CHAPTER XV

RIVER BARS

1. =Deltaic Rivers.=--When a river flows into a tideless sea its silt deposits and forms a shoal or bar. This shoal may in time extend and rise up to the water-level. The current of the river makes its way through it in various directions, and in this way a delta is formed and constantly extends seawards. This flattens the slope of the lower portions of the river, and causes raising of the bed in the reaches upstream, and this again may cause the water to break out further upstream and form fresh channels to the sea. The bars at the mouths of deltaic rivers are generally formed with great rapidity, and they are apt to form a complete hindrance to navigation. They are sometimes partly scoured away by floods in the river, but in this case the scoured material may deposit on the outer slopes of the bar. If a river which carries silt has no delta, it is probably because there is a littoral current, which prevents the silt from depositing. On the other hand, if a river brings down very heavy sediment, a delta may be formed even when tidal flow is not wholly absent. This occurs in the case of the Ganges.

The bars at the mouths of deltaic rivers cannot usually be kept down by dredging except at great expense. The usual method of dealing with them is to run out two parallel jetties, in continuation of the river banks, so as to bring the mouth of the river out to the bar. The river then scours a channel through the bar and, if the walls are not too far apart, the depth will probably become as great as in the river and sufficient for navigation. The river, however, tends to at once form a new bar further out. The rapidity with which the new bar forms will be greater or less as the specific gravity of the materials carried by the river is greater or less, and as the strength of any littoral current is less or greater. Clay is spread far out while sand quickly sinks. All deposits are, however, swept away if there is a strong littoral current. The steeper the slope of the bed of the sea away from the bar the longer the new deposit will take in forming a fresh bar. Also the less the discharge of the river the less the deposit will be. The branch of a deltaic river selected for improvement by having the bar at its mouth removed, should be one which has a small discharge and whose mouth is in a position where there is a strong littoral current. In the case of the Rhone, the branch selected was the eastern one, whose mouth was not exposed to any littoral current. Moreover, the other branches of the river were closed, and this increased the discharge of the branch which was left open. The work did not succeed. In other cases, the parallel jetty method has succeeded, and notably in the case of the Mississippi. In this case willow mattresses weighted with stones were used. The question of keeping down the discharge does not, however, appear to have always received sufficient attention. In the case of the Mississippi the “South pass” was selected for improvement. In order to remove a shoal its upper end was narrowed and its discharge reduced. The upper ends of the other “passes” were then obstructed so as to restore the discharges of all the passes to their former amounts. The wisdom of this step is questionable. It is desirable to keep down the discharge of the branch which is to be improved to the lowest limit consistent with free navigation.

If the width of the river near its mouth is greater than is desirable for the width between the jetties, the latter are sometimes made to converge though their outer ends are made parallel.

In the case of the Mississippi the jetties were made with a slight curve to the right. It would seem desirable always to make the jetties with quite a considerable curve. The jetty which was convex to the channel could then probably be shortened. In a case where there is a littoral current, say to the right, the curve of the jetties could be to the right, so that the stream on issuing would tend to merge into the current and assist it.

2. =Other Rivers.=--It often happens that the materials--sand, gravel, and shingle--of which a sea beach is composed shift gradually along the shore. This is known as “littoral drift.” It is by some supposed to be due to the action of the tides, and by others to the action of waves, the drift taking place in the direction of the prevailing winds, excluding those which are off shore. The latter cause is the more probable.

Most rivers have bars at their mouths. In the case of deltaic rivers the bar, as already stated, is caused by the heavy silt carried by the river, though it may be assisted by littoral drift. In the case of non-deltaic rivers flowing into tideless seas, the quantity of silt is not enough to form a bar, and the same is generally true in the case of tidal rivers where the volume of tidal water is usually much greater than that of the upland water. In both these classes of rivers the formation of the bars is due chiefly to littoral drift or to sediment brought in by the sea water. The bar, as in the case of deltaic rivers, may be partly scoured away by a flood in the river, and the scoured material may deposit on the seaward slope of the bar. Generally, the navigation channel across a bar of this kind can be kept sufficiently deep by dredging, but sometimes jetties, like those mentioned in the preceding article, have been constructed, and in this case there is the great advantage that the bar is not liable to form further out. If littoral drift tends to accumulate, the jetties, or at least the one on the side whence the drift comes, can be lengthened. This was done, as mentioned by Harcourt (_Rivers and Canals_, CHAP. IX.), in the case of the rivers Chicago, Buffalo, and Oswego, which flow into the Great Lakes of America. The same writer states that the jetties at the Swine mouth of the tideless river Oder were made to curve to the left, the convex or left-hand jetty being the shorter, but that this exposed the mouth to littoral drift coming from the left. The river, upstream of the jetties, had a slight curve towards the left, but this could have been corrected or, at all events, the jetties made to curve to the right.

