Rill and gully morphodynamics and sediment flux: Implications for drainage network development and landscape evolution
Gordon, Lee M.
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Rill and gully erosion, as conditioned by actively migrating headcuts, contributes greatly to overall sediment yield from landscapes in a range of environments worldwide. Accurate prediction of rill and gully development in space and time remains elusive because gaps remain in our understanding of morphodynamics of rill and gully formative processes and the associated sedimentology. This research tackles a number of fundamental questions in order to further understanding and provide the basis for improvement of prediction technologies. A number of experimental and analytical approaches are used to investigate rill and gully development through headcut erosion. Headcut morphodynamics in stratified soils are examined in detail and shown to exhibit steady-state behavior. An analytical model of headcut development and migration is shown to be capable of predicting headcuts in stratified soils. A numerical model is used to simulate ephemeral gully erosion driven by headcut migration over long time periods, under different agricultural management practices, in a range of environments. It is shown that ephemeral gully erosion repair through tillage operations greatly increases sediment yield over the no-till condition. Rill networks driven by exogenic forcing (headcuts created by baselevel drop) are examined in an experimental landscape using data acquisition techniques with very high spatial and temporal resolution. Detailed information on sediment budgeting through time and space is developed, as are longitudinal profiles, cross sections, and hydraulic geometry relationships. Headcut erosion is shown to be genetically linked to virtually all sediment detachment within the landscape. Baselevel adjustments resulted in peaks in sediment discharge as headcut-driven waves of degradation propagated throughout the landscape. These waves of degradation were quickly and effectively communicated through the drainage network. Rates of headcut migration were shown to be well correlated to discharge. Stream order indices and fractal dimensions indicate that the rill network pattern emerges relatively early and remains relatively unchanged, despite continued application of rainfall. Rill basins do not exhibit self-affinity observed for river basins – they are geometrically self-similar at a range of scales within the experimental geomorphic environment.