2005 – Hydrogeology of the Ross River Dam

Robert Virtue, Deryk Forster, Jon Williams and Sabina Fahrner

Basic pre-construction foundation investigations for the Ross River Dam were done in the late ‘60s to early ‘70s but a more detailed hydrogeological assessment was carried out to investigate and manage water logging and salinity, which developed immediately downstream in the late 1970s.

As part of the 2005 Stage 2 to 5 upgrade design, detailed conceptual and numerical hydrogeological modelling was required to predict aquifer response along the embankment and downstream. This required “data mining” and additional drilling and aquifer testing to fill in data gaps, with the filtered and re-interpreted data used to build a 3D conceptual model of the embankment and underlying geology, by a design team comprising specialist hydrogeologists, geologists, geotechnical and damsengineers. This was converted to a 10-layer, 2-million cell numerical model, to enable high-resolution modelling of groundwater behaviour for a range of aquifer properties, flood hydrographs and seepage management options. As well as a design tool, the model is a valuable monitoring tool in confirming the performance of seepage management systems and to provide early warning of seepage management failures.

The study emphasised the need to capture data for a wide range in aquifer stress, to have simple preliminary spreadsheet models to provide a “sanity check” and to collect data away from the embankment to allow a 3D interpretation of the geology, to the assumption of “layer cake” models.