scholarly journals Grain sorting in the morphological active layer of a braided river physical model

2015 ◽  
Vol 3 (3) ◽  
pp. 577-600
Author(s):  
P. Leduc ◽  
P. Ashmore ◽  
J. T. Gardner
Keyword(s):  
Grain Size ◽  
Numerical Modeling ◽  
Active Layer ◽  
Scale Model ◽  
Bed Load ◽  
Active Layer Thickness ◽  
Braided River ◽  
River Bed ◽  
Physical Scale ◽  
Bed Material

Abstract. A physical scale model of a gravel-bed braided river was used to measure vertical grain size sorting in the morphological active layer aggregated over the width of the river. This vertical sorting is important for analyzing braided river sedimentology, for numerical modeling of braided river morpho-dynamics and for measuring and predicting bed load transport rate. We define the morphological active layer as the bed material between the maximum and minimum bed elevations at a point over extended time periods sufficient for braiding processes to re-work the river bed. The vertical extent of the active layer was measured using 40 hourly high-resolution DEMs of the model river bed. An image texture algorithm was used to map bed material grain size of each DEM. Analysis of the 40 DEMs and texture maps provides data on the geometry of the morphological active layer and variation in grain size in three-dimensions. Normalizing active layer thickness and dividing into 10 sub-layers we show that all grain sizes occur with almost equal frequency in all sub-layers. Occurrence of patches and strings of coarser (or finer) material relates to preservation of particular morpho-textural features within the active layer. For numerical modeling and bed load prediction a morphological active layer that is fully mixed with respect to grain size is a reliable approximation.

scholarly journals Grain sorting in the morphological active layer of a braided river physical model

2015 ◽  
Vol 3 (4) ◽  
pp. 577-585 ◽  
Author(s):  
P. Leduc ◽  
P. Ashmore ◽  
J. T. Gardner
Keyword(s):  
Grain Size ◽  
Numerical Modeling ◽  
Active Layer ◽  
Bedload Transport ◽  
Scale Model ◽  
Active Layer Thickness ◽  
Braided River ◽  
River Bed ◽  
Physical Scale ◽  
Bed Material

Abstract. A physical scale model of a gravel-bed braided river was used to measure vertical grain size sorting in the morphological active layer aggregated over the width of the river. This vertical sorting is important for analyzing braided river sedimentology, for numerical modeling of braided river morphodynamics, and for measuring and predicting bedload transport rate. We define the morphological active layer as the bed material between the maximum and minimum bed elevations at a point over extended time periods sufficient for braiding processes to rework the river bed. The vertical extent of the active layer was measured using 40 hourly high-resolution DEMs (digital elevation models) of the model river bed. An image texture algorithm was used to map bed material grain size of each DEM. Analysis of the 40 DEMs and texture maps provides data on the geometry of the morphological active layer and variation in grain size in three dimensions. By normalizing active layer thickness and dividing into 10 sublayers, we show that all grain sizes occur with almost equal frequency in all sublayers. Occurrence of patches and strings of coarser (or finer) material relates to preservation of particular morpho-textural features within the active layer. For numerical modeling and bedload prediction, a morphological active layer that is fully mixed with respect to grain size is a reliable approximation.

3D numerical modeling of sediment handling techniques in a hydro power reservoir

2020 ◽  
Author(s):  
Diwash Lal Maskey ◽  
Dipesh Nepal ◽  
Daniel Herman ◽  
Gabriele Gaiti ◽  
Nils Rüther
Keyword(s):  
Numerical Model ◽  
Guide Vane ◽  
Scale Model ◽  
Bed Load ◽  
3D Numerical Modeling ◽  
Deposition Pattern ◽  
Flow Measurements ◽  
Reservoir Volume ◽  
Physical Scale ◽  
3D Numerical Model

