CHARACTERIZATION OF ERODED SEDIMENT FROM A LOESS SLOPE WITHIN THE RĀPAKI CATCHMENT, WHAKARAUPŌ | LYTTELTON HARBOUR, CANTERBURY, NEW ZEALAND

Erosion of Quaternary loess and loess-derived soils on the slopes of Banks Peninsula, Canterbury, New Zealand contributes to increased suspended sediment in Whakaraupō (Lyttelton Harbour). Sediment that reaches the harbor can be harmful to the marine ecosystem. When exposed to water, loess is prone to dispersion and rapidly loses shear strength. These characteristics make the soil susceptible to erosional processes that lead to the formation of tunnel gullies and surface gullies. Whaka Ora Healthy Harbour is trialing native revegetation as an erosion management technique at a loess slope study site in Rāpaki, New Zealand. To quantify the effectiveness of this revegetation, the characteristics and quantity of loess eroding from the slopes in the Rāpaki catchment need to be examined. In this study, laboratory drying, particle size analysis, and pXRF was undertaken on in-situ non-eroded and eroded samples. These data provide a baseline profile of eroded loess in the early stages of the revegetation project. The data indicate that the majority of sediment discharge occurs immediately after (e.g. in the few days following) rainfall events. Silt and clay size particles comprise over 65% of all samples. Clay size particles comprise up to 24% of runoff sample sediments, demonstrating the susceptibility of finer particles to remain in suspension. Chemical composition data indicate that loess sediment from harbor slopes is present in harbor sediments. While these data present an initial assessment of erosion at the study site, ongoing monitoring and analysis is required to investigate the effectiveness of revegetation in reducing erosion at the study site.