Technically the term “dam” relates to the barrier constructed across a stream, valley or similar natural depression for the purpose of impounding water, but popular usage includes the impounded water along with the barrier.
Apart from the obvious requirements of storage for community or stock water supply, the economic purposes served by impounding of water by dams are: land irrigation, generation of electricity by hydraulic power, amelioration of flooding by partial retention of flood waters, and improvement of natural waterway facilities for inland navigation. Of these, only the first two have been exploited to any extent in New Zealand. Some details of the more important dams are tabulated.
Since New Zealand's maritime climate ensures a generally abundant and reasonably uniform distribution of rainfall in most areas, dependence of the economy on land irrigation is not extensive; the use of dams of any but minor size is confined to Central Otago. The development of water resources for irrigation in Central Otago is a Government undertaking that was put into effect some years ago. None of the several dams in the system would be rated large, however, by standards elsewhere.
The major development of water resources by dams in New Zealand has been planned for electric power generation and controlled directly by the Government on a national basis. The assessment of power potential from hydrological and topographical investigations and the planning and construction of the required dams and other hydraulic works is a function of the Ministry of Works (formerly the Public Works Department), and is performed on behalf of the New Zealand Electricity Department, the latter having responsibility for the power plant and its operation and for the transmission of power to distribution centres.
There are some instances of a limited contribution by hydro-electric works to facilities for land irrigation; for instance, economical electric power from the Roxburgh station enables the pumping of water from local sources, and in the Hawea Lake control works provision has been made to supply water to a future irrigation project. The influence of hydro-electric dams on ameliorating major flood effects in low-lying areas, though not neglected, is unlikely to be conspicuous because it lies outside their primary function.
Geological features of the reservoir basin, and of the dam site in particular, figure large in determining the feasibility of a project and the type of dam. In New Zealand the basic structure of the country has been greatly affected by orogenic forces not only in remote ages but also in renewed activity lasting into a much later period (Quaternary, Pleistocene) than was the case in most other land masses. This later uplifting is apparent both in the extensive faulting, folding, and fracturing so widely manifest in the geological structure, and in the present volcanic activity. Weathering agencies have been active in sculpturing the present deep relief, a legacy from which, of course, is the present dam sites.
Where old rock formations (severely worked by previous orogenesis) are within reach for dam foundations, they prove usually to be not only extensively affected by much cracking and shattering but often are also deeply deteriorated by penetration of weathering effects (e.g., the schist rock of Central Otago and the sandstones and mudstones of South Canterbury). Younger formations of marine origin, such as those of the Wanganui region, though more uniform through having escaped much of the former working, remain soft and weak. Volcanic and hydrothermal activity has added to the complexity of the problem by leaving large areas of the North Island covered successively with variable formations lacking density and strength and having high permeability. This applies in particular to the region traversed by the Waikato River, one of the country's more important sources of hydro-electric power.
A further effect of the aggressive weathering and late volcanic activity is that deposits of dam construction materials (gravels and sand for concrete and soils for embankments) tend to be lacking in both uniformity and extent where they are most wanted. Effects of earthquakes (an aftermath of the recent orogeny) present another factor to be taken into account. While dam sites traversed by active faults or situated close to them are avoided for obvious reasons, and those traversed by dormant or long inactive faults are treated circumspectly, it is not possible to proceed very far in the development of New Zealand's water resources without having to take into consideration faulting and other tectonic weaknesses. The provision of seismic resistance in dams is not novel, however, and has figured as normal practice in several countries. This factor, though, bears on the suitability of certain types of dam. Although the geological structure of New Zealand thus poses problems for dam building, none has proved unique; problems of a similar type have been encountered elsewhere, often in worse degree individually, and have been successfully surmounted. In matters pertaining to dams, New Zealand shares in the experience of other countries through membership of the International Commission on Large Dams, a section of the World Power Conference.
For dams of any but minor size the traditional building material in older countries has been masonry, later succeeded by concrete; these materials in appropriate structural forms have been associated with the development of sites favourable to their support and economy. Comprised in this group are dam forms designated as mass gravity, massive buttress or hollow gravity, arch and arch-gravity types (both simple and vaulted or domed), thin multiple arch and buttress, slab and buttress or Ambersen dams, typically in relatively narrow valley sites. Not all of these types are represented in New Zealand.
Expanding development in New Zealand has forced attention on the wider sites and on those less favourable in respect of rigidity and strength of foundation rock; hence there has been a significant general trend to the embankment type of dam built of soils, alluvials, and rockfills variously compacted. Nevertheless, in some countries (notably Spain, Portugal, Italy, and France), the economic use of concrete arch forms for wide valleys is favoured, provided there are no foundation weaknesses.
