Study region
The Waiho River is a rapidly aggrading, levee-confined, glacier-fed braided river on the West Coast of New Zealand, crossing a steep proglacial alluvial fan where stopbanks protect the State Highway 6 lifeline corridor and adjacent community.
Study focus
This study quantifies how riverbed aggradation alters flood behaviour and levee performance and evaluates alternative levee strategies for the Waiho River. Using multi-temporal topographies (2016–2023), terrain-change analysis, and spatially distributed floodplain roughness derived from remote sensing, we apply a two-dimensional hydrodynamic model to simulate (1) breach sensitivity on both riverbanks under varying bed elevations and (2) levee reconfiguration options from partial to complete removal of the southern levee system.
New hydrological insights for the region
Results show that incremental levee heightening yields only short-term benefits; under continued aggradation, it elevates water levels against protected margins and exacerbates breach consequences. Breach behaviour is strongly conditioned by riverbed elevation, indicating that aggradation control and freeboard management must be planned jointly. Partial south-levee removal provides limited relief, whereas complete removal (or an equivalent setback) substantially lowers hydraulic loading on the opposite bank and redistributes flow and shear across the southern floodplain, promoting wider conveyance and sediment dispersion consistent with a reconnected system. Overall, the findings support an adaptive pathway prioritising restored floodplain connectivity, complemented by targeted reinforcements and risk-informed operations near critical assets (e.g., lifeline highway infrastructures).
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