scholarly journals On how sediment supply affects step formation, evolution and stability in steep streams: an experimental study

2020 ◽  
Author(s):  
Matteo Saletti ◽  
Marwan A. Hassan
Keyword(s):  
Sediment Concentration ◽  
Sediment Supply ◽  
Transport Capacity ◽  
Channel Stability ◽  
Step Frequency ◽  
Bell Curve ◽  
Experimental Campaign ◽  
Channel Outlet ◽  
Dynamic Channel ◽  
Formation Step

Abstract. We present results from an experimental campaign run in a steep flume subject to longitudinal width variations and different sediment feed rates. The experiments were designed to study how sediment supply influences step formation, step location, and step stability. Our results show that steps are more likely to form in narrowing areas (i.e., where the channel width is getting smaller moving downstream) because of particle jamming, and these steps are also more stable. Sediment supply increases particle activity generating a more dynamic channel morphology with more steps forming and collapsing. However, sediment supply does not inhibit step formation, since more steps are generated in experiments with sediment feed than without it. Time-series of step formation, evolution, and destruction show that the maximum number of steps is achieved for average values of sediment supply. We summarize this outcome in a conceptual model where the dependence of step frequency on sediment supply is expressed by a bell curve. Sediment yield measured at the channel outlet followed the sediment feed at the inlet closely, even when we fed 50 % more and 50 % less than the transport capacity. This outcome challenges the applicability of the concept of transport capacity to steep channels and highlights the key role played by sediment supply for channel stability and sediment transport. Finally, we detected a positive correlation between sediment concentration and step destruction, which highlights the key role played by granular jamming for step formation and stability.

scholarly journals Experimental study of sediment supply control on step formation, evolution, and stability

2020 ◽  
Vol 8 (4) ◽  
pp. 855-868
Author(s):  
Matteo Saletti ◽  
Marwan A. Hassan
Keyword(s):  
Sediment Yield ◽  
Sediment Concentration ◽  
Sediment Supply ◽  
Particle Interactions ◽  
Transport Capacity ◽  
Step Frequency ◽  
Bell Curve ◽  
Channel Response ◽  
Channel Outlet ◽  
Formation Step

Abstract. We present results from an experimental campaign conducted in a steep flume subject to longitudinal width variations and different sediment feed rates. The experiments were designed to study how sediment supply influences step formation, step location, and step stability. Our results show that steps are more likely to form in segments of the channel where the width narrows because of particle jamming, and these steps are also more stable. Sediment feed increases particle activity which generates a dynamic channel morphology with steps forming and collapsing. A comparison with experiments without sediment feed shows that sediment supply does not inhibit step formation. Time series of step formation, evolution, and destruction show that the maximum number of steps is achieved when the sediment feed is larger than zero but smaller than the transport capacity. We summarize this outcome in a conceptual model where the dependence of step frequency on sediment supply is expressed by a bell curve. Sediment yield measured at the channel outlet followed the sediment feed at the inlet closely, even when we fed 50 % more and 50 % less than the calculated transport capacity. This outcome challenges the applicability of the concept of transport capacity to steep channels and highlights the key role played by sediment feed in dictating sediment yield and channel response. Finally, we detected a positive correlation between sediment concentration and step destruction, which stresses the importance of particle interactions for step formation and stability.

scholarly journals Vegetative impacts upon bedload transport capacity and channel stability for differing alluvial planforms in the Yellow River Source Zone

2016 ◽  
Author(s):  
Z. W. Li ◽  
G. A. Yu ◽  
G. Brierley ◽  
Z. W. Wang
Keyword(s):  
Riparian Vegetation ◽  
Yellow River ◽  
Bedload Transport ◽  
Sediment Supply ◽  
Source Zone ◽  
Western China ◽  
Tibet Plateau ◽  
Transport Capacity ◽  
Channel Stability ◽  
Braided Reach

Abstract. The influence of vegetation upon bedload transport and channel morphodynamics is examined along a channel stability gradient ranging from meandering through anabranching through anabranching-braided to fully braided planform conditions along trunk and tributary reaches of the Upper Yellow River in western China. Although the regional geology and climate are relatively consistent across the study area, there is a distinct gradient in the presence and abundance of riparian vegetation for these reaches atop the Qinghai-Tibet Plateau (elevations in the study area range from 2800–3400 m a.s.l.). The hydraulic and geomorphic role of riparian vegetation varies as follows: trees exert the strongest influence in the anabranching reach, the meandering reach flows through meadow vegetation, the anabranching-braided reach has a grass, herb, and sparse shrub cover, and the braided reach has no riparian vegetation. A non-linear relation between vegetative cover on the valley floor and bedload transport capacity is evident, wherein bedload transport capacity is highest for the anabranching reach, followed by the anabranching-braided, braided and meandering reaches respectively. The relationship between the bedload transport capacity of a reach and sediment supply from upstream exerts a significant influence upon channel stability. Bedload transport capacity during the flood season (June–September) in the braid ed reach is much less than the rate of sediment supply, inducing bed aggradation and dynamic channel adjustments. Rates of channel adjustment are less pronounced for the anabranching-braided and anabranching reaches, while the meandering reach is relatively stable (i.e. this is a passive meandering reach).