The bolder use of embankment types of dam, often of considerable height, has been conditioned to an extent by the modern expansion of the science of soil mechanics. Progress in the development of efficient means for the excavating, transporting, and compacting of rock and earth materials has also exerted a profound effect. Whereas costs of all other forms of construction have risen severalfold in money value, the costs of earth handling have been kept to much the same levels as in the days of the pick and shovel, and even in some cases, lower. Thus it is now possible to plan such projects involving earth-works on an unprecedented scale. Moreover, with the increasing difficulty of finding dam sites favourable to the traditional forms of construction, the development of this new method is timely. The Benmore power project on the Waitaki River is one instance. A site below the confluence of the Ahuriri tributary was preferable because of the greater power from the added inflow, but the Waitaki valley is relatively wide there and basement rock is not of good quality, thus indicating an embankment type of dam. To give some idea of the work involved, it may be stated that in operations associated with foundation excavations for required structures and construction of the main wall in rolled earth and other materials, more than 20 million cu. yds. were handled, yet in its economic aspect in relation to the power output, this project can rank as one of the most favourable in the country.
| Dams for Power—Clutha River | |||
| Purpose and Group | Power—Clutha River | ||
| Name of dam | Roxburgh | Hawea Lake | Kawarau |
| Location | .. | .. | Lake Wakatipu, outlet |
| Type | Concrete gravity | Rolled earth central core | Multiple piers and gates |
| Foundation rock or bedding | Schist | Weathered chlorite schist | Schist |
| Maximum height (ft) | 180 | 98 | .. |
| Dam volume (cu. yd.) | .. | 560,000 | Small |
| Spillway type | .. | Gated undersluices | Gated weir |
| Discharge capacity (cu. ft./sec) | 162,000 | 11,000 | 36,000 |
| Reservoir area (sq. miles) | 1·25 | 46 | 113 |
| Effective storage volume (ac. ft.) | Small | 2,000,000 | 500,000 |
| Year completed | 1956 | 1959 | .. |
Typical features of various types of dams are depicted in the diagrams and illustrations. Two that come in for mention are the “outlets” and the “spillway”. The outlets provide for the draw-off of required water and often constitute the “intakes” of other installations associated with use of the water; in particular the intakes to pipelines or penstocks of hydro-electric power projects may be quite large. In some cases the intakes may be part of a structural unit that forms a separate dam from the main barrier. Similarly, the facilities for the discharge of floodwaters may sometimes be provided in a separate structure where circumstances are appropriate. Since power development on a national scale involves damming of the larger catchments, the flood spillage provision must usually be correspondingly large. Often the spillway is a part of the dam where the crest is made lower than it is elsewhere; for thin dams (e.g., arch type) the outflowing cascade of water may be arranged to spring clear of the structure (e.g., Manorburn concrete arch dam), though it is more usual for the spillway lip and outflow surface to be specially shaped to conform to the natural overflow jet profile, known as an “ogee” type.
Spillway openings in dam crests (or as separate structures) may be relatively narrow and deep and fitted with gates—the more usual case in modern practice; or they may be made much wider and shallower and be unobstructed, these being known as the free-overflow type (e.g., Waitaki dam).
It is not only the artificial barrier that has to be watertight but also the country formation underlying the dam and surrounding the body of water. In certain instances—not in New Zealand—reservoirs have failed to fill because of undiscovered seepage paths underground. Foundation and abutment treatments, sometimes extending also to parts of the lake's rim, to tighten and strengthen these zones are an important feature of dam construction; such treatments (e.g., by pressure injections of fluid mixtures of cement and water and of other materials) may often extend in depth to hundreds of feet and can become quite elaborate and expensive. Foundation treatments at Karapiro and Roxburgh dams extend actually below sea level. Moreover, saturation of the foundation materials, whether of earth or rock, through ingress of lake water and the ensuing development of “pore pressures”, can endanger the stability of dams owing to the effects of partial flotation. Hence to tighten the formation in the vicinity of the water-face is not enough. It is also necessary to provide means for drainage from under and from within the body of the structure or embankment. Typical examples of these provisions are depicted in the diagrams.
Referring to hydro-electrical development, while some dams are provided only for the control of natural water storage (e.g., at Taupo, Cobb, Pukaki, and Hawea lakes), the primary object of others is to create the difference in water level necessary for the production of hydraulic power. The principle is typified in the chain of projects on the Waikato River: Taupo lake water, controlled by a gated weir, is fed to a series of dams downstream that develop the available “head” in successive reaches of the river. The diagram reveals that little of the gross head between lake and sea remains unutilised.
The height to which a dam can be built is determined by a number of factors. In the case of storage dams, the required reserve of water in relation to probability of incidence, and duration of, “dry” periods, together with the cost increment involved in the increase in height of a dam, are aspects that come in for consideration. In the case of hydroelectric dams essentially for head, the higher the dam the more effective the water, hence the aim is to build to as great a height as is practicable. Where characteristics of the dam site or of the valley and its existing economic usage do not interpose a natural limit, a consideration of incremental cost of the increased head, as compared with cost of obtaining the additional power from other sources (including the possibility of another dam on the same river), becomes the deciding factor.
by William Eric Sisson, B.E.(CIVIL), A.M.I.STRUCT.E., Inspecting Engineer (Power), Ministry of Works, Wellington.