<p>Sedimentation of small as well as large water storage reservoir has become a major issue. Due to the fact that we observe a 1% decrease of reservoir volume every year due to sedimentation and that the largest part of the reservoirs have been built between 70 and 40 years ago, many HPPs are confronted with the threatening scenario that soon the active storage and therefore their lifetime is dramatically diminished. Due to the above mentioned combination, active and sustainable sediment management has become the last option to retain or preferable enlarge the left-over reservoir volume. There are several options for a sustainable sediment handling, each for a different boundary condition, which must be evaluated carefully in order to be successful. For a successful choice, design and conduction of a sediment handling technique, usually a physical scale model will be conducted. Physical scale model have the advantage that there is a lot of experience in conducting these models and that they are illustrative. The disadvantage of scale models is that there are restrictions in the use of certain sizes of sediments due to scaling issues and that they are rather expensive.</p><p>This study attempt to use a 3D numerical model to overcome the above mentioned disadvantages and to serve as an additional source of alternatives in finding the right sediment handling techniques in reservoirs with high discharges of suspended and bed load. The goal is to simulate several flood events in order to gain insights in the current situation as well as to have a better understanding of the physical processes in the reservoir. This will support and positive influence the sustainable design of sediment handling techniques. The numerical model will be verified with flow measurements a physical model study and with bathymetry measurements from field observations. Based on the actual deposition pattern and the given input data, different sediment handling techniques are planned and conducted by means of the numerical model. The results show that the 3D numerical model is able to simulate sediment transport deposition pattern, bed load guide vane structures, as well as bed load diversion structures.</p>

scholarly journals Evidence of, and a Proposed Explanation for, Bimodal Transport States in Alluvial Rivers

2017 ◽  
Author(s):  
Kieran B. J. Dunne ◽  
Douglas J. Jerolmack
Keyword(s):  
Grain Size ◽  
Shear Stress ◽  
Critical Shear Stress ◽  
Bed Load ◽  
Bed Sediments ◽  
Channel Geometry ◽  
Alluvial Rivers ◽  
River Bed ◽  
Tentative Evidence ◽  
Near Threshold

Abstract. Gravel-bedded rivers organize their bankfull channel geometry and grain size such that shear stress is close to the threshold of motion. Sand-bedded rivers on the other hand typically maintain bankfull fluid stresses far in excess of threshold, a condition for which there is no satisfactory understanding. A fundamental question arises: Are bed-load (gravel-bedded) and suspension (sand-bedded) rivers two distinct equilibrium states, or do alluvial rivers exhibit a continuum of transport regimes as some have recently suggested? We address this question in two ways: (1) re-analysis of global channel geometry datasets, with consideration of the dependence of critical shear stress upon site-specific characteristics (e.g. slope and grain size); and (2) examination of a longitudinal river profile as it transits from gravel to sand-bedded. Data reveal that the transport state of alluvial river-bed sediments is bimodal, showing either near-threshold or suspension conditions, and that these regimes correspond to the respective bimodal peaks of gravel and sand that comprise natural river-bed sediments. Sand readily forms near-threshold channels in the laboratory and some field settings, however, indicating that another factor, such as bank cohesion, must be responsible for maintaining suspension channels. We hypothesize that alluvial rivers adjust their geometry to the threshold-limiting bed and bank material – which for gravel-bedded rivers is gravel, but for sand-bedded rivers is mud (if present) – and present tentative evidence for this idea.

scholarly journals Experimental and numerical study on the design of a deposition basin outlet structure at a mountain debris cone

2013 ◽  
Vol 1 (4) ◽  
pp. 3169-3200
Author(s):  
B. Gems ◽  
M. Wörndl ◽  
R. Gabl ◽  
C. Weber ◽  
M. Aufleger
Keyword(s):  
Numerical Study ◽  
Retention Volume ◽  
Transport Processes ◽  
Flood Protection ◽  
Scale Model ◽  
Bed Load ◽  
Outlet Section ◽  
Efficient Design ◽  
Physical Scale ◽  
Debris Cone

