The Pamphlet Collection of Sir Robert Stout: Volume 26
Mr. George M. Barr's Report
Mr. George M. Barr's Report.
13th january, 1874.
Robert Gillies, Esq.
Sir—In response to the desire conveyed to me by one of the members of the Committee appointed to circulate information with reference to the deepening of the Upper Harbour, Dunedin, and of which you are Chairman, that I should, in common with other members of the Engineering profession in Dunedin, state my opinion with regard to the scheme suggested by the late Mr. Balfour for improving the navigation, I now beg to submit the following remarks thereupon, and upon the general question under discussion, regretting at the same time that pressure of business has somewhat delayed this matter, and has also precluded my going so much into detail as the importance of the subject deserves.
It is quite unnecessary to discuss the general principles in Harbour improvements laid down by Mr. Balfour, as they have been established by long experience, and are such as are acted upon by the leading engineers in their most important and successful works; therefore all that is called for at present is a few practical considerations upon this particular case.
|1st.||Are their main features such as, if carried into practice, would be likely to counteract the tendency of the Upper Harbour to shoal?|
|2nd.||Is Mr. Balfour's scheme practicable?|
|3rd.||Can it be executed within a cost not too great in proportion to the benefits to be derived? and|
|4th.||If not, can any modification of his own larger scheme, either as proposed by himself or in any other manner, be substituted for it, giving the main advantages sought at an expenditure within the means of whatever public body may undertake these improvements.|
First, then, as regards the relation between Mr. Balfour's main scheme and the form of the Upper Harbour and its banks. The wide extent of the Harbour from the Islands upwards, compared to the narrow passages between these, has doubtless been the original cause of the shoaling of that part; for immediately the tidal water has got through the narrow passages the slacking of the current has allowed the deposit of such solid material as may have been carried by the water from the Lower Harbour; while the gentle nature of the outflowing tide, from the same wide dispersion and unassisted by any fresh water-flow of any consequence, has been insufficient to carry to any distance the heavier portions of the matter derived from the sides of the Bay, which has thus assisted in the general shoaling. The other evil in this part of the Harbour, much dwelt upon by Mr. Balfour—viz., the bifurcation of the channel above Sawyer's Bay—is a result of the joint action of the widening of the tidal waters and the configuration of the shores upon both sides. With the inflowing tide, the action of the projecting point above Sawyer's Bay has induced a deflection of the current towards the other side, and in the direction of the cross channel, but probably in the direction of the beach upon the eastern side, from Bare Head southward, has been the chief cause in the original formation and subsequent maintenance of that channel, in so far as it has directed the force of the outflowing current, and prevented the deposit of material in that line as upon the contiguous banks. Possibly also the similarity between the inclination of the ground upon the two sides of the Harbour has had some effect in the maintenance of the two channels, by preventing the operation of those laws according to which rivers creep along under the steepest sides of a valley, though this being an arm of the sea, and not a fresh water current, such could have but little more than a theoretical effect. Viewing Mr. Balfour's proposal for a central training wall from the Islands up to opposite Dunedin in connection with these two principal evils in our Harbour, I think there can be no doubt, that if carried out, the result would be highly beneficial, for the tide would be confined within such limits as to secure that its effect would be powerful as a scouring agency, while the complete blocking up of the cross channel would remove the other objectionable feature of the present condition of the Harbour.
