The variability in the morphological active width: Results from physical models of gravel‐bed braided rivers

Abstract

The morphological active width, defined as the lateral extent of bed material displacement over time, is a fundamental parameter in multi‐threaded gravel‐bed rivers, linking complex channel dynamics to bedload transport. Here, results are presented from five constant discharge experiments, and three event hydrographs, covering a range of flow strengths and channel configurations for which morphological change, bedload transport rates, and stream power were measured in a physical model. Changes in channel morphology were determined via differencing of photogrammetrically‐derived digital elevation models (DEMs) of the model surface generated at regular intervals over the course of ~115 h of experimental runs. Independent measures of total bedload output were made using downstream sediment baskets. Results indicate that the morphological active width increases with total and dimensionless stream power and is strongly and positively correlated with bulk change (total volume of bed material displaced over time) and active braiding intensity (ABI). Although there is considerable scatter due to the inherent variability in braided river morphodynamics, the active width is positively correlated with independent measurements of bedload transport rate. Active width, bulk change, and bedload transport rates were all negligible below a dimensionless stream power threshold value of ~ 0.09, above which all increase with flow strength. Therefore, the active width could be used as a general predictor of bulk change and bedload transport rates, which in turn could be approximated from total and dimensionless stream power or ABI in gravel‐bed braided rivers. Furthermore, results highlight the importance of the active width, rather than the morphological active depth, in predicting volumes of change and bedload transport rates. The results contribute to the larger goals of better understanding braided river morphodynamics, creating large high‐resolution datasets of channel change for model calibration and validation, and developing morphological methods for predicting bedload transport rates in braiding river systems. Copyright © 2018 John Wiley & Sons, Ltd.

Evolution of grain size distributions and bed mobility during hydrographs in gravel‐bed braided rivers

“…From a visual inspection of the DoDs, the morphological active width expands and becomes more continuous along the channel with increasing flow strength (and see Peirce et al, 2018). Plots of the active width against discharge show a general threshold of 1.14 L s -1 , below which the active width is narrow (<3% of the wetted width) and relatively constant, and above which the active width expands linearly with increasing discharge to over 20% of the wetted width at high discharge.…”

“…the morphological active width) increases rapidly with discharge in braided rivers (Ashmore et al, ; Lugo et al, ; Peirce et al, ). The lateral adjustment of the morphological active width with discharge is a significant component of bed material transport in braiding rivers (Bertoldi et al, ; Williams et al, ; Peirce et al, ) that aids in the accessibility of a wide range of bed material particle sizes, both laterally and from the subsurface. These processes differ from more‐stable, single‐threaded channels, which can have immobile areas of the bed that persist for years (Haschenburger and Wilcock, ) and in which bedload mechanics are dominated by particle exchange at the bed with limited active layer depth (Church and Haschenburger, ).…”

Rates of planimetric change in a proglacial gravel‐bed braided river: Field measurement and physical modelling

“…Combining these kinds of studies with planimetric measurements may yield valuable insights and reliable predictions of the morphodynamics of gravel braided rivers. The empirical correlation between planimetric change and bedload has a physical basis because the planimetric change and total mobilized mass of bed material are correlated through coincident areas of change and related active layer depth (Ashmore et al, 2018;Peirce et al, 2018a). The empirical correlation between planimetric change and bedload has a physical basis because the planimetric change and total mobilized mass of bed material are correlated through coincident areas of change and related active layer depth (Ashmore et al, 2018;Peirce et al, 2018a).…”

“…Application of a dilation filter modified the simple threshold map by reducing noise and increasing continuity between areas of change. The dilation filter used a binary mask from the threshold analysis to include areas of change of less than 3σ that are within a radius of 15 cells (22.5 mm) (based on trials with different radii) adjacent to change areas above the threshold (see also Peirce et al, 2018a).…”

Short Communication: Challenges and Applications of Structure-from-Motion Photogrammetry in a Physical Model of a Braided River

“…An alternative to using custom scripts would be the software program Geomorphic Change Detection or ArcGIS, although a comparison of techniques was not completed for this research. In addition to estimating changes in the morphological active width (Peirce et al, 2018), the data can be used for many applications, including estimates of water depth and bed surface texture. .…”