Debris flows and bridge losses at Waterfall Creek, SH6 at Lake Wanaka, New Zealand

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McSaveney, M.J. 1995 Debris flows and bridge losses at Waterfall Creek, SH6 at Lake Wanaka, New Zealand . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 95/21 18 p.

Abstract: Waterfall Creek alongside Lake Wanaka was bridged with a single-lane, 10-m span structure of wood over steel beams for 50 years without incident. A two-lane, concrete replacement was destroyed by a large debris flow in January 1983, within months of completion. Within several months of the loss, construction-triggered slope instability required removal of 40,000 cubic m of rock from above the northern bridge approach. A temporary Bailey bridge was replaced with another two-lane, concrete bridge in April 1989, but this replacement too was destroyed by an even larger debris flow, in January 1994, despite a small shift in the bridge site to reduce the risk. The power of this debris flow at peak discharge was estimated at 350 Mw (460,000 horsepower or equivalent to about 1.4 tonnes of TNT per second). The waterway now is bridged with a two-lane, heavily armoured, concrete and steel culvert intended to withstand the impact of future debris flows. Expenditure to date on replacing the original minor bridge on a lightly travelled road has been $(NZ)1.4 M (about $(US)0.94 M) over 13 years. The engineering and geotechnical investigations to date have been to high standard for a small bridging project, but the work has been very much ad hoc and not without errors of fact and judgement. Contractors were seen as adversaries, not members of the project team sharing a common goal. This resulted in delays, cost over-runs, and ultimately led to a very costly slope-instability problem which continues today (although the instability may have been unavoidable). Work to mitigate floodwater jetting onto the bridge, by smooting and widening the watercourse to speed water flow, also sped powerful debris flows, causing an immensely worse hazard to jet onto the site. Had the first replacement bridge been built 4 m higher, water could not have jetted onto its deck, construction may not have triggered slope failure, and the bridge probably would have survived at least the debris flow of 1983. By taking an ad hoc approach to the problem, successive teams changed the natural environment to accommodate preferred engineering structures, locking future teams into ever more costly ad hoc attempts at solutions. There was nothing at the start of the minor project, however, to suggest that the simple approach taken was not appropriate for the task. (auth)