Reach-scale morphodynamics: insights from 20 years of observations and model simulations

Authors: AL-GHORANI, NISREEN - University Of British Columbia; HASSAN, MARWAN - University Of British Columbia; Langendoen, Eddy

Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2022
Publication Date: 10/15/2022
Citation: Al-Ghorani, N.G., Hassan, M.A., Langendoen, E.J. 2022. Reach-scale morphodynamics: insights from 20 years of observations and model simulations. Geomorphology. 413, 108375. https://doi.org/10.1016/j.geomorph.2022.108375.
DOI: https://doi.org/10.1016/j.geomorph.2022.108375

Interpretive Summary: Sediment budgets play an important role in determining the physical integrity of stream systems. However, quantifying sediment inputs from all sediment sources and by size fraction is problematic when both land use and reach geometry are not static. ARS researchers at Oxford, MS, in collaboration with scientists from the University of British Columbia, Canada, used a comprehensive 20-year record of stream flow and sediment transport data collected by the USDA, ARS, National Sedimentation Laboratory in the Goodwin Creek Experimental Watershed in combination with surveyed reach geometry and computer simulation to study sediment budget dynamics for two reaches during the period 1982-2002. For the two reaches, large gains in fine sediment storage were predominant and dictated the pattern of total sediment, whereas both reaches displayed contrasting patterns of their sand and gravel storage. Medium and long-duration flow events were found to have a major control on in-channel fine sediment dynamics, while large changes in sand and gravel were related to their availability within the channel and the presence of competent flows. Sediment control measures significantly reduced sediment transport during the 1990s, from both hillslopes and channel boundaries. The study shows that sediment budgets, which adequately quantify sediment inputs from all sources, can help evaluate the impact of land management practices on sediment transport processes, and therefore identify appropriate policies for future land use while mitigating environmental consequences.

Technical Abstract: Understanding morphodynamic processes and their controls has long been the focus of geomorphologists. Studying channel adjustments with respect to fractional sediment (clay + silt, sand, and gravel) is often hampered by the lack of both long-term measurements and detailed information about their transport. Taking advantage of a 20-year detailed dataset from the Goodwin Creek, Mississippi (USA), this study explores temporal patterns of fractional sediment dynamics in two consecutive reaches with contrasting geomorphic settings: one with tributaries and gravel-dominated bed (Middle Reach) and the other without tributaries and sand-dominated bed (Lower Reach). Floods associated with large erosional/depositional events were examined to evaluate their geomorphic effectiveness. Though large deposits in fine sediment were predominant within the two reaches, both reaches exhibited contrasting behaviors of sand and gravel dynamics. We noted that substantial changes in fine sediment storage tend to occur during medium, long-duration flood events. Their effectiveness, however, depends strongly on the stability state of the riverbank and the conditions of near-bank sediment storage inherited from historical flows. Among the three sediment fractions, only sand exhibited high activities during peak events = 1.4-year flood. The presence of high proportion of sand in bed materials prevented the development of a clear relationship between peak flow and gravel dynamics. The channel’s geomorphic settings, hence, sediment supply have been found to control the variations in fractional sediment dynamics. Episodic sediment supplies from upstream, tributaries, and banks determined the changes in fine sediment storage, whereas changes in sand and gravel storage were related to their availability within the reaches. In Goodwin Creek, the sustainable delivery of sand from upstream enhanced bed mobility from the Middle Reach downstream to the Lower Reach creating opposite, albeit small, gravel dynamics. Though sediment control measures significantly reduced sediment supply during the 1990s, the streambanks of the Lower Reach were found to be a major source of fines, contributing as much fines as that produced by 84% of the watershed area. This result has direct implications for the management of similar reaches in the bluff hills region, Mississippi. The findings highlight the importance of considering variations in sediment storage when predicting likely adjustments of channels with erodible banks based on hydraulic thresholds.