GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project E-1 E-1 Model Description A water balance model was used to assess how water gains change over the life of the mine and to check that proposed infrastructure for conveyance, storage and treatment will be adequate. The GoldSim water balance model described in this section was first developed in 2012 as an initiative of the site environmental team. The objective in building the model was to have a tool to forecast storage requirements in the TSF’s in the future (i.e. to check embankment construction was aligned) and as an ongoing check that the site water management infrastructure as a whole had capacity for ongoing mine development. The model was also used to predict water treatment requirements post closure as a component of annual bond calculations. Given the model has been calibrated against the site water balance and in use for some time there is confidence that it does represent well the quantities of water generated from the different water sources that require treatment. The model has been recalibrated against 2015 to 2017 site operation data and modified to represent the changes to the mine plan development timeframe and additional water quantities requiring treatment under the Waihi North Project. E-2 Introduction to GoldSim GoldSim (refer www.goldsim.com) is a software package designed to run Monte Carlo simulations for probabilistic analysis of dynamic systems. The software package essentially provides a visual interface for an excel spreadsheet type programming environment. Within the visual interface, elements are created to represent processes and events using equations and logic based decisions. GoldSim allows user interfaces (dashboards) to be created which are used to specify model inputs and variable data before or during simulations. Models developed in GoldSim with appropriate dashboards can be exported as an executable program that can be run with the freely available GoldSim player software. GoldSim includes features which make it a suitable tool for water balance modelling. These include; user specified time stepping, dimensional awareness and a range of programmed elements that can be implemented. Model updates can be scheduled to occur at user specified time steps for the duration of a simulation and these can be specified to complement input data resolution such as rainfall or river flow. To improve modelling accuracy unscheduled model updates are also included to capture dynamic processes such as the time at which a reservoir begins to overflow or a discrete event is triggered. Dimensions are specified for all input data and an internal database is applied to unit conversions to ensure consistency throughout the model. Within the GoldSim interface programmed elements are implemented to simulate a variety of dynamic and discrete events. The range of elements that are useful for water balance modelling include; Containers – allow a model to be subdivided into sub-systems, for example, individual catchments or processes. Inputs – this category of elements allow initial conditions to be specified and lookup tables to be defined containing information such as daily rainfall. Stocks – these elements capture how the state of a system changes with time and can represent reservoirs, ponds or other stores. Functions – these elements define processes or decisions at each model update, for example, calculating catchment runoff or the current discharge quality. Scripts can also be written to simulate complex behaviour or iterative processes.
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