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Ohinemuri Regional History Journal 21, June 1977

By H. R. (Hal) THORP.

In the previous Journal [see Journal 20: The Draining of a Swamp - E] I wrote of the drainage of the swamp and how the structure of the land changed when consolidation of the peat occurred. This change in structure presented new difficulties in obtaining satisfactory drainage.

On our farm the land containing old buried timber drained out satisfactorily because as the timber became dry, it rotted and was pulled out and burned. After many years of hard work this land was brought into productive pasture, but the pure peat remained a problem still to be solved. The obvious answer was more drainage, but how? - open drains at closer intervals, or underground drains at very close intervals.

Tile drains were tried shortly after the first World War and several lengths of drain were opened up. The idea was to follow conventional methods with tile drain spade and scoop. It was found that the peat could not be extracted without leaving a lot of slurry in the bottom of the drain and, with a cross section of just a few inches, many channel blockages occurred. Then a wider drain was dug - one wide enough for a man to stand in the bottom, but the amount of mud that was left made it impossible to place tiles securely. In an endeavour to improve this, the workman stood on a 9 x 1 plank, one length being prepared ready for laying the tiles the next morning. A mare and foal were in the paddock and during the night the foal fell into the prepared drain, destroying about 1 chain of drain before it was removed. The problem was then to bypass this length. To make the job easy, one side of the drain was trimmed down straight and a ledge dug on the correct level on which to place the tiles.

This was found to be a satisfactory solution and set the practice for all future operations. In consolidated peat a 3 inch diameter tile was found sufficient for a distance of up to 10 chains, although many miles of 2½ inch diameter tiles were laid with success. After a drain was dug, the tiles were laid out on the spoil bank within reach of the tile layer. He commenced at the top end of the drain, and with a No. 3 spade cut a vee into the bank at water or mud level. This he could do for as far as he could reach without moving his feet. Then he would place the tile into the vee and make it secure by a kick with boot or palm of hand. The reason for not moving his feet was to avoid making more mud (or the least mud possible).

Tiles, in this type of soil, had a space between them of about ¼ of an inch or more. It had been found that a seal of a fungoid nature would form on tile spacings of less than ¼ inch. The tile laying would continue down the drain, following the water gradient to the outlet. A very even gradient was maintained and there was very little mud or slurry in the drain, and what was most important, no mud in the tiles. If possible, back filling was delayed for several days or even weeks, so that the spoil banks would dry out a bit and not be an impervious mess when pushed back into the drain. It was found that tile draining was more satisfactorily done in the drier months of the year. On the advent of the hydraulic digger, the same method of laying was used. It did not matter if the drain bottom was uneven, because the vee dug on the water gradient governed the success of the procedure.

There is a popular conception that tiles will not remain even, or on an even gradient, in peat country, but this has been disproved. After 50 years, tiles have been found to be in exactly the same place as when they were laid. The reason for this is simple: If no water is removed from below the tiles there is no material or ground shrinkage, therefore no alteration takes place where the tile was laid. Of course, this might not be the case if a road or a stop bank was constructed across the line of a tile drain. Another popular belief is - that large peat swamps will sink if deep drains are dug through them. This is a misconception, because the water table gradient in any type of peat is very steep. The wetter the climate, the steeper the water table, and in our rainfall of about 50 inches per year it has bean found that a gradient of about 1 in 30 in the water table can be obtained after many years. This means that even a canal 12 ft. deep will not drain peat for many chains from its banks.

At first tile drains were placed about 3 chains apart because of the cost, but after 10 years it was found that these drains only gave efficient drainage for less than one chain width, leaving a strip of very wety country between each line of tiles. Another drain was placed midway down this strip and this gave good drainage. In the process of consolidation, the land assumed a hump and hollow appearance, but the hollows were the driest part of the land and the humps the wettest. After 20 more years, the humps tended to disappear, and to the uncritical eye, the land was practically level.

The reduction in levels of this swamp was as much as 6 feet, and though it was necessary to produce land that was clear of timber and of a stable nature, it also took it down to a very dangerous level as far as floods were concerned. Until the first river works were completed, flooding was frequent and aggravated by the tailings from Mines on the Ohinemuri River. This was very disheartening to the settlers. The large drains dug in conjunction with the Waihou and Ohinemuri River Scheme, and the protection given by the stop banks, kept the land free from floods from the rivers, but did not improve surface (internal) flooding from rain. In fact, the internal floods were higher after the river works were completed than before. This was because the river waters were confined, and rose to greater heights. Consequently, the internal floods were deeper, covered more country and remained on the land for longer periods. This was demonstrated very dramatically when the stop bank at Tirohia was breached in 1928, and later in the prolonged flood of 1954.

