Predicting fluvial-deltaic aggradation in Lake Roxburgh, New Zealand. Test of a Water and Sediment Routing Model

Sedimentation upstream of the Roxburgh Dam in central Otago, New Zealand, has created a delta of similar to50 x 10(6) m(3) since commissioning of the dam in 1956. Growth of the delta has decreased hydroelectric capacity and possibly aggravated flooding in the town of Alexandra at the upstream end of the lake. One mitigation strategy is to drop water levels in Lake Roxburgh during flood events and erode the delta, but it is difficult to know a priori how the system will behave under these conditions. Here, a coupled water/sediment routing model is proposed for heterogeneous size-density sediments (MIDAS) that predicts reasonably well temporal changes in bed elevations of up to 10 m and grain-size coarsening of almost an order of magnitude for a 33-year interval of delta growth and flushing. Model-data comparison also shows that the rate of downstream fining of the delta increases during aggradation, corroborating the suggestion of others that fining rates in ancient alluvial deposits reflect basin subsidence rates as well as distance to source terrain.