Managing geomorphological drivers of river hazards in the rapidly aggrading Waiho River, Franz Josef / Waiau, Aotearoa New Zealand

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Aggradation in the Waiho River has been the subject of research for over 40 years (e.g. Mosley, 1983; Hoey, 1990; Davies, 1997; Davies et al., 2003). Where the Waiho emerges from confinement at the Southern Alps rangefront it has formed a large alluvial fan. Development on this fan in the form of the Franz Josef / Waiau tourist township, State Highway 6 and its bridges, and pastoral agriculture has resulted in artificial confinement of the active portion of this fan, principally using stopbanks (flood walls / artificial levees). Unable to distribute its bedload across the fan surface, the river has responded by aggrading its bed. In turn, stopbanks have been raised regularly, perching the river, which now sits ~2 m above the level of the township, elevating flood risk. Application of Geomorphic Change Detection (GCD) using LiDAR surveys acquired in 2016, 2019, 2023, January 2024 and July 2024 demonstrate a remarkable rate of aggradation equating to 0.2 m yr-1 in the vicinity of the township, that appears to have been ongoing since about 1960. A combination of recent storms and glacier retreat appears to have increased sediment delivery in the Waiho proximal to the Franz Josef Glacier. GCD analysis reveals that sediment is being pulsed through to the Waiho Fan through this relatively confined proglacial reach. On the fan, an avulsion has cut through to the adjacent Tatare River to the north, which is now rapidly infilling with bed calibre material. As the avulsion incises, more flow is captured and a full switching of the Waiho into the Tatare is a possibility.The situation is complicated by the high probability (75% in 50 years) of an extreme earthquake in the area, that will damage stopbanks and severely aggravate aggradation over years to decades. This event is so likely that all but short-term flood risk management strategies must consider it. These significant flood and avulsion hazards pose extreme risk to life and property in the vicinity of Franz Josef / Waiau and are in urgent need of mitigation. The current management practice of raising stopbanks and repairing rock-lined edges is setting the system up for catastrophic failure given the rates of change we observe. A ten-year programme allowing for managed retreat and release of the Waiho to the south is proposed. It is anticipated that this will reduce the rate of current riverbed aggradation and allow a staged relocation of the township in the longer term. ReferencesDavies, T. (1997). Long-term management of facilities on an active alluvial fan - Waiho River Fan, Westland, New Zealand. Journal of Hydrology (NZ), 36, 127–145.Davies, T., McSaveney, M., Clarkson, P. (2003). Anthropic aggradation of the Waiho River, Westland, New Zealand: microscale modelling. Earth Surface Processes & Landforms, 28, 209-218.Hoey, T. (1990). Aggradation in the Waiho River. Final Report to the West Coast Regional Council, 23p.Mosley, M.P. (1983). Response of the Waiho River to variations in Franz Josef Glacier, Westland, New Zealand. Internal Report WS 858, Hydrology Centre, Christchurch, NZ, 17p.

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