Channel widening quantification under laboratory conditions

2020 ◽  
Author(s):  
Chao Qin ◽  
Fenli Zheng ◽  
Robert Wells
Keyword(s):  
Flow Velocity ◽  
Sediment Supply ◽  
Channel Width ◽  
Sediment Discharge ◽  
Inflow Rate ◽  
Sediment Delivery ◽  
Negative Power ◽  
Manual Sampling ◽  
Channel Widening ◽  
Bed Slope

<p>Channel widening constitutes about 80% of total soil loss, especially in the presence of a plow pan which manifests a less or nonerodible soil layer. Channel bank erosion quantification is prerequisite to couple effectively the bank sediment supply system with fluvial sediment transport fluxes. The objectives of this study were to: 1) describe and evaluate methods for monitoring and data post-analysis of channel widening and 2) investigate how inflow rate, slope gradient and initial channel width affect channel widening processes in the presence of a non-erodible layer. Technology was developed to capture 5-cm spaced cross-sections along a soil flume at 3-s time intervals. Two off-the-shelf digital cameras were positioned 3-m above the soil bed and controlled by a program to trigger simultaneously and download images to the computer. Methods utilizing color differences in images and elevation differences in DEMs were applied to detect discontinuities between channel walls and the soil bed. Channel widths were calculated by differentiating the coordinates of these surface discontinuities. A volumetric method was used to calculate flow velocity with measurements of flow depths obtained from ultrasonic depth sensors. Sediment concentration was determined by manual sampling.</p><p>The results showed that different channel width calculation methods exhibited comparable outcomes and achieved satisfactory accuracy. Sediment discharge showed a significant positive linear correlation with channel widening rate, while exhibiting a 5 to 25-s time lag compared to the peak of channel widening rate. Total sediment discharge calculated by photogrammetry was 3.1% lower than that calculated by manual sampling. Flow velocity decreased with time and showed a significant negative power correlation with channel width. Sediment delivery and channel width increased with the increase of inflow rate, bed slope and the decrease of initial channel width. Exponential equations were used to predict the channel width time series. Toe scour, crack development, sidewall failure and block detachment and transport, in sequence, were the four main processes of channel widening. Basal scour arc length, tension crack length and width decreased with initial channel width and increased with time, flow discharge and bed slope. Basal scour arcs were divided into three patterns according to different shapes in comparison to the failure arcs. Sediment delivery equations based on the disaggregation of concentrated flow entrainment and mass failure were also fitted. Advantages of the described methodology include automated high spatial and temporal monitoring resolution, semi-automated data post-processing, and the potential to be generalized to large scale river/reservoir bank failure monitoring. This study provides new insight on improving channel widening measurements and prediction technology.</p>

scholarly journals Alluvial cover controlling the width, slope and sinuosity of bedrock channels

2017 ◽  
Author(s):  
Jens Martin Turowski
Keyword(s):  
Theoretical Explanation ◽  
Rate Of Change ◽  
Sediment Supply ◽  
Sediment Flux ◽  
Channel Width ◽  
Bedrock Channel ◽  
The Pacific ◽  
Storm Frequency ◽  
Bed Slope ◽  
Bedrock Channels

Abstract. The problem of bedrock channel meandering is closely related to the problems of channel width and slope. Active meandering occurs when the channel walls are eroded, which also drives channel widening. Further, for a given drop in elevation, the more sinuous a channel is, the lower is its channel bed slope in comparison to a straight channel. It can thus be expected that studies of bedrock channel meandering give insights into width and slope adjustment and vice versa. The mechanisms by which bedrock channels actively meander have been debated since the beginning of modern geomorphic research in the 19th century, but a final consensus has not been reached. Remote sensing studies of the Pacific Arc islands show that regional channel sinuosity scales with storm frequency and inversely with the erodibility of the substrate. However, no mechanisms are known that effectively reduce sinuosity and keep it at a constant value, and a coherent theoretical explanation for the field observations is lacking. It has long been argued that whether a bedrock channel meanders actively or not is determined by the availability of sediment relative to transport capacity, a notion that has also been demonstrated in laboratory experiments. Here, this idea is taken up by postulating that the rate of change of both width and sinuosity over time is dependent on bed cover only. Based on the physics of erosion by bedload impacts, a scaling argument is developed to link bedrock channel width, slope and sinuosity to sediment supply, discharge and erodibility. It is shown that this simple model built on sediment-flux driven bedrock erosion yields the observed scaling relationships of channel width and slope with discharge and erosion rate, can explain why sinuosity evolves to a steady state value and predict the observed relations between sinuosity, erodibility and storm frequency.

