THE DRAINAGE OF THE FLAT.

Otago Daily Times, Issue 4723, 7 April 1877, Page 1 (Supplement)

THE DRAINAGE OF THE FLAT.

The following is Messrs Barr and Oliver's report upon this subject :—

To Messrs Bridgman and M'lntyre. In accordance with request from you, we have made a series of surveys and observations with, t'ue view of ascertaining the best means of preventing the extensive floods which from time to time are the cause of much damage, inconvenience, and certain amount of risk, to the inhabitants living upon the ground between Caver sham and Porbury Range and the head of Duuedin Harbour, and have now the honour to submit the following report for the consideration of the Conference of Suburban Municipalities repressnted by you.

We find that the chief contributors to the floods, and the causes which must be dealt with, in the first instance, are the Caversliam, Glen or Mornington, and Corstoiphine or Sidey's Creeks, and, in a minor degree, a quantity of water which is derived from many rivulets rather thau in one stn am from the loose ground above the Caversham to Forbury Eoad at its junction with the Macandrew Eoad. A smaller body of water is brought down from the gully near the Immigration Barracks, but the dealing with this, though it will form part of the detail in carrying out a general scheme of drainage, will not, in the meantime, affect the decision upon which is the best. Upon the south-east or Musselburgh por. tion of the Flat some water finds its way from the high ground between it and Anderson's Bay, but the avea draining in this direction is comparatively small, while the ground i* of such a nature both as regards slope and character of soil as to prevent the rapid ran off which is ao marked a feature in the gathering of the waters from the heights about Mornington and Oaversham. At any rate, its distance from the other water courses and its contiguity to the Har bour, decide at once that its proper treatment will be separate from them.

The areas from which the separate creeks are derived are approximately as follow :— Gavorsham, 300 acres ; The Glen, 2-15 acres ; Corstorphine, 150 acres ; head of Macandrew road, about 70 acres.

Tho area draining down by the Immigra^ tion Barracks is included with C.iversham.

The area of the flat upon which these waters spread over amounts to about 730 awes, or rather loss than the higher ground whoso drninage it receives. The chief part of the high land is stepp, and therefore favourable for running oft' the water quickly, and in considerable streams, to the flat ; while its escape therefrom is delayed partly by its uivii level, or altuosb level, surface, and small elevation above the tide water. The present provision for the di cnarge of the waters is by ditches alongside the roads leading to the Harbour, haviug their outlets at or near low water, si that the discharge is almost stopped at high water, and durine; the greater part of the time of rise and fall of the tide the outflow is slow and inconsiderable. Tt thus happeno that the ditches are too small to store the water until it runs off, and there is a consequent flooding of streets and private property,

Tho heights of the different creeks at the' highest pjuts which their waters could be lifted above high water spring tides, and the distances of the same are the following : — HeiL'ht above h.w. Diatanc". liar- Uoo.m II r- Ocean li i«ic. htauh. hour. Bbacli. it. ft. ihs. clis. Civevsham Creek .. 27.70 24 00 via Main Head and Southern Trunk Ruilivav ~ 0fi.35 ICO via C inrill Read.. lOiai The G!en .. .. 3.44 001 via Soutli c m Trunk llailway 50 via Church Luids and Cargill Road G3 Corfctorphiue .. 10.20 7.4 SI 50 7!) Water at head of | M&Cfiudrcw Road 5.12 5.32 Si 00 53 From the heights above the high water at the Ocean Beach it would be necessary to deduct 2ft., for it would not be advisable to place the outlet so low as ordinary high water of spring tides, exposed, as it would be, to heavy storms and shifting sand. Even making this allowance, it will' be seen that it would be quite practicable, so far as levels are concerned, to take the Cavershain, Corstorphine, and Macandrew Road creeks to the Ocean Beach, and allow them there to have a constant discharge. Indeed, from the fact that nothing would be lost by a rapid drop from the Corstorphine Creek to the ground as there is in carrying it to the harbour, the gradient at which a channel or covered conduit could be formed to the Ocean Beach would be more favourable than the one leading to the harbour. This favourable feature, however, is not sufficient to counterbalance certain difficulties in construction which would raise the cost to such an amount as to be its tho meantime at least prohibitory. The narrow width of the road

between the creeks and the Ocean Beach would prevent an open channel beiug placed upon its aiea, while the loose nature of the ground upon the high side and private rights upon tho lower would preclude its being made upon either side except at great cost. Tho only alternative by this route would be to take it in pipes or built conduit, which would not cost less than £5400 for the accommodation of Caversham, Corstorphine, and Macandrew Road creeks. With the possibility of cheaper mocks of disposal of tha flood waters, this one may be at once excluded.

