Download AEBR 343 Cyclone Gabrielle: tracing river-sediment source contributions to marine sedimentation

Cyclone Gabrielle (13–14 February 2023) caused widespread severe damage in the Te Matau-a-Māui/Hawke’s Bay and Tairāwhiti/Gisborne regions. Soil erosion caused by intense rainfall deposited sediment and woody debris in river valleys and in the sea. This project measured the proportional contributions of major rivers to the cyclone sediment deposits in the sea, and describes the physical characteristics of these deposits.

River sources accounted for more than 98% of these mud-rich deposits. In northern Tairāwhiti, deposits were dominated by the Waiapu River. In Hawke’s Bay and Poverty Bay, cyclone sediment deposits mainly originated from the Mohaka and Wairoa-Nuhaka Rivers. These findings demonstrate that sediment plumes from Hawke’s Bay rivers were transported north along the coast, all the way around around Mahia Peninsula and deposited in Poverty Bay. The overwhelmingly land-derived origin of the marine deposits highlights the vulnerability of the East Coast to soil erosion and marine sedimentation.