scholarly journals Alluvial cover controlling the width, slope and sinuosity of bedrock channels

2018 ◽  
Vol 6 (1) ◽  
pp. 29-48 ◽  
Author(s):  
Jens Martin Turowski
Keyword(s):  
Bedload Transport ◽  
Rate Of Change ◽  
Sediment Supply ◽  
Sediment Flux ◽  
Channel Width ◽  
Transport Capacity ◽  
Bedrock Channel ◽  
Bed Slope ◽  
The 19Th Century ◽  
Bedrock Channels

Abstract. Bedrock channel slope and width are important parameters for setting bedload transport capacity and for stream-profile inversion to obtain tectonics information. Channel width and slope development are closely related to the problem of bedrock channel sinuosity. It is therefore likely that observations on bedrock channel meandering yields insights into the development of channel width and slope. Active meandering occurs when the bedrock channel walls are eroded, which also drives channel widening. Further, for a given drop in elevation, the more sinuous a channel is, the lower is its channel bed slope in comparison to a straight channel. It can thus be expected that studies of bedrock channel meandering give insights into width and slope adjustment and vice versa. The mechanisms by which bedrock channels actively meander have been debated since the beginning of modern geomorphic research in the 19th century, but a final consensus has not been reached. It has long been argued that whether a bedrock channel meanders actively or not is determined by the availability of sediment relative to transport capacity, a notion that has also been demonstrated in laboratory experiments. Here, this idea is taken up by postulating that the rate of change of both width and sinuosity over time is dependent on bed cover only. Based on the physics of erosion by bedload impacts, a scaling argument is developed to link bedrock channel width, slope and sinuosity to sediment supply, discharge and erodibility. This simple model built on sediment-flux-driven bedrock erosion concepts yields the observed scaling relationships of channel width and slope with discharge and erosion rate. Further, it explains why sinuosity evolves to a steady-state value and predicts the observed relations between sinuosity, erodibility and storm frequency, as has been observed for meandering bedrock rivers on Pacific Arc islands.

MODELING THE EFFECT OF SEDIMENT SUPPLY ON CHANNEL WIDTH CHANGE ALONG ELWHA RIVER, WASHINGTON

2017 ◽  
Author(s):  
Kathryn Grace De Rego ◽  
◽  
Brett Eaton ◽  
J. Wesley Lauer ◽  
Marwan Hassan
Keyword(s):  
Sediment Supply ◽  
Channel Width ◽  
Elwha River ◽  
Width Change

scholarly journals Amplified Impact of Climate Change on Fine-Sediment Delivery to a Subsiding Coast, Humboldt Bay, California

2021 ◽  
Author(s):  
Jennifer A. Curtis ◽  
Lorraine E. Flint ◽  
Michelle A. Stern ◽  
Jack Lewis ◽  
Randy D. Klein
Keyword(s):  
Climate Change ◽  
Climate Change Impacts ◽  
Baseline Period ◽  
Climate Impacts ◽  
Sediment Supply ◽  
Balance Model ◽  
Climate Projections ◽  
Sediment Delivery ◽  
Beneficial Reuse ◽  
Coastal Margin

AbstractIn Humboldt Bay, tectonic subsidence exacerbates sea-level rise (SLR). To build surface elevations and to keep pace with SLR, the sediment demand created by subsidence and SLR must be balanced by an adequate sediment supply. This study used an ensemble of plausible future scenarios to predict potential climate change impacts on suspended-sediment discharge (Qss) from fluvial sources. Streamflow was simulated using a deterministic water-balance model, and Qss was computed using statistical sediment-transport models. Changes relative to a baseline period (1981–2010) were used to assess climate impacts. For local basins that discharge directly to the bay, the ensemble means projected increases in Qss of 27% for the mid-century (2040–2069) and 58% for the end-of-century (2070–2099). For the Eel River, a regional sediment source that discharges sediment-laden plumes to the coastal margin, the ensemble means projected increases in Qss of 53% for the mid-century and 99% for the end-of-century. Climate projections of increased precipitation and streamflow produced amplified increases in the regional sediment supply that may partially or wholly mitigate sediment demand caused by the combined effects of subsidence and SLR. This finding has important implications for coastal resiliency. Coastal regions with an increasing sediment supply may be more resilient to SLR. In a broader context, an increasing sediment supply from fluvial sources has global relevance for communities threatened by SLR that are increasingly building resiliency to SLR using sediment-based solutions that include regional sediment management, beneficial reuse strategies, and marsh restoration.