It coa3oi now to bo considered whether this water will be carried to the Harbour by channels discharging at high or low water— either being practicable—and, in considering this, we must view the waters dtrived from the high grounds in conjunction with the rain which ialis upon the Flat, and which of necessity must be discharged under high water level—scarcely any of the Flat being high enough, t) jnrrnib of s.-.iything els<\ The advantage of discharging at the low level >vould be simply that the one system of channels or drai-s would serve for both waters; but against this may bs set that as already stated with regard tc the existing drainage—the outlets would bs blocked during a great pivt of the day by the tide, and an amount of space required to be provided in the chanusls or sewors to give room for storage of a large quantity of water thus acoumulaliug. To secure the single advantage jnstrefcired to, it would bo necessary to limit the bjitoin of tho channel to a fall of about one foot pjr mile, which means a, slow discharge, an-1 the necessity tor uniform and well maintained channels with wide beds. So far as merely passing off the waters of the high grounds and the flat, a channel, or channels, equivalent to a total capacity of 13 feet wide by 3 feet deep would be sufficient constantly flowing, but to give ample storage during high, water a capacity more than double this would be required. The mere excavation would not be costly, but to this would have to be added the price ef land necessary to be acquired from private parties, the road lines being too narrow to allow any of their breadths being devoted permanently to the purpose of deep open drains, even though distributed over throe of them. Unless taken over the flat in several streams, it is evident also that the sole advantage of a lowlevel drain would be lost to the bulk of the residents who were placed at any distance away, the cost of the cross connections beiug too great, and in many esses the levels un° suitable. But to all these, we must add this consideration, of 20 much importance to the public health, that these drains would unavoidably become the receptacles of all kinds of filtb, while, under ordinary circumstauces, the water would be too scanty and slowflowing to carry it away, and only, plentiful enough tc promote putrescence. This could be only slightly mitigated, not cured, by the admission of the tide. There would be only one remedy for this, aud that wouid be the conversion of these open ditches into covered sewers, so that in a very sho>:t time after their construction, the enormous expenditure for about three to six miles of large sewers would have to be undertaken by the district. The chief part of the size of these sewers would be due to tbe water coming from the upper lands, and to the necessity for storage during the high times of the tide. As this water would be devoid oC anything injurious to health, it could, without h?.rm to anyone, be carried along in open chauneh to the harbour ; aud the levels being favourable, it could be discharged at high water, whereby the underground drainage would be greatly cheapened. This water could be turned into the underground conduits, if such were wanted, for the purpose of flushing at any time. Two facts would contribute towards their reduction—first, that the rain water would gather but slowly to them from the flat ground ; and, secondly, that their inclination could be greater than that of a large sewer. In view of the above facts, and the general circumstances of the case, we would recommend the separate treatment of the high and the low rain waters, and with the latter associate the house drainage. Caversham Creek, so far as distance and levels are concerned, may be taken either to join the (Jlen or the Corstorphine waters before entering the harbour.

