Controls on the uncertainty of sediment transport thresholds and the implications for interpreting river processes
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Authors
Feehan, Scott Albert
Issue Date
2023
Type
Dissertation
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Abstract
The transport of sediment by fluid flow shapes landscapes through the redistribution of material as a response to changes in imposed flow. Rivers are a primary example of this process as sediment sourced from the headwaters is transported downstream by flowing water that is highly variable in space and time. When and where this sediment will be entrained or deposited is relevant to biota, human lives and infrastructure in river-adjacent areas. Despite the importance of sediment transport, the process of entraining sediment is still poorly understood. As a result, entrainment is commonly predicted based on a critical stress related to local flow conditions. This technique has been used for nearly a century; however attempts to refine this framework have done little to collapse the multi-order magnitude scatter of observations on the threshold for sediment motion in both laboratory experiments and natural systems. A necessary next step is to reframe the entrainment problem as one that directly incorporates inherent properties of the sediment and describes the conditions for entrainment as a flow metric rather than a state of stress, such that both can be directly measured and the processes associated with sediment transport can be better understood. I use a combination of existing theory and observations from natural systems to define a new scaling relationship between sediment grain size and flow velocity necessary for entrainment and investigate how entrainment reframed in terms of these properties can be leveraged to further investigate more complex processes. This framework is unique in that the incorporation of these observations provides an explicit account of natural uncertainty in the parameters controlling sediment entrainment. With this approach, I seek to (1) collapse and explain multi-order magnitude scatter in entrainment observations. I show that observed variability of parameters that both promote and resist entrainment of sediment explains nearly all variability in observations in laboratory experiments and natural systems. I will also (2) determine how variability in entrainment thresholds is expressed in natural rivers. I demonstrate that the grain size-flow velocity scaling relationship explains a large portion of data relating formative river discharges and the size distribution of sediment in gravel-bedded rivers. Lastly, I (3) use entrainment thresholds applicable to common river systems to estimate flow conditions of unconstrained high magnitude floods. I use the refined scaling parameters to quantify a set of outburst floods from glacial Lake Tahoe to test the limits of applicability of the scaling relationship. The results suggest that entrainment is a fundamentally variable process but it can be constrained with additional observations and used to interpret and explain critical processes.
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Creative Commons Attribution 4.0 United States