Dynamics of capillary-driven liquid–liquid displacement in open microchannels

2014 ◽  
Vol 16 (44) ◽  
pp. 24473-24478 ◽  
Author(s):  
D. Yang ◽  
M. Krasowska ◽  
C. Priest ◽  
J. Ralston
Keyword(s):  
Flow Velocity ◽  
Channel Width ◽  
Liquid Front ◽  
Liquid Displacement

For capillary-driven liquid–liquid displacement in rectangular open microchannels, the square of the position of the liquid–liquid front increases linearly with time, whereas the flow velocity decreases with increasing channel width.

EXPERIMENTAL STUDY OF BAR FORMATION IN SAND BED CHANNEL

2016 ◽  
Vol 78 (5-3) ◽  
Author(s):  
Duratul Ain Tholibon ◽  
Junaidah Ariffin ◽  
Jazuri Abdullah ◽  
Juliana Idrus
Keyword(s):  
Experimental Study ◽  
Flow Rate ◽  
Experimental Work ◽  
Large Scale ◽  
Sediment Supply ◽  
Hydraulic Characteristics ◽  
Flow Rates ◽  
Physical Mechanisms ◽  
Bed Slope ◽  
Bar Formation

A large number of studies both theoretical and experimental have been devoted to understand the physical mechanisms underlying the bar formation. This can be investigated by carrying out an experimental work in an erodible sand bed channel using a large-scale physical river model. The study included the various hydraulic characteristics with steady flow rates and sediment supply. An experimental work consists of four matrices of flow rate and channel width with other variables namely grains size and bed slope were kept constant. Details of bar profile development that generated using Surfer, a software used for 3D elevation plots are included.

Theoretical Analysis of Flame Propagation in Meso and Microscale Channels

2003 ◽  
Author(s):  
Yiguang Ju
Keyword(s):  
Flow Velocity ◽  
Heat Loss ◽  
Flame Propagation ◽  
Critical Value ◽  
Flow Speed ◽  
External Heat ◽  
Channel Width ◽  
Recent Experimental Data ◽  
Quenching Distance ◽  
Large Flow

Extinction and flame propagation in a meso and microscale channels are investigated analytically. Emphasis was paid to the coupling of wall heat loss, wall preheating, external heat loss and chemical reaction. The results showed that, wall thermal properties, channel width and flow velocity have dramatic effects on the flame propagation and lead to multiple flame regimes and extinction limits. With the decrease of channel width, flame reaches its first quenching limit, the so called critical quenching distance. However, with a further decrease of channel width, the results show that there exists a slow burning flame. With the increase of wall heat loss the speed of the slow burning flame slightly decreases and eventually reaches its second burning limit. With the change of the flow velocity, the results show that sub-limit flame can only exist at flow velocity larger than a critical value. At moderate flow velocity, flame speed increases with the increase of flow speed. At very large flow velocity, flame will be blown off. The above results are confirmed from the recent experimental data.

Revised sediment transport model for estimation of suspended sediment flux and chemical composition of the Irrawaddy and Salween rivers

2020 ◽  
Author(s):  
J. Jotautas Baronas ◽  
Edward T. Tipper ◽  
Michael J. Bickle ◽  
Robert G. Hilton ◽  
Emily I. Stevenson ◽  
...  
Keyword(s):  
Chemical Composition ◽  
Organic Carbon ◽  
Suspended Sediment ◽  
Transport Model ◽  
Sediment Concentration ◽  
Sediment Flux ◽  
Sediment Discharge ◽  
Anthropogenic Pressures ◽  
Modeling Framework ◽  
Sediment Delivery