2nd. As regards now the practicability of Mr. Balfour's scheme. I am of opinion that, given unlimited funds, it is not impracticable; but the cost set down by him I consider inadequate for the purpose; and, indeed, throughout the whole course of his Report, he appears to under-estimate the difficulty which would be experienced in completelypage break page 15
blocking up the channel and forming a watertight wall to restrain and direct so large a body of water as requires to be dealt with. In the following portion I shall submit estimate of this scheme for the same length of wall, but with these differences, that I would not at first attempt the construction of the wall either in its length or at its crossings of the channels of a watertight nature, believing that the same results, in the main, may be attained by one otherwise made, while the risk in execution would be much diminished. Instead of the wall with clay hearting and rubble facing, I would suggest that the work be commenced with a simple rubble wall, the stones being pitched in and allowed to form their own slope and bed; care being taken that they are of such sizes as to be of sufficient weight to resist the force of the current at the different places; heavy stones being used across channels, and lighter ones upon the banks and shallow parts. It is true that some of the water would pass laterally through the wall, and thus be of less effect for scouring in the channel; but on the other hand the ease of construction would, I consider, more than compensate for the loss of the comparatively small quantity of water at first. Probably, the result, before long, would be the silting up of these interstices between the stones, and a watertight wall in all parts, except where the current was particularly strong, obtained at less risk than with the works contemplated by Mr. Balfour. Very satisfactory results have been obtained upon the Clyde, from the River Cart downwards, with rubble walls; but, as in this case, watertight walls could ultimately be constructed at but little comparatively additional cost. I would recommend these at those parts where the strongest scour would be required—viz., where dredging would be necessary in order to attain the depth of water required. At those parts it would be advisable, shortly after the deposit of the rubble wall, and when the current has been so far controlled, to deposit waterproof material upon the Peninsula or eastern side of the wall; and experience of the nature of the material in the bed of the Harbour has shown me that no better than it could be obtained for this purpose. The cheapest and best mode of deposit would be that already employed in the dredging operations below Black Jack's Point and other parts—viz., by the use of the discharging apparatus which has been proved to possess undoubted advantages over the system of punting. Hitherto the greatest length the material has been carried by this means in our Harbour has been a little over one hundred feet from the buckets of the dredge, but for the purposes of these works a greater length of carriage would be necessary, but still not beyond the power of similar, though larger, machinery to effect. The extra cost thus involved has been allowed for in the following estimates. It would not be prudent to deposit in that situation any more than absolutely necessary to prevent the lateral flow of the water from one side of the wall to the other, as all the space thus taken up between high and low water is so much deducted from the tidal water coming into the Upper Harbour; but even bearing this in view, the most economical and best mode of construction would simply be to allow the material to take its natural slope about one vertical to four or five horizontal, in which position no facing of stone on the eastern side would be required to prevent its being washed away. This has already been found so in different parts of the Harbour, except with the fine silt and soft mud in the neighbourhood of Dunedin, the mixture of shells and clay forming the bulk of the bed, possessing a tenacity and solidifying power which enables it to consolidate within a very short time after deposit. Moreover, only a very faint current would wash along this part, as the main part would keep nearer the Peninsula side. It may be urged against this, that considerable bodies of water would find their way through those parts not rendered watertight, and in the case of the inflowing tide travel in the direction of the head of the Bay, meeting the advancing tide also rising upon the Peninsula channel, and thus to a certain extent retard the force which we wish to encourage. This would be true to a certain extent, but not to a degree such as could materially affect the success of the scheme, but would be, roughly stated, proportional to the relations between the interstices of the rubble and the area allowed for the free flow along the new channel as formed by the training walls. The whole practical effect would be, that probably the deposit of material, which invariably results at the head of any bay or current, instead of being close to the back of the wall at the Islands, would be at some distance nearer Dunedin, where the two currents met. Even this would disappear as the rubble got silted up, as probable; but, at the worst, if this were found a practical evil, and spontaneous silting did not lessen it year by year, it would be much easier to form these walls watertight after the body of the water was directed by the rubble, than it would be at the first off going to carry out works of that strong and watertight nature contemplated by Mr. Balfour. In dealing with water, abrupt modes of treatment should never be resorted to, unless a more gradual course be impossible; for in the latter case the forces we seek to control can often be utilised for the purpose of constructing our works; while in the former, the same forces, intensified by our interference, would possess more than their natural intensity.
3rd. Concerning the cost, I would submit the following, as an estimate of Mr. Balfour's length and locality of walls in his larger scheme, but with mode of construction as shown in Sections A B and C.page 16
In placing the above estimate before you, it will be necessary to remark upon several of the items, and more especially upon the dredging and reclamation, taken both separately and conjointly. In Mr. Balfour's Estimate No. I., in his Report of 7th September, 1864, no account is taken of the dredging which would be necessary in order to obtain the full advantages of his scheme. But in his final Report, before leaving the Province (15th November, 1865), he has stated both a quantity and expenditure upon it, which would probably be required before a channel twenty-one feet deep and one hundred and fifty feet wide could be obtained to Dunedin. As regards his quantity, I find that calculations I have made coincide very closely with his; but in price, I consider that he is in excess, whether it be viewed in connection with the cost at which dredging has already been done in this Harbour, or with the cost at which it is being done in the Clyde and Tyne in Britain. From my own experience in connection with dredging operations here, I can state that hitherto the price has ranged from 3.1d. per cubic yard up to 11d., according to depth, nature of material, and distance to deposit ground. In one case, for a small quantity raised and deposited under most disadvantageous circumstances, the cost per yard was about three times the highest price stated above, but this was so exceptional as to be altogether out of the question in considering an average.
