For over 30 km (20 miles) along the North Island Main Trunk (NIMT) railway line, the Ongarue River generally flows adjacent to the railway embankment, and has caused many erosion and slope stability concerns at various locations. The purpose of our recent assessment was to aid KiwiRail in prioritising where to implement river stabilization measures in order to increase the NIMT’s long-term resiliency. In the study area, the Ongarue River is approximately 30 m (100 feet) wide and characterised by large meanders cutting through pasture-converted floodplains. The maximum recorded flowrate in the river is approximately 570 cubic meters per second (20,000 cubic feet per second).
ENGEO performed a desktop study, topographic survey, site assessment, hydraulic modelling, and slope stability modelling to determine specific risk ratings for 25 locations of concern. Based on the risk level for each location of concern, we provided recommendations including visual and automated monitoring; “soft” non-engineered remediation solutions such as reducing train speeds, installing fences to limit grazing and riverbank erosion, and planting deep-rooted vegetation; and engineered remediation solutions such as rip rap rock along the riverbank, tied-back H-beam and concrete lagging retaining walls, and palisade walls (buried reinforced concrete piles).
Given the high number of sites spread out over a large geographic area, ENGEO created a GIS web portal to display points of concern, survey data, aerial photographs, flood limits, KiwiRail assets, previous geotechnical investigations, and other relevant site information. Each location icon on the web portal map can be clicked to view its corresponding risk level, recommendations, site characteristics, site description, and site photographs.
Due to the relatively remote location of the railway line, steep terrain, and close proximity to the river, construction of potential remediation solutions will be challenging. ENGEO performed a Multi-Criteria Analysis (MCA) which included considerations regarding resiliency, constructability, cost, effort to consent / permit, and historical issues in order to help KiwiRail prioritize which high risk sites to stabilize first.
