MARINE MODELLING IN SUPPORT OF KOOLAN ISLAND MINE RECOMMENCEMENT AND EVENTUAL DECOMMISSIONING

Abstract

Koolan Island is located in a remote macrotidal (9m mean spring range) area of the Buccaneer Archipelago of NW Australia. High-quality hematite has been mined from the island since the 1940s, with direct export by ore carriers from a dedicated marine terminal. Mining in recent decades has focused on an open pit immediately adjacent to the ocean. The initial operator decommissioned the site in 1993, at which time a channel connecting the pit to the sea was excavated, the pit was flooded and a productive marine ecosystem developed within the photic zone of the former mine. The site was subsequently acquired by Mt. Gibson Iron, which drained the pit in 2007 and constructed a protective seawall to facilitate additional ore extraction through the extension of the pit both in depth as well as closer to the historical shoreline. A catastrophic failure of a section of the seawall in 2014 re-flooded the pit, which resulted in no injuries as the site was successfully evacuated in response to instrumented movements in the seawall structure. Construction of a strengthened seawall has recently been completed, and pit dewatering activities will be initiated during the week of the ICCE conference to enable in-Pit mining activities to recommence by the end of 2018. The presentation will present a summary of the unique history of the site, as well as supporting numerical modelling performed to describe a) the complex regional and local macrotidal dynamics, b) sediment spillage in connection with the reconstruction of the seawall following the 2014 failure, and c) coupled 2D/1D model calculations optimizing channel geometry to ensure adequate flushing of the pit following its eventual re-flooding in connection with final decommissioning.