ARAMOHO DRAINAGE.

Wanganui Herald, Volume XLVII, Issue 13701, 7 June 1912, Page 8

ARAMOHO DRAINAGE.

MR. MANSFIELD'S SCHEME. An Extended Report. At the Wanganui Borough Council meeting on Tuesday evening, Mr W. T. Mansfield, Borough Engineer, presented a report in extension of his previous report on a drainage scheme for Aramoho. It will be remembered that the estimated cost of the scheme drawn up by the late Mr Russell was 111722, while the estimated cost of Mr Mansfield’s scheme is J!7840. The difference of .£6123 iT produced, Mr Mansfield says, mainly by three factors:- —01 Greater depths, (2) pipes of lareer 'diameter, and (3) concrete under pipes.' ”

Ur Mansfield goes on to say; Thefe clcrabn(h arise wholly in consequence of a difference of opinion between my views as to what is reasonable and proper as a scheme of sewerage for the area in question and that which has been so considered in the scheme upon which the loan was raised. The estimated total cost of the complete scheme was put at £8540, and it was computed that the scheme involved the use of the following pipe goods:—l2-inch, 847 pipes; 9-inch, 2220 pipes; 6-inch, 17,094 pipes. It will thus be seen that the scheme is mainly a 6-inch one.

The Engineer goes on to say that any serious variation from the scheme as designed moans, in pipes alone, apart from any other added cost, a very considerable increase in the estimates. The distances to he covered by the several pipe lines dealt with in this report aggregate 11,564 lineal feet. He then gives a very comprehensive set of tables, showing in detail the variations proposed, as also the works contemplated in the scheme pul before the ratepayers. The scheme already approved commences with a 12-inch pipe at lift s(in below the level of River Bank Road at the intersection of Russell Street, while his (Mr Mansfield's) suggested variation proposes an 18-inch pipe at the same point at a depth of 20ft 3in. The tables show the following in Mr Mansfield’s scheme; (1) That the length of the lines has been added to,

(2) That the pipe’s sizes have been considerably increased, (3) That the depths are much greater, and

(4) That the discharging capacities have been augmented.

The Engineer continues; —As to (1), it will be further seen (i.e., from the tables) that the added length is produced by the pipe line along the Kaikokopu Road, counterbalanced in a small degree by the omission of Gurr Street. That portion of the Kaikokopu Road sewer between Ballance .Street and Murray Street proposed by me is at once necessary in order to effectively reach Murray Street, and provide for probable developments along the Kaikokopu Road, inasmuch as the existing Kaikokopu Road sewer is wholly inefficient above Reliance Street, for the reason that at that point it is too shallow to be capable of effective extension. Had its grade been flattened it could have been by reason of its size coupled with the difference of level without impairing the “desired” efficiency. Tire other portion below Ballance Street down to the River Bank Road has been plotted with a view of retaining the existing sewfr as a stormwater sewer only, but if for financial reasons it is thought advisable, this latter portion can be eliminated for the present by a slight adjustment of the levels in Cameron Street, not of a vital character; and a reversal of direction of flow along a portion of the River Bank Road. This could be effected at no ultimate detriment to my proposals or to the efficiency of the scheme, and will serve to reduce my estimates bv some TOGO.

As to group 2, 3, and 4, the Engineer says that it is not efficient gravity practice to employ grades giving a full or half full velocity of less than 2Aft per second. With the exception of the 12-inch pipe and a short length of 6-inch, none of the grades proposed in the adopted scheme are anywhere near that, for the sizes contemplated. To obtain the requisite velocity for the plotted 6-inch pipes grades of 1 in 100 are necessary. The inefficient grades are the direct results of the depth at which the 12-inch pipe is plotted at its point of commencement, inasmuch as to have given them the requisite fall would have brought the pipes much above the surface of the ground, which, it is evident, is not practical.

The area dealt with in this report is flat, and therefore difficult to deal with. Owing to the flatness and the necessity for discharging at least at or above the level of high-water ordinary spring tides, the available falls for the "purpose of velocities are limited entirely by the difference in level between the lowest points of file area—minus cover over pipes—and that of high-water ordinary spring tides. Mr Mansfield then gives figures relating to tide levels and the effect of the latter upon the different sections of the drainage system, and draws the conclusion therefrom <hat it is, therefore, a ease cither of extreme depths with a 9-inch pipe, and ineffective grades from the head, or else larger pipes with flatter grades. The latter is, in his opinion, the proper course, and he has plotted his starting point accordingly, so ns to ensure not only free discharge at all times during dry weather, but at all times except high floods; and wicn a larger pipe (18-inch) he has secured a flatter and equally efficient grade, but not more than is requisite and necessary, i.e., one foot in 400. The difference in the price of 15-inch and 18-inch pipes, not in position, is only about £75, and he thinks it well worth while to have the added capacity for the slightly added cost of pipes. The report concludes with the following remarks: The employment of the larger-sized pipes will permit of storm water from the road surfaces being carried off by them for the time being, in this instance a matter of necessity, owing to the flatness and to the absence of suitable creeks or watercourses; and they can ho employed for that purpose until the grwth of the district or other necessity arises, when the storm-water can he disconnected, and led away by storm-water sewers. In the table I have shown the minimum cover provided over the pipes in the approved scheme, a cover in my opinion wholly inadequate for their proper protection from traffic—even on hard ground. This cover, over a distance of 33ft to the boundary of the property, from which 4-inch pipe connections would bo necessary would further reduce the cover by thereabouts a foot, and would, in cases, necessitate the pipe being above the ground on private property. Quite apart from the impropriety of surh a proceeding, it would in such cases involve the

construction surrounded in concrete, an expense which, I imagine, the owner would regard with considerable disfavour. The scheme as outlined, in addition to the serious disabilities already referred to, serves an area of some 63 acres only, whereas it should serve the full area; and is incapable, by reason of its depths at its terminating points in relation to the low points yet to be served, of being extended to serve these points; whereas it should be so capable. In effect, it means that if to-morrow Mr Jones or Mr Eussell, whoever the owners may be, determined to so act that their lands came into use for suburban residential occupation, it would be necessary, in the interests of the health of the community, to furnish the land with sewers capable of discharging sewage from those lands. In that event it would be requisite to construct identically the scheme I now propose as a variation. I have, therefore, provided for the contingency. With this report before it the Council has one of two courses open to it to pursue, either of which, subject to direction, 1 am prepared to follow: (1) To carry out the scheme as approved, in which case the cost will be not much in excess of the amount, provided, or, (21 If it is satisfied that the proposed variations are necessary and requisite, then it is essential, in view of the requirements of other portions of Aramoho, that further loan moneys he raised.

I regret that time lias not permitted of my being in a position to now report on the whole of flic scheme.