An inventory of deep-seated landslides in the Waipaoa and Waimata catchments

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Page, M.J.; Lukovic, B. 2011 An inventory of deep-seated landslides in the Waipaoa and Waimata catchments. Lower Hutt, N.Z.: GNS Science. GNS Science report 2011/08 75 p.

Abstract: An inventory of deep-seated landslides in the Waipaoa and Waimata catchments on the East Coast of the North Island, was compiled to provide statistics on their type, features, size (area and volume), and distribution in relation to a number of factors. This data set will be used to extrapolate spatially and temporally the results of more detailed studies and, together with information from a number of environmental records, inform numerical models. The purpose is to identify the role that deep-seated landslides play in landscape evolution and their associated contribution to long-term (~18 ka) sediment flux, as part of a FRST funded programme “Terrestrial Landscape Change: MARGINS Source-to-Sink New Zealand”. Large deep-seated landslides are common and catchment-wide. A total of 1026 landslides were recorded, covering 28,725 ha, and occurring mainly within Tertiary and Quaternary terrain. For a further 17,668 ha, mainly in the East Coast Allochthon, where practically the entire terrain is affected by landsliding, individual landslides were not recorded. Combining the area of mapped landslides with this area of East Coast Allochthon terrain gives a total of 46,393 ha or 18.5% of the combined Waipaoa and Waimata catchments affected by deepseated landsliding, or 21.4% of the hilly terrain. Landslide density (excluding the East Coast Allochthon) is 0.49/km2. Forty-five percent of the mapped landslides are slide/flow complexes in Miocene-Pliocene mudstones and sandstones, and 76% are 105 - 106 m2 in area. The major factors influencing the location of landslides are; lithology, slope steepness, and stream incision. The largest landslides are mainly dipslope failures. Landslide connectivity to the stream network is very high (83%). However, upstream sediment accumulation due to channel blockage by landslides was only recorded in 3% of cases, indicating that channel blockages are relatively short-lived, or that accumulated sediments have been subsequently eroded. Under the present mainly pastoral land use, the high sediment yield of the Waipaoa River is dominated by gully and shallow landslide-derived sediment. However, prior to human deforestation, deep-seated landsliding was a relatively more important processes delivering sediment to stream channels, and has exerted a major control on hillslope morphology. The distribution of these landslides in relation to the position of knick points in stream gradients, suggests that oversteepening of slopes and removal of toe support by river incision, has primed hillslopes for failure. While historically only high intensity rain storms have triggered deep-seated landslides in the catchments, elsewhere in New Zealand they have also been triggered by earthquakes. Combining this landslide data set with paleoearthquake and paleostorm records and landslide ages offers the possibility of adding greater temporal resolution to the long-term sed iment flux/budget for the Waipaoa and Waimata catchments. An initial estimate, based on an average depth of 20 m and average Sediment Delivery Ratio (SDR) of 0.25, indicates that deep-seated landslides may have contributed ~2.3 km3 or ~10% of the post-18 ka sediment budget, and that this is ~40% of the contribution from shallow landsliding. Large, deep-seated landslides are a recognised hazard. In the Waipaoa and Waimata catchments, their number, size and occurrence under both forest and pasture, indicates that there are associated risks hitherto under-appreciated in this type of landscape, and that these risks will increase with increasing rural development. (auth)