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Controls on knickpoint migration and consequences for landscape evolution: experimental and numerical modelling

Periodic Reporting for period 1 - WaterfallModel3D (Controls on knickpoint migration and consequences for landscape evolution: experimental and numerical modelling)

Reporting period: 2016-06-01 to 2018-05-31

Bedrock river channels mediate the response of the landscape to changing boundary conditions, such as tectonics and climate through migrating ‘knickzones’ or ‘knickpoints’, often in the form of waterfalls. As they migrate upstream, adjoining hillslopes tend to become longer and steeper due to the sudden drop in the channel elevation downstream of the knickpoint, potentially increasing the rate of sediment transport and the susceptibility to landsliding on hillslopes. Such a concept of an ‘upstream incision wave’ also occurs in geomorphological systems beyond mountain rivers such as soil erosion on hillslopes, rill/gully formation and the erosion of sediment following dam removal. Thus, understanding the response of bedrock river channels, and knickpoints in particular, to external perturbations is vitally important for the understanding of landscape evolution across multiple spatial and temporal scales. However, the complexities of these processes are often ignored in studies of landscape evolution, in favour of simple relationships between erosion rate and parameters such as catchment drainage area. This project aimed to explore and quantify the complexities of knickpoint erosion processes and the dynamics of knickpoint retreat to improve the understanding of how landscapes evolve in response to an ‘upstream incision wave’ induced by changes in tectonic and climatic forcing.
The work performed in this project fell into three main areas; (i) the development and application of an experimental flume facility for the study of bedrock channel and knickpoint erosion processes, (ii) numerical modelling to identify and explore the hydraulic-geomorphic interactions that can lead to rapid change in river systems, and (iii) study of a natural landscape (New Zealand) that has recently undergone rapid erosion by knickpoint processes following a change in climate.
The experimental flume facility was developed at the start of the project and a successful laboratory campaign was completed that considered the relative effect of the following factors on knickpoint erosion in a controlled environment: bedrock strength, water discharge and sediment flux. Additional experiments considered the role of sediment flux in setting bedrock channel width, and the role of sediment flux and slope in driving the transition of a channel from fully alluvial to incised bedrock conditions. Data showing the evolution of the topography during the experiments was collected at two-minute intervals using a terrestrial laser scanner (TLS), and were analysed to calculate the rates of knickpoint retreat, the evolution of the river long profiles and the evolution of channel width and slope.
Results from the experimental part of the project have been published in two peer-reviewed journals. The first, in Scientific Reports, identified that the strength of the bedrock material is the key factor in setting the retreat rate of knickpoints. Importantly, these experiments also showed that the river discharge does not control the knickpoint retreat rate, which is contrary to existing theory and many common landscape evolution modelling studies. River channels adapt their width to the river discharge (wider channels under higher discharges) which maintains a constant shear stress acting on the bed and constant erosion rate at the knickpoints. In the second publication, published in Geology, the experiments identified the physical process that leads to the transition of river channels from a wide, fully-alluviated state, to narrow, deeply incised bedrock conditions. Critically, the results identified a multi-stage erosion process that included both a downstream and upstream incision wave component. The erosion into bedrock and the onset of the upstream incision wave is triggered under supercritical flow conditions, a geomorphic-hydraulic interaction that had never been previously identified in large-scale rivers.
The hydraulic-geomorphic interaction leading to bedrock incision and knickpoint development was identified using the output from a numerical hydrodynamic model. This allowed a detailed investigation of the previously impossible to recognise role of interactions between channel hydraulics and channel form in determining the evolution of the system. Further investigation using the numerical model has validated the dynamics of water flow in plunge pools at the base of waterfalls, and work is ongoing to implement an erosion law for the evolution and retreat of knickpoints into the morphodynamic part of the model.
The results from the Rangitikei river area of New Zealand support the experimental results from part 1 of the project. The region is a unique natural laboratory for the study of the interactions between sediment flux, bedrock river morphology and knickpoint dynamics, due to the uniform lithology of the region, with the exception of the headwater tributaries where some supply hard, coarse, gravel to the river channel. As a result, some rivers in the region have high sediment fluxes while others have no coarse sediment flux, allowing a spatial comparison of the impact of sediment on channel geometry and also the rate and style of knickpoint erosion. Rivers containing coarse sediment are systematically wider than those without coarse sediment. A paper exploring the role of sediment flux in setting channel width, combining data from experiments and the Rangitikei region is currently being prepared for publication. Results combining experimental data from part 1 and data from the Rangitikei region will also be published showing that sediment has an important role in setting the morphology and retreat rate of waterfalls. Waterfalls in channels containing sediment, either experimental or in a natural system, are characterised by a series of steps in a narrow incised canyon while those without sediment are characterised by a single vertical drop in a wider, open, canyon. These results provide insights into important factors controlling both the morphology of bedrock river channels, the complexities of waterfall/knickpoint erosion processes and the drivers of rapid landscape change over multiple spatial and temporal scales.
The work undertaken in this project has identified physical processes that drive landscape change that had not previously been recognised in the scientific community. The results demonstrate the importance of channels self-organising their width for understanding how river channels evolve in response to external forcing that may be driven by changes in climate or tectonic activity. These results have implications for future studies that use the physical understanding of earth surface processes to model the evolution of landscapes, such as how landscapes respond to changes in climate and tectonics over long timescales. Additionally, the quantitative understanding of channel and knickpoint dynamics over short timescales (e.g. response to changes in sediment flux) has implications for important societal issues such as flood risk, as changing channel geometry due to sediment can limit the ability of river channels to convey peak flows without flooding.