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>

Compiling and Analysing Bedrock River Data Across the USA to Unpick Bedrock River Geomorphology

2021 ◽  
Author(s):  
James Buckley ◽  
Rebecca Hodge ◽  
Louise Slater
Keyword(s):  
Shear Stress ◽  
Landscape Evolution ◽  
Critical Shear Stress ◽  
Sediment Supply ◽  
Small Samples ◽  
Alluvial Rivers ◽  
Bedrock Rivers ◽  
Mountainous Regions ◽  
The Usa ◽  
Channel Properties

<p>Active incision of bedrock rivers exerts a vital control on landscape evolution in upland areas. Previous research found that bedrock rivers were typically steeper and sometimes narrower than alluvial rivers. However, most of the literature on partially-exposed bedrock rivers has employed small samples mostly from mountainous regions, so their geomorphological properties remain poorly understood. In contrast with the existing literature, a large-sample analysis of bedrock river channel properties would allow the controls on bedrock river width and slope to be unpicked and reveal whether or not the existing literature is biased towards pristine, mountainous bedrock rivers. Overall, such an analysis could improve the reliability of upland landscape evolution models.</p><p>Here we present an analysis of 1,924 river sites from the EPA National Rivers and Streams Assessment to assess the geomorphological differences between bedrock and alluvial rivers. The influences of lithology and uplift on bedrock channel properties are examined using external datasets. We find bedrock rivers to be significantly steeper and wider than alluvial rivers. Sedimentary bedrock rivers were seen to be significantly wider than igneous/ metamorphic bedrock rivers, consistent with findings from Ferguson et al. (2017). We estimated shear stress and critical shear stress for each river site and assessed correlation with bedrock exposure. We found that exposed bedrock could not always be explained by local sediment transport exceeding local sediment supply, indicating that bedrock exposure may be controlled by other factors in some bedrock rivers. Currently, uplift data are being compiled for further analysis.</p>

Responses of gravel-bed rivers to periodic climate change

2021 ◽  
Author(s):  
Fergus McNab ◽  
Taylor Schildgen ◽  
Jens Turowski ◽  
Andrew Wickert
Keyword(s):  
Transport Processes ◽  
Sediment Supply ◽  
Sediment Flux ◽  
Alluvial Channels ◽  
Gravel Bed ◽  
Fluvial Terraces ◽  
Wide Range ◽  
Gravel Bed Rivers ◽  
Lag Times ◽  
Climatic Signals

<p>Periodic variation in Earth's orbit leads to variation in temperature and precipitation at its surface that are expected to exert a profound influence on landscape evolution. Indeed, cyclical fluctuations in sediment yield and grain size are a ubiquitous feature of the geological record, and recurrence times of geomorphological features such as fluvial terraces and alluvial fans often appear to reflect orbital periodicities. However, making quantitative interpretations of these records requires a detailed understanding of the ways in which sediment is transported from mountainous source regions along alluvial channels to depositional sinks. Sediment transport processes may dampen (i.e. buffer, 'shred') or amplify climate signals, such as changes in channel elevation or sediment flux, and may introduce a lag between them and the responsible external forcing. Recent modelling studies, mostly focused on the potential transmission of climatic signals to sedimentary archives, have predicted a wide range of behaviour and have proven challenging to test in the field. Here, we aim to clarify this discussion and also consider the potential preservation of climatic signals by fluvial terraces along alluvial channels. Our starting point is a recently developed model describing the long-profile evolution of gravel-bed rivers. This model is the first of its kind to be derived from first principles using physical relationships that have been extensively tested in laboratory settings, and takes a non-linear diffusive form. We employ perturbation theory to obtain approximate analytical solutions to the relevant equations that describe how channel elevation and sediment flux vary in response to periodic fluctuations in discharge and sediment supply. Our solutions contain expressions for response amplitudes and lag times as functions of downstream distance, system 'diffusivity' and forcing frequency. Lag times can be a significant fraction of the forcing period, implying that care is required when interpreting the timings of terrace formation in terms of changes in discharge or sediment supply. We also show that at the onset of periodic forcing, or a change in the dominant forcing period, alluvial channels undergo a transient response as they adjust to a new quasi-steady state. Importantly, this result implies that suites of fluvial terraces can be preserved without the need for significant local base-level fall. Since the expressions presented here are defined in terms of fundamental properties of alluvial channels, they should be readily applicable to real settings.</p>

scholarly journals Inferring Sediment Transport Capacity from Soil Microtopography Changes on a Laboratory Hillslope