In both these instances, more than 2,000 acres of land was under water for up to two weeks. This area was therefore turned into a ponding zone, and ever since the completion of the works, has been flooded two or three times a year. The drainage in normal times has improved considerably because normal river levels have been reduced by about three feet. This was brought about by the construction of the Koutu and Naharahi cuts, which reduced the length of the Waihou river by almost five miles. The bed of the Waihou was lowered by 3 ft. 6 in. at the top end of the Naharahi cut.

To facilitate the drainage of this area, the Public Works Dept. dug two main canal drains - one to Tirohia and the other to connect the existing drainage system to the south and the Rotokohu area. In the first instance, these canals connected and then entered the Waihou at the end of Mill Road. Later, the whole system was taken across the old channel of the Waihou and on to the Ohinemuri, some distance above the present junction of the two rivers. This gave about 4 miles of new drain in the area. At the same time, Cooper's drain was enlarged and deepened to maximum depth.

During the years 1920-25 when no local authority had control of this area, and the work was done by individual settlers, there was no co-ordination and the canals deteriorated rapidly. After repeated representation from the settlers, the Public Works Dept. agreed to do a reconstruction job if the area was formed into a drainage district and a drainage authority was promulgated. The settlers appointed a committee to take the necessary steps and a Drainage Board was set up and gazetted in 1928. The area took in all the country between the two rivers, except part of the borough of Paeroa and some Maori land on the Ohinemuri river. All service drains were taken over and classification for rating purposes was completed. The Board consisted of five trustees who were elected from the ratepayers.

About 15 miles of drains were put under the control of the Board. Nearly 5 miles were of machine construction and were originally designed by the Public Works Dept. The cross section was 8 feet bottom with depth up to 18 feet at the flood gate culverts. The width at the top of the drains was up to 44 feet. A fall of ½ in. to the chain was designed, but was not able to be obtained because after the first mile from the river, the drain bottom would have been almost level with ground level. Therefore, machine operators were instructed to dig on a dead water basis. So the drains were dug just as the settlers had dug them, by taking in water levels until the required depth had been obtained.

The rest of the drains taken over by the Board had been dug by hand by the settlers and needed only maintenance or a small amount of reconstruction, with the exception of the Kouiti stream. Although the Public Works Dept. had put a large 3 barrel flood gate on this stream, it had not been considered to be a satisfactory outlet. The reason for this was, the outlet was into the Ohinemuri river where even small floods closed the flood gates very frequently, causing the stream to flood across country and into the low lands to the north. The Board considered that it was worth while to reconstruct this stream, so a loan was raised and with a subsidy from the Government, the stream was widened and straightened for a distance of 90 chains. This assisted with the land drainage, but the flooding problem was only slightly abated.

At that time, all the Board drains were maintained by hand, and men with shovels, slashers and drain drags were employed, mostly at contract rates. In the large canals, it was found very hard to make a good job because of the difficulty of removing the spoil and debris. With the growth of raupo and pouarua grass causing obstruction, these canals slowly deteriorated and it was found necessary to use draglines again. Machine cleaning was very costly, and it was not until the advent of the wide weed bucket and later, the back blade on the wheel tractor, that it became possible to maintain drains up to a good standard of efficiency. In this area, every inch of drainage by gravity counts. In the early days of the Board there were no subsidies for maintenance, but subsidies could be obtained for reconstruction work. It was not until the Soil Conservation and Rivers Control Act came into force and Catchment Boards were formed, that maintenance subsidies were paid for drainage works, which was a great help to this Board.

The works constructed by the Public Works Dept. and later by the Ministry of Works were limited concerning the Waihou and Ohinemuri rivers. The design was to a minimum capacity; proved in floods soon afterwards. The Hauraki Catchment Bd. produced a plan to cover Waihou river catchment - approved by National Authority and the Govt. Two construction seasons saw the work and when completed, reduced flood levels. Production from farm land in the area should then be considerably increased.