Channel (hydrology)

“ If an estuary is not funnel-shaped, if, for instance, it widens out very rapidly, the tidal flow is much less effective in keeping the channel open. In this case, training works, which would give the necessary funnel shape, are indicated rather than dredging. If an estuary is narrow at the entrance, the flow is much less powerful, unless the narrow part is of greater depth, but even then the force of the tide is reduced owing to the change in the shape of the channel. ”

“ Action on the Sides of a Channel.—It has been seen that the laws of silting and scour on the bed of a channel depend on the ratio of the depth to the velocity. The same laws probably hold good in the case of a gently shelving bank, so that here again V ought to vary as D·64. The velocity near the angle where the slope meets the water surface seems to decrease faster than D·64. At all events, silt tends to deposit in the angle and the slope to become steep. ”

“ Training or canalising should not be effected in any reach of a stream without regard to other reaches. A mere local lowering of the water-level by dredging may accentuate the effect of a shoal at the upper end of the reach.
When the water-level is raised by a weir or by narrowing the channel—though in the latter case the raising may not be permanent—it is generally best to commence the work from the upstream end. The raising of the water-level will then not interfere with the execution of the rest of the work. ”

“ This may be an open channel of earth, masonry or wood, laid at as small a slope as is consistent with the delivery of the necessary supply of water, or it may be a closed cast or wrought-iron pipe, laid at the natural slope of the ground, and about 3 ft. below the surface. In some cases part of the head race is an open channel, part a closed pipe. The channel often starts from a small storage reservoir, constructed near the stream supplying the water motor, in which the water accumulates when the motor is not working. ”

“ Silting and scouring are generally regular or irregular in their action according as the flow is regular or irregular, that is, according as the channel is free or not from abrupt changes and eddies. In a uniform canal fed from a river, the deposit in the head reach of the canal forms a wedge-shaped mass, the depth of the deposit decreasing with a fair approach to uniformity. Salient angles or places alongside of obstructions are most liable to scour, and deep hollows or recesses to silt. Eddies have exceptionally strong scouring power. ”

“ A river channel, indeed, generally suffices approximately to carry off the average flow of the river, which, whilst comprising considerable fluctuations in volume, furnishes a sufficiently constant erosive action to maintain a fairly regular channel; though rivers having soft beds and carrying down sediment erode their beds during floods and deposit alluvium in dry weather. As the velocity of a stream increases with its fall, the size of a channel conveying a definite average flow varies inversely with the fall, and the depth inversely with the width. ”

“ Sometimes they are employed in spreading still farther the barren crusts of their peculiar deposit, and sometimes probably in filling up the old channels, after which of course the stream has to find a new one, but as it is rather disposed from the form of the ground to spread over the surface, than to find its way in a single channel, an artificial one has been made for it down to the Anio. ”

“ If the water be charged with sediment to its normal supporting capacity, it can not take up more unless the rate of current be increased. Caving banks are caused wholly by the alternations in the velocity of the current. Alternations are inseparable from a curved channel, because the current in the bend is usually more rapid than on the point; but, if the channel be nearly uniform in width, the caving caused by the curves will be very trifling. ”

“ In carrying on works of this kind, wherever we find that there is a tendency to make a channel, and with it a strong current, it must be checked gradually from the upper end, so that the quantity of water passing through it may be decreased, and this channel will soon fill up. When a certain space has been naturally or artificially raised to the level of a fine green marsh, provided that there be a sufficient quantity to pay the expense, it should be embanked entirely from the sea. ”

“ The first stream captured in this way must necessarily be the nearest to the large stream. The diversion of the considerable volume of headwaters, H, to the channel of the small subsequent branch, G, causes it to deepen its channel rapidly; the same effect is perceptible in H for a distance above its point of capture and diversion: the increased load of sediment thus given to G will be in great part dropped in a fan-delta where it enters the flat valley of the master stream, A, (fig. 6) . ”

“ The bed has to be heavily protected in any case, and the proper principle is to fix a velocity which is considered to be safe and, the maximum discharge being known, to determine the area of the waterway accordingly. In the case of a very wide river like the Nile, with a well-defined channel, it is inconvenient to make the distance between the abutments of a work much less than the width of the channel, but so far as velocity is concerned, the floor need not usually be lower than the bed. ”

“ With the deepening of the channels, there comes an increase in the number of gulleys on the slopes of the channel; the gulleys grow into ravines and these into side valleys, joining their master streams at right angles (La Noë and Margerie) . With their continued development, the maturity of the system is reached; it is marked by an almost complete acquisition of every part of the original constructional surface by erosion under the guidance of the streams, so that every drop of rain that falls finds a way prepared to lead it to a stream and then to the ocean, its goal. ”

“ Where, at a considerable distance from the shafts, the water has not formed for itself a definite channel, it percolates over a wide area. The denudation in such cases is more generally spread, and a very extensive shallow trough or basin is formed. Again, where the pumping stations are close together, or in the same line, the various rivulets or streams of brine converge into one broad and deep channel, in which the denudation proceeds with great rapidity. ”

“ Where the current is gentle, and on ponds and lakes, the opening of the channel and canals is not attended with much difficulty. Where the current is swift, as on the large rivers, practical skill is required in opening up the field. The pressure of the current makes it difficult to open the channel and canals, and the shore ice is in danger of giving way. Stays and braces are sometimes employed to anchor the shore ice, and to prevent the channels from closing. ”