scholarly journals Vegetative impacts upon bedload transport capacity and channel stability for differing alluvial planforms in the Yellow River source zone

2016 ◽  
Vol 20 (7) ◽  
pp. 3013-3025 ◽  
Author(s):  
Zhi Wei Li ◽  
Guo An Yu ◽  
Gary Brierley ◽  
Zhao Yin Wang
Keyword(s):  
Riparian Vegetation ◽  
Yellow River ◽  
Bedload Transport ◽  
Sediment Supply ◽  
Source Zone ◽  
Tibet Plateau ◽  
Transport Capacity ◽  
Channel Stability ◽  
Upper Yellow River ◽  
Braided Reach

Abstract. The influence of vegetation upon bedload transport and channel morphodynamics is examined along a channel stability gradient ranging from meandering to anabranching to anabranching–braided to fully braided planform conditions along trunk and tributary reaches of the Upper Yellow River in western China. Although the regional geology and climate are relatively consistent across the study area, there is a distinct gradient in the presence and abundance of riparian vegetation for these reaches atop the Qinghai–Tibet Plateau (elevations in the study area range from 2800 to 3400 m a.s.l.). To date, the influence of vegetative impacts upon channel planform and bedload transport capacity of alluvial reaches of the Upper Yellow River remains unclear because of a lack of hydrological and field data. In this region, the types and pattern of riparian vegetation vary with planform type as follows: trees exert the strongest influence in the anabranching reach, the meandering reach flows through meadow vegetation, the anabranching–braided reach has a grass, herb, and sparse shrub cover, and the braided reach has no riparian vegetation. A non-linear relation between vegetative cover on the valley floor and bedload transport capacity is evident, wherein bedload transport capacity is the highest for the anabranching reach, roughly followed by the anabranching–braided, braided, and meandering reaches. The relationship between the bedload transport capacity of a reach and sediment supply from upstream exerts a significant influence upon channel stability. Bedload transport capacity during the flood season (June–September) in the braided reach is much less than the rate of sediment supply, inducing bed aggradation and dynamic channel adjustments. Rates of channel adjustment are less pronounced for the anabranching–braided and anabranching reaches, while the meandering reach is relatively stable (i.e., this is a passive meandering reach).

scholarly journals The Response of Turbidity Maximum to Peak River Discharge in a Macrotidal Estuary

Water ◽  
2021 ◽  
Vol 13 (1) ◽  
pp. 106
Author(s):  
Yuhan Yan ◽  
Dehai Song ◽  
Xianwen Bao ◽  
Nan Wang
Keyword(s):  
River Discharge ◽  
Recovery Time ◽  
Morphological Evolution ◽  
Spring Tide ◽  
River Estuary ◽  
Sediment Concentration ◽  
Ocean Model ◽  
Turbidity Maximum ◽  
Sediment Supply ◽  
Three Dimensional Model

The Ou River, a medium-sized river in the southeastern China, is examined to study the estuarine turbidity maximum (ETM) response to rapidly varied river discharge, i.e., peak river discharge (PRD). This study analyzes the difference in ETM and sediment transport mechanisms between low-discharge and PRD during neap and spring tides by using the Finite-Volume Community Ocean Model. The three-dimensional model is validated by in-situ measurements from 23 April to 22 May 2007. In the Ou River Estuary (ORE), ETM is generally induced by the convergence between river runoff and density-driven flow. The position of ETM for neap and spring tides is similar, but the suspended sediment concentration during spring tide is stronger than that during neap tide. The sediment source of ETM is mainly derived from the resuspension of the seabed. PRD, compared with low-discharge, can dilute the ETM, but cause more sediment to be resuspended from the seabed. The ETM is more seaward during PRD. After PRD, the larger the peak discharge, the longer the recovery time will be. Moreover, the river sediment supply helps shorten ETM recovery time. Mechanisms for this ETM during a PRD can contribute to studies of morphological evolution and pollutant flushing.

