The February 2004 Landslide Event in Geomorphic Perspective

<p>In February 2004 a severe storm impacted the lower half of the North Island, New Zealand. Intense rainfall during the storm triggered extensive landsliding throughout the Tertiary hill country of Wanganui, Manawatu, and Wairarapa. The storm event also produced floods estimated to have a return period of 100 years. Flooding impacted on many communities, destroying homes, drowning livestock, and ruining crops. Because the effects of flooding were more immediate, and affected a greater number of people, landsliding damage received little coverage in the news media. However, the importance of these large rainfall-triggered, multiple landslide events that occur periodically in New Zealand should not be underestimated. New Zealand is losing valuable hillslope soil through erosion processes at a rate far in excess of the development of new soil. Landsliding is the most obvious and active hillslope erosion process operating in the hill country of New Zealand today. This study examines the impact of the February 2004 landslide event from a geomorphic perspective, addressing questions such as: what changes to landforms were produced by this event, and, how much geomorphic work (volume of material, moved a given distance in a given time) was done by landsliding during the event. The proposition underlying this study is that it is not just the magnitude of the triggering event that determines the geomorphic response in terms of landform change and work done, but also that the nature of the terrain influences the magnitude (e.g. landslide densities, volumes, areal extent) of the landsliding produced. In order to test this hypothesis the study was undertaken in two parts. The first, a catchment-based study using mostly field methods to produce a sediment budget and landform change measurement. Secondly, a regional analysis of four areas which experienced the most severe landslide damage were analysed in terms of terrain and landslide characteristics. From the methodologies employed in these studies it is demonstrated that terrain characteristics are highly influential in determining the type and severity of landsliding. To determine the geomorphic significance of the event in terms of the history of similar New Zealand landslide events, a frequency-magnitude analysis comparison was conducted, and the results compared with studies of previous rainfall-triggered, multiple landslide events. The results of the catchment-based study, the regional study, and the frequencymagnitude analysis show that the February 2004 event is likely to be the most geomorphically significant event of its type (rainfall-triggered) to have occurred in New Zealand over the past 100 years. The area affected (16,000 [square kilometer]) and number of landslides produced (~70,000) are greater than previously documented events. Landslide densities are also amongst the highest recorded in New Zealand. Although the majority of landslides were shallow regolith failures, large scars from deep-seated, rotational landslides will be visible in the landscape for hundreds of years. Material eroded from hillslopes during the event is estimated (conservatively) to be in excess of 20 million tonnes. While the majority of this eroded material remains within the hillslope system (depositional slopes and fans), a considerable proportion (an average of 25 % in the study catchment) is transferred to fluvial systems via fluvial coupling and removed from hillslopes permanently.</p>