To join the former of these a route, either by the Main South road or through private property, would be necessary, the latter being cheaper, so far as cost of construction is concerned ; but this advantage would be more than counterbalanced by the large sums which would have to be paid to owners of ground. By the main rosd, a covered culvert cr sewer 34A chains long and an open channel 5 chains long would be requisite, bringing the cost to the junction with the Glen Creek alone up to £2700. This large sum is due partly to the cost inseparable from all large underground culverts, and partly to the excejitionaliy deep cuttings, which would be necessary along the road line. It is true that some much needed road improvements might be effected along with Ihe3e works ; but, if so, the total sum which would hav3 to be expended at once by tho district would probably be felt to be a serious burden. Notwithstanding the shorter distance then, we reject the route via. the Glen Crock in favour of the one vi;t the Corstorp'.iino. We have shown upon the accompanying tracing the lines of drains we would recommend for tlie high and low waters, which are :— The Caversham Creek to join the Covstorphine at the head of Cargill road, and that the two be carried to high water at the Harbour by the water ehanuels on each side. The water at Ihe head of Macandrew road by its water channels to high water at the Harbour. The Glen water preferably by the Cbu'ch lanrfs, Southern Trunk Railway, and Church lauds, section 10, block VII , Town district; but, if not, by streets in Chnroh lairis to Cargill road, and along it to Harbour. For house, and the drainage of the Flat, pipes to be put in each of the present ditches, and made to discharge at low water. We show upon tracing the width and depth requisite for the water chaumls to carry off the water of a ilood such as that of tho sth and 6th of last month, ami the general formation of the roads. The fall per mile would be from ISin. to 2ft. Gin., according to tho ground, care being taken to suit that as much as possible. It will be observed that even with a much greater flood than that referred to private property would etill be protected by the elevation of the back of the footpaths, and even though the water spread over the whole width of the road, which could not happen except with floods altogether greater than have yet been experienced, the slow current would not cause any injury to the surface. • Silt tanks would be necessary at the bottom of the fall on to the flat at the top of Cargill •Road, and also at the outfall to the Harbour, but tl:eie would not be expensive.

Each 12in. pipe under the footpaths would allow for the discharge of the setvage of a popnlation of 750 at 30 gallons p'ir day, or 3000 in all, still leaving capacity sufficient for the conveyance of flood water as quickly-ag

it could find its way to the pipes from the ground in the locality of Cargill and Macandrew roads.

We estimate the works as follows :— Open channel and culverts to carry the Caversham Creek to join the Corstor>ilune £130 Conveyance of the two creeks to Harbour— Doubly channel, 3ft by lft, 5 chains 45 do do as in sketch, 70 do 830 bt.-eet crossings, culverts, and raud tanks 150

Making up roadway and footpaths 70 chains .. I'ipes in present ditches, 9in. and 12in., 152 chains

Total for Cavcrsham and Corstorphino creeks and flat ground in neighbowliooq of Cargill rojci ".. ..£2307 If the Glen creek be taken to join this near the Government Workshops the channels would be wider—for which add over a length, 10 chains .. .. ~ .. .. ~ 120 Conveyance of Glen creek from the culvert under Southern Trunk Railway to Cargill road 24 chains Xl 6 Total for Caversham, Corstorphine, and Glen crooks,with pipes for drainage, of flat ground in neighbourhood of Cargill road .. ..£2513 Macundrew roud : Double channel, S2 chains .. .. £402 Silt-tanks and street crossings .. 120 — 612 Footpath and road formation 123 9in. and 12in. pipes under footpaths .. .. 1025 Total £4303 The above sum provides, as you will observe, for joining the Glen creek with the Caversham and Corstorphine one near the harbour, while in our preceding remarks we have indicated that it would be better to carry it by a more direct route to the harbour, via the railway. We would prefer keeping it separate from the others if possible, because if it is joined to them, it throws the whole waters on to Cargill road, which means either enlarged channels or greater fall, both of which are objectionable. At the same time the shorter route is more expensive, and also open to this objection, that in order to provide for the drainage of the railway formation, the channel would either have to discharge under high water, or if at j it, then a rubble drain would have to be placed immediately under. The cost under these circumstances would be £450, or £114 more than the above.

It will be observed that the above estimate includes certain works which, will be necessary in road and footpath formation, and properly to be charged thereto. We have endeavoured to describe in the above report a scheme which will meet uresent requirements within a moderate expenditure, aud at the same time be capable of development and extension as the needs of the population increase.

The only parts which will require enlargement are the underground draius, but as you will perceive from the figures we give above, these are proposed to be of such a size as will meet tbe wants of a largely increased population. When the time does como for increasing these, extra pipes laid alongside, without disturbing the original ones, will give all that is required, and by th-au devoting one set to the rain watar and another to house drainage, tbe advantages of the separate system of treatment will be secured. On the other hand, should it ever be reEolved to carry the united sewage of Dunelm and suburbs to Ocean Beach instead of to the harbour, which is not unlikely when the amount of it is found to creafce'a nuisance upon the shores in the neighbourhood of the town, the pipss now laid can still carry the rain water as proposed, and the new ones be laid to suit the general and extended scheme.—We have, &c, B,vßr, & Oliver, C.E.'s. Dunedin, 24th Ma-xh, 1877.