Shoreline erosion on Lake Hawea, New Zealand, caused by high lake levels and storm-wave runup

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 t hat represents a problem area due to homes constructed there. In 1961 the l ake 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 fr onting beach that is now sufficiently wide to reduce further cliff erosion except during times when high lake levels coincide with the generation of w aves by strong winds blowing along the length of the lake. Such an event oc curred during December 1995 through January 1996, resulting in significant cliff erosion and the destruction of a revetment that had been built follow ing a similar occurrence in 1984. The 1995/96 erosion is analyzed using a m odel that evaluates total water levels achieved by the runup of waves super imposed on measured lake levels. The total water levels are compared with g eomorphic features associated with this extreme event, including wave-cut n otches in the eroded cliff and elevations of accretional beach ridges that were overtopped by the high water levels. A plan has been developed to redu ce future erosional impacts, one that includes building a wider beach in cr itical areas by beach nourishment. The construction of groynes would restri ct movement of the nourished gravel and trap additional sediment from longs hore transport, material that otherwise has been a problem because of its a ccumulation adjacent to the dam on the reservoir.