- Clyde.—9 3-100d. per cubic yard. Average over all the dredging, some of it deposited on land, and some carried about 40 miles to Loch Long.
- Tyne.—5 7-10d. per cubic yard. Carried on an average seven miles and dropped in the sea.
- Dee.—9 64-100d. per cubic yard. This was deposited by crane at the back of a new Quay wall.
The great disparity between the rates upon the Clyde and Tyne, where the most extensive operations are carried on, arises chiefly from the fact that on the former river considerable quantities were deposited on land at the cost of 1/ per cubic yard for deposit alone, as against 3d. when carried to Loch Long; whereas on the Tyne, the rate quoted applies only to the sea-borne material, scarcely any being placed upon the land. Where it has been done, however, the cost, including both dredging and deposit, has been 11d. per cubic yard. On review of these figures, then, both as regards our own Harbour and the ones in Britain, I consider the rate of ½ per cubic yard as a fair average at which we may expect the dredging and deposit to be effected here in the main channel, and 1/ per yard along the wall A A A A. In previous part of this Report I have page 17 indicated how a portion of the dredged material would be deposited behind the rubble wall, but the main bulk of it I would recommend being carried to those parts of the beach which Mr. Balfour has shown as proper to be reclaimed, and there dropped out of the punts when the tide was suitable. This mode of deposit would be cheaper than carrying to sea, the length of haulage being very much shorter; while it would possess the main advantage of that mode—viz., the getting quit of it without the necessity of handling, if punts of construction similar to those in use upon our own Harbour for the last four years were adopted. This would all be so much gain towards the reclamation of that area, though both for sanitary and financial reasons it might be better to complete it with material derived from the land. Of the total quantity of dredging, probably about 70,000 cubic yards would be laid behind the rubble training wall, leaving 1,012,000 cubic yards available for reclamation, which, to an average depth of 3 feet, would cover 209 acres; and supposing this area covered to a further depth of 3 feet with material from the land at 1/9 per cubic yard, the 209 acres would be rendered fit for building, &c., at a cost of £88,550, in addition to the cost above set down for dredging. Probably this is too large an area for the reclamation to be undertaken at once, so that it might be well to undertake only a smaller portion, as it would be unadvisable, in a sanitary view, partly to reclaim same and leave it lying alternately dry and covered by the tide. If the dredged material were used to fill up the ground to its permanent level, of course the cost of deposit would be somewhat greater, and the question would arise as to whether to do so or use material from the kind—a question which can only be dealt with when the actual facts of the case come to be decided upon, and which may be presently omitted from a Report of this general nature, it being sufficient merely to indicate that the dredging should be made to assist the reclamation as far as possible.
The cost of plant necessary to execute the dredging in, say five years, would be about £45,000—which includes three dredges, one tug, fifteen punts, and the self-acting discharging apparatus. Probably, at the completion of the works, one dredge and a few punts could be retained, and the remainder sold and realise half the above.
In the calculation of the quantities of materials to be dredged, it has been assumed that the high water level, after these improvements are accomplished, will be the same as at present, though, probably, it would be higher to an appreciable extent, though how much of course cannot at present be more than conjectured. From levels recently taken, I find that the tide rises about three feet higher on the Ocean Beach than at the head of the Harbour; and it is not unreasonable to expect that of this quantity some of it would be gained by the improvements contemplated, though looking at the great obstructions which the Islands are to the rising tide, we cannot probably expect to gain more than from one to two feet. I do not think it would be unreasonable to look forward to this, provided the location of the wall be judiciously chosen, and an easy run made for the tide, so far as that could be done at moderate cost; but it would be improper to count upon this in any estimates formed at present, founded as these necessarily are upon the meagre facts presently known as to the figure of the bottom of the Harbour, and the slope of the high and low water lines. Making no allowance in the meantime for the probable gain will keep these estimates upon the safe side in this particular.
