Lists (None yet)

Login to create lists

Tagged (None yet)

Add Tags

Comments (None yet)

Add New Comment

5 corrections, most recently by lakesneedwater - Show corrections

THE MURRAY BARRAGE.

REPORT BY EXPERTS.

Messrs. T. W. Keele (Principal Engineer of Harbors and Rivers of New South Wales), W. Davidson (Inspector-General of Public Works of Victoria), and A. B. Mon- crieff (Engineer-inChief) have submitted the following joint report to the Govern- ment on the proposed Murray River bar-

rage:

We have the honor to inform you that by your direction we have investigated certain proposals for the construction of a barrage in the vicinity of the mouth of the River Murray, by visiting the sites of the several sections of which it is suggested it shall be composed, the examination of sections and preliminary plans, and the con- sideration of former suggestions of a similar nature. The scheme now to be dealt with has for its immediate object the exclusion of sea water from Lakes Alexandrina and Albert, which it has been urged have suffered much from the influx of salt water during recent years. This subject was enquired into at some length by the Inter State Royal Commission on the River Murray. On page 34 of the Commission's report the fol lowing appears:-"Several very positive statements were made in evidence by residents in the neigh- borhood of Lake Alexandrina of the increasing saltness of the lakes at the Murray mouth. Apart from verbal statements, the evidence of facts is against the hypothesis that there has been any in- crease of saltness in the Murray lakes by reason

of diversions, of water from the river channel. We are of opinion that the proper course to main- tain a supply of fresh water is the construction of a work at the outlet to exclude the sea, so de- signed as to discharge river floods over its crest." The Commission did not suggest any scheme by which this might be accomplished, but at least it has put before the authorities a definite issue. The former suggestions referred to appear to have been made almost entirely in the interests of the River Murray navigation. So far back as 1856 Mr. G. W. Goyder, Surveyor-General, proposed to cut the rock bar in the Mundoo channel, and so concentrate action of river water for scouring a channel through the bar at the river month. In 1857 Mr. George Abernethy proposed by piling to direct tile tidal scour more effectively across the bar; and in 1874 Mr. H. C. Mais recommended the cutting of a canal direct to the sea just be- low Goolwa. In 1876 a breakwater and other works for the establishment of a port inside the bar at the river mouth was recommended by Mr. Hickson at a cost of £1,500,000. Sir John Coode's project in 1879 was for the construction of two breakwaters projecting from the shore and parallel to one another, which, with their contin- gent works, were estimated to cost not less than £2,000,000. It is obvious that by neither of these would the lakes' condition have been altered. But in 1890 a scheme was formulated by a member of this board which, while providing for the main- tenance of facilities for shipping in the river, con- templated more particularly the stoppage of the inflow of sea water to the lakes. It appears to the board that in any obstruction that may be erected against the passage up the river of sea water, the shipping interests, trifling as they are at present below Goolwa, should receive recog- nition, and in the recommendations which are sub- mitted herewith provision therefore will be found.

THE SITE OF THE BARRAGE.

The points in the scheme as placed before the board which demanded early and special con- sideration were those of site. For reasons that need not be dwelt on at any length, it was con- sidered to be impracticable to construct a barrage to exclude tidal effects acrcss the actual mouth of the river; the alternative being in the blocking of the several channels from the lakes which con- verge on the river mouth. In respect of such blocking work on the most important of these the Goolwa channel-two sites were examined, the board deciding that all the elements of safety which were lacking in the lower, are to be ob- tained in the upper one about half a mile below Goolwa township. And it may here be noted that it is proposed to insert in this section of the barrage a lock for the passage of river vessels. The board considered the advisableness of so design- ing or locating the barrage on the eastern side as to exclude the Coorong from the Alex andrina Lake system, and decided that the circumstances of the Murray River volumes to be anticipated would not warrant the inclusion of an evaporating area of 90 square miles additional to that of the lakes for what at best would be an extremely problematical, if any, advantage. The flood waters of the River Murray, before reaching the sea, spread over 284 square miles of'low-lying land, thus forming Lakes Alexandrina and Albert, having an average depth of about 6 ft. The discharge from these lakes is between prominent headlands known as Point Sturt and Point McLeay, and thence through seve- ral channels, which pass between and around low lying islands, and finally conjoin about seven miles south-east of Goolwa, emptying through one open- ing in the sandhills into Encounter Bay. The discharge of fresh water from the River Murray to the sea in the flood of 1890 reached the enormous total of nearly 4,000,000 cubic feet of water per minute, but in a dry season falls off to such an extent that the sea water, meeting no opposing stream, flows in through the channels referred to above, and dif- fuses the salt water over the lakes, the tidal in- fluence being felt at Milang, where the reduced level of low water is 106.50, and is marked as zero on the gauge by which the rise and fall of the lake waters is measured. The datum is 100 ft. below low water at Port Adelaide.

WHAT THE SCHEME IS TO DO.

