Quantifying the Delivery & Dispersal of Landslide-Derived Sediment to the Dart River, New Zealand

Following heavy rainfall on the 4/1/14, a major debris flow at Slip Stream (44.59 S 168.34 E) introduced >10^6 m^3 of sediment to the Dart River valley floor in NZ Southern Alps (S. Cox, pers. comm). Runout over the existing fan dammed the Dart River causing a sudden drop in discharge downstream. This broad dam was breached quickly, however the loss of conveyance has since impounded a 4 x 1 km lake with depths that exceed 20 m.

This event presents a rare and unprecedented opportunity to study the impacts of a discrete, high magnitude 'sediment pulse', remarkable in its capacity to dam a large river in flood (peak discharge during the event was recorded at ~790 m^3/s). Quantifying the impact of this disturbance on the form and stability of the receiving body, the Dart River, will advance our understanding of how such low frequency geophysical events shape the evolution of large alpine rivers and will create a vital baseline for future research that seeks to test theories of how such large bed wave propagates and disperse sediment downstream.

The impact of this pulse also elevates the risks posed by natural hazards in the region. Enhanced sediment transport has the potential to raise riverbed levels, destabilise floodplain assets, reduce standards of flood protection, increase the risk of channel avulsion and impact on freshwater and riparian ecology with a legacy that long outlasts the initial disturbance. Locally, this event may result in rapid advance of the Dart-Rees delta into Lake Wakatipu threatening the lakeshore communities of Glenorchy and Kinloch. The assessment of how large fluvial sediment pulses migrate, disperse and condition such hazards will offer key insights that may be transferable to other dynamic alpine settings. However, in order to constrain this event effectively, an initial topographic and sedimentological survey must be undertaken urgently, in the immediate aftermath of the event, to enable robust quantification of the sediment pulse and the existing channel morphology.

This research aims to advance this goal by seeking to: develop a unique baseline dataset that will be used to quantify the delivery and dispersal of sediment inputs from the Slip Stream landslide, from its source at Te Koroka to its sedimentary sink in Lake Wakatipu. Using a combination of aerial, terrestrial and bathymetric surveying, we will acquire two synoptic, system-wide snapshots of this highly charged sediment cascade that record the 3d morphology and sedimentology of the interlinked components of the sediment transfer system. Surveys will be undertaken in April 2014 and then one year later in March 2015, following the annual summer floods that dominate fluvial sediment transport in the region. The first survey will establish the initial state of the system and so create the opportunity to quantify the downstream pattern of sediment storage and transport through comparison with the second and any subsequent re-surveys direct differencing of Digital Elevation Models. The simultaneous bathymetric surveys of the upstream impounded lake and the delta morphology will provide constraints on sediment flux across the boundaries of the study area, enabling closure of the coarse sediment budget.

The combined results of these two survey campaigns will create an unparalleled dataset to help frame and test hypotheses that seek to explain the dispersal of major sediment pulses within rivers.

The natural environment and geophysical processes of the Dart River catchment sustain a wide range of ecosystem services that impact significantly on the regional economy, cultural heritage and social welfare of the residents of the Wakatipu basin and beyond. In addition, this specific geophysical event also elevates the potential risks associated critical natural hazards - both on-site (at the landslide) and off-site (through impacts upstream in the backwater lake and the downstream events triggered by the sediment pulse).

The complex chain of processes now set in action presents a series of major challenges for the local inhabitants of the region and its wider social, economic and cultural stakeholders. These concerns have both immediate consequences and future impacts which are yet to be realized and include:

1) The recent activity at Slip Stream threatens a sacred source of the precious nephrite jade, known as 'pounamu' and treasured by the Maori communities of New Zealand. The local caretakers of the site, or Kaitiaki, our project partners Te Runanga o Ngai Tahu, are anxious to understand the processes that are undermining Te Koroka, which due to its history of limited human occupation and unspoilt archaeology is unique within New Zealand.

2) The new lake formed upstream of Slip Stream has submerged approximately 3 km of an internationally renowned hiking trail, the Dart-Rees Track. While the track has temporarily re-routed, continuing instability poses serious questions over the continuing access to this popular trail which is a major source of tourist revenue for the nearby communities of Glenorchy, Kinloch and Queenstown.

3) The downstream reworking of the sediment pulse has the potential to raise riverbed levels on the Dart with a series of co-related impacts. These include: a reduced standard of protection associated with existing flood defences at Kinloch and Glenorchy; elevated risk of major avulsion that could threaten critical infrastructure (roads and bridges) and farmland; the potential for rapid progradation of the Dart River delta, that could impact seriously on the tourist-based township of Glenorchy, posing a major flood hazard and drowning the local wharf facilities.

4) Aggradation of the braided channel also has consequences for the activities of a major commercial jet boat business operating from both Queenstown and Glenorchy (Dart River Safaris - an internationally renowned tourist enterprise and major component of the regional economy) and other public users of the river.

5) Riverbed aggradation and increases in the flux of fine sediment may also have a profound impact on the quality and sustainable of the regional fisheries, directly in the Dart River, but also for the wider Lake Wakatipu. Fishing for the trout and salmon are a major draw to the region both nationally and internationally.

6) Finally, there is mounting concern (see the letter of support from PP GNS Science) that the increased availability of fine sediment on the exposed fan and bars of the Dart River, might contribute to major dust-storms the could cause respiratory problems for vulnerable inhabitants.

The importance of these impacts on the environment is exacerbated by the regional importance of tourism to the local economy of region, prized for its pristine mountain scenery. However, while these consequences are largely regional in scale, an improved understanding of the processes that link catastrophic sediment pulses to the dynamics of the receiving rivers, has broader implications for the management of alpine rivers worldwide, many of which occupy valley floors with considerable densities of population (e.g., European Alps). Advancing the science base to help understand and predict the consequences of such landslides into rivers is therefore urgently required to support strategic and emergency planning.