A case (fig. 69) where parallel jetties were recently constructed in a tidal sea is that of the mouth of the Richmond River, New South Wales (_Min. Proc. Inst. C.E._, vol. clx.).

In the case of a bar at the mouth of an estuary, parallel jetties would be too far apart. In such cases converging breakwaters (fig. 70) are sometimes made, especially if the tidal capacity of the estuary is small. The entrance is generally 1000 to 2500 feet wide. If made narrow, it would reduce the tidal flow too much. The space inside the breakwaters adds to the tidal capacity, and thus induces scour at the bar. The case is similar to that of the Mersey estuary (CHAP. XIV., _Art. 5_), the breakwaters assisting scour at the bar, though perhaps slightly interfering with the tidal flow in the estuary.

Converging breakwaters also tend to stop littoral drift, and the space inside them acts as a harbour of refuge in storms and as a sheltered place where dredgers can work (_Rivers and Canals_, CHAP. XI.). They have to be heavily built and are very expensive, and they are generally adopted only when there is an important seaport, and when they can be put to all the uses above indicated.

APPENDIX A

=Fallacies in the Hydraulics of Streams= (CHAP. I., _Art. 4_, and CHAP. VI., _Art 2_).--In an inundation canal in India the supply during floods was excessive. Orders were given that a flume be made at the head, as shown in fig. 71. The sides were to be revetted, as shown in fig. 19 (CHAP. VI., _Art. 3_); the length, excluding the splayed parts, was to be 200 feet, and the floor was to be a mattress well staked or pegged down. The order stated that “by this means we cannot get into the canal much more than its true capacity.” With 9 feet of water, a surface fall of 4 inches in 300 feet would give a velocity of some 6·5 feet per second, and a further fall of about 8 inches would be required at the head of the flume to impress this velocity on the water. The flume would reduce the depth of water in the canal by 1 foot, _i.e._ from 9 feet to 8 feet. This would not be in anything like the proportion desired. Moreover the flume, unless the bed was extremely well protected, would be destroyed. The above is a case of exaggerating the effect of an “obstruction.”

Again, on a branch canal it was observed that “wherever cattle crossings exist there is a deep silt deposit which practically blocks the branch.” The deposit exists because the sides of the channel are worn down. A wide place always tends to shoal (CHAP. IV., _Art. 9_). If the deposit obstructed the flow of water there would be a rush of water past it, and it could not exist.

The Gagera branch of the Lower Chenab Canal--the left-hand branch in fig. 72--was found to silt. It was proposed to make a divide wall (fig. 72) extending up to full supply level. The idea is unintelligible. The silt does not travel by itself but is carried or rolled by the water. As long as water entered the Gagera branch, silt would go with it. The authorities, who had apparently accepted the proposal, altered the estimate when they received it, and ordered the wall to be made as shown dotted and of only half the height. This was done. The idea seems to have been that the wall would act as a sill and stop rolling silt. This is intelligible, but see CHAP. IV., _Art. 2_, last paragraph. Moreover, there was a large gap, A B, in the wall. The work is said to have proved useless, and proposals have been made to continue the wall from A to B. In this form it is conceivable that it may be of use.

In a river, the rises and falls at different places are not, of coarse, the same, even when they are long continued. In the river Chenab, at the railway bridge at Shershah, the rise from low water to high flood is generally a foot or two more than the rise at a point 25 miles upstream. It has been suggested that the railway embankments, which run across the flooded area, cause a heading up of the stream. If this were the case, to any appreciable extent, there would be a “rapid” through the bridge, which, if it did not destroy the bridge, would at least be visible and audible.