Further preliminary surveys have been made and tidal observations recorded to form the basis for the preparation of a scheme to meet existing four-fold requirements.

1st. Of shutting out the salt water of the sea from Lakes Alexandrina and Albert and the chan- nels leading therefrom.

2nd. Of retaining fresh water from the river in these lakes and channels approximately up to R.L. 109, or 2' 6" above zero on the Milang gauge.

3rd. Of allowing floodwaters to pass equal in volume to the 1890 flood at least; and,

4th. Of providing for navigation in such a man- ner as to admit river steamers to pass through the barrage to and from the sea.

When the subject was previously considered, the   depletion of the fresh water supply for irriga- tion works in New South Wales and Victoria had not been undertaken, and one part of the proposi- tion was to close the Coorong from tidal action, thus keeping it fresh as long as possible after each heavy flood, but as there is no southern outlet to the Coorong the salt water could never be completely scoured out. In view of the limiting of the quantity of fresh water in dry seasons it is desirable to exclude the Coorong, not from the sea, but from the evaporating area of the lakes, the whole of which it is desired to render, available as a fresh water storage; and this branch of the question is more fully dealt with in the evidence given to the River Murray Inter-State Royal Commission-vide answer to question No. 5,215, in which the reason is given for the neces- sary raising of the level of the stored water to 109.00 R.L. The exclusion of the Coorong from the evaporating area necessitates the closing of five openings, or channels, as shown on the ac- companying plan, through which the sea water at present reaches the lakes, and through which, in addition to the overflow of the adjoining islands, -the floodwaters. overcoming the tidal action, ulti- mately reach Encounter Bay. Owing to the vary- ing physical conditions, the erection of the bar- rage will require individual treatment in regard to each of these openings, care being taken to maintain the uniform reduced level of 109.00 as the top of the crest, which is intended to secure the impounding of the fresh water.

A LOCK.

The most important of these outlets is that known as the Goolwa channel. At the site se- lected by the board and shown on the plan as suitable for the erection of the barrage the width is 2,680 ft. between rather prominent headlands. The reduced level of high water is 109.00, and the rise and fall of the tide is 2 ft. 6 in. The river at this place contains a large deposit of sand and mud, there being two clearly-defined channels of a maximum depth of 20 ft, "one at either side of the stream, divided by a bank, which is covered to a depth of only a few feet at low water. At this place it is proposed to erect a sheet-pile structure, protected with rip-rap; and as this channel provides the most suitable site for a lock to admit of the passage of river steamers, it is suggested that this accommodation should be provided at the northern bank of the river, the dimensions of the said lock to be 150 feet by 50 feet, providing a depth for the passage of boats of, say, 5 feet draught, which, we are given to under- stand, will be sufficient for the vessels to be con-

sidered.

THE OTHER OPENINGS.

The opening between Mundoo and Hindmarsh Islands is the next in importance, being the most direct outlet from the lakes to the sea. The width of this opening at the place chosen for the barrage is 1,900 feet the maximum depth at low water be- ing 4 feet; but in this case, owing to position and the influence of westerly winds, the tide rises to R.L. 110.70, or, considerably higher than at Goolwa. The foundations for the whole width of the river in this instance would be solid rock.

At this place it is proposed to erect a construc- tion of sheet-piling, but the main portion of the structure must be planted in holes drilled into the rock, and wliile R.L. 109.00 must be maintained as the crest for the outflow of fresh water, it will be necessary to provide shutters to keep out the salt water from the lakes to about R.L. 110.70. These shutters have been designed as hinged to the crest of the main structure, so that by simply re- moving the catches the outflow of the fresh water can be secured immediately in case of flood.

The channel known as Boundary Creek is com- paratively insignificant, being only 950 feet wide. The foundations would be on the reef of hard rock. The rise and fall of tide at this place are very similar to those in the Goolwa channel, and it is proposed to put in an earthen wall, pitched with stone to the proper grades, so that the sea water may be excluded and the fresh water retained to R.L. 109.00 by a simple structure.

The openings between Tauwitcherie and Ewe Is- lands are shallow throughout, and should be treat- ed on the same principle as at the Goolwa channel. The longest opening is the channel between Tau- witcherie Island and Pelican Point. In this open- ing there is an average depth at low water of 1 ft., and the total width amounts to about 8,500 ft. For a considerable portion of its length the foundations would be on the reef of limestone, which crosses Boundary Creek and Mundoo Chan- nel, but for the remaining distance the rock is absent and the foundations are of a treacherous character. The treatment proposed for this open- ing is the erection of a composite structure, partly as in the Goolwa channel and partly as in Boun- dary Creek, admitting of the overflow of the fresh water at R.L. 109.00.

BARRAGES. NOT TO BE WATERTIGHT.

