Prior to 1941 river-control work in New Zealand was mostly carried out in piecemeal fashion with little appreciation of the causes of the more frequent and damaging floods and of the need for overall planning. River boards were established in 1884 to protect the more important rural and urban areas against flooding and did much good work as far as their limited resources would allow. The authority of boards usually extended over limited lengths of river; raising the necessary finance was difficult, and there was no assurance of Government subsidies. In these conditions it was difficult to plan and carry out works along the best lines to meet the increasing demands for better protection; consequently, works were often designed to meet the immediate problem. Threats to communications and other public utilities were usually met by the Ministry of Works, Railways Department, or county councils carrying out their own protective works.
The increase in flood damage, the deterioration of river channels, and general lack of progress in river control to meet increasing development, coupled with the growing awareness of the problems of soil erosion and deterioration of our hill country, led in 1941 to the passing of the Soil Conservation and Rivers Control Act, which included amongst its primary objectives the control of rivers, prevention of damage by floods, the control of erosion, and the promotion of soil conservation. The Act made provision for the establishment of the Soil Conservation and Rivers Control Council as the national authority in these matters, responsible to the Government through the Minister of Works. The Council consists of 14 members representing catchment boards, catchment commissions, county councils, municipalities, river and drainage boards, Federated Farmers, and Ministry of Works, Agriculture, Lands and Survey, Forest Service, and Treasury Departments, with an independent chairman.
Broadly, the function of the Council is to establish policy; to initiate the carrying out of surveys and investigations; to collect and publish information; to set up standards of work; to allocate finance; and generally to coordinate the activities of Departments, local bodies, and other organisations throughout the country in so far as they are concerned with soil conservation and river control. The work of the Council is administered through the Ministry of Works and the Department of Agriculture, and sufficient funds are appropriated annually for the Soil Conservation and Rivers Control vote to cover administration and works programmes.
The Act also provides for the establishment of catchment boards and catchment commissions as district executive authorities working under the general supervision of the Council on soil conservation and river-control work. Catchment districts normally cover a number of complete catchments with a community of interests and may vary in area from 1,500 to 13,000 square miles. Finance is allocated to these authorities by the Council annually to enable them to function, and boards and commissions submit proposals to the Council for checking of technical and economic aspects and consideration of subsidy. The same functions are also performed by the Waikato Valley Authority which, set up under a special Act, is responsible directly to the Minister of Works instead of to the Council.
Most of the old river boards and some of the drainage boards within catchment districts were abolished soon after the new authorities were set up, their powers and functions being taken over by catchment boards. Those internal drainage authorities which remain come under the supervision of catchment boards and the Waikato Valley Authority. Catchment boards have both elective and nominated members representative of local and Government interests. The members of the catchment commissions and Waikato Valley Authority are nominated from both local bodies and Government Departments. The 13 catchment boards, the Waikato Valley Authority, and one catchment commission already established, cover approximately 70 per cent of the country. Three additional catchment commissions are in the process of being set up, and practically the whole of the country will ultimately be covered by these special authorities. A limited amount of river control and a considerable amount of drainage work (concerned with specific local problems) is carried out by river and drainage boards which also qualify for financial assistance from the Soil Conservation and Rivers Control Council.
New Zealand is favoured with a generous and well distributed rainfall exceeding 45 in. over about 70 per cent of the country and with only a very small area with less than 25 in. In some districts, such as the west coast of the South Island and the western central plateau of the North Island, annual rainfall exceeds 100 in. with extremes over 200 in. on the West Coast and Fiordland.
The topography and soils over most of the country generally favour a high run-off both in total flow and in peak flood discharges. There are, however, in the North Island some areas where flood peaks are greatly reduced by porous pumice soils, while on some river systems the natural lake storage in headwaters helps to control flooding. The general condition of the country has had an appreciable effect on the magnitude and incidence of flooding, due in some areas to such factors as the clearing of steep hill country, excessive burning, overgrazing by domestic animals, and grazing and browsing by animal pests.
Severe storms are likely to occur at any season of the year, arid extreme falls of 12 in. to 20 in. in 24 hours and up to 40 in. over four to five days have been experienced in certain areas. It is therefore not surprising that river control is a major problem in New Zealand, with total run-off and peak flood discharges being among the highest in the world.
