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

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 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 a gravel-dominated bed (Middle Reach) and the other without tributaries and a 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 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 proportions 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.