Waiohine River - Geomorphic Trends Assessment and its Application to River Management
Annual Conference
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The Waiohine River is located in the Wairarapa Valley and joins the Ruamāhunga River just south of Greytown. The Waiohine River has been actively managed in its current location since 1890 by a number of agencies to prevent it from flooding Greytown, and is now managed by Greater Wellington Regional Council. As part of the development of the Waiohine Floodplain Management Plan, representatives of the Greytown community and Greater Wellington sought to understand the character and behaviour of the Waiohine River to guide current and future management actions.
Our investigation found the Waiohine River has had a change in river behavior following the end of the last glacial maximum, switching from a braided system to a wandering system. As the climate warmed, and there was an increase in precipitation, hillside vegetation cover increased, reducing sediment supply. Increases in precipitation also increased sediment transport. The Waiohine River is still displaying a slight incision trend, which suggests it may still be responding to paleo-climate changes. Infrequent episodic events have also triggered changes in river behavior. Following the 1855 rupture of the Wairarapa fault, the Waiohine River avulsed, and occupied numerous channel locations on the true right floodplain. The earthquake also induced large-scale landsliding in the upper catchment, and head-cut erosion where the fault scarp crossed the bed of the Waiohine River, creating a short-term pulse of sediment into the system.
Today, the Waiohine River is managed using a combination of rock groynes, gravel management and willow planting. It is likely the gravel management is removing the coarse surface armour layer on gravel beaches, enabling more frequent transport of coarse material into downstream reaches, and an increase in fine grained sediment supply. The three reaches identified during the investigation also show different responses to river management and sediment inputs. A tendency towards aggradation in one of the reaches, and the numerous flood channel on the true right floodplain, has created an elevated risk for channel avulsion. This risk will increase under predicted climate change flood scenarios.
Climate change also poses additional challenges for the Waiohine River. Predicted seasonal reductions in precipitation, but increases in temperature may reduce vegetation condition on the hillslopes. This may increase sediment inputs into the river. Sea level rise may change the erosional and depositional areas in the Ruamāhunga River, which could lead to bed aggradation in the lower reaches of the Waiohine River.
High magnitude events may also have different impacts on slope processes, and delivery of sediment. The frequency and timing of these events will largely determine the channel response. Some events may trigger aggradation in upper reaches, and a potential ‘sediment starvation’ incision response downstream, while other events may be more successful at transporting sediment pulses into downstream reaches.
Our findings highlight the importance of understanding landscape scale behaviours in order to effectively manage New Zealand rivers at a reach scale.