The lacustrine geomorphology of Lakes Manapouri and Te Anau.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis studies the lacustrine geomorphology of Lakes Manapouri and Te Anau. The shorelines of the two lakes are described and mapped. The type of shoreline produced is a product of the local geology and the effects of subaerial and lacustrine processes. Approximately three quarters of the shorelines are hard rock walls the remainder being deltaic and morainic pavements and beaches, arm head beaches and pocket sand beaches. Low lake levels have resulted in beach slumping and degradation, on the pocket sand beaches where surficial sediments are thin and beach basement materials fundamentally different from the surficial deposits. The beach and nearshore relief may be likened to an ideal wave worked continental shelf in terms of both profile geometry and shore normal sediment distribution. The wind and wave climates are described and wave forecast curves prepared; the low energy and narrow range of wave forms restricts the range of morphologies produced. Both lakes have a wide natural range of lake levels ensuring that lake levels control the elevation at which an extremely limited energy resource is dissipated on the beach face. The distinctive shelf profile has been formed during low levels when wave action is highly effective in disturbing material on the outer shelf. Since the lakes have come under control the natural range of levels has been reduced and lake levels lowered, wave effectiveness on the outer shelf has been increased accordingly. The wakes produced by passing launches modify the natural wave climate particularly at beaches exposed to short fetch lengths. The magnitude, frequency and direction of wave approach are all adversely affected. The discussion of beach morphology shows the profile form of the beaches to be in equilibrium with energy and level conditions; only minor shore normal transfers of sediment are required to maintain this balance. On beaches exposed to a wide range of wave approach directions and particularly those confined by well defined barriers changes in both the profile form and plan shape of the beach are necessitated to maintain equilibrium. Sustained periods of zero wave energy prevail, resulting in the stranding and drowning of beach forms during periods of fluctuating levels. Changes in beach morphology in response to fluctuating lake levels follow the Bruun effect, whereby there is a net shoreward movement of material on falling levels and net lakeward movement on rising levels. The erosion and deposition on the nearshore shelf takes the form of a wedge of material widest closest inshore, rather than that of a uniform change in elevation as suggested by Bruun. As such this provides one of the first field verifications of the Bruun effect beyond the breaker zone. Tracer experiments confirm that long periods of inactivity occur and that on both fine and coarse grained beaches the largest sediment transfers take place under and landward of the breaker. Sediment dispersal lakeward of the breaker on the outer shelf is slow and is not usually manifested in measurable morphological changes. Tracer experiments point to the nearshore shelf being a wave worked deposit formed during storm conditions at low lake levels exceeded greater than 87% of the time in the natural record. Recommendations for operational control of the two lakes confirm that the guidelines drawn up by the Guardians of Lakes Manapouri and Te Anau could not be improved on and if adhered to should maintain the stability of the lakeshore beaches.