Search

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]