Mass movement hazard to infrastructure, Arthur's Pass to Greymouth.
Thesis DisciplineEngineering Geology
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
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.