LAND DRAINAGE.

Hauraki Plains Gazette, Volume XXXIII, Issue 4424, 7 June 1922, Page 4

LAND DRAINAGE.

NEM ZEALAND AND AMERICA!. MR. J. B. THOMPSON’S REPORT. No. X GENERAL PRACTICE. In considering a new project in 'either country extensive engineering surveys, data as regards precipitation and run-off,, and other details are called for, and it is only after, deliberate analysing of all information and fl’iance proposals that the works should proceed. This Is the approved method, but it is not general. Engineers complain that the layman does not appreciate the necessity fpr. spending money on collection of data, and this especially applies to land drainage. Large sums of money arc expended in the assembling of data considered necessary for power installations, dams, etc., and business people realise the necessity.

In the United States much of the way ’s cleared for the drainage engineer, as practically .the .whole of the States ar covered by meteorologi - cal and hydrographic surveys. The precipitation, run off, gauging of all streams, their velocities, and so on, are available, and thus a drainage coefficient is at once known for a particular district. This data enables the engineer to design with some measure otj exactitude such channels, outfalls, and laterals as will meet the requirements of the area he is dealing with.

This points markedly to the disadvantages engineers in New Zealand labour under, due to a complete absence of systematic observations of precipitation and run-off. Flood discharges of our rivers apd streams are sadly wanted, and can be only arrived at under largely supposititious conditions, and the employment, of formulae which lead to indifferent approximation. Actual observations extending over a period of years are essential. To assist the Land Drain* age branch ip this matter several flow-recorders and current-fmeters were obtained, and will shortly come to hand. CLASSES OF DRAIINAGEI Drainage may be said to come unQer one or the other of the following headings: (1) Gravity drainage; (2) salt marsh drainage ; (3) drainage by means of pumping. Fortunately, the major portion of the swampy areas in the United States can be declaimed by gravity drainage. This also ap’ plies to New Zealand. All the above classes were closely looked into and deductions made. Gravity System.—This consists of the excavation of channels, outfalls, laterals, and sub-laterals leading *o some arranged point or points ofi 1 final discharge. The necessary tide anl flood-gates forming a, component of the scheme are constructed either or timber or concrete. Generally the same system is world-'wide, but many small features—in the aggregate large—were observed, and will be utilised as opportunities occur in our own practice.

Salt Marsh Drainage.—Tne amount of work done in this connection is quite trifling compared with gravity drainage proper. The systems closely resemble each other, but much greater precision has to be observed, and these undertakings are rather shied at. Their success depends on a good range of tides being available, secure impervious levees, and really good tide gates or sluices. Observations made on ground lead one to the conclusion that, given a good range of, say. Bft between mean low and mean high-water marks, marsh land at, say. mean level can be successfully dealt with. The drainage c:’ areas with less range than abovementioned has not been ar. unqualified success by any means, and has in some cases been subsequently as- , sisted by pumping. ' Dramage by Pumping. Very considerable areas throughout the United States have been reclaimed by pump-

ing. The Mississippi Valley offers many examples of areas of from 1000 acres up .to 7000 acres. Apparently the latter area is a good size h) handle. In Louisiana several drained tracts were visited, and the inspection was very satisfying. In North Carolina a very large tract, now known as New Holland, but locally as Lake Matamaskeet, was visited, and proved, most instructive. The total watershed, Comprising lake and foothills;, is about J 0 acres. The lake, originally 3ft to sft deep, is to be drained completely, and was at date of visit practically non, existent. The pumping plant is capable of handling 1800 cusecs, and comprises the latest centrifugal pumps, whicn are steam driven. The pump* ing plant cost £50,000 ; annual maintenance 1 ., £lO,OOO. Areas were also visited in Sacramento Valley, it being observed that the general lay-out of. pumping plants, and areas was similar to .those seen in Louisiana. The majority of schemes have low pumping heads. In addition, . the huge pumps in use in New Orleans, known as the wood-screw pump, were seen. They are 12ft in diameter, and have a capacity of 800 cusecs each. These pupipr are for low heads, and came into use during times of; continued heavy rains, and control the drainage of many miles of coiintry surrbundthg New Orleans.

Exceptional precautions !hgve to be taken in designing pumping units and lay-outs. The practice universally adopted is for the engineer to prepare all hydraulic data, amount of power available, the special features of the .job, etc., and then invite quotations from manufacturers of pumps for a suitable installation. This Jesuits in obtaining guaranteed. efficiency plants. The practice of allowing the civil engineer .to design the plant is entirely discouraged,. and 'rightly so. Great development has taken* place in connection with lowlift pumps, and is ever increasing. Tile Draining.—ln many parts of the United States tile, draining is rapidly coming to the front, and its use has been most successful. Opan ditches are a disability to any farm, and their maintenance is a continual worry and expense. Then, again, a smooth tile will discharge water with much less friction than is possible with an open ditch. Open ditches formerly 6 r t deep have been replaced with tiles of 48in diameter, and then the old ditch backfilled. There can be no doubt o? the advantages of the system. Pipes of 36in diameter were observed running 75 per cent, bore.