Ohinemuri Regional History Journal 20, June 1976

By H.R. (Hal) THORP

Before the arrival of the European, only natural forces affected the river flows and the rise and fall of the tides in Hauraki swamp lands south of the Gulf. About 1840, a change came about; firstly by the small endeavours of the settlers themselves and later by Government and Local Authorities. Drains were dug, mainly because of the necessity to construct roads and bridges. This work was pushed on vigorously when gold was found in the Coromandel range.

THE TIROHIA-ROTOKOHU AREA. This is typical of most of the Hauraki swamp lands. It has the same sea mud foundation with river silts superimposed on it near to the main river and stream channels. It had its cover of Kahikatea on the clay areas, red tea-tree on the higher peat dome and raupo and flax on the sunken timber swamps. It was known by the Maoris as Rotokohu or "valley of mist". To the south and east it is bounded by the Coromandel range and to the north and west by two rivers. It is thus separated from the Hauraki swamp lands but vitally affected by all the river and flood protection works.

The first parts settled were on the banks of the Ohinemuri river and on the lower slopes of the range. The Maori had their Pas and cultivations on the river banks and the lower foothills. They were wise and built their houses and food stores only on land well above flood level. The first white settlers were able to buy small portions of the river bank lands after much negotiation. The swamp lands were much more easily obtained because apart from eel fishing, they were useless to the Maori. Both the rivers are tidal: the Ohinemuri for about 6 miles above the junction, and the Waihou up to Tirohia. They were both used by boats drawing up to 5 feet of water.

The few settlers who wished to extend their farm land out into the swamp found drainage very difficult because the land near the river banks was always higher than it was further out. This necessitated a very deep drain near the river in order to collect the water in the lower areas. These drains proved disastrous in flood time because they let more water into the swamp than they let out, necessitating the construction of flood gates which helped to contain the waters of the rivers.

The first major drainage works were done by the Thames County Council then the district local authority. Its first concern was the drainage of the main access road to Te Aroha and the rail head. About the same time, Cadman Road drain was dug in preparation for the railway to Thames. Cooper's drain was dug soon afterwards by a very enthusiastic settler.

The Thames County Council opened up the main outlet known as the Kaouiti stream to the south end of the swamp which was almost a lake. Construction then began on a road to Te Aroha, across the eastern end of the swamp. The practice was to dig two drains, one on either side of the road and to put the spoil in the centre to form a foundation for the road; but where there was no clay found in the drains, it had to be carted in by horse and dray from the foothills. A swamp containing a lot of timber makes a very unstable foundation for a road. It was a long and laborious process. Any metal that was found in the stream beds was used, but the bulk of it was broken on the road-side by hand. In some places a clay road would suffice for many years.

The draining of this swamp was influenced by many things. No matter how eager the settlers were to carry out the work on their separate holdings, it was found that they could only do the job in a piecemeal fashion because of the restrictions imposed by the conditions of the two tidal rivers, which did not give a very good outfall for drainage for most of this area. Frequent flooding often caused water to flow over on to land that the settlers were endeavouring to develop, flooding being accentuated by the rapidly rising hill-streams. Serious floods caused losses of stock and crops, yet this did not deter operations. Drains were pushed on right through the area, but much of the land was difficult. There were areas of Kahikatea bush, and old sunken forests besides large areas of peat swamp covered with tea-tree scrub.

Drainage resulted in piles of timber and fires swept across the country with devastating results, though they cleaned up much surface rubbish and burned the peat to varying depths ranging from two feet near the drains to only a few inches further away. Fires were almost impossible to control, but by strenuous efforts in clearing breaks and digging trenches, homes and plantations were protected.

The practice was, to surface sow the burns with grass seed. The resultant pasture was very good and when fenced, the area temporarily carried many hundreds of cattle and sheep, but loss of freeboard meant drains had to be deepened and the whole process gone through again. Where the clay was near the surface and the land was more stable, it was more easily kept clean, especially near the river banks and foothills. In other areas of Kahikatea timber, when the ground sank the timber came up, sometimes in such profusion that it was impossible to put a foot down without standing on timber, in places to a depth of 12 to 15 feet. The drainage of such country was easy because of its permeability, but the digging of drains was difficult. It was all hand work with shovel and axe and a lot of dynamite was used. As the land was drained it was reduced in level by the shrinkage of the peat and the rotting of the timber. Unfortunately the levels could not be reduced very much because thousands of acres had a clay level of much below river and tidal levels. The idea was to reduce the peat and timber country until it was on or near clay levels. The result was a basin into which winter floods flowed and sometimes remained there for many months.

Rivers were also deteriorating by the growth of willows then deposits of mining tailings, following the advent of mining operations on the Ohinemuri River. This led to major flooding occurring from the early part of the century, and culminating in the large floods of 1907 and 1910. A Royal Commission was set up in 1910 to consider river and drainage improvement, vitally necessary because railways and roads were being washed away - the town of Paeroa flooded and covered with silt. The Commission's findings resulted in improvements being started by the Public Works Department.

The first work was the shortening of the Waihou River by cutting a canal about 85 chains long, reducing it by nearly 5 miles and also taking the junction with the Ohinemuri about 2 miles further down stream. The canal was dug in the first instance by hand and proved to be quite troublesome because it was 200 ft. wide by about 6 ft. deep. The work was started 1912 and some smaller drainage works were carried out, with a combination of hand work and walking dipper dredges. The conventional dragline was not used until a later date. The 1914-18 war slowed down work, but some surveying and designing was done. After the war, work started in earnest with the advent of suction dredges and large steam draglines (as well as smaller machines). 1928 was supposed to be completion date, but many miles of river works and stop banks remained to be completed. Many men were employed with wheelbarrow and shovel to build stopbanks and to sheath with clay the sand stopbanks constructed by the suction dredges. Thousands of yards of clay were shifted in this way. Twelve barrows to the yard were required which had to be wheeled some distance from the pits and then to the banks up to 15 feet. Strong men made good wages at prices from 6d. to 18d. per yard, plus I penny per yard lead money over a certain distance.

Digging small drains, prices were: 4d. to 6d. per yard, larger drains 6d. to 1/6d per yard, depending on the depth and the type of material to be shifted, e.g.: it took a good man to lift with a shovel, 20 yards a day from a drain 5 or 6 ft. deep. Where timber was present it was difficult to put a fair price on the job. In many cases a compromise was arrived at, where the owner or local authority supplied half the gelignite used to blast out the timber. Many miles of drains were dug in this manner. In the pure peat areas there was no Kahikatea, but the first few feet had some red tea-tree that had been buried for many hundreds of years and was preserved in the peat. It was very hard, up to 9 ins, in diameter, and made excellent fire wood when dry.

Originally, the peat swamp was a mixture of sphagnum moss, wiwi rush and tea-tree scrub. The surface was so spongy that it was difficult to walk on, but once it was dried out by draining, was easily burnt off. Drainage was also fairly easy, because of the open texture which contained many tea-tree sticks. Therefore, a water table of a reasonable gradient could be obtained with drains about 20 chains apart. When consolidation occurred, it was found that there were many feet of very tight, clean peat before the clay subsoil was reached. It was very hard to drain and water tables would rise very steeply from drainage levels. In many places it was found that a water table would run from 4 ft. to zero in a distance of 30 feet, and this would not improve over the years. There are many thousands of acres of this type of peat on the Hauraki Plains.

(To be continued) [see Journal 21: Drainage of the Plains - E]