Hydrology and Sediment Modelling in a South Pacific Environment

A proposed mine development project located in the South Pacific required predictions of sediment loads and concentrations that could be discharged from site and how this varied over the life of the mine. The site location was in extremely rugged terrain with a high annual rainfall of approximately 4.7 m and also subject to cyclonic activity. There were two major sediment ponds proposed for the site to reduce sediment discharges. Total Suspended Solids (TSS) concentration was the key criterion that was required to be modelled as this was referenced back to the discharge water quality guidelines. A sediment and hydrology model was developed in Goldsim to provide daily estimates of runoff flows and TSS concentrations. A modified model of the Australian Water Balance Model (AWBM) was developed and incorporated into the Goldsim model and calibrated using recorded field data.

The sedimentology module in the Goldsim model was developed based on the Revised Universal Soil Loss Equation (RUSLE). As RUSLE is an annual prediction of soil loss over a catchment, modifications were required to provide daily estimates of TSS concentrations. This was achieved by proportioning the annual R value over the year with the assumption that higher rainfall (intensity) days produced proportionally higher rainfall erosivity. Using RUSLE, a land-use type erosion vector was developed for 19 different land-use types (for example natural ground, open-pit, waste dump, etc.). For each sub-catchment (57 in total), the proportion of land-use type was calculated and then used to produce a sub-catchment erodibility vector. This vector was multiplied by the daily rainfall erosivity factor to estimate the daily soil loss from each sub-catchment. The estimated soil loss and the modelled runoff produced predictions of daily TSS concentrations. Finally a sediment settling module was used to predict the daily efficiency of the sediment ponds and hence discharged TSS concentrations.

Ind, M. 2014. “Hydrology and Sediment Modelling in a South Pacific Environment,” in: Hydrology & Water Resources Symposium 2014, Perth, Australia, February 24-27, 2014. Perth: Engineers Australia.

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