How do sand-bed meandering rivers co-construct their own bankfull discharge and bankfull channel geometry? Analytical and numerical study

Alluvial meandering rivers;Bankfull channel geometry;Flow duration curve;River morphodynamic modeling

Naito, Kensuke

关键词: Alluvial meandering rivers;    Bankfull channel geometry;    Flow duration curve;    River morphodynamic modeling;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/104767/NAITO-DISSERTATION-2019.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Alluvial rivers are authors of their own geometry. Given water and sediment, a river constructs its own well-defined channel and floodplain. This channel is characterized by a bankfull width and depth, longitudinal slope, and bankfull discharge when the flow spills out onto the floodplain. No specific discharge is sufficient, however, to describe how the channel maintains itself. Above-bankfull flows increase the height of the floodplain and deepen the channel. Modestly below-bankfull flows drive channel shift across the floodplain, shaving it down and making the channel shallower. Now consider a stable alluvial river reach with a well-defined floodplain. Suppose the sediment supply to the reach, or the range of flows to which it is subjected, changes. How will the river evolve to a new equilibrium?Here I present a morphodynamic model that captures these processes. A simple modeling framework is developed for the co-determination of bankfull discharge and corresponding bankfull channel geometry (width, depth, and longitudinal channel slope) of a natural alluvial meandering river. I inquired as to how an alluvial meandering river selects bankfull discharge among the wide range of discharges to which it is subjected. Here, I provide a physically-based predictor of bankfull discharge that goes beyond the simple empirical assumption that bankfull discharge is equal to the 1.5-year flood discharge. I do this with the aid of physically-based sub-models for channel and floodplain processes. I show that bankfull discharge and bankfull geometry are established as a result ofi) floodplain vertical accretion due to overbank deposition,ii) inner bank migration and outer cut bank migration,iii) net removal of floodplain sediment and reduction in average floodplain height due to lateral channel shift, andiv) in-channel downstream bed material transport. The flow duration curve is casted so as to quantify the effect of these processes, as well as to account for the flow variability. This new model captures the spatiotemporal evolution of bankfull discharge, depth, width, and down-channel slope toward equilibrium for specified flow duration curve and watershed characteristics. This new framework can be used for various purposes, including the assessment of the river response to climate change, land-use change and river restoration, indeed any change that can be quantified in terms of change in sediment supply or flow duration curve. I then apply the model to the Minnesota River, USA, which has experienced rapid widening associated with a changing hydrologic regime. The model, which captures flow variability in terms of the flow duration curve, allows study of the effect of altered hydrologic regime on bankfull discharge and channel characteristics. Here I further examine the importance of extreme flows relative to base flows and moderately high flows on long-term bankfull channel evolution to equilibrium. This analysis suggests that altered flow duration curve can play significant role in rapid (decadal-scale) bankfull channel widening observed in the Minnesota River. In this case, increase in the 5%-50% exceedance flows in the specified flow duration curve are more responsible than the increase in the 0.1% exceedance large flow or the 90% exceedance low flow for substantial bankfull channel enlargement. In the case of the Minnesota River, the model predicts equilibrium bankfull discharge to be nearly identical to the 1% exceedance flow of a specified flow duration curve, regardless of its shape, indicating the potential presence of the relation to specify the position of the equilibrium bankfull discharge as a function of model feeding parameters such as ones for the bank processes.