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.