“ The distribution of the water between the diversion and the old channel really depends on their relative discharging capacities. If the required quantity of water does not flow down a diversion it can be dredged.
Sometimes a long spur is run out to send the water towards the off-take of a diversion. The effect of this is very small—it merely causes a set of the stream,—unless its length is so great that it amounts to something like a closure dam. It is sometimes said that it is easier to lead a river than to drive it. ”

“ Canalization of Rivers.
Rivers whose discharge is liable to become quite small at their low stage, or which have a somewhat large fall, as is usual in the upper part of rivers, cannot be given an adequate depth for navigation by regulation works alone; and their ordinary summer level has to be raised by impounding the flow with weirs at intervals across the channel (see Weir) , while a lock (see Canal and Dock) has to be provided alongside the weir, or in a side channel, to provide for the passage of vessels (fig. 8) . ”

“ When a channel is scoured, any regulator in it can be kept partly closed so as to reduce the surface slope in the reach upstream of the regulator and encourage the deposit of silt. A table should, in such cases, be drawn up giving the gauge readings to be maintained at the tail of the reach corresponding to given readings at the head.
Various methods of protecting banks are described in River and Canal Engineering, Chapter VI. The growing of plants on the inner slopes of channels whose sides fall in, needs special attention. ”

“ If rivers were always nearly of an equal fulness, the best means of diminishing their rapidity, and confining them within their banks, would be to enlarge their channel; but as almost all rivers are subject to increase and diminish, to confine them we must retrench the channel, because in shallow waters, if the channel is very broad, the water which passes in the middle hollows a winding bed, and when it begins to swell follows the direction it took in this particular bed, and striking forcibly against the banks of the channel destroys them and does great injuries. ”

“ In rivers which always have a fair fresh-water discharge, or a small fresh-water flow combined with a tidal flow and ebb, the channel sometimes has its direct outlet closed, and is deflected parallel to the shore till it reaches a weak place in the line of beach, through which a new outlet is formed; or, where the current is strong enough to keep the outlet open, a bar is formed across the entrance by the littoral drift, reducing the navigable depth. ”

“ The stream may also have to be trained, by means of guide banks or spurs, so as to remain in one channel and flow past the canal head and not form shoals against it. Where the river is unstable, it may shift its course so as to strike the weir obliquely and this may cause excessive heading up at one side of the weir. ”

“ BRAIDED CHANNELS. A river actively engaged in aggrading its valley with coarse waste builds a flood plain of comparatively steep gradient and often flows down it in a fairly direct course and through a network of braided channels. From time to time a channel becomes choked with waste, and the water no longer finding room in it breaks out and cuts and builds itself a new way which reunites down valley with the other channels. Thus there becomes established a network of ever-changing channels inclosing low islands of sand and gravel. ”

“ With ordinary soils a mean velocity of 3·3 feet per second in the channel is generally safe as regards scour of the sides. Any velocity of more than 3·5 feet per second may give trouble. A velocity of less than 1 foot per second is likely to give rise to deposit on the sides.
In channels in alluvial soils the falling in of banks is sometimes said to occur more when the stream is falling than at other times. This has been noticed on both the Mississippi and the Indus. ”

“ The stream is a unit; most streams have a single channel as the last goal of their descent. Others may have double channels, which contend with each other for the superiority. If they are double only for a part of the whole length of the river, and in the upper or middle course flow together, and form one main channel, they can be called twin head-streams. We have an example in the Danube and Inn, which are equally long and equally large. Other instances are the Rhone and the Saone, the Volga and Kama, the Missouri and Mississippi, the Blue and White Nile, the Ganga and Jumna. ”

“ A culvert which is liable to run full and which has a steep approach channel (fig. 52) may become suddenly drowned on the upstream side. As soon as the water rises to the crown of the arch, the wet border of the culvert increases and this reduces the velocity and discharge. The water coming 136 down the approach channel then rises abruptly, and the increased section of the stream causes a reduced velocity of approach, and this further reduces the discharge through the culvert. ”

“ An existing canal or distributary may need remodelling for various reasons, and in various degrees. If the velocity is too high and the bed has scoured, or the sides have fallen in, it may be necessary to raise the crests of falls, or to construct intermediate weirs, or to widen the channel and reduce the depth. ”

“ The navigable channel from the open sea to the canal entrance could only be maintained in an efficient state by means of continual dredging, owing to the silt. The channel passed close to the lighthouse at the end of the Mole, and then in a fairly direct line, for a distance of three-quarters of a mile, to a position midway between the extremities of the two piers marking the canal entrance. Thence the deep water channel passed slightly to the westward of the central line between the piers. ”

“ There is no doubt that it is always feasible to carry training walls right through an estuary, or at least down to a point where deep water is reached, and if a proper funnel shape is given to the channel the reduction of the tidal flow and silting up of the spaces behind the walls need not cause any trouble. Training the complete estuary was carried out in the case of the Tees, where, however, the estuary was not of great length, and was not of a good shape for keeping itself open. Any affluents entering the estuary can be provided with separate trained channels. ”

“ I again ordered each captain to pick his own channel, and having a strong breeze and all oars working we managed to get above. In this place I discovered by experience that what we had found to be the best channel on that day, was the worst a few days after. The water falling six inches to a foot every day, continually changed the rapids, making a bad place better and a good place bad. Above this cataract we had thirteen miles of what we called smooth water with a current of from three to five miles an hour. ”