Sedimentation rates and processes in the Te Whaiau / Otamangakau system, Central North Island, New Zealand

Abstract

This study assesses how, where, and at what rate sediment is accumulating in the Te Whaiau/Otamangakau System. This system consists of two canals (Te Whaiau Canal and Otamangakau Canal) and two reservoirs (Lake Te Whaiau and Lake Otamangakau). It was constructed as part of the Tongariro Power Scheme to transport water diverted from rivers which drain the western slopes of the mountains within the Tongariro National Park. Entrained in this water are sediments which require trapping. The system also traps fine sediment to reduce silting in Lake Rotoaira downstream, the main storage reservoir. Computer modelling of cross-section and profile changes over time has shown that sediment is accumulating at the rate of 34 700 ± 5000 m3 a-1. Approximately 88% of the sediment is considered to be diverted in origin, with the largest contributor being the Whakapapa River. Sediment depo-zones form dominantly at the upstream end of the system where 21% of the surface area is trapping 73% of the sediment. Aggradation rates are highly variable, from a maximum of 77 mm a-1 in Lake Te Whaiau to a minimum of 5.5 mm a-1 in Lake Otamangakau (excluding the delta). Sediment is being supplied to the Te Whaiau/Otamangakau System at a rate of 199 t km2 a-1 and this represents between 30% and 40% of the suspended sediment being carried in all the contributing rivers combined. The remainder escapes the system when flood flows overtop the river bed intakes. Accumulation is such, that if dredging ceased, flow out of the system would be severely reduced in approximately twenty years, and cease in fifty years. On the basis of textural analysis five sedimentary environments are recognised. The dominant grain size deposited is sand, particularly fine sand. 1 Well to poorly sorted sands with rare gravel and mud which originate from the diverted catchments and represent the bulk of sediment entering the traps. These are all deposited at the head of the system. 2 Poor to very poorly sorted fine silts and clays which also originate from the diverted catchment and are being trapped in the Main Basin of Lake Otamangakau. 3 Very poorly sorted mixtures of sediment comprising of fine material from the diverted catchment and coarse material from the natural catchment. This is deposited on the shallow shelves in the Main Arm and adjacent to the delta in lake Otamangakau. 4 Very poorly sorted gravel/sand/mud mixtures derived from the natural catchment and deposited in the North and South Arms of Lake Otamangakau. 5 Poorly sorted gravelly sands deposited on beaches and where streams enter Lake Otamangakau along western shores. These sediments are eroded from the Taupo Tephra Formation which mantles the natural catchment Sediment supply is proportional to discharge and is greatest in spring and early summer, when melting snow combines with high rainfall. Sediments deposited in channelled areas show a clear relationship between water velocity and particle size that agrees with the established model of Sundborg (1956). Within these zones water velocity is the primary influence on sediment deposition and dispersion. Outside these channelled areas, in Lake Otamangakau, influences such as wind, thermal stratification, and aquatic macrophytes, modify sediment distribution.