Can Bankfull Discharge and Bankfull Channel Characteristics of an Alluvial Meandering River be Cospecified From a Flow Duration Curve?

Abstract

We present a simple modeling framework for the codetermination of bankfull discharge and corresponding bankfull channel geometry (width, depth, and longitudinal channel slope) of an alluvial meandering river. We specifically consider a sand‐bed river whose floodplain is capped by a mud‐rich layer. We inquire as to how the wide spectrum of flows to which the river is subjected leads to the establishment of specific values for bankfull discharge and associated bankfull geometry. Here we provide a physically based predictor of bankfull discharge that goes beyond the simple assumption of the 1.5‐year flood discharge. We do this using physics‐based submodels for channel and floodplain processes. We show that bankfull discharge and bankfull geometry are established as a result of (i) floodplain vertical accretion due to overbank deposition, (ii) migration of the inner bank and outer cut bank, (iii) net removal of floodplain sediment and reduction in average floodplain height due to lateral channel shift, and (iv) in‐channel downstream bed material transport. The flow duration curve is employed to quantify the effect of these processes, as well as to account for flow variability. Our model captures the spatiotemporal evolution of bankfull discharge, depth, width, and down‐channel slope toward equilibrium for specified flow duration curve and watershed characteristics. Our new framework can be used for assessing long‐term river response to change in sediment supply or flow duration curve. A model implementation is presented for the case of the Trinity River, TX, USA, to demonstrate the use of the model and its behavior.

Adjustment of self‐formed bankfull channel geometry of meandering rivers: modelling study

“…We explore applications of a modelling framework for the co-determination of bankfull discharge and corresponding bankfull channel geometry (i.e., bankfull width, depth and down-channel slope) of a sand-bed meandering river developed by Naito and Parker (2019), for the case of the Minnesota River, MN, USA.…”

“…As pointed out by Naito and Parker (2019), the model is not designed for prediction over short spatial or temporal scales (e.g. a single bend), and the analysis has limitations to its use.…”

“…This study implements a new framework developed by Naito and Parker (2019) for the spatiotemporal co-evolution of the bankfull characteristics (i.e., bankfull discharge Q bf , bankfull width B bf , bankfull depth H bf and down-channel slope S c ) of freely meandering sand-bed rivers. A river that is the target of this model should exhibit active lateral channel shift, and the floodplain should consist of two distinct sediment layers: a lower layer with non-cohesive coarse material, which corresponds to bed material; and an upper layer with cohesive fine material, which is referred to here as floodplain material (supporting information, Figure S1).…”

“…Prior to the main analysis, we calibrate the model, as is required for model implementation (see Naito and Parker, 2019). The calibration parameters are characteristic slump block residence time T SB , characteristic vegetal encroachment rate c VEG , fractional efficiency of overbank deposition ϕ e , and a calibration parameter in the Engelund-Hansen total load relation α EH .…”

“…It should be kept in mind that a sensitivity analysis of the four calibration parameters suggests that predicted B bf is sensitive to bank process parameters (i.e., T SB and c VEG ) and that S c is somewhat sensitive to the sediment transport parameter α EH , respectively (Naito and Parker, 2019).…”