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.