Waipaoa River Flood Plain, Gas Pipeline Soil Cover Erosion – Geohazard Assessment and Remediation

PDP assisted First Gas Limited to assess the mechanism and provide remedial design for a 29 m long erosion feature that exposed the gas pipeline near Gisborne when the Waipaoa River over-topped its stop banks during ex-Tropical Cyclone Gabrielle in 2023. The erosion was found to have been caused by flood waters flowing through an old railway cutting located more than 600 m from the main river channel. Geotechnical remediation successfully addressed site challenges including finding suitable local material to infill the erosion feature and reinstate pipeline cover, improving site resilience by designing a buried concrete block wall to protect the pipeline from future flood flows, and installing a drainage network to ensure the area remains dry for farming activities.