Since the passing of the Soil Conservation and Rivers Control Act, considerable progress has been made with river control throughout the country. Action has naturally been focused first on the major river valleys where there has been a history of severe flood damage and where the property owners have been able to meet their share of the cost. Schemes are now in hand, or planned, to deal with the lower reaches of most of the major problem rivers, and they range from channel clearing, training works, and bank protection with partial flood protection up to floods of 5–10-year frequency, to complete protection with stopbanks up to floods of 100-year frequency. Such schemes may involve expenditure up to £2,500,000, and Government subsidies vary generally from £1–£1 to £3–£1 or more depending on the economics of the proposed works and the capacity of the property owners to meet their share of the cost.
The policy of the Soil Conservation and Rivers Control Council is to encourage soil conservation work as complementary to river control. In the case of four large river schemes and several smaller schemes, planning has included the whole catchments. As farmers become aware of the benefits of soil conservation work, comprehensive planning for other river catchments will follow.
There are many rivers in New Zealand where major control schemes cannot be justified at the present time but where a great many smaller individual works covering clearing, training works, bank protection, minor stopbanking, and the like are being carried out. Such works primarily serve to hold the river in check and prevent serious deterioration until such time as more comprehensive planning can be justified. Type and cost of works have to be related to the financial capacity of the individual to pay. Subsidies are generally £2–£1 but may be up to £3–£1. Local share of the cost is generally met by individual cash payments.
As continuity of control and maintenance is of the greatest importance in river work, the Council therefore encourages the planning of comprehensive schemes where rating districts are established to ensure the meeting of capital charges and adequate future maintenance. Rating for works is based on benefits and with larger works it is usual to classify the land benefiting into six classes to conform to the degree of benefit estimated for defined areas. Rating is normally based on capital value, but there is also provision for rating on an acreage basis.
Since 1942 a total length of approximately 2,600 miles of river channel has been improved, 1,400 miles of banks protected, and 250 miles of diversions and 850 miles of stopbanks completed. The present annual expenditure on river works is approximately £1,500,000. On schemes that are well advanced there have already been not only substantial savings in flood losses but also major increases in production due to the protection afforded.
The high rainfall and run-off factors indicated above, combined with topography and soils, have created drainage problems in many areas, particularly in the lower river valleys where river gradients are flat, or in coastal areas. Drainage works are often carried out in conjunction with river-control schemes and include provision for floodgates and pumping stations. Particularly in the North Island, there are large areas of peat swamps that pose a special problem of development because of shrinkage and the necessity for water-level control. Some of the largest and most difficult areas have been mainly Crown land and have been developed by the Department of Lands and Survey for settlement. Only one area now remains under the Department's control, other areas having been handed over to local authority control in accord with Government policy.
Drainage work is not confined to the lower valleys or coastal areas, but extends into pockets or basins in tributary valleys where development is complementary to the improvement of adjacent hill country. Community drains, serving more than one property, qualify for subsidy from the Soil Conservation and Rivers Control vote. Other small open drains, tile drains, mole drains, etc., are considered as “in farm” drains for which an advisory service is available through the agricultural colleges or the Department of Agriculture. New community drains and improvements, if planned and carried out by a recognised drainage authority, usually receive a £1–1 subsidy but higher subsidies may be granted in special cases. Subsidised drainage works must be properly coordinated with river works. The maintenance of community drains, if serviced by catchment boards or by a recognised drainage authority within catchment districts or by the Waikato Valley Authority, qualifies for subsidy, the usual basis being £1–2.
Drainage work is carried out by many different authorities, primarily by drainage boards and catchment boards, but also to a less extent by river boards, county councils, and the Ministry of Works.
The total area of developed or partially developed land under the control of recognised drainage authorities is approximately 1,000,000 acres. The total length of drains dug or reconstructed since 1942 is approximately 1,600 miles, and the length of drains under a regular maintenance subsidy arrangement totals 2,174 miles. Present annual expenditure on drain construction is approximately £180,000, and that on maintenance £240,000.
by Arthur Raymond Acheson, B.E., formerly Chief Soil Conservation and Rivers Control Engineer, Ministry of Works.