Abstract. Mountain debris cones in the Alpine region often provide space for dense population and cultivation. Hence, a great number of buildings are exposed to torrential hazards. In order to protect the settlement areas against flooding and overbank sedimentation, torrent defence structures are implemented directly at the debris cones. In many cases, these protection measures include a deposition basin at the head of the debris cone and/or a confined channel that passes or tracks through the settlement. The work presented within this paper deals with the effect of specific outlet structure layouts, situated at the lower end of a selected deposition basin, on bed-load transport processes and flood protection. A case study analysis was accomplished comprising of a 3-D-numerical model (FLOW-3D) and a physical scale model test (1:30). The subject of investigation was the deposition basin of the Larsennbach torrent in the Austrian Northern Limestone Alps. The basin is situated on a large debris cone and opens out into a paved channel. Since the basin is undersized and the accumulation of sediment in the outlet section reduces the available cross section during floods, adjoining settlements are considerably endangered of lateral overtopping of both clear water and sediment. Aiming for an upgrade in flood protection, certain layouts for a "closing-off structure" at the outlet were tested within this project. For the most efficient design layout, its effect on flood protection, a continuous bed-load output from the basin and the best possible use of the retention volume are pointed out. The simple design of the structure and the key aspects, that have to be taken into consideration for implementation, are highlighted.

scholarly journals Using ground-penetrating radar, topography and classification of vegetation to model the sediment and active layer thickness in a periglacial lake catchment, western Greenland

2016 ◽  
Vol 8 (2) ◽  
pp. 663-677 ◽  
Author(s):  
Johannes Petrone ◽  
Gustav Sohlenius ◽  
Emma Johansson ◽  
Tobias Lindborg ◽  
Jens-Ove Näslund ◽  
...  
Keyword(s):  
Numerical Modeling ◽  
Layer Thickness ◽  
Active Layer ◽  
Ground Penetrating Radar ◽  
Vegetation Classification ◽  
Active Layer Thickness ◽  
Sediment Thickness ◽  
Catchment Scale ◽  
Scale Models ◽  
Ground Penetrating

Abstract. The geometries of a catchment constitute the basis for distributed physically based numerical modeling of different geoscientific disciplines. In this paper results from ground-penetrating radar (GPR) measurements, in terms of a 3-D model of total sediment thickness and active layer thickness in a periglacial catchment in western Greenland, are presented. Using the topography, the thickness and distribution of sediments are calculated. Vegetation classification and GPR measurements are used to scale active layer thickness from local measurements to catchment-scale models. Annual maximum active layer thickness varies from 0.3 m in wetlands to 2.0 m in barren areas and areas of exposed bedrock. Maximum sediment thickness is estimated to be 12.3 m in the major valleys of the catchment. A method to correlate surface vegetation with active layer thickness is also presented. By using relatively simple methods, such as probing and vegetation classification, it is possible to upscale local point measurements to catchment-scale models, in areas where the upper subsurface is relatively homogeneous. The resulting spatial model of active layer thickness can be used in combination with the sediment model as a geometrical input to further studies of subsurface mass transport and hydrological flow paths in the periglacial catchment through numerical modeling. The data set is available for all users via the PANGAEA database, doi:10.1594/PANGAEA.845258.

scholarly journals Uniform grain-size distribution in the active layer of a shallow, gravel-bed, braided river (the Urumqi River, China) and implications for paleo-hydrology

2018 ◽  
Author(s):  
Laure Guerit ◽  
Laurie Barrier ◽  
Youcun Liu ◽  
Clément Narteau ◽  
Eric Lajeunesse ◽  
...  
Keyword(s):  
Grain Size ◽  
Size Distribution ◽  
Grain Size Distribution ◽  
Active Layer ◽  
Size Distributions ◽  
Gravel Bed ◽  
Surface Samples ◽  
River Bed ◽  
Braided Stream ◽  
Grain Size Distributions