Water ◽  
2021 ◽  
Vol 13 (7) ◽  
pp. 929
Author(s):  
Sayjro Nouwakpo ◽  
Chi-hua Huang ◽  
Laura Bowling ◽  
Phillip Owens ◽  
Mark Weltz
Keyword(s):  
Experimental Data ◽  
Sediment Transport ◽  
Transport Processes ◽  
Transport Capacity ◽  
Erosion Modeling ◽  
Subsurface Hydrology ◽  
Concentrated Flow ◽  
Erosion Models ◽  
Flow Hydraulics ◽  
The Impact

In hillslope erosion modeling, the Transport Capacity (Tc) concept describes an upper limit to the flux of sediment transportable by a flow of given hydraulic characteristics. This widely used concept in process-based erosion modeling faces challenges due to scarcity of experimental data to strengthen its validity. In this paper, we test a methodology that infers the exceedance of transport capacity by concentrated flow from changes to soil surface microtopography sustained during rainfall-runoff events. Digital Elevation Models (DEMs) corresponding to pre- and post-rainfall events were used to compute elevation change maps and estimate spatially-varying flow hydraulics ω taken as the product of flow accumulation and local slope. These spatial data were used to calculate a probability of erosion PE at regular flow hydraulics intervals. The exceedance of Tc was inferred from the crossing of the PE = 0.5 line. The proposed methodology was applied to experimental data collected to study the impact of soil subsurface hydrology on soil erosion and sediment transport processes. Sustained net deposition occurred under drainage condition while PE for seepage conditions mostly stayed in the net erosion regime. Results from this study suggest pulsating erosion patterns along concentrated flow networks with intermittent increases in PE to local maxima followed by declines to local minima. These short-range erosion patterns could not be explained by current Tc-based erosion models. Nevertheless, Tc-based erosion models adequately capture observed decline in local PE maxima as ω increased. Applying the proposed approach suggests a dependence of Tc on subsurface hydrology with net deposition more likely under drainage conditions compared to seepage conditions.

scholarly journals SEDIMENT TRANSPORT PROCESSES AND COASTAL VARIABILITY ON THE ALASKAN NORTH SLOPE

1980 ◽  
Vol 1 (17) ◽  
pp. 81
Author(s):  
E.H. Owens ◽  
J.R. Harper ◽  
D. Nummedal
Keyword(s):  
Sediment Transport ◽  
Wave Energy ◽  
Energy Levels ◽  
Transport Processes ◽  
Sediment Supply ◽  
North Slope ◽  
Storm Events ◽  
Shore Zone ◽  
Ice Content ◽  
Alaskan North Slope

Shoreline development and shore-zone sediment transport on the Alaskan North Slope are dependent upon levels of wave energy, sea ice conditions, and the ice-sediment characteristics of eroding tundra cliffs. Considerable variation exists between the coastal processes and the shore-zone morphology of the Chukchi and Beaufort Sea beaches, (respectively west and east of Point Barrow). The supply of coarse sediments (sands or gravels) and the volumes of material eroded from tundra cliffs are a function of the initial character of the cliff sediments and of the ice content of the exposed cliffs. As cliff heights decrease, the ice content of the cliff increases, erosion rates increase but the sediment supply rates decrease. Wave-energy levels are relatively high and maintain a constant level on the Chukchi coast. The transport system on this coast is continuous and is augmented by storm events. On the Beaufort coast, energy levels are much lower, transport processes discontinuous, and storm events are therefore more significant. Sediments supplied to the coastal zone on the Chukchi coast are derived largely from the erosion of tundra cliffs and the barriers are continuous, linear, and stable. Rivers are the primary source of coastal sediments on the Beaufort coast and the more variable energy levels produce unstable barriers that are subject to aperiodic transport processes.