4. The fourth inquiry now remains to be investigated—viz., as to whether any modification of Mr. Balfour's principal scheme can be suggested which would secure the same, or nearly the same, advantages at less cost. His own suggestion, as you are aware, was the shortening of the training wall, so that it would be only about half the length of the one in his original suggestion, his chief reason for this evidently being to meet the objection which would be urged against his scheme of cutting off the Peninsula settlers from direct communication with Port Chalmers. I am not inclined to give the same weight to that objection, under the present circumstances, as he was when he propounded his scheme, though in the face of the considerable expense which the larger scheme would involve, I would suggest for your consideration the construction of a wall from Bare Head of Captain Stoke's chart up to opposite the centre of Dunedin, an estimate for which you will find below. It may be asked, if my reason for this suggestion be an economical one, why not adopt Mr. Balfour's alternative one, which, being shorter, would be correspondingly cheaper; but, in reply, I think I may urge that there are grave objections to this. Principally these are, that its direction being nearly parallel to the general direction of the channel between Green Point and Dunedin, would probably lead the current across the bank towards the Peninsula side, and possibly scour it to such an extent as to form a channel corresponding in its nature and effects to that already referred to further down the Harbour; neither would it block up that same channel which Mr. Balfour recognises as one of the main evils in our Harbour; and, lastly, it would not improve the scour at some parts of the dredged channel where such improvement would be most necessary for its maintenance. In the position I suggest no risk of the formation of a new channel would be likely, seeing that the current would naturally cling to the side opposite the bank, on account of its concavity, while the present one would be crossed, and, therefore, stopped by the wall at Bare Head.page 18
The estimate is as follows :—
The saving effected by this would be about £12,470; but as it is upon the cost of the Training Wall alone, the Circular Wall and the Dredging being the same, it is really more than at first appears in relation to the whole sum. In an engineering point, too, this possesses the advantage of offering no impediment to the inward flow of the tide at the Islands, as the long wall would to a certain amount, by being directly across its direction immediately the water gets inside.
In either of these schemes, it would be well that these works should be carried out as a whole, though if it were an object to save a few thousands in first cost, probably only portion of the Circular Wail and portion of the Dredging in front might be undertaken in the meantime. By this a sum of possibly £10,000 to £12,000 in works and plant might be saved.
I cannot close these remarks upon Harbour Improvements without reference to the subject of the disposal of the sewage of Dunedin, the more so as this subject must be decided one way or another within a few years, unless the health of the inhabitants is to suffer to a probably alarming extent. At this stage, however, I am more in a position to urge what not to do with the refuse, than what to do with it, which, after the expenditure of much money and years of experiment in Britain, is just the position that scores of municipal corporations find themselves in with regard to their sewage. The only general and indubitable conclusion that has been come to is not to discharge it in its unpurified state into any harbour or river, and that is the only extent to which I would feel justified in giving a decided opinion at present with regard to our own case; but as the subject of reclamation of some of the tidal beaches in the neighbourhood is treated of in connection with these Harbour Improvements, I shall only just throw out a few hints as to how this part of the proposed improvements may be made to fit into the system of sewage disposal which has been found most successful in the older towns of Britain. A little more than three hundred years ago the system of utilising the sewage of Edinburgh, by irrigating lands in the neighbourhood, was commenced, and the latest findings of the Commissioners appointed to inquire into the disposal of the Sewage of Towns and into the Pollution of Rivers, are to the effect that no other system is to be preferred to it, provided that the crops grown, the areas irrigated, and the distribution of the sewage are adapted to each other.
The relation then between the Harbour Improvements and the sewage, which I would at this stage wish to indicate is, that portion of the reclamation area either about Pelichet Bay or towards Anderson's Bay should be prepared for the receipt of the sewage, as it would cost scarcely any more to do so than to prepare it for any other purpose, the only difference being that it would be better that say three feet of the ground should be made up of a material more porous than the clay taken from the bed of the Harbour, and for this purpose the sand in the neighbourhood of the Ocean Beach or Anderson's Bay would be admirably adapted.page 19
If we take the case of Merthyr Tydvil, where the system of intermittent filtration and irrigation has been tried by the recommendation of the Commissioners on the Pollution of Rivers, an area of about ten acres would be sufficient for the present population of Dunedin. We have seen above that material more than sufficient for this will be taken from the Harbour under any circumstances, so that only the cost of the sand deposit would be charged against the disposal of the sewage. No fear need be entertained by the residents in the neighbourhood of the irrigation ground that such a scheme would prove a nuisance, as, under proper arrangements, not the slightest unpleasant odour could be detected; and indeed, in the case of Croydon, the localities in the closest contiguity to the irrrigation are those in which the smallest death-rate occurs.
After the above remarks, it is scarcely necessary to point out that the three most important portions of these Public Works—viz., Harbour Improvement, Reclamation, and Sewage—should be carried out as a whole, as they are so intimately linked together and fit into each other as parts of the same system.
I have the honour to bo, Sir,
Your obedient servant,
George M. Barr, C.E.