The effects of an Alpine Fault earthquake on the Taramakau River, South Island New Zealand.

An Alpine Fault Earthquake has the potential to cause significant disruption across the Southern Alps of the South Island New Zealand. In particular, South Island river systems may be chronically disturbed by the addition of large volumes of sediment sourced from coseismic landsliding. The Taramakau River is no exception to this; located north of Otira, in the South Island of New Zealand, it is exposed to natural hazards resulting from an earthquake on the Alpine Fault, the trace of which crosses the river within the study reach. The effects of an Alpine Fault Earthquake (AFE) have been extensively studied, however, little attention has been paid to the effects of such an event on the Taramakau River as addressed herein. Three research methods were utilised to better understand the implications of an Alpine Fault Earthquake on the Taramakau River: (1) hydraulic and landslide data analyses, (2) aerial photograph interpretation and (3) micro-scale modelling. Data provided by the National Institute of Water and Atmospheric Research were reworked, establishing relationships between hydraulic parameters for the Taramakau River. Estimates of landslide volume were compared with data from the Poerua landslide dam, a historic New Zealand natural event, to indicate how landslide sediment may be reworked through the Taramakau valley. Aerial photographs were compared with current satellite images of the area, highlighting trends of avulsion and areas at risk of flooding. Micro-scale model experiments indicated how a braided fluvial system may respond to dextral strike-slip and thrust displacement and an increase in sediment load from coseismic landslides. An Alpine Fault Earthquake will generate a maximum credible volume of approximately 3.0 x 108 m3 of landslide material in the Taramakau catchment. Approximately 15% of this volume will be deposited on the Taramakau study area floodplain within nine years of the next Alpine Fault Earthquake. This amounts to 4.4 x 107 m3 of sediment input, causing an average of 0.5 m of aggradation across the river floodplains within the study area. An average aggradation of 0.5 m will likely increase the stream height of a one-in-100 year flood with a flow rate of 3200 m3/s from seven metres to 7.5 m overtopping the road and rail bridges that cross the Taramakau River within the study area – if they have survived the earthquake. Since 1943 the Taramakau River has shifted 500 m away from State Highway 73 near Inchbonnie, moving 430 m closer to the road and rail. Paleo channels recognised across the land surrounding Inchbonnie between the Taramakau River and Lake Brunner may be reoccupied after an earthquake on the Alpine Fault. Micro-scale modelling showed that the dominant response to dextral strike-slip and increased ‘landslide’ sediment addition was up- and downstream aggradation separated by a localised zone of degradation over the fault trace. Following an Alpine Fault Earthquake the Taramakau River will be disturbed by the initial surface rupture along the fault trace, closely followed by coseismic landsliding. Landslide material will migrate down the Taramakau valley and onto the floodplain. Aggradation will raise the elevation of the river bed promoting channel avulsion with consequent flooding and sediment deposition particularly on low lying farmland near Inchbonnie. To manage the damage of these hazards, systematically raising the low lying sections of road and rail may be implemented, strengthening (or pre-planning the replacement of) the bridges is recommended and actively involving the community in critical decision making should minimise the risks of AFE induced fluvial hazards. The response of the Taramakau River relative to an Alpine Fault Earthquake might be worse, or less severe or significantly different in some way, to that assumed herein