Circulation and sedimentation in Lake Benmore, New Zealand

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Abstract
Benmore is the largest artificial lake in New Zealand. The lake has two arms which have quite different inflows in terms of volume, temperature, and suspended particulate matter (SPM) loadings. The Ahuriri Arm is fed by natural runoff from unglaciated catchments, whereas the Waitaki Arm receives the outflow of three large glacial-fed lakes which filter out much of the SPM and restrict the temperature range of water entering Lake Benmore. Geographical isolation of the two arms allows each to develop its own annual cycle of lake/ river interactions. The Ahuriri water, which is more responsive to seasonal temperature changes, underflows the warmer Waitaki water in winter and overflows in summer. Upstream lakes entrap most of the bedload from the upper catchment and much of the coarse silt fraction in the suspended load. Consequently, there has been little delta growth with most of the suspended load being clay, which is swept further down the lake. Since forming in 1965, the shoreline (42%) has been altered by limnic processes, with beaches forming in sands and gravels, steep shores being cliffed, prefill subaerial slope failures being rejuvenated, and bedrock and low slopes left unaffected. However, shoreline erosion has only contributed about 8% of the sediment infilling the lake with more than 85% originating from river-borne SPM. Sedimentation rates (3 mm/year) are lower than in the natural lakes upstream, and sediment deposition should not create any problems to the efficient running of the power station during its design life.

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