scholarly journals Relative changes in sediment supply and sediment transport capacity in a bedrock-controlled river

2001 ◽  
Vol 37 (12) ◽  
pp. 3307-3320 ◽  
Author(s):  
W. J. Young ◽  
J. M. Olley ◽  
I. P. Prosser ◽  
R. F. Warner
Keyword(s):  
Sediment Transport ◽  
Sediment Supply ◽  
Transport Capacity ◽  
Sediment Transport Capacity

scholarly journals How urban stormwater regimes drive geomorphic degradation of receiving streams

2020 ◽  
Vol 44 (5) ◽  
pp. 746-778 ◽  
Author(s):  
Kathryn L Russell ◽  
Geoff J Vietz ◽  
Tim D Fletcher
Keyword(s):  
Sediment Transport ◽  
Bedload Transport ◽  
Urban Streams ◽  
Sediment Supply ◽  
Relative Role ◽  
Bed Sediment ◽  
Transport Capacity ◽  
Channel Incision ◽  
Sediment Transport Capacity ◽  
Urban Catchments

For streams draining urban catchments, sediment transport capacity is the key driver of physical impacts including bed sediment removal and channel incision. The main unanswered question is the relative role of flow alteration compared to sediment supply in influencing sediment transport capacity. With this objective, we computed sand and gravel bed sediment transport capacity using the Wilcock and Kenworthy two-fraction bedload transport relation for nine streams in catchments covering a gradient of urbanisation. Computations were done for typical natural bed surface material, based on conditions in the least urban study streams. We compared transport capacity distributions and cumulative transport capacity over one-year between the streams. Transport capacity was up to three orders of magnitude higher in urban streams than in forested-catchment streams. This was driven overwhelmingly by the urbanisation-induced alterations to the flow regime, with only minor feedback from channel form changes. Transport capacity was two to three orders of magnitude greater than measured bedload transport in all but the least urban streams. This excess bedload transport capacity mobilises and removes bed sediment, produces channel incision and enlargement and reduces channel complexity. Rebalancing transport capacity with sediment supply therefore requires significant flow mitigation towards pre-urban conditions. Other responses, which may theoretically help to regain this balance – channel widening, grade control, increasing roughness, sediment augmentation – are either inappropriate or only feasible following flow mitigation measures.

Long-profile evolution of transport-limited gravel-bed rivers: Implications for sediment and landscape dynamics

2020 ◽  
Author(s):  
Andrew Wickert ◽  
Taylor Schildgen
Keyword(s):  
Sediment Transport ◽  
Water Supply ◽  
Sediment Supply ◽  
Attrition Rate ◽  
Catchment Scale ◽  
Alluvial Rivers ◽  
Gravel Bed ◽  
Base Level ◽  
The World ◽  
Gravel Bed Rivers

<p>Gravel-bed rivers cross and sculpt Earth's upland regions. Field, flume, and theoretical studies together provide governing equations for these rivers. Building upon this rich background, we quantitatively link catchment-scale hydrology, sediment transport, and morphodynamics into a model of river long-profile change over time. We focus on the transport-limited case (i.e., alluvial rivers), as most rivers around the world expend the majority of their geomorphic work by moving sediment rather than eroding the underlying substrate. Morphologically, this "transport-limited" category includes all alluvial rivers as well as those bedrock rivers for which bedrock erosion is easy relative to sediment transport. This model provides predictions for how such systems respond to changes in water supply, sediment supply, and base level – which are often linked to climate, land use, and tectonics. After deriving the central equation for long-profile evolution, we demonstrate that river concavity is strongly determined by the attrition rate of gravel, which can occur by either hillslope weathering or downstream fining. This dependency creates the potential for significant feedbacks between climate, tectonics, lithology, and river morphology. Furthermore, the equation predicts that oscillations in sediment and water supply will lead to net river incision when compared to steady means of both quantities. If true, this theoretical prediction could help to explain the near-ubiquitous presence of river terraces around the world.</p>

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