KIVKU KM:INKRIUN<!. 



RIVER ENGINEERING. 



108 



" 1 tTwUoh attention" will be called hereafter. 



m . of the material* of the bed a river may flow over will 



( I)* ulismjral quality, and therefore the value, of it* waters for 

 ueii purrx-** a* irrigation or waU-r supply; and this, t.-, whether 

 the material* taken up be so taken in solution, or merely in mechanical 

 P^. In WATIJI Si l-l-l.T. attention will U- cull.-.! to the chemical 

 portion of this inquiry ; but at pra-Mit it is advisable to dwell ii|>u 

 ibe merely ' -ti effect* produced, for the outlines of the beds 

 of riven ids-pcnd mainly upon them. Now the longitudinal proiile of 

 a rirtir ahnost alway* (unn* a ].rcie f paraU'lie curve, whose a|>ex is 

 liigh land*, and whoe baae i* at the nbooohun : * that the 

 : flow i* greater near the aource than it is near the sxaboaohure, 

 UM> tnUHBortinK and the erosive powers being also diminislie.l iu the 

 UM proiortioo. By the exprrimenU of Du Hint we learn that the 

 following material* will be carried forward by waters flowing with the 

 eiocitie* named in connection with them in the table : 



RlTTT mod, MBt Juld .ill 



Bran poitcrr eUy 



... 



YUo Madjr loon . 

 Cammou rim Mn<! 



. Speed per tccond feet S Inchet. 

 i 11 s t .. 







8} 



Caanebtllait .... 

 Km .huirlr, 1 Inch diameter . . 

 Lar* ihinfle .... 

 AajraUr Ilmu, tlxc of tiro's erg* . 



oft KhUi(r raelu 



RocU, with dtaUnct tntiflcatkm 



Hard rocks 





 

 1 

 S 



3 

 . 



I 

 4 

 4 

 I 



It throe* follow* that when the velocity of the river attains any of 

 the above rates, it will be able to attack and remove by erosion the 

 material* uf iU bank*, which would remain in susjwnsion at those 

 velocities. In designing a system of river defences, however, attention 



t be paid exclusively to the maximum, not to the artrayc velocity 

 of the stream ; because in flood time not only is the volume, but also 

 the velocity, greater than at any other period, and the materials of 

 which the bonks are formed must be able to resist the transporting 

 power of the flood*. However, in consequence of their variable trans- 

 l-.rtin:,' power, rivers in their normal state flow over materials gradually 

 decreasing in volume as they leave the steeper ports of their courses 

 and approach their oil t fall* ; ami a* those materials necessarily become 

 at last very fine, especially in l"iii; Hat plains, they offer little resistance 

 to occasional floods, as may be observed in the deltas of such rivers as 

 the Nile, Danube, Ganges, &c., which are composed of the fine mud 

 I rout-lit down from the uplands, and in which the course of the 

 utrcam continually shifts. 



Some curious effects may be observed to take place with regard to 

 the deposition of alluvial matters near the junction of streams draining 

 area* of country situated under marked differences of thermometrical 

 or hydrographical conditions; because the floods (or as they are 

 technically called lJu frithrti) frequently occur at distinctly different 

 epoch*, and thus, by reason of the freshet of one affluent ponding back 

 UK other, a deposition of the matters in suspension in the retarded 

 train will be produced. It often happens that, under such circum- 

 stance*, the accumulation of alluvial matters tokos place in a manner 

 to direct the local current against the banks in a direction essentially 

 different from the main line of flow, the effect of which would bo to 

 divert the river from it* original bed, to a greater or lesser extent, 

 according to the resistance of the banks. There is another law with 

 reapect to the mode in which the alluvial matters brought down by 

 floods or fremheU takes place, namely, that when the waters overflow 

 their banks they throw down those matters in the order of their 

 " : gravities, the heavier ones being thrown down on the edge of 







i deep stream, where the velocity is greater, and the lighter ones 

 being thrown down on the edge of the shallow water where on account 

 nf the friction the velocity is the least. The transverse section of a 

 valley subject to freshet* will be in fact something like the sketch 

 here given. The bank* will form slightly elevated ridges close to the 



water's edge, and the surface of the- alluvial deposits will fall away gra- 

 dually on either aide: thi* condition may remain xtalile for wmie time, 



.re is always a danger from the tendency of the stream t. 