Abstract. The grain-size distribution of ancient alluvial systems is commonly determined from surface samples of vertically exposed sections of gravel deposits. This method relies on the hypothesis that the grain-size distribution obtained from a vertical cross-section is equivalent to that of the river bed. We report a field test of this hypothesis on samples collected on an active, gravel-bed, braided stream: the Urumqi River in China. We compare data from volumetric samples of a trench excavated in an active thread and surface counts performed on the trench vertical faces. We show that the grain-size distributions obtained from all samples are similar and that the deposit is uniform at the scale of the river active layer, a layer extending from the surface to a depth of approximately ten times the size of the largest clasts.

scholarly journals Experimental and numerical study on the design of a deposition basin outlet structure at a mountain debris cone

2014 ◽  
Vol 14 (2) ◽  
pp. 175-187 ◽  
Author(s):  
B. Gems ◽  
M. Wörndl ◽  
R. Gabl ◽  
C. Weber ◽  
M. Aufleger
Keyword(s):  
Numerical Study ◽  
Retention Volume ◽  
Transport Processes ◽  
Flood Protection ◽  
Scale Model ◽  
Bed Load ◽  
Outlet Section ◽  
Efficient Design ◽  
Physical Scale ◽  
Debris Cone

Abstract. Mountain debris cones in the Alpine region often provide space for dense population and cultivation. Hence, a great number of buildings are exposed to torrential hazards. In order to protect the settlement areas against flooding and overbank sedimentation, torrent defence structures are implemented at various locations within catchments. Directly at the debris cones, these protection measures often include a deposition basin at the fan apex and/or a confined channel that passes through the settlement. The work presented within this paper deals with the effect of specific outlet structure layouts, situated at the lower end of a selected deposition basin, on bed-load transport processes and flood protection. A case study analysis was accomplished comprising a 3-D numerical model (FLOW-3D) and a physical scale model test (1 : 30). The subject of investigation was the deposition basin of the Larsennbach torrent in the Austrian Northern Limestone Alps. The basin is situated on a large debris cone and opens out into a paved channel. Since the basin is undersized and the accumulation of sediment in the outlet section reduces the available cross section during floods, adjoining settlements are considerably endangered of lateral overtopping of both clear water and sediment. Aiming for an upgrade in flood protection, certain layouts for a "closing-off structure" at the outlet were tested within this project. For the most efficient design layout, its effect on flood protection, a continuous bed-load output from the basin and the best possible use of the retention volume are pointed out. The simple design of the structure and the key aspects that have to be taken into consideration for implementation are highlighted.

scholarly journals Numerical Modeling of the Active Layer Thickness and Permafrost Thermal State Across Qinghai-Tibetan Plateau

2017 ◽  
Vol 122 (21) ◽  
pp. 11,604-11,620 ◽  
Author(s):  
Yanhui Qin ◽  
Tonghua Wu ◽  
Lin Zhao ◽  
Xiaodong Wu ◽  
Ren Li ◽  
...  
Keyword(s):  
Numerical Modeling ◽  
Tibetan Plateau ◽  
Layer Thickness ◽  
Active Layer ◽  
Thermal State ◽  
Active Layer Thickness

scholarly journals Bedload Equation Analysis Using Bed Load-Material Grain Size

2013 ◽  
Vol 61 (3) ◽  
pp. 241-249 ◽  
Author(s):  
Arman Haddadchi ◽  
Mohammad H. Omid ◽  
Amir A. Dehghani
Keyword(s):  
Grain Size ◽  
Sediment Transport ◽  
Field Data ◽  
Transport Equations ◽  
Bed Load ◽  
Gravel Bed ◽  
Equation Analysis ◽  
Bedload Sediment ◽  
Material Grain Size ◽  
Bed Material

Abstract Twelve predictive bedload sediment transport equations are rated against 14 sets of gravel-bed river field data collected by handheld bedload sampler in Narmab River, northeastern Iran. To evaluate these formulas two types of grain size namely bedload and bed material were used. The results show that the equations of Engelund and Hansen, Van Rijn and Einstein perform well with bed material grain size, while Shocklitsch, Meyer-Peter and Mueller, and Frijlink yield the best results using the bedload grain size.

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