The exaggerated ideas which often prevail regarding the tendency of a river, when in flood, to scour out a new channel, have been mentioned in CHAP. IV., _Art. 8_. Spring, in his paper on river control, admits, when mentioning Dera Ghazi Khan, that there was little danger, but in mentioning the Chenab Bridge at Shershah he quotes, without disputing it, an opinion of the opposite kind (_Government of India Technical Paper_, No. 153, “River Training and Control on the Guide Bank System”).

For some other fallacies, see _Hydraulics_, CHAP. VII., _Arts. 9_ and _15_.

APPENDIX B

=Pitching and Bed Protection= (CHAP. VI., _Art. 3_, and CHAP. X., _Art. 2_).--Any scour upstream of a weir is merely due to the eddies formed upstream of the crest (_Hydraulics_, CHAP. II., _Art. 7_), and is not serious. And, similarly, as to scour upstream of a pier. A hole formed alongside a pier or obstruction, if there is no floor, may work upstream. The chief use of a floor extending far upstream is to flatten the hydraulic gradient (CHAP. X., _Art. 3_).

For pitching of the sides, monolithic concrete is not very suitable, because it may settle unequally and crack. For heavy pitching, concrete blocks can be used. They can rest on a layer of 3 to 6 inches of rammed ballast or gravel. The toe wall, as shown in fig. 13, page 65, is sometimes dispensed with, the pitching being merely continued to a suitable depth below the bed, and the bottom edge being at right angles to the slope instead of horizontal. The portion below the bed may be of concrete.

INDEX

Abrupt changes in streams, 28.

Alterations in a channel, upstream effect, 4.

Aqueduct, Kali Nadi, 149.

Available rainfall, 9.

Bank protection, 60.

-- -- artificial weeds, 70.

-- -- berms, 70.

-- -- bushing, 68.

-- -- fascining, 66.

-- -- heavy stone pitching with apron, 71.

-- -- on the Adige, 67.

-- -- reinforced concrete, 70.

-- -- rolls of wire-netting, 70, 140.

-- -- staking, 69.

-- -- trees, 68.

-- -- twig revetment, 68.

-- -- Villa system, 69.

Banks, 92.

-- Bell’s guide, 137.

-- continuous lining of, 64.

-- dimensions of, 93.

-- guide, 137.

-- side slopes of, 92.

Barrage of the Nile, Assiut, 117.

Bars, river, 203.

Bed, protection of, 58.

-- -- Villa system, 59.

Bell’s guide banks, 137.

Bends, effect of, 44.

-- short cuts of, 44.

Bengal Dooars Railway, bridge and floods, 139.

Bifurcation of a channel, 53.

Birmingham water supply, 173.

Borrow-pits in bed of channel, 53.

Breakwaters, converging, 207.

Bridges, 132.

-- foundations or floor, 132.

-- on Indian rivers, 134.

-- piers and abutments, 132.

-- protection of, 137.

-- at Wazirabad, 134.

British Rainfall Organisation, 9.

Canalisation of rivers, 84.

Canals, 92.

-- fall, 113.

-- headworks, 54.

-- navigation, 93.

-- rapid, 113.

-- Ship, 95.

Catchment area, “yield,” 9.

Channel, alterations in, 4.

Chanoine falling shutters, 121.

Chenab River at Shershah, 210.

Choice of types of work, 3.

Closures of streams, Colorado River, 80.

-- -- cradle for, 78.

-- -- Tista River, 81.

Collection of information concerning streams, 18.

Colorado River, closure of, 80.

Conduits, 92, 100.

Cradle for closing streams, 78.

Culverts, 135.

-- flooding of, 136.

Dams, culvert of, 180.

-- design of masonry, 181.

-- earthen, 174.

-- masonry 181.

-- -- construction of, 185.

-- -- design, 181.

-- -- failures of, 185.

-- -- stresses in, 185.

-- pitching of, 179.

-- reservoir, 162.

-- Sidhnai Canal, 117.

-- tower for culvert, 180.

Dee estuary, 201.

Dera Ghazi Khan, Indus at, 71.

Discharge curves, 23.