If a new bed in the lower level. On the Po, where the banks 

 have been carefully ma. 1 Centuries, the accumulation of the 



alluvial matter* has taken place in the manner above sketched, but to 



n extent a* to create a real source of danger ; for the banks of 



IT have been gradually so rawed by the accumulation of alluvial 

 matter*, a* to be in some cane* as much as 40 feet above the extreme 

 ilsyiisisiiill [Ai.i.L vu M, in NAT. Jli.vr. Div.j 



A few final general remarks with respect to n<>n tidal rivers may 

 here be inserted, namely, that their volumes, even when they are of a 

 comparatively stable character, vary within very wide limits. Thus 

 the Rhone at Lyon, at times, flows with a volume of 7000 cubic feet 

 per minute, at others with a volume of 208,000 cubic feet ; the Rhino 

 at Strasbourg varies between the limits of 9,500 and 10-1,000 cubic feet 

 per minute; the Thames at Teddington between 5,400 and 15,000 

 cubic feet per minute. For the practical purposes of navigation, a 

 current is considered to be slow when it does not exceed 1 foot 6 inches 

 per second ; a velocity of between 2 feet and 3 feet 6 inches per second 

 is considered to be an average one ; beyond the latter limit the velocity 

 of the current is considered to be decidedly unfavourable to an upward 

 navigation, and if the stream should flow at the rate of 6 or 7 feet per 

 second, no boats can economically be made to work against it. When 

 the rate of inclination of the bed of the river attains between o and 6 

 in 10,000, the velocity of the current becomes too great to allow of the 

 navigation being carried on against it by the ordinary modes of trac- 

 tion ; but by the use of steam tugs worked by high-pressure exp - 

 engines, the Rhone has been navigated against the stream, even though 

 iU inclination has been on the average of its course from 7 to 8 in 

 10,000. Rivers which do not present a permanent depth of 1 foot 2 

 inches, with an available width of at least 14 feet, are not adapted in 

 receive any kind of navigation, and they can only be rendered available 

 for commerce by being canalized, or by being converted into what the 

 continental engineers call floatable rivers. Streams of this kind occur 

 in mountainous and woody districts, and are much used for the pur- 

 pose of floating down rafts of timber, or such natural productions as 

 are not likely to be injured by being occasionally submerged. When 

 the width and depth of a river exceed respectively 18 feet and 18 

 inches, a barge navigation may be established, provided of course that 

 the rate of flow be not unfavourable ; and when the width and depth 

 of the stream become respectively 60 feet and 8 feet, the river becomes 

 navigable for sea-going vessels. The modes of propulsion to be adopted 

 must depend upon local circumstances. In still water animal power 

 is the most economical ; steam is frequently used in deep water canals 

 and river navigation ; whilst sails, oars, and tides are frequently re- 

 sorted to for this purpose iu rivers adapted to their use. 



In the lower parts of the courses of rivers discharging their waters 

 into the ocean, and even to some extent in those discharging into tidal 

 inland seas, a very marked interference takes place in the conditions of 

 the currents, in consequence of the periodical tidal action. The tides 

 themselves are produced by the periodical elevations and depressions of 

 the level of the sea, caused by the combined action of the sun and 

 moon, and under such circumstances they are naturally of greater 

 amplitude on the shores of the larger bodies of water ; on the Pacific, 

 for instance, they are greater than they are upon the Atlantic; upon 

 the latter, again, the tides are greater than upon the Baltic or the 

 Mediterranean. The rising tide is called the food ; the falling tide, 

 the ebb ; and the returns of the tide take place twice in each con- 

 secutive interval comprised between the returns of the moon to the 

 upper meridian. The mean length of this interval is 1*03505 day, so 

 that the mean interval between two successive high waters is 0'51 7525 

 day, and the mean time of low water divides this interval again into 

 two nearly equal portions. As in the case of all quantities susceptible 

 of a maximum and a minimum, the increase or the diminution of the 

 tides towards their limits is proportional to the squares of the time 

 elapsed between the high and low tides. The height of the full tide 

 on the sea-shore varies, normally, every day according to the phases: 

 of the moon ; it is greatest at the gyzigies, and least at the quadra- 

 tures ; but it is to be observed that iu every place there is a, species of 

 retardation in the period of the high tide, as compared with the exact 

 astronomical periods of the moon's changes, which is technically known 

 by the name of the vulgar ettaUMment of the place, and which 

 i - upon laws to be noticed under TIDES. The rise of the high 

 tides is usually proportional to the fall at low tides, but there is an 

 irregularity between the heights of the two tides of the same clay ; the 

 rise and fall at the syzigie,. is about double those of the quadratures, 

 the former being called the spring tides, the latter the mtija; but it 

 may be added, that the former are much increased when the moon is 

 in perigee. An augmentation in the rise of the tide also takes place 

 when the sun's declination is zero, or at the period of the equinoxes ; 

 and the greatest normal tide occurs when a new or full moon happens 

 to occur near the equinox, when the moon is in perigee; and her 

 action would be still further increased if her node should hapi- 

 coincide with the perigee. The other causes of the variations in the 

 height of the tides may be neglected for the present, with the exceptii >n 

 of those which are likely to affect their conditions iu and upon rivers 



Thus, in cases where the tidal undulation is confined within a 

 narrow gulf it becomes considerably increased in amplitude; and its 

 reflection from the opposite shore may also increase that amplitude, 

 especially in the recesses of such coasts. It is on this account that 

 whilst at St. Helena in mid Atlantic, the rise is only 3 feet, and in the 

 Northern Atlantic it never exceeds 10 or 12 feet, it is in the Bay of 

 Mont St. Michel, 46 feet; in the Bay of Fundy, 50 feet; and in the 

 Wye, at Chepstow, 60 feet. High winds, and especially the equinoc- 

 tial gales, when they coincide with the spring tides may also give rise 

 to great irregularities in the amplitude of the tidal range, and instances 

 have even been recorded in which the tides of the quadratures have 



