THE SOUTHLAND DRAIN.

Hawke's Bay Tribune, Volume I, Issue 152, 13 June 1911, Page 11

THE SOUTHLAND DRAIN.

At yesterday’s meeting of th.? Hawke’s Bay County Council, Mr. U. D. Kennedy, C.E., reported £« follows:— I have gone over the proposed deviation to the north, through part of the borough, having its outlet at the washpool. If the country traversed by this proposed line were not subdivided into small sections with numerous roads it would have been a better line to adopt, as it follows the natural slope’ of the country and saves distance; owing to close settlement and the cost and inconvenience of taking a large drain through Hastings and its suburbs, I regret to say that this route is not practicable.

I have also given further consideration to the District Engineer’s proposals, and had some holes dug on the old Ngaruroro river bed along and near his proposed line ; the water rose freely in the holes to within a few inches of the surface, with a perceptible movement down stream, and this at a point where it was necessary to make a drain over five feet deep. I reluctantly .advise you to abandon this scheme.

There now remains the present line of drain. I am told that last

year the drain was in a very foul condition, not having been cleaned for over three years, and also that the rainfall was abnormal, very' heavy rain falling while the ground was thoroughly' saturated. The drain was cleaned last spring and is at ptesent being cleaned in a very thorough manner, but the work is only' partly done. During last week’s very heavy rain I inspected the locality and found that the drain was acting well and meeting requirements.

The drain is uneven in sections, the width on the bottom varying from 4 to 8 feet, and the side slopes from vertical to f horizontal to 1 vertical. Three quarters of a mile through sandy ground have been planked to a height of 4ft 6in ; much of this is decaying and requires renewing; in addition, a similar distance requires, support. I consider it advisable to u'se concrete slabs in futpre. Culverts across the Maraekakaho road and the railway' line do not provide sufficient waterways and must be enlarged. Numerous slips have I taken place since the last heavy i rains. The notes and sections on the attached plans supply' further details.

Messrs. Sheath and W. Dinwiddie were appointed a visiting committee.

It is hardly ‘ reasonable to expect provision to be made. for a drain that will immediately carry away the whole of an abnormal rainfall, but it is necessary to provide drains of sufficient capacity, to carry the whole of me water during moderately heavy rains and to provide quick relief after extraordinary rains. In calculating the capacity of drains in the fen districts of Lincolnshire, the oki engineers provided for the drainage due t-o the water arising from a continuous rainfall of a quarter of an inch of rain in 24 hours, making no deductions for soakage or evaporar tion. This calculation was alsc adopted by Sir John Hawtcshaw for drainage of the East Fen and by Sir John Coode for the drainage of the North Level. In Holland the quantity varies from | to £ of an inch of rain in 24 hours. In Ireland for the Rathdowny drainage provision was made in the channel for a discharge of 600 cubic feet per square mile per minute, equal to a continuous rainfall of about | of an inch in 24 hours —the soil being a deep alluvium.

Owing to the torrential rain these allowances would not suffice in this country, and it would not be advisable to allow for a rainfall

■of less than | of an inch in 24 J hours. Making no deduction for I soakage or evaporation, the presI ent drain at its crossing with the road has to dis- ! charge the water from about 4000 I acres; f of an inch of rain in 24 hours over this area is equivalent to 7500 cubic feet per minute, which would be accomplished by a drain having a fall of five feet in a mile, six feet wide on the bottom, having side slopes of | to 1 and running 4ft 6in deep. If a drain of these dimensions was kept thoroughly' clean it would in my opinion carry all the rainfall during a period of heavy rain and afford , quick relief after abnormal rain.

I estimate the cost of enlarging the drain to a uniform capacity as suggested above, replacing decaying timbering with concrete slabs and further similar work where necessary, at £lBOO.

In addition to this provision should be made for an annual expenditure of £2OO for repairs and to enable the drain to be kept, thoroughly clean. The report was referred to the Chairman and Cr. Crosse.