<p>A large portion of freshwater and sediment is exported to the ocean by just several of the world's major rivers. Many of these mega-rivers are under significant anthropogenic pressures, such as damming and sand mining, which are having a significant impact on water and sediment delivery to deltaic ecosystems. However, accurately measuring the total sediment flux and its mean physicochemical composition is difficult in large rivers due to hydrodynamic sorting of sediments. To account for this, we developed an updated semi-empirical Rouse modeling framework, which synoptically predicts sediment concentration, grain size distribution, and mean chemical composition (organic carbon wt%, Al/Si ratio) with depth and across the river channel.</p><p>We applied this model to derive new sediment flux estimates for the Irrawaddy and the Salween, the last two free-flowing mega-rivers in Southeast Asia, using a newly collected set of suspended sediment depth samples, coupled to ADCP-measured flow velocity data. Constructing sediment-discharge rating curves, we calculated an annual sediment flux of 326 (68% confidence interval of 256-417) Mt/yr for the Irrawaddy and 159 (109-237) Mt/yr for the Salween, together accounting for 2-3% of total global riverine sediment discharge. The mean flux-weighted sediment exported by the Irrawaddy is significantly coarser (D<sub>84</sub> = 193 ± 13 µm) and OC-poorer (0.29 ± 0.08 wt%) compared to the Salween (112 ± 27 µm and 0.59 ± 0.16 wt%, respectively). Both rivers export similar amounts of particulate organic carbon, with a total of 1.9 (1.0-3.3) Mt C/yr, contributing ~1% of the total riverine POC export to the ocean. These results underline the global significance of the Irrawaddy and Salween rivers and warrant continued monitoring of their sediment fluxes, given the increasing anthropogenic pressures on these river basins.</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>

Flood hazard in mountain streams: the key role of geomorphic processes during high magnitude events

2020 ◽  
Author(s):  
Nicola Surian ◽  
Andrea Brenna ◽  
Marco Borga ◽  
Marco Cavalli ◽  
Francesco Comiti ◽  
...  
Keyword(s):  
Hazard Assessment ◽  
Flood Hazard ◽  
Channel Width ◽  
Mountain Streams ◽  
Channel Network ◽  
Channel Dynamics ◽  
High Magnitude ◽  
Channel Widening ◽  
Large Floods ◽  
Geomorphic Processes

<p>Although channel dynamics (i.e. channel lateral mobility, intense sediment and wood transport) are commonly dominant processes in mountain streams during high-magnitude floods, hazard assessment still mostly focuses on water flooding only. Therefore, there is a need to include river geomorphological hazard to produce reliable flood hazard mapping and define effective mitigation measures. This work deals with the “Vaia” storm that occurred in the Eastern Alps (Italy) on 27-30 October 2018. Our aims are (i) to improve the understanding of geomorphic processes in response to large floods and (ii) to improve the prediction capability of the reaches more prone to undergo intense channel dynamics (e.g. channel widening, in-channel sedimentation) during such events.</p><p>An integrated approach was deployed to study the flood event in the Cordevole river catchment (876 km<sup>2</sup>). The approach includes (i) analysis of geomorphological processes, by comparing remote sensing data acquired before and after floods and field survey (e.g. recognition of different flow types); (ii) hydrological and hydraulic analysis (collection of rainfall and streamflow data, estimation of peak discharges at multiple sites in ungauged streams, and model-based consistency check of rainfall and discharge data); (iii) landslide mapping and analysis of sediment delivery to the channel network.</p><p>Intense sediment and wood transport took place. A wide range of transport processes (i.e. debris, hyperconcentrated and water flows) was recognized in the channel network and notable channel aggradation occurred at specific location (e.g. in channelized reaches). Channel widening was the most relevant geomorphic response along the fluvial network. Width ratio (i.e. channel width after / channel width before the flood) reached up to 2.1 and 4.4, respectively in the Cordevole and in its tributaries. Locally, the valley slopes were eroded (e.g. slope retreat up to 14 m). This means that the lateral channel dynamics affected not only large portions of the valley floor (e.g. forested floodplain) but also the valley slopes, especially if made of Quaternary deposits or soft bedrock.</p><p>These results have several implications in terms of flood hazard assessment in mountain streams. Since channel widening is a major process (streams may take up the whole floodplain and, locally, erode the valley slopes), so-called “river morphodynamic corridors” need to be defined and integrated into flood hazard maps. During high-magnitude floods the sediment mobilization may take place through mechanisms (e.g. hyperconcentrated flows) that can be different from those expected for ordinary water floods. Since major channel changes commonly occur during large floods, their prediction is needed and should accompany flood hydraulic modelling to obtain reliable flood event scenarios.</p>

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