Shoreline Erosion on Lake Hawea, New Zealand, Caused by High Lake Levels and Storm-Wave Runup

Abstract

Lake Hawea lies within a glacial valley on the South Island of New Zealand. The southern shore of the lake consists of easily eroded glacial moraine that represents a problem area due to homes constructed there. In 1961 the lake was converted into a reservoir and its level was raised by 20 m with an operating range of about 8 m. The raised water levels produced erosion of the moraine to form a nearly vertical cliff. The released gravel forms a fronting beach that is now sufficiently wide to reduce further cliff erosion except during times when high lake levels coincide with the generation of waves by strong winds blowing along the length of the lake. Such an event occurred during December 1995 through January 1996, resulting in significant cliff erosion and the destruction of a revetment that had been built following a similar occurrence in 1984. The 1995/96 erosion is analyzed using a model that evaluates total water levels achieved by the runup of waves superimposed on measured lake levels. The total water levels are compared with geomorphic features associated with this extreme event, including wave-cut notches in the eroded cliff and elevations of accretional beach ridges that were overtopped by the high water levels. A plan has been developed to reduce future erosional impacts, one that includes building a wider beach in critical areas by beach nourishment. The construction of groynes would restrict movement of the nourished gravel and trap additional sediment from longshore transport, material that otherwise has been a problem because of its accumulation adjacent to the dam on the reservoir.