The April 2017 flooding of the Fitzroy River, Queensland, was an insignificant sediment source to the Gladstone Harbour system

The Fitzroy River discharges into Keppel Bay, immediately north of the Gladstone Harbour system. In April 2017, rainfall from ex-Tropical Cyclone Debbie caused the Fitzroy to flood. This research quantified the potential sediment flux from Keppel Bay through “The Narrows”, into the harbour by: i) taking detailed measurements associated with the rainfall event, using moored instruments to measure oceanographic and sediment parameters and ii) interpreting the key sediment transport processes, based on detailed measurements. Sediment transport in the Narrows is overwhelmingly dominated by the semi-diurnal tides, combined with estuarine processes during the wet season. Especially important are “overbank” tides, which drive ebb-directed sediment transport to the south towards the harbour, at instantaneous rates up to ~500 kg/s and net rates of up to ~20 tonnes per flood-ebb cycle. Smaller “within-channel tides” produce only 1 - 20% of this flux, with variable net direction. Over the 9-tide measurement period, there was a net southward sediment flux of ~60 tonnes, 60% of which resulted from only 2 flood-ebb tidal cycles. Extrapolated to an annual basis, the net sediment flux through the Narrows is <5,000 tonnes southwards, and may be only ~1,000 tonnes. For context, modelling indicates that i) the Calliope and Boyne rivers together may deliver ~65,000 tonnes/yr to Gladstone Harbour and ii) tides and wind re-suspend ~15,000,000 tonnes/yr of sediment within it. The relative contribution of sediment transport through the Narrows into the harbour is inconsequential, even during river flood events.