The erosion of an ideal gully under steady state conditions
Gullies are responsible for as much as 40% of the accelerated sediment reaching the GBR, con tributing to poor water quality and impacting the health and resilience of the Reef. Improving water quality on the GBR is essential to provide the best opportunity to recover from past events and meet the challenges of climate change. The Australian and Queensland Governments aim to reduce sediment delivery to the Reef by 25% as part of the Reef 2050 Water Quality Improvement Plan 2050. Given the disproportionate contribution of gullies to the total sediment budget, reducing gully erosion will be an important part of meeting this target. The Prosser Report (2018) has identified process based models of gully erosion as important to support the strategic remediation of gullies. Previous models have employed empirical or conceptual approaches, which are not well suited to be adapted to representing interventions to provide localised decision support. This pa per presents a first step in the development of a locally focussed, process based model of gully erosion that is capable of representing gully interventions. The focus on water quality impacts on the GBR motivates the modelling focus on the concentration of sediment within the water column rather than on the evolution of the gully system. Gully erosion is therefore modelled using conservation of mass arguments, with entrainment of sediment from the gully walls and bed acting as a sediment source and deposition of sediment as a sink. The rate of entrainment is determined by balancing the power available to do work on the gully bed with the power required to entrain. The power available to do work on the gully is due to the stream and waterfall power of the system resulting from the loss of potential energy of the flow as it loses height. Adapting the approach of Hairsine and Rose (1992a,b) we introduce the concept of a soil cohesiveness factor, which represents the power required to overcome the cohesion of the sediment to enable it to be entrained. Together with a static friction term, this represents the streamflow threshold, the minimum amount of power required for entrainment to occur. A key feature of this model is the inclusion of a depositional layer and the concept of re entrainment after the model of Hairsine and Rose (1992a,b) Steady state solutions for the area encompassed by the deposi tional layer are explored, which demonstrates that the power available to erode cannot be independent of the concentration and flow conditions. A sediment carrying capacity is therefore introduced, providing a simple mechanism to represent the energy requirements of transporting sediment in suspension. The adapted model is explored under the assumptions of a highly erosive bed, and shown to provide the expected behaviour. Ex ploration of the model demonstrates that the assumptions of a depositional layer, particularly at steady state, requires further consideration. The aerial approach to modelling the depositional layer, although consistent with observations at the laboratory scale, appears not to extend to natural high flow events. This analysis suggests that an alternative modelling framework to capture the mechanism of re entrainment is necessary. The developed model is suitable to explore gully erosion impacts on water quality and the potential benefits of different interventions. We illustrate how different interventions can be represented in the model, however parameterisation to different interventions is yet to be undertaken. Future work will focus on improving the representation of the depositional layer, validating the model against observations, and parameterising the model including the representation of interventions.