Effects of episodic sediment supply on experimental step-pool channel morphology, bedload transport and channel stability

2021 ◽  
Author(s):  
Jiamei Wang ◽  
Marwan A. Hassan ◽  
Matteo Saletti ◽  
Xingyu Chen ◽  
Xudong Fu ◽  
...  
Keyword(s):  
Grain Size ◽  
Morphological Changes ◽  
Mass Movement ◽  
Channel Morphology ◽  
Bedload Transport ◽  
Sediment Supply ◽  
Primary Control ◽  
Channel Stability ◽  
Flume Experiments ◽  
Small Magnitude

<p>Steep step-pool streams are often coupled to adjacent hillslope, directly receiving episodic sediment supply from mass movement processes such as landslides and debris flows. The response of step-pool channels to the variations in sediment supply remains largely unexplored. We conducted flume experiments with a poorly sorted grain-size distribution in an 8%-steep, 5-m long flume with variable width at the University of British Columbia, to study the effects of episodic sediment supply on channel evolution. After a conditioning phase with no feed, the channel was subjected to sediment pulses of different magnitude and frequency under constant flow discharge. High-resolution data of hydraulics, bedload transport, bed surface grain size, and channel morphology were collected every 10-20 minutes and an additional time at the end of each pulse.</p><p>In response to sediment pulses, we recorded an increase in bedload transport rates, channel aggradation, bed surface fining, and continuous step formation and collapse. In between pulses, bedload rates dropped by several orders of magnitude, net erosion occurred, the bed surface gradually coarsened, and steps became more stable. The small-magnitude high-frequency pulses caused smaller but more frequent spikes in bedload transport, bed surface evolution, and thus step stability. Instead, the large-magnitude low-frequency pulses cause larger changes but provided a longer time for the channel to recover. This suggests that in step-pool channels pulse magnitude is a key control on channel rearrangement, while pulse frequency controls how fast and strong the recovery is.</p><p>The frequency and stability of steps varied as a function of local channel width, showing that channel geometry is a primary control on step formation and stability even under episodic sediment supply conditions. Instead, the effect of sediment pulses is less important because the total number and average survival time of steps were similar among runs with different pulses. The critical Shields stress decreased following sediment pulses, then increased immediately after, and fluctuated until the next pulse. The variations in sediment supply caused cycles in bedload transport rate, surface and bedload texture, thus controlling the variability in the threshold for motion.</p><p>Our results indicate that episodic sediment supply is a primary control on the evolution of step-pool channels, with sediment feed magnitude affecting mostly morphological changes, and sediment feed frequency controlling channel stability.</p>

Dynamics of sediment delivery to mangrove forests of the Ganges-Brahmaputra-Meghna Delta

2020 ◽  
Author(s):  
Richard Hale ◽  
Alexandra Garnand ◽  
Carol Wilson
Keyword(s):  
Sediment Load ◽  
Monsoon Season ◽  
Sediment Concentration ◽  
Water Velocity ◽  
Sediment Supply ◽  
Tidal Range ◽  
Mangrove Forests ◽  
Southern Coast ◽  
Sediment Delivery ◽  
The Ganges

<p>The Ganges-Brahmaputra-Meghna Delta (GBMD) is among the largest in the world, nourished by the ~1 Gt/yr sediment load of its titular rivers. Approximately 75% of this sediment load is debouched to the Bay of Bengal, with ~180 Mt subsequently reworked by tidal processes across the southwestern portion of the delta. This region includes this Sundarbans National Reserve Forest (SNRF), which is the words’ largest continuous mangrove stand. In addition to global sea level rise and the enhanced subsidence intrinsic to deltas, ongoing and proposed alterations to the upstream fluvial sediment supply threaten the future viability of this important ecological and cultural resource.</p><p> </p><p>In this study, we use data collected in situ by acoustic and optical instrumentation to examine the physical processes controlling sedimentation in the mangrove forest along the southern coast during both the monsoon (October 2019) and dry seasons (March 2020).  These data are then compared with sedimentation rates measured using sediment elevation tables and marker horizons, as well as observations made 100 km further inland near the northern extent of the SNRF. At this inland site, sediment supply, inundation depth, and salinity have been identified as important factors controlling sediment deposition to the mangrove platform, which ranges from ~1 cm during the dry season (November – June), to > 2 cm during the monsoon (July-October). Data from the second location along the coast are vital for understanding the regional nature of the various threats to delta viability.</p><p> </p><p>Preliminary analysis of the 2019 monsoon season data from the southern coast reveals the relative importance of water depth, water velocity, and mangrove pneumatophore density on modulating both water velocity and suspended sediment concentration. Previous studies have identified that while the inland location features a larger tidal range (~5 m vs. ~3 m), frequent cyclone activity likely impacts sedimentation at the coastal site. Data collected in March 2020 will address how these variables impact controls on sedimentation both seasonally and regionally. Results from this study demonstrate the importance of providing regional context to sedimentation studies, as delta communities adapt to dynamic forcing conditions.   </p>