The abovenamed works, if erected, will keep the sea water from entering the lakes, and the perma- nent crest of each portion of the barrage being R.L. 109.00, will retain the fresh water to the required level, equal to a depth of 2' 0" above zero, on the Milang gauge. It should be understood that the proposed structures form a barrage only, and not an absolutely watertight dam, which the board considers unnecessary, and which would be extravagantly costly. The placing of the barrage across the outlets will form occasionally submerged weirs, and, therefore, lead to the raising of the level of floodwaters to an appreciable extent. Cal- culations which have been made show that the total length of outlets available for floodwaters of the River Murray, after the completion of these works, would be about 35,000 ft., or over six miles; but this provides not only for utilising the over- flow weirs, but for the floodwaters to rise over Pelican Point, Tauwitcherie, and Ewe Islands, and part of Mundoo Island, as it did during the flood periods of 1871 and 1890.

SOME STARTLING RESULTS EXPECTED.

The carrying out of such a work as that de-

scribed above may. bring about some rather start-

ling results, which, apart from the strictly engi-   neering enquiry, may be referred to for the infor- mation and consideration of the Commissioner. The first and most apparent of these re- sults will be the permanent raising of the level of the fresh water over lands closely adjoining the lakes and outlet channels, always supposing fresh;

water to be available to supply evaporation; but, it is stated that the damage to property due to this cause would not be serious. The influence of so raising the water would probably be felt as far up the main river as Wellington, but in periods of extreme flood, based upon information obtain- able for the year 1890, there would be a temporary submergence of land, as approximately indicated on a plan which the Engmeer-in-Chief will pre- pare to accompany this report. The only possible arrangement lor preventing the submergence of this additional land would be the erection of very,

costly failing weirs which in periods of flood   would leave all the existing channels uninter-   rupted, and the board has to express the opinion   that the additional expense would in no sense be justifiable. Another point of view to which we would call attention is the fact that the low-ly- ing flat islands will be more frequently used as a by-wash then heretofore. These islands are com-

posed of clay, covered for the most part with samphire, but having occasional pickings of good grass for sheep. The tenure of these islands is now leased from the Crown, and the value of these leases would undoubtedly be decreased; but this is a question which can more properly be dealt with by the Surveyor-General. The construction of a weir or dam in the tidal compart- ment of a river has invariably been found to re- sult in the shoaling not only of the portion of the river immediately above the dam, but also be- low it. In the case under consideration, how- ever, the area of the lakes is so immense that shoaling above the barrage would not be appre- ciable for a very long period of time. Below the weirs the effect would be more quickly notice- able, especially as the sandhills which separate the river from the sea below the proposed site for

the barrage in the Goolwa channel are drifting   badly. During dry periods, when there would be little or no discharge over the weirs, the Murray,

mouth would become so shoal as to be unnavi-   gable; indeed, were it not for the long stretch of water in the Coorong which would enable the tide to propagate itself up a distance of many miles, the Murray mouth after the barrage is constructed might be expected to shoal up completely.

A DISQUIETING POSSIBILITY.

The most important of the possible results of the erection of the barrage is the effect which may be produced under possible conditions in the lakes and channels in which it is sought to en- sure a continuously fresh supply of water. At present, all the channels being open, the water in

the lakes is fresh when the supply is sufficient' to overcome tidal action, and when the supply of

fresh water falls off the sea water takes its place.  

Supposing the barrage to have been erected, if     the supply of fresh water is not sufficient to pro- vide for evaporation, the surface level of the lake» will fall below that which is now maintained by the influx of the sea, large areas of the foreshore may be left dry, and the remaining submerged areas become little better than salt swamps. If the supply of water from the upper reaches of

the river is not sufficient to keep the lakes reason-   ably full, the effect of the erection of the barrage

as described above would be serious injury to the ' condition of the lakes instead of being an im- provement.

In view of the experience of the recent drought, there can be no doubt that if the barrage referred to had been erected five years ago it would have required nearly the whole available flow of the river to keep the lakes sweet and open for naviga- tion. In this relation the physical feature must

not be overlooked, that the subsoil water of the   lakes is itself salt and that soakage waters from the banks of the river have been shown to be off

such a character that' continued evaporation would   render the resulting supply unfit even for stock.

Vide evidence given before the River Murray Inter-     State Royal Commission, page 21.  

It is submitted, therefore, that these possible re  

suits require the most careful consideration, and the establishing of a definite policy in regard to   a regular supply of fresh water from the river be-

fore such work as that of erecting the simple bar-   rage described above should be undertaken.  

ESTIMATES OF COST.       The estimated cost of the erection of a barrage

at the mouth of the River Murray, as described in

the preceding report, forming a composite struc-  

ture of timber sheet-piling and earthen-pitched     embankment, is £113,300, which may, however,    

be modified as a result of closer investigation,    

after further surveys and borings. Such a struc- ture would, doubtless, entail expense in mainten-

ance after a series of years.      

Estimated cost of barrage and lock, £98,130.

Engineering, supervision, &c. 5 per cent, £4,906    

= £103.036.

Contingencies, 10 per cent, £10,303.         Total, £113,339; say, £113,300.      

Zoom

plus
thumb
minus
left
thumb
right
up
thumb
down