title: Mangahauini / Tokomaru Bay Landslide Dam: Initial scientific observations, survey results, and breach inundation modelling 
Creators: name: Wolter, Andrea
name: Rosser, Brenda J.
name: Cave, M.
name: Morgenstern, Regine
name: Farr, Jason
name: Massey, Chris I.
Publisher: name: GNS Science
Year: 2023

description: As part of a GNS Science (GNS) landslide response to Cyclone Gabrielle (12–16 February 2023), GNS scientists were asked by the National Emergency Management Agency (NEMA), via a request from Gisborne District Council (GDC), for help to investigate the Tokomaru Bay / Mangahauini landslide dam (‘Mangahauini dam’). The Mangahauini dam formed during Cyclone Gabrielle when a landslide blocked the Mangahauini River, approximately 3 km upstream of Tokomaru Bay township. The landslide dam was identified by GDC as being a potential hazard that could pose a risk to life, property and infrastructure downstream if an outburst flood (from a sudden dam breach) were to occur. The hazard that such dams pose can be highly uncertain given the large number of factors that influence their stability and subsequent debris inundation if/when they breach. The landslide response consisted of two site visits, surveying and landslide dam-breach inundation simulations. Prior to an initial aerial reconnaissance survey of the dam on 20 February 2023, the dam had partially breached and a spillway had formed. Nonetheless, following this visit, numerical modelling was undertaken to indicate the extent of dam-breach inundation if the remaining dam material and lake were to fail suddenly. An additional field survey was carried out on 3 March 2023 and included Global Navigation Satellite System (GNSS) and Remotely Piloted Aircraft System (RPAS)-mounted LiDAR (Light Detection and Ranging) surveys of the landslide, landslide dam and lake to estimate their volumes more accurately. Dam-breach models were re-run with the updated volume estimates, and the results were provided to GDC. At the time of surveying (3 March 2023), the Mangahauini River was flowing through the dam along a relatively wide spillway, partly developed along State Highway 35, and there was only a small, impounded lake behind the dam with an estimated volume of ~80,000 m3 . Based on our field surveys and numerical simulations, two potential end-member scenarios regarding the hazard (not the risk) posed by the dam were identified: 1. Lake continues to gradually drain via the spillway through the dam – the current scenario, and thought to be the ‘best case’ scenario, with the following conditions and impacts:The lake continues to drain through the existing channel without sudden and complete failure of the dam and lake; Water gradually erodes and incises the existing dam debris and spillway; Floodwater and debris inundation downstream outside of active river channel less likely to occur. 2. Re-damming of the river due to remobilised landslide debris or wood, perhaps in a future storm – thought to be the ‘worst case’ scenario, with the following conditions and impacts: Up to 320,000 m3 of potentially unstable debris remains on the slope above the Mangahauini River valley, and; If this debris from the slope above the dam or farther upstream were to remobilise and re-dam the river, then the lake could increase in size, which; Could result in potential for larger (rapid) dam failure, which could lead to inundation downstream by dam-breach floodwater and debris; This scenario is highly uncertain, as it depends on the volume of debris remobilised, which affects the volume and geometry of any potential dam and impounded lake, and the (unknown) stability of the dam material.