Analysis of geometric relationships of bedrock and alluvial channels: a comparison between rivers from the Scottish Highlands and San Gabriel Mountains (USA)

2021 ◽  
Author(s):  
Mel O. Guirro ◽  
Rebecca A. Hodge ◽  
Fiona Clubb ◽  
Laura Turnbull
Keyword(s):  
Sediment Transport ◽  
Transport Processes ◽  
Sediment Supply ◽  
Spatial Distributions ◽  
Alluvial Channels ◽  
Transport Capacity ◽  
Alluvial Rivers ◽  
Bedrock Rivers ◽  
San Gabriel Mountains ◽  
Scottish Highlands

<p>Sediment transport in rivers depends on interactions between sediment supply, topography, and flow characteristics. Erosion in bedrock rivers controls topography and is paramount in landscape evolution models. The riverbed cover indicates sediment transport processes: alluvial cover indicates low transport capacity or high sediment supply, and bedrock cover demonstrates high transport capacity or low sediment supply. This study aims to evaluate controls on the spatial distributions of bedrock and alluvial covers, by analysing scaling geometric relations between bedrock and alluvial channels. A Principal Component Analysis (PCA) was conducted to evaluate correlations between river slope, depth, width, and sediment size. The two principal components were used to implement a clustering analysis in order to identify differences in alluvial and bedrock sections. Spatial distributions of mixed bedrock-alluvial sections were investigated from two datasets - Scottish Highlands (Whitbread 2015) and the San Gabriel Mountains in the USA (Dibiase 2011)-, representing different environmental conditions, such as erosion rates, lithology, tectonics, and climate. The rock strength of both areas is high, and therefore it is excluded as a factor that explains the difference between the areas. The results of the cluster analysis were different in each environment. The main sources of variation among river sections identified by PCA were slope and width for the San Gabriel Mountains, and drainage area and depth for the Scottish Highlands. The rivers in the Scottish Highlands formed clusters that differentiate bedrock and alluvial patches, showing a clear geometric distinction between channels. However, the river analysis from the San Gabriel Mountains showed no clusters. Bedrock rivers are typically described as narrower and steeper than alluvial rivers, as demonstrated by rivers in the Scottish Highlands (e.g. slope was around 0.1 m/m for bedrock sections and 0.01 m/m for alluvial sections). However, this may not be always the case: both bedrock and alluvial sections in San Gabriel Mountains presented similar slope around 0.1 m/m. The inability to demonstrate significant geometry differences in bedrock and alluvial sections in the San Gabriel Mountains may be due to the frequency and magnitude of sediment supply of that region, which are influenced by tectonics and climate. A major difference in the supply of sediment in rivers of the San Gabriel Mountains is the frequent occurrence of debris flow. Non-linear interactions between hydraulic and sediment processes may constantly modify the geometry of bedrock-alluvial channels, increasing the complexity of analysis at larger temporal and spatial scales. This study is part of the i-CONN project, which links connectivity in different scientific disciplines. A sediment connectivity assessment in different environments and scales may be useful to evaluate the controls on the spatial distribution of bedrock and alluvial rivers.</p><p> </p><p>Dibiase, R.A. 2011. Tectonic Geomorphology of the San Gabriel Mountains, CA. PhD Thesis. Arizona State University, Phoenix, 247pp.</p><p>Whitbread, K. 2015. Channel geometry data set for the northwest Scottish Highlands. British Geological Survey Open Report, OR/15/040. 12pp.</p>

The Analysis and Discussion of Sediment Transport Capacity Formula

2012 ◽  
Vol 188 ◽  
pp. 259-263 ◽  
Author(s):  
Wei Yi Zhang ◽  
Gui Ying Lu ◽  
Xi Shan Pan
Keyword(s):  
Diffusion Equation ◽  
Carrying Capacity ◽  
Sediment Concentration ◽  
Two Dimensional ◽  
Transport Capacity ◽  
Sediment Transport Capacity ◽  
Sediment Data ◽  
New Formula ◽  
Sediment Carrying Capacity ◽  
High Application

Combined with the equilibrium condition, the formula of vertical average sediment concentration has been derived from the vertical two-dimensional suspended sediment diffusion equation. Based on analyzing and comparing two hydrodynamic formulas, a new formula of sediment carrying capacity has been deduced. Some river and inshore sediment data has been collected to validate the formula and find that the new formula has higher accuracy and applicability comparing with the two hydrodynamic formulas, it shows that the new formula has a high application value.

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