-- observations, 21.

-- tables, 24.

Divide wall, Gagera branch canal, 209.

Drainage, 141.

Dredging and excavating, 84, 88.

Drift, littoral, 205.

Eddies, scouring power of, 28.

Embankments, 156.

-- design of, 157.

-- Holland, 159.

-- Irrawaddy, 159.

-- Rhine, 159.

-- slips in, 177.

Estuaries, Dee, 201.

-- Mersey, 202.

-- Seine, 200.

-- tidal, 197.

Fall, canal, 113.

Fallacies in hydraulics, 5.

Falling shutters, Chanoine, 121.

-- -- Fouracres, 121.

-- -- Khanki, 124.

-- -- Thénard’s, 121.

Flood discharge, estimating, 148.

Floods, 141, 146.

-- Bengal Dooars Railway, 139.

-- formulæ for, 147.

-- prediction, 150.

-- prevention, 153.

Flowing stream, closure of, 75.

Ford in a river, 43.

Forests and vegetation, influence of, on rainfall, 14.

Formulæ for floods, 147.

Groynes or spurs, 58, 60, 61.

-- on the Indus, 53.

Guide banks, 137.

Headworks of a canal, 54.

Holland, embankments, 159.

Hurdle dykes, 79.

Hydraulics of open streams, 4.

-- -- fallacies in, 209.

Important works, precautions at, 130.

Indian rivers and bridges, 134.

Indus, groynes on, 53.

Information concerning streams, collection of, 18.

Inundation canal, flume in, 209.

Irrawaddy, embankments, 159.

Irrigation channels in embankment, 52.

Irwell, weir on, 124.

Jetties, in continuation of river banks, 204.

-- Mississippi, 205.

-- Richmond River, New South Wales, 206.

Kali Nadi, aqueduct, 149.

Khanki, falling shutters at, 124.

Leakage, stoppage of, 78.

Lockage, 98.

Locks, 96.

-- in flights, 98.

Mersey estuary, 202.

Mississippi jetties, 48.

Narora weir, 109.

Navigation canals, 93.

Needles, regulator, 117.

New South Wales, available rainfall, 12.

Obstruction, effect of, 5.

Obstructions in streams, 28.

Okla weir, 112.

Open streams, hydraulics of, 4.

Perishable materials, use of, 4.

Permanent régime of streams, 29.

Pitching, 64.

Prediction of floods, 150.

Prevention of floods, 153.

Protection of banks (see Bank protection).

Rainfall, 6.

-- available, 9, 26.

-- -- New South Wales, 9.

-- -- Sudbury River, Massachusetts, 12.

-- -- various countries, 11.

-- British Organisation, 9.

-- “catchment area,” “basin,” 9.

-- distribution of, 7.

-- driest year, 7.

-- evaporation, 9.

-- heavy falls in short periods, 15.

-- influence of cultivation, 15.

-- -- of forests and vegetation, 14.

-- local figures, 8.

-- measurement of, 13.

-- observations, period of, 7.

-- statistics, 6.

-- variation of, 6.

Rain-gauge, 8, 13.

Rapid, canal, 113.

Regulators, 118.

Reservoirs, 162.

-- capacity, 167.

-- compensation water, 162.

-- leakage, 163.

-- waste weir, 164.

Résumé of the subject, 1.

Rhine, embankments, 159.

Richmond river, jetties, 206.

-- weir at, 119.

River bars, 203.

-- training groynes, 85.

-- -- walls, 85.

Rivers, deltaic, 203.

-- floods in, 146.

-- non-deltaic, 205.

-- tidal, 192.

-- training and canalisation, 84, 89.

Run-off in small streams, 143.

Salt water, effect of, 29.

Sand separator, 34.

Sandbanks, 47.

Scour (see Silt and scour).

Seine estuary, 200.

Set of stream, effect of, 5.

Ship canals, 95.

Shutters, self-acting, Switzerland and Bavaria, 127.

Silting and scouring, 27.

Silt and scour, action at bends, 42.

-- -- -- on sides of channel, 40.

-- -- effect of regulator or movable weir, 49.

-- -- in the Sirhind Canal, 32, 54.

-- -- increasing or reducing, 48.

