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.

Scale‐Free Structure of Surface‐Water Connectivity Within a Lowland River‐Floodplain Landscape

“…This also highlights the robust‐yet‐fragile nature of scale‐free surface‐water connectivity within lowland river landscapes. Finally, the concept of bankfull discharge, traditionally used to differentiate between in‐channel and floodplain processes, has been questioned recently because hydraulic and geomorphic conditions can vary longitudinally and complicate the application of localized flow properties to other portions of the corridor (Czuba et al, 2019; Hudson et al, 2013; Lindroth et al, 2020; Tockner et al, 2000). On the other hand, scale‐free networks have been found to exhibit emergent synchronization where the elements of a system are operating in unison (Restrepo et al, 2006).…”

“…Our findings indicate that the concept of synchronization in surface‐water connectivity can be used in place of the bankfull concept because it is more of a landscape‐ or reach‐level description and not a single point‐ or cross‐section‐based description that is extrapolated over a river landscape/corridor. This is because the bankfull flow is typically used to define the transition between strictly in‐channel and floodplain processes (Lindroth et al, 2020) and synchronization in surface‐water connectivity can be used to more accurately represent the gradual nature of this transition.…”

“…Surface-water connectivity from overbank flooding enables the largest and most abrupt material exchanges between patches composing the river-floodplain landscape (Junk et al, 1989;Poff et al, 1997). Nevertheless, widespread flood inundation within a lowland river floodplain can occur with river flows below bankfull conditions (Czuba et al, 2019;Hudson et al, 2013;Kupfer et al, 2014;Lindroth et al, 2020;Meitzen et al, 2013;Phillips, 2013;Tockner et al, 2000). The connections between landscape patches induced by flood inundation create a dynamic network whose size, configuration, and function will depend on the magnitude of river flows.…”

The importance of threshold in alluvial river channel geometry and dynamics

“…Data compilations of hydraulic geometry typically report, for each site, average values of bankfull depth and width from a few cross-sectional surveys, a longitudinal profile of the river bed, and a statistically random sampling of the river-bed grain-size distribution 101 . Variation and ambiguities exist in methods for determining bankfull conditions as topographic indices are not always prevalent or multiple indicators exist 102,103 ; the data used here generally represent river reaches where morphological breaks in channel cross section indicate the bankfull conditions. An under utilized source in such compilations is the vast network of stream gages maintained by the United States Geological Survey (USGS), in which 'stage' and discharge are determined for a wide range of flows in surveyed channel cross sections.…”

Bankfull shear velocity predicts embeddedness and silt cover in gravel streambeds

“…These data were georeferenced using a Trimble R10 real-time kinematic global positioning system. At each cross section, the bankfull elevation was determined as the lower of the two channel banks (Lindroth et al, 2020). At this bankfull elevation, we computed a bankfull hydraulic radius and averaged the three values together at each site.…”