Reconnaissance report on landsliding caused by the 19-20 June 2015 rainstorm in the Taranaki-Wanganui-Manawatu region

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Page, M.J.; Rosser, B.J.; Townsend, D.B.; Carey, J.M.; Ries, W.F. 2015 Reconnaissance report on landsliding caused by the 19-20 June 2015 rainstorm in the Taranaki-Wanganui-Manawatu region. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/47 50 p. + appendices

Abstract: Heavy rainfall on the 19–20 June 2015 caused landsliding in the Taranaki-Wanganui-Manawatu hill country, over an area of about 8,900 km2. GNS Science carried out two reconnaissance flights as part of a GeoNet Landslide Response, to gain an initial assessment of the landslide damage using oblique aerial photography. Horizons Regional Council and Taranaki Regional Council have engaged GNS Science to provide a regional perspective of landslide distribution and severity in their regions. This reconnaissance report is based on the photography obtained on those flights, and satellite imagery available at the time. GeoNet, a geohazards monitoring programme operated by GNS Science, funded the reconnaissance flights and access to satellite imagery. A map of landslide distribution was produced to show approximate landslide extent and severity, using a severity scale of slight (<1% of hill slopes affected), moderate (1–10%) and severe (>10%). Areas of severe landsliding generally corresponded with areas where 48 hour rainfall totals exceeded 150 mm, and occur in the lower and mid reaches of the Whanganui, Whangaehu and Turakina catchments in the Horizons Regional Council area. In the Taranaki Regional Council area, areas of moderate to severe landsliding occur in the lower and mid reaches of the Whenuakura and Waitotara catchments, throughout the Patea catchment, in the hill country between Toko and Whangamomona, and in the upper Waitara catchment. Within these areas the steep dissected terrace-lands between Patea and Turakina were the most severely affected. The June 2015 storm affected much of the same area between Waverley and Woodville as the February 2004 storm, which was the largest on record. While the 2015 landsliding was not as severe across the region, locally some farmers report landsliding was worse than in 2004. Landsliding was largely confined to areas in pasture, or recently planted or logged forest. Landsliding was infrequent in areas of indigenous forest and scrub, and closed canopy exotic plantation forest. Most landslides were shallow (1–2 m deep), and occurred on steep, north facing slopes between 20 and 35°. There were several large deep-seated landslides triggered by the storm in the Taranaki region. Streambank erosion was locally severe in the mid and lower reaches of the larger rivers. The outlet channels of two prehistoric landslide-dammed lakes eroded during the storm, leading to a drop in lake level of about 6 m (equivalent to a loss of ~500 000 m3 of water) in one case, and the complete draining of the lake in the other. These events highlight a need to investigate the potential for other barrier failures and resulting downstream flood hazard, associated with the many landslide-dammed lakes in the Taranaki and Horizons regions. Exposed landslide debris and lake-bed sediment may provide opportunities to identify the ages of these landslides and to compile a storm history for the region. (auth)