The landslide response to the 14 May 2015 high intensity rainstorm in the Kapiti Coast-Hutt Valley area

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Page, M.J.; Rosser, B.J. 2015 The landslide response to the 14 May 2015 high intensity rainstorm in the Kapiti Coast-Hutt Valley area. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/20 70 p.

Abstract: Following a high-intensity rainstorm in the Kapiti Coast-Hutt Valley area on the 14 May 2015, GNS Science undertook a reconnaissance helicopter flight to provide an overview of the distribution of landslides and other erosion processes. Rainfall for the storm was not exceptional, being in the range of 80-150 mm in 24 hours, although one hour maximums of between 20 and 30 mm were recorded in some areas. While this caused severe localised flooding and several road closures, the limited number of reported landslides indicated that rainfall was near the threshold for landsliding on the largely greywacke hill country terrain. Landslides generally occurred on slopes greater than 25°, with most landslides occurring on slopes between 30° and 40°. There appeared to be a threshold slope angle of about 18° for landslide initiation during the storm, while the average slope at the area of initiation of landslides was 37°. Almost all observed landslides occurred on hill country in pasture, or in recently logged pine forest plantations. There were also a number of instances where, despite the lack of catchment landsliding, rainfall intensity and runoff was sufficient to initiate incision (gullying) in hill slope swales/ephemeral channels. These are sites where sediment has been accumulating for 100s to 1000s of years, and are a component of hill slopes throughout the Kapiti Coast-Wellington area. As such they represent a potential hazard to infrastructure located at the catchment outlet. In several cases the resulting debris floods were the cause of road closures, including State Highway 1 between Pukerua Bay and Paekakariki, which was similarly affected by storms in 2003, 2004, 2005 and 2006. (auth)