UBC Theses and Dissertations
Effect of alternate stopbank alignments on the Waiho River, Westland, New Zealand : a microscale modelling investigation Beagley, Rosemary Patricia Jane
The Waiho River in Westland, New Zealand has been rapidly aggrading its bed as a result of lateral confinement by a stopbank network which restricts the river to 30% of its natural fan accommodation space. The ongoing aggradation has prompted the need to repeatedly raise the crest level of the stopbanks. This had led to the bed and stopbank elevation reaching unpreceded and dangerously high levels, putting the surrounding land, infrastructure and Franz Josef community at even greater risk than before should the stopbanks fail. This thesis investigates an alternative solution to the current management practice. Using a microscale model it tests the response of an experimental Waiho River and fan to the removal of the Southern stopbank and replacement with two alternatives which allow the river greater access to its Southern fan surface. In addition, the study allowed for an exploration of several microscale modelling techniques. The results found that an experimental fan in a state of dynamic equilibrium would not aggrade when confined as previously thought. Only when the fan was already aggrading did it continue to aggrade when confined. In this instance, when the confinement was removed it did not result in degradation to lower elevations. Aggradation continued, albeit at a reduced rate. This suggests that the Waiho was already in a state of aggradation prior to human interference, and that confinement exacerbated the rate. This result has implications for the future management of the Waiho. If the current aggradation trend is to continue, then increasing stopbank crest height is not a viable solution, however releasing the river to the South will reduce the rate of aggradation as well as the pressures on the Northern stopbanks which protect the Franz Josef township. Effectively, this buys time for more drastic action (i.e. relocation of the township) to be taken. In addition to these results, the experiments found that measurement tools and model materials used previously in other microscale models produced unreliable fan behaviour and results. That they have failed in this study, motivates the need for further investigation into the underlying principles of microscale modelling and its practice.
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