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Research title Controls over Bedrock Channel Incision. Summary of research Bedrock rivers plays a fundamental role in landscape history by setting the boundary conditions for the landform evolution. Bedrock channels determine local base level and the lowering rate of bedrock channels controls the rates of erosion and transport processes and the forms of the adjacent hillslopes. Thus, physical and chemical erosion processes of incision into bedrock control the overall erosion rate of the landscape. However, most previous studies on bedrock rivers have concentrated on the morphology of the bedrock forms, and are mainly descriptive although they can be used to infer erosion processes. The physical processes that generate bedrock channel morphology have so far been generally neglected. Flume studies have been used for process investigations as an analogy to provide insight into erosion process in bedrock rivers. However, in many cases flume study results are difficult to apply directly to bedrock rivers, because the appropriate laws to scale from flume to field are unknown. As a result of the incompleteness of understanding of the processes in bedrock channels, progress in the numerical modelling of the evolution of bedrock channels is also limited. Abrasion is a widespread erosion process in bedrock rivers, but functional relation between sediment transport and erosion is yet to be quantified. The role of grain size on erosion rate is also unknown. In my Ph.D study, these relations are investigated with a newly formulated modification of a channel bed incision model with abrasion, and are tested with physical modelling using a tumbling mill. Longitudinal profile changes with various settings are simulated and tested with cosmogenic nuclide analysis (10Be).
The structure of the modified numerical model of bedrock incision has three separate terms representing unit erosion by individual impact kinematics of the sediment particles, the number of sediment particles as a function of bedload transport rate and transport mode; and relative bedload supply rate. The modifications also involved calibration of the impact force as a function of sediment particle size, changes in saltation characteristics, and the introduction of an effective sediment flux term. The numerical simulations suggest that increasing sediment flux causes increasing erosion rate to a relative sediment flux (Qs/Qt) of 0.5. Erosion rates subsequently decrease with increasing sediment flux. Physical modelling with a tumbling mill suggested that the erosion rate increases with increasing sediment flux at low sediment flux and that the rate of increase declines with further increase in sediment flux. The overall pattern of weight loss change in resistant lithologies is consistent with prevous results using a different abrasion test facility. However, declining patterns in erosion rates with high sediment loads (>4kg) are different for different lithologies. When the sediment flux is increased, different lithologies respond in different ways. Detailed inspection results with 3 dimensional laser scanning and scanning electron microscopy suggests that microstructure of the bedrock controls spatial patterns of abrasion. Numerical simulation results also suggested that changing sediment particle sizes results in complex changes in erosion rate. As the sediment particle size increases, the impacting force also increases rapidly, but with fixed sediment flux the number of particle impacts against the channel bed decreases. As a result of this complexity, the erosion rate is slightly decreased. However, with the same number of sediment particles, increases in the sediment particle size increases the erosion rates. However, it was found through the physical modelling that increases in sediment particle size produce higher erosion rates for a given particle size. This result does not fully support the numerical simulation result, which could be due to the limitations of the particle velocity model used in the numerical simulation. It may mean that the effect of increasing sediment particle size is more influential than is suggested by numerical simulation, even though the physical modelling tested a limited range of conditions.
The result of the numerical simulations also suggested that transport stage (t*/t*cr) is the primary control over channel bed incision. The transport stage represents excess shear stress and is more sensitive to slope than to discharge. This suggestion is consistent with studies on the role of m and n in the unit stream power incision law. The non-linearity of erosion rates with slope was not found. So, it seems that the channel slope is a most important controlling factor in bedrock channel incision.
Longitudinal profile changes by abrasion process were simulated numerically to test the impact on long profile development of downstream fining of sediment particles, tectonic movement, lithologic characteristics, and tributary inputs. It is suggested that the spatial pattern of tectonic movement is the most influential factor controlling the erosion rate change along a river. Uplift at the downstream end of a reach, changing lithology and tributary inputs all produced knickpoints. The knickpoint also affects the erosion rates both upstream and downstream. To test the compatibility of the modification of channel bed incision model with abrasion for longterm landscape change studies, the longterm evolution of longitudinal profiles in a knickpoint reach were simulated. The simulated longterm erosion pattern within a knickpoint reach is compared with measured erosion rates derived from cosmogenic nuclide analysis. Five samples were collected from the active bed of the River Etive, Scotland, to measure the fluvial erosion rate within a knickpoint reach using cosmogenic 10Be analysis. The erosion rates upstream of the knickpoint were similar to the basin-wide postglacial erosion rate estimated from sedimentation rate in Loch Etive. The results from the cosmogenic 10Be analysis confirmed the erosion pattern suggested by numerical simulation and suggest that there is an abrupt increase in erosion rate at and about the knickpoint.
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