In the rivers 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 rivers wherein the disturbing causes are more simple ; and bar rivers, although extremely dangerous (luring stormy weather, are sometimes less inconvenient for the ordinary purposes of commerce than are those which, like the Thames, the Scheldt, or the Seine, are exposed to the Injurious effects of the shifting sands thrown down at their embouchure.. It is, moreover, to be observed that In the majority of canes 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 reulove; and there is on this aecouut a tendency in all such lays to silt up, a tendency which may distinctly be traced in the three rivers just mentioned. The maintenance of a navigable channel in these MACS 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 effurts of hydraulic engineers for this purpose should therefore be directed rather to concentrate the flow of the water in one well proportioned ehanuel, than to secure merely a Large volume of tidal water, without reference to the conditions of velocity it may be possible to impress upon the latter.. For this reason it. is preferable, as a general rule, in designing works for the improvainent of the navigable channels of rises, discharging their waters in tidal seam, to limit the width of the water-way In the lower portion, so as to drive the tidal wave as far as possible ; and fur the same reason every interference with the direct transmission of the tide is objectionable. Many illustra tions of the truth of these prupositieus might be found on our own shores; but the success of the works executed for the improvement of the Clyde, Tyne, Seine, must suffice to prove that the most im pertant condition fur the maintenance of a clear tidal navigation is, that the water should always lodes a uniform velocity throughout its longitudinal and its transverse sections. In awes like the one which occurs at the mouth of the Tyne, where the passage through which the flood tide enters is contracted by the long bar extend itig from one shore nearly the whole width of the river ; and where the thee', once admitted, can suddenly expand itself over a wide lay, it will be almost Impossible to prevent the formation of allu 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 &t;/re on- 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 tho Amazouas 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 S 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, iu the bay of Havre. Boite.) The practical rules for the improvement of a river must evidently, from what has been said above, ho susceptible of endless varieties, according to the nature of the stream, and the purposes to which it is intended to be applied. Even if no direct commercial benefit is anticipated from such operations, it iuust 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 fur the surface drainage. There are, however, few rivulets which might not be made to render service for irrigation purposes, and directly the flow of water becomes sufficient to allow the establishment of water-wheels, working occasionally or continuously, the streams become of great economical value. It is difficult to discover how the right of property in running waters has been established; and it is Is 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 streams. The precise objects aimed at in such operations are mainly to retain the waters in their beds, and to regularise the conditions of the flow 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 coining 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 duce' in our own colonies, and especially in South Africa and Australia. In blase IlVOIRS 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 Frisfs work upon ' Torrents,' to Valles, Savigny, Suzell, Hun, Bibb°, liabinet, &c., who have written at length on the inundations of France. For the present it may suffice to add, that when the rivers Irene() of sufficient volume to cause dangerous inundations, there seems to be less danger when the banks of the river are dressed off at the height of the ordinary winter flood lines (so as to allow the waters when they exceed that level at once to flow over the country), than when the waters are confined within narrow banks raised to a considerable height above the surrounding lands. In some places on the banks of the Po, a river subject to violent floods, the banks are made of two descriptions ' in froldi," when they are raised to the necessary extreme height at once, and on the margins of the stream ; and " im goleno " when they are raised at a groat distance from the margins, so as to interfere as little as may be with the spread of the waters. Tire former course is adopted when the valley of the river in narrow and the foundation, are good ; the latter, when the valley widens out, and there is any danger of the uudermining of the banks. A double system of dykes, such as the one adopted in Piedmont, might perhaps save the dwellings upon the banks, of the Rhine and of the Maas, from the periodiad returns of their disostrous inundations ; at any rate it would materially increase the security of the banks themselves when ice floats down in !ergo quantities, as the up of the hummock is ice more injurious to tho light earthen or (mewed embankments' of those rivers than even the floods themselves.
The embankment, thrown up for the purpose of protecting the subinersable 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 tightuese. The surface exposed to the wash of the current must be protected if the embankment should be com posed of easily transportable materials, by stone pitching, planking, concrete, wattling, cribwork, or by fascines, tonnages, or clayonnages, in the manner practised by the Gentian, French, and Dutch engineers. As a general rule the tops of these banks are made wide enough to carry a read ; and it is essential to make provieion for removing from the landward side the drainage waters which may there accumulate; it would be better 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 well as those connected with the materials to be used in the formation of the bank itself.
ease navigation purposes the improvement of smell streams in thinly peopled. mountainous, and woody districts, is almost forcedly limited 7verting them Into heatable rivers ; and this is often effected by threwing a set of dams serves the rivers, so as to form as it were locks s f still water, in which the timber can be made into floats, and moved ts's anis the sluices formed in the slams, which are opened whenever there is a sufficient accumulation of water to float the timber to the nest pond. The width of these sluices is usually made about 20 feet, and the fall varies between 3 and 4 feet; but as the sluicegates are opened suddenly and the waters cramps as it were with a flash, it is not desirable to make the fall great. There are some very good works this description on the Yonne, above Auxerre ; and the dimensions of the one at Itagilbert may perhaps be added, as a model for our own colonial engineers. The passage and sluice are formed by carrying out walls from both banks, in a speciea of funnel shape, with its wider mouth towards the upstream, of 233 feet in width, and a length of about 21n feet from its commencement to the paissage, which is 20 feet in width and 33 feet in length to the sluice. Beyond the sluice the wing walls open ant for a distance of 20 feet, and at the end to the down stream they have an opening of 24 feet; a floor, for the purpose of obviating the inconvenience of the cataract produced by the flash of water, is continued beyond the wing walls; the water is pondel up to a height of 4 feet, and the sluice is opened, and in a fit state to lass a raft, within about eight minutes. Occasionally it as desirable to combine sluices of this description with locks, or, in fact, to combine a navigable and a (testable system. In those cases the lock should be placed in the atilt water, on the opposite side to the flashing elides; the latter being itself near the head of the lock chamber, and the tail of the lock being placed at a sufficient distance from the end of the dens to obviate any danger from the back-water, or cataract of the gluier.