practised by the German, French, and Dutch cnp 

 An a general rule the tops of these banks are made wide enough to 

 carry a rood; and it is essential to make provision for removing from 

 the landward side the drainage waters which may there accumulate ; 

 it would be letter to effect this operation by lifting the drainage 

 waters over the bank instead of passing them under it. Local con- 

 siderations must, however, regulate the choice of all these details, as 



