skip to main content


Title: Spatial Variability in Bankfull Stage and Bank Elevations of Lowland Meandering Rivers: Relation to Rating Curves and Channel Planform Characteristics
Abstract

Mutual adjustment between process and form shapes the morphology of alluvial river channels, including channel banks. The tops of banks define the transition between the channel and adjacent floodplain, which corresponds to the level of incipient flooding. Despite the geomorphological and hydrological importance of this transition, few, if any, studies have extensively examined spatial variability in bank elevations and its influence on bankfull stage. This study uses an objective method to explore this variability at two spatial resolutions along three alluvial lowland meandering rivers. Results show that variability in bankfull stage is inherent to all three rivers. The mean variability of bankfull stage about the average downstream gradient in this stage is 10% to 20% of mean bankfull depth. Elevations of channel banks exhibit similar variability, even after accounting for systematic variations in heights of inner and outer banks associated with river meandering. Two‐dimensional hydraulic simulations show that the elevation range of mean variability in bankfull stage overlaps considerably with the elevation range of high curvature on rating curves, confirming that variability in bankfull stage influences the shape of these curves. The simulations verify that breaks in channel banks allow flow to extend onto the floodplain at stages below the average bankfull stage. The findings provide fundamental insight into the variable nature of bankfull conditions along meandering rivers and the role of this variability in channel‐floodplain connectivity. The results also inform river‐restoration efforts that seek to re‐establish the natural configuration of channel banks.

 
more » « less
Award ID(s):
1911321
NSF-PAR ID:
10450203
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Water Resources Research
Volume:
56
Issue:
8
ISSN:
0043-1397
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    High‐resolution topography reveals that floodplains along meandering rivers in Indiana commonly contain intermittently flowing channel networks. We investigated how the presence of floodplain channels affects lateral surface‐water connectivity between a river and floodplain (specifically exchange flux and timescales of transport) as a function of flow stage in a low‐gradient river‐floodplain system. We constructed a two‐dimensional, surface‐water hydrodynamic model using Hydrologic Engineering Center's River Analysis System (HEC‐RAS) 2D along 32 km of floodplain (56 km along the river) of the East Fork White River near Seymour, Indiana, USA, using lidar elevation data and surveyed river bathymetry. The model was calibrated using land‐cover specific roughness to elevation‐discharge data from a U.S. Geological Survey gage and validated against high‐water marks, an aerial photo showing the spatial extent of floodplain inundation, and measured flow velocities. Using the model results, we analyzed the flow in the river, spatial patterns of inundation, flow pathways, river‐floodplain exchange, and water residence time on the floodplain. Our results highlight that bankfull flow is an oversimplified concept for explaining river‐floodplain connectivity because some stream banks are overtopped and major low‐lying floodplain channels are inundated roughly 19 days per year. As flow increased, inundation of floodplain channels at higher elevations dissected the floodplain, until the floodplain channels became fully inundated. Additionally, we found that river‐floodplain exchange was driven by bank height or channel orientation depending on flow conditions. We propose a conceptual model of river‐floodplain connectivity dynamics and developed metrics to analyze quantitatively complex river‐floodplain systems.

     
    more » « less
  2. Abstract

    In meandering rivers, interactions between flow, sediment transport, and bed topography affect diverse processes, including bedform development and channel migration. Predicting how these interactions affect the spatial patterns and magnitudes of bed deformation in meandering rivers is essential for various river engineering and geoscience problems. Computational fluid dynamics simulations can predict river morphodynamics at fine temporal and spatial scales but have traditionally been challenged by the large scale of natural rivers. We conducted coupled large‐eddy simulation and bed morphodynamics simulations to create a unique database of hydro‐morphodynamic data sets for 42 meandering rivers with a variety of planform shapes and large‐scale geometrical features that mimic natural meanders. For each simulated river, the database includes (a) bed morphology, (b) three‐dimensional mean velocity field, and (c) bed shear stress distribution under bankfull flow conditions. The calculated morphodynamics results at dynamic equilibrium revealed the formation of scour and deposition patterns near the outer and inner banks, respectively, while the location of point bars and scour regions around the apexes of the meander bends is found to vary as a function of the radius of curvature of the bends to the width ratio. A new mechanism is proposed that explains this seemingly paradoxical finding. The high‐fidelity simulation results generated in this work provide researchers and scientists with a rich numerical database for morphodynamics and bed shear stress distributions in large‐scale meandering rivers to enable systematic investigation of the underlying phenomena and support a range of river engineering applications.

     
    more » « less
  3. 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.

     
    more » « less
  4. Abstract

    Floodplain inundation has been viewed as a type of binary process set by the relative elevation between river stage and levee crest. However, recent reports in the literature show that this perception may have limited applicability. In particular, through‐bank channels, conduits that cross the main river levees or banks, facilitate conditions for an “inundation continuum,” or inundation for a range of sub‐bankfull flows. Moreover, through‐bank channels and their networks provide a direct hydraulic connection between the main river and the floodplain interior. We analyzed through‐bank channel structure and floodplain topography and compared them to river surface elevation to provide greater insight on floodplain inundation processes. Results show that well‐developed levees with through‐bank channels facilitate frequent through‐bank inundation. Where levees are poorly developed, floodplain inundation occurs by overbank flow. Therefore, for a given discharge through‐bank and overbank inundation may occur simultaneously. For the Congaree River floodplain, we infer that this dichotomy of inundation processes leads to temporally and spatially complex inundation flow paths for a given river stage. Further, our analyses reveal that the inundation continuum concept should be considered in the context of having vertical, longitudinal, lateral, and temporal components.

     
    more » « less
  5. Abstract

    Scroll bars across a 65‐km stretch of the Trinity River in Texas, USA were studied using LiDAR data as well as with a series of 11 trenches spread out across the survey area. We conclude that scroll bars are levees that are deposited along the inner banks of these meandering river bends. Scroll bar crests were found to have similar elevations to those of outer bank levee crests, implying that they are constructional features that create positive topographic relief above the elevation of the floodplain. Trenches reveal that scroll bars are built from reworked suspended sediment, with common ripple‐scale cross stratification, planar laminations and muddy bioturbated layers – characteristics often associated with levee sedimentation in other systems. LiDAR observation of the erosion of scroll bars by bed material transport during flood implies that scroll bar spacing is an imperfect proxy for estimating overall channel migration rates. In addition, interspersed lenses of coarser sediment with dune‐scale cross stratification represent the stratigraphic record of these erosional events and suggest that erosion of the channel‐ward edge of the scroll bar is not uncommon. Preservation of scroll bars is unlikely, given that they are responsible for an average of only the uppermost 12% of the total inner bank relief. We suggest that misidentification of point bar lateral accretion surfaces as scroll bars is common and can lead to issues with reconstructing channel properties due to systematic differences between point bar and scroll bar planform geometries. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.

     
    more » « less