Large grains matter: contrasting bed stability and morphodynamics during two nearly identical experiments

Evolution of bed material mobility and bedload grain size distributions under a range of discharges is rarely observed in braiding gravel‐bed rivers. Yet, the changing of bedload grain size distributions with discharge is expected to be different from laterally‐stable, threshold, channels on which most gravel bedload theory and observation are based. Here, simultaneous observations of flow, bedload transport rate, and morphological change were made in a physical model of a gravel‐bed braided river to document the evolution of grain size distributions and bed mobility over three experimental event hydrographs. Bedload transport rate and grain size distributions were measured from bedload samples collected in sediment baskets. Morphological change was mapped with high‐resolution (~1 mm precision) digital elevation models generated from close‐range digital photogrammetry. Bedload transport rates were extremely low below a discharge equivalent to ~50% of the channel‐forming discharge (dimensionless stream power ~70). Fractional transport rates and plots of grain size distributions indicate that the bed experienced partial mobility at low discharge when the coarsest grains on the bed were immobile, weak selective mobility at higher discharge, and occasionally near‐equal mobility at peak channel‐forming discharge. The transition to selective mobility and increased bedload transport rates coincided with the lower threshold for morphological change measured by the morphological active depth and active width. Below this threshold discharge, active depths were of the order of D90 and active widths were narrow (< 3% of wetted width). Above this discharge, both increased so that at channel‐forming discharge, the active depth had a local maximum of 9D90 while active width was up to 20% of wetted width. The modelled rivers approached equal mobility when rates of morphological change were greatest. Therefore, changes in the morphological active layer with discharge are directly connected to the conditions of bed mobility, and strongly correlated with bedload transport rate. © 2018 John Wiley & Sons, Ltd.

Section: Discussionmentioning

confidence: 99%

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

Peirce

Ashmore

Leduc

2018

“…However, as supported by Piedra and Haynes (2011) data in this article also indicates that there is no unique equation with fixed parameters capable of describing bedload transport behaviour for gravel channels which have been exposed to differing inter-flood flow periods. This study has shown that the finest and coarsest fractions are most responsive to inter-flood flow duration and hence more realistic entrainment models might consider using these fractions to define bed stability (Carling, 1987(Carling, , 1988Ashworth and Ferguson, 1989;Eaton and Church, 2004;Tamminga et al, 2015;Eaton et al, 2017;MacKenzie and Eaton, 2017). This study has shown that the finest and coarsest fractions are most responsive to inter-flood flow duration and hence more realistic entrainment models might consider using these fractions to define bed stability (Carling, 1987(Carling, , 1988Ashworth and Ferguson, 1989;Eaton and Church, 2004;Tamminga et al, 2015;Eaton et al, 2017;MacKenzie and Eaton, 2017).…”

Section: Effect Of Inter-flood Duration On Bed Stabilitymentioning

confidence: 96%

The impact of inter‐flood duration on non‐cohesive sediment bed stability

Ockelford

Woodcock

Haynes

2019

Limited field and flume data suggests that both uniform and graded beds appear to progressively stabilize when subjected to inter‐flood flows as characterized by the absence of active bedload transport. Previous work has shown that the degree of bed stabilization scales with duration of inter‐flood flow, however, the sensitivity of this response to bed surface grain size distribution has not been explored. This article presents the first detailed comparison of the dependence of graded bed stability on inter‐flood flow duration. Sixty discrete experiments, including repetitions, were undertaken using three grain size distributions of identical D50 (4.8 mm); near‐uniform (σg = 1.13), unimodal (σg = 1.63) and bimodal (σg = 2.08). Each bed was conditioned for between 0 (benchmark) and 960 minutes by an antecedent shear stress below the entrainment threshold of the bed (τ*c50). The degree of bed stabilization was determined by measuring changes to critical entrainment thresholds and bedload flux characteristics. Results show that (i) increasing inter‐flood duration from 0 to 960 minutes increases the average threshold shear stress of the D50 by up to 18%; (ii) bedload transport rates were reduced by up to 90% as inter‐flood duration increased from 0 to 960 minutes; (iii) the rate of response to changes in inter‐flood duration in both critical shear stress and bedload transport rate is non‐linear and is inversely proportional to antecedent duration; (iv) there is a grade dependent response to changes in critical shear stress where the magnitude of response in uniform beds is up to twice that of the graded beds; and (v) there is a grade dependent response to changes in bedload transport rate where the bimodal bed is most responsive in terms of the magnitude of change. These advances underpin the development of more accurate predictions of both entrainment thresholds and bedload flux timing and magnitude, as well as having implications for the management of environmental flow design. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.

“…The idea that channel stability is related to D 50 can be traced to the modified concept of a threshold channel proposed by Parker (), which asserts that self‐formed channels will adjust such that their banks are at the threshold of motion of D 50 , while bedload is transported along the centre of the channel. However, there is wide‐ranging evidence to suggest that mobility of D 50 has little to do with the stability of a channel (e.g., MacKenzie and Eaton, ).…”

Section: Channel Stabilitymentioning

confidence: 99%

“…). Based on our own experimental observations (e.g., MacKenzie and Eaton, ) and those documented in previous studies (Carling, , Ashworth and Ferguson, ; Eaton and Church, ; Tamminga et al , ; Eaton et al , ), we propose that the threshold between dynamically stable and unstable channels is characterized by the onset of full mobility of the D 84 , which occurs at τ 2 = 2 τ c 84 . These flows can mobilize 85% or more of the surface of a typical gravel bed, leaving only 15% of the surface grains (by area) in place (Figure ).…”

Section: Channel Stabilitymentioning

confidence: 99%

“…If we assume that D 50 plays a fundamental role in mediating the channel dynamics, and we observe that both channels develop approximately the same bed surface D 50 , we would expect both to develop a similar channel morphology. We tested this assertion experimentally (MacKenzie and Eaton, ) and instead found the resultant channel morphologies to be fundamentally different (Figure ). Although the surface median grain size of two distributions were practically identical (1.8 mm vs. 1.9 mm), only the channel with slightly less coarse material (GSD 1 in Figure ) was laterally active, developing a distinct suite of high‐amplitude bars, pools and riffles.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Breaking from the average: Why large grains matter in gravel‐bed streams

MacKenzie

Eaton

Church

2018

Self Cite

While the influence of large grains on the morphodynamics of gravel‐bed rivers has long been recognized, nothing dominates our collective efforts to model such rivers like the bed surface D50, which turns up in virtually all the relevant equations. While researchers interested in flow resistance have recognized the relative importance of large grains and have modified flow resistance equations accordingly, there have been few attempts to quantify the effects of large grains on gravel‐bed river morphodynamics. However, there is little evidence that D50 exerts first‐order control over the physics occurring along the channel boundary, and its prevalence seems to be primarily based on the untested, a priori assumption that the best description of a distribution is the mean or median value. This commentary questions the long‐standing assumption that D50 is the best choice for characteristic grain size, and uses evidence from previous studies to show that mobilization of the largest grains in the bed likely controls morphological stability, and possibly sediment transport. © 2018 John Wiley & Sons, Ltd.