Reduction of water and bed levels in the lower Waikato River
Wo, Y. G. (1994). Reduction of water and bed levels in the lower Waikato River (Thesis, Doctor of Philosophy (PhD)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/10713
Permanent Research Commons link: https://hdl.handle.net/10289/10713
The major aims of this thesis are to determine the historical qualitative and quantitative reduction in water and bed levels of the lower Waikato River, to identify probable factors influencing the reduction, and to predict the future water and bed levels at Huntly in relation to the Huntly Power Station management Sediments in the Waikato River are composed of gravelly pumiceous sands. In terms of the mean and median grain sizes, generally sands dominate the lower river b~ from the river mouth to the cross section at about 100 km upstream of the mouth, while gravels dominate the bed upstream onward. A hiatus in the longitudinal mean and median grain size distributions from sand to gravel is apparent around Horotiu. In the Waipa River, the texture of bed materials was found to be similar but probably finer to those in the lower parts of the Waikato River. On the basis that ∂n / ∂Q= ∂W/ ∂Q =∂S / ∂Q=0, where n, W, S and Q are, respectively, Manning's coefficient, width, energy slope and discharge, water level of each gauging can be adjusted to one for an index flow of 350 m³/s ( WLQ₌₃₅₀ ). Further assuming that ∂n / ∂T = ∂W / ∂T = ∂S / ∂T=0 where T is time, variations in the time series of WLQ₌₃₅₀ represent the mean bed level changes over time. These assumptions seem to be acceptable for those gauging data within a certain range of discharges in the lower Waikato River. A quantitative analysis of the available river survey data, water level profile measurements, and gauging records has indicated that, in general, there was a continuous trend of reduction in water and bed levels on the lower Waikato River since the 1960s. The reasons for these reductions likely include sand mining operation along the river, consequent disturbance of the river bed surface, long term sand extraction around Mercer and further downstream, upstream effects of the significant bed level lowering at Mercer, and downstream effects of the 1947 Karapiro Dam closure. Analysis of river bulk volume changes suggested that the amount of bedload transported into the river downstream of Ngaruawahia was about 160 000±24 000 m3/yr between 1964-1989. Nearly two-thirds of this was contributed by the bed materials stored in the upstream Waikato River course by an analysis of data from 1974-1989, and the remainder by the Waipa River and the catchment yield from the Hamilton basin. Two disparate time scales for water and bedload movements in practice result in singularperturbation characteristics of the system. With a quasi-steady flow approach, linearization of the water and bedload movement system produces a hyperbolic equation or even a parabolic equation (uniform flow). The parabolic equation is a good approximation of the hyperbolic equation under the condition of large values of time or a large distance from the original disturbance. Variations along the Waikato River of an average mean bed level within a certain length of channel are expected to be small. Therefore the linear models can be applied. In the domain between 48.25-94.45 km upstream of the Waikato River mouth, the parabolic model has been used numerically to predict the future bed levels at Huntly in December 2040 for different scenarios. Potential effects on operation of the present cooling water system have been assessed for given discharges at that time. The ratio of river width to water depth for the formation of alternate bars at the Huntly Railway Bridge has been found to be exceeding 100 by an analysis of the gauging data at this site and at the Ngaruawahia Cableway. This critical width-depth ratio, 100, is much bigger than those suggested in the literature. The corresponding conditions of discharge and mean water depth are, respectively, less than about 350 m³/s and 2.30 m. However further research is required to confirm these conclusions.
University of Waikato
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