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.

Sustainable management of river systems involves balancing multiple objectives. These include alleviating flood hazards and the risks they pose to people and critical assets whilst promoting good ecological status by supporting healthy biological communities and enhancing habitat diversity. In regions with high rates of coarse sediment supply to rivers, management often involves addressing the issues associated with the progressive accumulation of gravel within channels. Such sedimentation can raise riverbeds levels, resulting in reduced flood capacity which in turn may result in an increased probability of flooding and a reduction in the standards of protection associated with existing defences. One approach to manage this hazard is through the extraction of riverbed gravels to restore flood capacity, correct river alignments, prevent bank erosion and reduce the threat of catastrophic course changes. The extracted river gravels are also not without value and represent an important source of aggregate for the construction industry. So much so, that gravel-bed rivers close to urban areas are often considered ideal mines of readily available sediment. This situation can, therefore, be presented as a potential win-win game. As long as gravel extraction is balanced against the naturally occurring upstream sediment supply, an adaptive management regime can be devised to maintain flood capacity whilst generating a key commercial resource. However, it is now well-established that estimating this balance incorrectly and over-extracting gravels can lower the riverbed, steepen the channel gradient, leading to enhanced bank erosion and paradoxically reduce flood protection by destabilizing existing flood control measures. Additionally, removal of the typically coarse surface layer of riverbed gravels can alter the bed sediment composition creating a flush of fine sediment that degrades invertebrate and fish habitat. Plans to dredge rivers to enhance flood capacity, so prominently popularized by the recent events in the Somerset Levels, must therefore be based on cautious, scientifically-informed and evidence-led strategies to plan, implement and review interventions. Traditionally, sediment management plans have been based on data from sparse networks of river cross-sections. These provide a basis for monitoring trends in bed levels through periodic resurveys. The resulting data can also be used to determine a morphological gravel transport rate and estimate the background rate of sediment supply. Recent research has shown that the river level and gravel transport estimates based on section data, which is effectively blind to the river morphology between sections, can incorporate significant bias giving rise of 2-3 order of magnitude uncertainties the key data used to drive management strategies. Advances in remote sensing offer a solution to alleviate this bias by estimating channel changes through the comparison of 3D elevation models through time. Differences between these models provide reliable measures of elevation change and can be integrated to assess regional trends. Historically, the high costs of acquiring dense topographic data to create these models has prohibited their use for routine monitoring. Continuing developments, most notably in photogrammetry methods have recently and dramatically reduced the cost of these data and removed a bottleneck preventing their adoption. In this project we will work with a group of stakeholders from national and local government in the UK and NZ to develop a software tool that can support routine channel monitoring using these new streams of dense 3D topographic data. The resulting tool will facilitate simplified workflows that can be easily implemented by agency and authority staff and used to present the results within a statistical uncertainty framework that accounts for errors in the underlying topographic data.