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The Wairau River is one of the most engineered river systems in the Marlborough region. Historical river control works, particularly the blocking of the Opawa Breach and the subsequent construction of the Wairau Diversion, have had a significant impact on flow and sediment transport in the Lower Wairau. Currently, the Lower Wairau is aggrading as a result of an increase in sediment concentration and a decrease in the duration for which flows exceed the critical threshold to keep sediment entrained. These changes are attributable to the substantial reduction in flow in the Lower Wairau that is a result of over half the total Wairau flow now passing down the Wairau Diversion. During the previous period, following the blocking of the Opawa Breach up until the construction of the Wairau Diversion, the opposite occurred, with significant increases in flow down the Lower Wairau and widespread erosion. This paper reviews the geomorphic setting of the Wairau deposition zone and the impacts of the Opawa Breach and the Wairau Diversion on the Lower Wairau by examining historical cross-sectional data (1927 to present) and assessing the distinct flow and sediment transport regimes driving the aggradation and erosion. The effects on channel slope, width and bed level through the different regimes are also examined, along with their implications for sustainable management decisions. [ABSTRACT FROM AUTHOR]

Most New Zealanders reside in coastal regions and many communities are situated on active floodplains. In the last decade there have been many storm events with rainfall annual exceedance probabilities of 1/150 or rarer and there have been many flood-related disasters. Insurance claim statistics suggest that the frequency of floods is increasing. Such statistics are a concern for local government authorities, insurance companies and populations in low-lying areas. Some underlying physical and hydrologic causes of the flood disasters are investigated. It is found that the present numbers of rare rainfall events are not unexpected and there does not appear to be any significant trend evident in the occurrence of river floods. For the areas studied, the river floods appear to cluster in certain decades. The clusters do not occur at the same time in different parts of the country and high rainfalls (required to cause floods) in the southwest of New Zealand are associated with El Niño conditions. Recently there has been more flooding in the north, which is where more of the population lives. The general increase in population has also seen more houses built in locations prone to flooding. Thus the increase in flood-related insurance claims is attributed to more floods in populated areas and to more people getting in the way of floods, rather than to any increase in the number of floods that have occurred. Wise regulation of future infrastructure development will be required to prevent more flood disasters in the years to come. [ABSTRACT FROM AUTHOR]