Reaction to change: erosion, sedimentation and baselevel change in the Upper Karori Reservoir, Wellington, New Zealand

Abstract

The Upper Karori Reservoir, Wellington, New Zealand has recently been decommissioned as a water storage reservoir for the Wellington Regional Council. The sediments deposited over the life of the dam (some eighty five years) provide an excellent record of sedimentation and erosion history for the catchment The decommissioning of the dam and draining of the reservoir present an opportunity to test the actual sediment yield of the catchment against predicted sediment yields.. The estimate of 2154t/km2/yr for the catchment erosion rate, based on eighty years of record, is an order of magnitude larger than rates calculated by empirical formulae by Streeter (1982) 295t/km2/yr, McSaveney (1978) 230t/km2/yr, and Griffiths (1982) 256t/Km2/yr. This figure is high when compared to other catchments within the Wellington Region, for example Morton dam 27t/km2/yr; Pauatahanui 98t/km2/yr and Pauatahanui (Curry 1981) 37t/km2/yr. The reasons for some of these differences may stem from the methods of yield calculation, catchment size (and storage potential) and frequency, intensity and spatial distribution of sediment producing storms. Other North Island catchments have erosion rates that do compare well with the rate at Karori, although these catchments are much larger. Rates of ground lowering are comparable to other catchments in the Wellington region. The Karori reservoir has undergone two baselevel changes in its eighty five year history. The current regime has resulted in active down cutting of the river channels, and a rapid response within the catchment to large rainfall events. Both fine and coarse sediments are transported in the streams draining the upper Karori Reservoir catchment, however, coarse material (bedload size) is the prominent sediment type in transport during major storms. Fine lake sediments above the dam are only transported when the lake level has been reduced from its current operating level. Future use of the upper catchment has still not been finalised, however there has been a proposal to create a wildlife sanctuary. This will pose some resource management issues, such as what effect this development will have on the environment. In terms of the sediment transport regime and yield, however, any major construction work within the catchment is likely to have a negative impact on the catchment in the short term, as erosion rates increase during construction.