Catchment-scale LiDAR-based SedNetNZ modelling in the Upper Waitomo catchment

The Waikato Regional Council (WRC) contracted Bioeconomy Science Institute Maiangi Taiao (BSI) to model erosion and suspended sediment loads in the upper Waitomo catchment using a LiDAR-based version of the SedNetNZ model. The project involved building a new digital stream network from a 5 m digital elevation model (DEM) to better represent the characteristics of the Karst landscape. A high-resolution landslide susceptibility layer was also developed to predict rainfall-induced shallow landslide erosion. These layers were used to model contemporary suspended sediment loads as well as three future, and a retrospective erosion mitigation scenario for the catchment. The project also assessed load reductions required to meet National Policy Statement for Freshwater Management (NPS-FM 2020) visual clarity attribute targets and included an economic analysis of cost-effectiveness of erosion mitigations.

The model estimated a contemporary suspended sediment load of 5.7 kt yr–1 delivered to the catchment outlet, with shallow landslides being the largest source of sediment in the catchment, followed by surface erosion and riverbank erosion. Under the retrospective scenario, where all erosion mitigations were removed, the modelled sediment load increased by 49%, highlighting the importance of existing erosion control mitigations. Future scenario modelling shows that mitigations including afforestation/bush retirement, space-planted trees, and riparian fencing could reduce sediment loads by 24–45% across the catchment compared to the contemporary baseline. Even under full implementation of the most ambitious mitigation scenario, further reductions would still be needed to meet the national bottom line for visual water clarity at the catchment's state of the environment monitoring site.