| Summary: | The purpose of this study was to investigate the threats posed by mass movement to
infrastructure between Arthur's Pass and Greymouth. This large area has been split
into three sections; Arthur's Pass to Jacksons, Jacksons to Greymouth via Kumara,
and Jacksons to Greymouth via Moana. The geology, topography, vegetation and
land use vary considerably within this area, and the main infrastructure elements
investigated are roads, the railway, and smaller townships.
Two of the specific objectives of the study were to develop an historical mass
movement inventory, and to create mass movement hazard maps. The West Coast
Historical Mass Movement Inventory was compiled from current and archived files of
infrastructure management organisations, and incorporates an inventory started by
West Coast Regional Council in 2002. Mass movement hazard maps were created
through aerial photograph interpretation and field mapping. They incorporate a fourtier
hazard ranking system, where red is high hazard or active in the last 5 years,
orange is moderate hazard or active in the last 50 years, blue is low hazard or not
active in the last 50 years, and non-coloured areas are considered to be very low
hazard areas. The series of seven 1:10,000 maps included in the thesis cover the
area along the infrastructure alignments east of Harris Swamp and Rotomanu, and
west of Arthur's Pass village.
The database can be interrogated with built-in searches to retrieve information on
specific sites, dates, and triggers. Other searches can be built, in the database,
which select different information. From the information contained in the database
and the mass movement hazard maps, key mass movement features, in particular
bedrock failures, colluvium failures, debris fans and alluvial fans, have been
identified. The threats to each route segment and township are compared on the
basis of how much damage they have caused in the past, and how recently they
have been active.
Another of the objectives of the study was to investigate triggering of mass
movements. Precipitation and seismicity are identified as the key triggers of
damaging mass movement events, with precipitation by far the dominant one in the
short time-frame assessed. However, the triggering relationships are complex and
thresholds for rainfall quantities that trigger mass movements, or above which mass
movements cause damage, were not able to be determined with any reliability.
The three main forms of damage to infrastructure in the study area are impact
damage, blockage of waterways at culverts and bridges, and erosion. The two most
common incidents of damage recorded in the mass movement inventory are debris
depositing on the road and/or railway, and culverts being blocked causing diversion
of streams and resulting in erosion and/or deposition elsewhere. Erosion by the
major rivers, the Taramakau River, the Otira River and the Grey River, also occurs
frequently. Remedial measures for these types of damage are briefly discussed,
particularly with reference to debris fans and debris flows, which are a major, and at
times underestimated, threat through the study area.
This thesis presents a method of assessing mass movement hazard to an
infrastructure corridor, and highlights problem sites along the route investigated. An
opportunity for further research exists in developing this method further, particularly
to investigate aggradation damage from large earthquakes.
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