-- -- materials carried in suspension, 3.

-- -- methods of investigation, 33.

-- -- practical formulæ and figures, 37.

-- -- production of, 48.

-- -- rolled materials, 29.

-- -- sand separator, 34.

-- -- scrapers or harrows, 48.

-- clay and sand, 36.

-- deposit, production of, 51.

-- in river Sutlej, 35.

-- in river Tay, 35.

-- quantity and distribution of, 35.

-- upstream of weirs, 103.

Sidhnai Canal, dam, 117.

Sirhind Canal, silt and scour in, 32, 54.

Slips in embankments, 177.

Sluice gates, Stoney’s, 119.

Sluices, 102.

Small streams, floods in, 141.

Soundings, 21.

Spurs or groynes, 58, 60, 61.

Stream gauges, 19.

-- diversions of, 73.

-- general tendencies of, 45.

-- information concerning intermittent, 25.

-- -- -- small, 24, 25.

-- overflow, 46.

-- shifting, 20, 47.

Sudbury River, Massachusetts, available rainfall, 12.

Surface slope observations, 22.

Survey of a stream, 21.

Sutlej River, silt in, 35.

Syphons, 132, 135.

Tay River, silt in, 35.

Teddington, weir at, 119.

Tidal estuaries, works in, 198.

-- river, diagrammatic route-guide, 196.

-- rivers, 192.

-- works in, 196.

Tide-gauges, 192.

Tides, 190.

Tista River, closure of, 81.

Training of rivers, 84.

Types of work, choice of, 3.

Upper Jhelum Canal, syphons to carry torrents, 144.

Velocities which enable a stream to scour, 40.

Villa system of bank protection, 69.

-- -- bed protection, 59.

Waste weir, hydraulic problem, 165.

Water supply, Birmingham, 173.

Waterway, area of, in regulators, 129.

Wave, travel of, down a stream, 152.

Waves, effect of, 29.

Wazirabad, bridge at, 134.

Weeds, artificial, 70.

-- growth of, 29.

Weirs, 102.

-- adjustable, 126.

-- Bear Trap, 123.

-- drum, 127.

-- frame, 120.

-- general design of, 105.

-- Narora, 109.

-- oblique, 103.

-- Okla, 112.

-- on sandy or porous soil, 106.

-- Richmond, 119.

-- silt deposit, upstream of, 103.

-- types of, 111.

-- waste, 164.

-- with sluices, 115.

Wire-netting for bank protection, 70, 140.

Works, design and execution of, 3.

Zifta Regulator, 109.

PRINTED BY NEILL AND CO., LTD., EDINBURGH.

FOOTNOTES:

[1] _Min. Proc. Inst. C.E._

[2] _Engineering News._

[3] _Encyclopædia Britannica._

[4] The paper by Spring--in size it is a book--will repay perusal by engineers engaged on railway bridges over large shifting rivers. London Agents, Constable & Co.

[5] _Hydraulics with Working Tables._ Spon, 1912.

[6] Irrigation canals are dealt with in _Irrigation Works_ (Spon, 1913).

[7] See also Appendix A.

[8] The regulator runs across the canal head; the under-sluices are a continuation of the weir, between the divide wall and the regulator.

[9] See also Appendix B.

[10] On Indian canals the term “regulation” is applied to the control of the discharge at the regulators or off-take works.

[11] See also Appendix B.

[12] _Irrigation Works_, CHAP. I., _Art. 4_.

[13] _Min. Proc. Inst. C.E._, vols. lx. and lxxxv.

[14] _Rivers and Canals_, Harcourt.

[15] _Rivers and Canals_, Harcourt.

[16] _Rivers and Canals_, Harcourt.

[17] The foundations of piers and abutments should be deep enough to allow of this.

[18] _Report on the Revised Estimate, Upper Jhelum Canal._

[19] _Revised Estimate of the Upper Jhelum, Upper Chenab, and Lower Bari Doab Canals._

[20] See also Note on p. 161.

[21] The height of a wave is supposed to be 1·4√(fetch), but this allows nothing for splashing.

[Transcriber’s Note:

Obvious printer errors corrected silently.

Inconsistent spelling and hyphenation are as in the original.]