Estuaries are the confluence point at which rivers carrying freshwater and terrestrially derived sediments and nutrients meet the marine environments of the sea. They are extremely dynamic and are constantly changing and adapting to variations in climate, sediment source, and anthropogenic activities. Estuaries hold great ecological, social, cultural, and economic importance, with more than half of the world's population living along estuaries and the coast. Ecologically, they provide an environment where freshwater and saltwater meet and mix, are usually high in nutrients and shallow enough for photosynthesising benthic habitats and coastal ecosystems to grow and thrive. From a socio-economic perspective, estuaries provide sheltered natural harbours for establishing trade routes, accessing natural resources, and facilitating tourism and recreation. However, due to their dynamic location, they are vulnerable to the impacts of extreme weather events, climate change, and urbanisation, which can result in changes to their health, quality, and ability to continue to support ecological and socio-economic aspects.

In the Australian region of southeast Queensland, estuaries are fed by complex river systems sourced by groundwater extrusion and rainfall occurring east of the Great Dividing Range. The climate of this region is subtropical with distinct dry and wet seasons and is further driven by climatic drivers occurring in and above the Pacific Ocean (El Niño Southern Oscillation - ENSO). During periods of El Niño, the region experiences less than average rainfall, higher than average temperatures, and the risk of drought and fire is increased. During periods of La Niña, higher than average rainfall is likely, often resulting in major flooding and the oversaturation of river catchments. The likelihood of cyclones and east coast lows are also increased during this time, which can result in short duration but high intensity rainfall. Climate change is also having an impact on the duration and severity of weather events associated with these climate drivers, resulting in extremes in salinity during drought and inundations of freshwater during flood. Flooding rains following periods of drought can generate increased pulses of sediments where accumulated sediments are washed downstream into estuaries in high concentrations.

Our current understanding of the impacts of flooding and significant mobilisation and redistribution of sediment on estuaries in the region is relatively limited; however, there are various methods available and utilised globally to measure and assess the redistribution of sediments during flooding events. To determine these approaches, a comprehensive review of literature (Chapter 2) was conducted to consolidate the existing knowledge of the physical processes within estuaries, historical studies examining the impacts of flooding on water quality and sediment distribution, calculations of sediment transport in sub-tropical estuaries during drought, flood, and ambient conditions, and common catchment management practices as they relate to mitigating the impacts of flood-driven sediment loss and redistribution at a global and national scale. A local case study on the Gold Coast, in Queensland, Australia (Chapter 3) provides an example to explore changes during flooding and possible adaptations to management. Here, the current knowledge of sediment distribution, geology, hydrodynamics, and catchment management was examined in the case study area. Studies that use methods to calculate and estimate sediment transport and redistribution were reviewed and referenced to develop a site-specific approach for estimating sediment redistribution in the study area (Chapter 4). The literature identified significant wealth of knowledge in methods for tracing sediments, monitoring redistribution, and further assessing the impacts of flooding events on benthic ecosystems. However, there was a significant gap in the knowledge available for the Gold Coast, likely due to the paucity of the data available and as most studies were performed to meet specific objectives usually linked to a development project, which limited the granularity of data and the frequency at which data were collected. This indicated that there was a need to improve the understanding of how the rivers are contributing sediments to the estuary during high rainfall events, to assist with more proactive management of the system.

Two methods were tested using the Gold Coast case study: the utilisation of existing, publicly-funded, water quality monitoring program data (Chapter 5), and a targeted field sampling program that analysed benthic sediments for their physico-chemical characteristics prior to and following a significant flooding event (Chapter 6). An ambient baseline condition of the water quality in the rivers was able to be broadly established through the analysis of the available data. This baseline indicates that the case study system had low concentrations of mobile sediments during period of low and no rainfall. However, the analysis of the long-term water quality datasets demonstrated that the coarseness of long-term, multi-catchment water quality datasets is not appropriate for estimating sediment yields or spatial distribution where other data requirements (i.e., river discharge, soil characteristics, and spatial data) are not available. Sediment tracing studies and the development of catchment fingerprints from field measured data showed that high rainfall events have a significant impact on the redistribution of sediments in the system, as measured data showed notable changes in both the particle size of sediments present before and after major flooding, as well as in the concentrations of metals analysed. This study also determined that tracing studies are useful, low-cost, targeted methods for estimating sediment redistribution during different types of rainfall events.

These findings of the studies were applied in the context of informing potential adaptations to management practices in the river catchments and the estuary (Chapter 7). While the results of the redistribution analysis represent a single event, they are useful in determining the impact of similar high rainfall events in systems that are affected by climatic seasonality. The impacts are necessary for enabling proactive and effective management of catchments and estuaries and can inform targeted approaches that reduce the risk of ecosystem loss and reduced public amenity.