Gravel-bed Channels Research Articles

Convective accelerations and boundary shear stress over a Channel Bar

Alternate bars are important features in alluvial channels as they determine flow and transport patterns. They appear fundamental to selection of meander wavelengths and the geometry of bends. Bend flow has been studied extensively: far less study has been made of flow over alternate bars. Field results from Solfatara Creek, a 5.2‐m‐wide, 0.2–0.7‐m‐deep gravel bed channel where flow exits an upstream bend and shoals over a bar in a straight reach, are used to examine patterns of flow and the fluid forces determining the flow field. Large cross‐sectional area changes, tied primarily to variation in depth, force large stream‐wise accelerations and substantial cross‐stream flow off the central bar. The topographically driven downstream and cross‐stream accelerations are sufficiently large that their influence upon the balance of forces is of the same order as the pressure gradient and the boundary shear stress. The importance of convective accelerations in the downstream flow equation in this straight reach concurs with bend flow results, but the similar importance of convective accelerations in the cross‐stream equation contrasts with results from bend flow. While part of the difference may be attributed to the lower stage conditions herein, in the absence of significant curvature change the cross‐stream force balance depends upon the flow going over and around the bar. Local boundary shear stress estimated from the law‐of‐the‐wall and a roughness algorithm decreases out of the upstream bend, increases over the bar top to values approaching the threshold for motion, and then decreases in deeper flow. Strong bed surface coarsening maintains the topography in a stress field that would otherwise lead to planation of the bar top and filling of the deeper regions.

Flume Experiments on the Transport of Heavy Minerals in Gravel-Bed Streams

ABSTRACT Heavy-mineral transport and deposition by shallow unidirectional flows in a gravel-bed channel were studied in a water-recirculating sediment-feed flume 6 m long and 0.15 m wide. The sediment was poorly sorted gravel with mean size 3 mm and 3% by weight of finer magnetite, lead, and tungsten. The heavy minerals became concentrated into a thin layer (the heavy infralayer) composed of nearly 100% heavy minerals lying beneath a thin surficial layer of low-density sediment (the light supralayer). Heavy minerals were not transported past a given location on the bed until the heavy infralayer was fully developed there. Heavy minerals were transported on the upper surface of the heavy infralayer only when local and temporary erosion of the light supralayer, caused by large clasts, clast jams, or downstream-moving dunelike bedforms, exposed the heavy infralayer. In two runs, one with sediment feed and one without, overall degradation of the bed was forced by slow lowering of a tailgate at the downstream end of the channel. In the sediment-feed run a downstream-prograding heavy infralayer formed as in the nondegrading runs. In the run without feed, the surficial heavy-mineral concentration increased gradually at all points along the channel to the end of the run. In light of these results, we hypothesize that ultimately the surficial heavy-mineral concentration would reach a steady state and the bed slope would change until the flow can transport all of the heavy-mineral sediment supplied by local exhumation. Although no runs were designed for aggradation, a too-small initial slope caused overall bed aggradation in the early parts of two runs. Heavy minerals in the feed sediment were transported only a short distance downstream to form a zone of higher but downstream-decreasing heavy mineral concentration extending vertically through the deposit. We hypothesize that the downstream length of this zone would increase as the heavy-mineral feed rate increases relative to the bed aggradation rate.

CRITICAL TRACTIVE FORCE AND BED-LOAD DISCHARGE IN GRAVEL-BED CHANNEL WITH STEEP SLOPE

CRITICAL TRACTIVE FORCE AND BED-LOAD DISCHARGE IN GRAVEL-BED CHANNEL WITH STEEP SLOPE

An assessment of bed load sediment transport formulae for gravel bed rivers

The performance is tested of 12 bed load sediment transport formulae developed for use in gravel bed channels. The formulae are applied in the manner intended by the original authors. To this end, the test data are restricted to ones obtained in approximately steady flow when the material in motion was similar to that present on the bed; that is, transport was not size selective. This represents the nearest approach to “equilibrium sediment transport” that can be realized with available data. Four sets of river data and three sets of flume data were chosen for the test, covering a range of eight orders of magnitude in unit bed load transport rate. The test data were not used in the development of the formulae. The analysis separates mean bias and local bias. No formula performs consistently well. Limitations of the test data, the constraints imposed by an operationally realistic test, and reasons based upon the physics of the transport phenomenon all may be adduced for this. To estimate the magnitude of transport with limited hydraulic information, stream power equations should be used because they provide the most straightforward scale correlation of the phenomenon. In particular, the approach of Bagnold deserves further study. When local hydraulic information is available, a formula should be selected that is sensitive to bed state or grain size distribution and, in this context, the formulae of Einstein, Parker, and Ackers‐White‐Day bear continued examination.

River Channels: Environment and Process

1. Rivers: environment, process and form: Keith S.Richards (Department of Geography, University of Cambridge) 2. Spatial adjustments to temporal variations in flood regime in some Australian rivers: Robin F.Warner (Department of Geography, University of Sydney) 3. The effect of active tectonics on alluvial river morphology: Daniel I.Gregory (Water Engineering and Technology Inc., Colorado) and Stanley A.Schumm (Department of Earth Sciences, Colorado State University) 4. Modelling fluvial systems: rock, gravel and sand bed channels: Alan D.Howard (Department of Environmental Science, University of Virginia) 5. River Channel adjustment - the downstream dimension: D.Knighton (Department of Geography, University of Sheffield) 6. Hydraulic and sedimentary controls of channel pattern: Rob Ferguson (Department of Environmental Science, University of Stirling) 7. Bed forms and clast size changes in gravel bed rivers: B.J.Bluck: (Department of Geology, University of Glasgow) 8. Mechanics of flow and sediment transport in river bends: William E.Dietrich (Department of Geology and Geophysics, University of California) 9. Channel boundary shape - evolution and equilibrium: T.R.H. Davies (Agricultural Engineering Department, Lincoln College, Canterbury) 10. Small and medium scale bedforms in gravel bed rivers: Pamela S.Naden (School of Geography, Leeds University) and Andrew C.Brayshaw (BP PLC, Brittanic House) 11. Measuring and modelling bedload transport in channels with coarse bed materials: James C.Bathurst (NERC Water Resource Systems Research Unit, Department of Civil Engineering, University of Newcastle-upon-Tyne) 12. The classification and characterization of rivers: M.P.Mosley (Ministry of Works and Development, New Zealand) 13. Bed stability in gravel streams, with reference to stream regulation and ecology: P.A.Carling (Freshwater Biological Association, Cumbria) 14. Applied fluvial geomorphology: river engineering project appraisal in its geomorphological context: K.S.Richards, (Department of Geography, University of Cambridge) with D.Brunsden, (Department of Geography, King's College London) D.K.C.Jones, (Department of Geography, LSE) and M.McCaig (formerly Geomorphological Services Ltd., Bucks.).

CHANNEL FORM AND STREAM ECOSYSTEM MODELS1

ABSTRACTA system is proposed to classify running water habitats based on their channel form which can be considered in three different sedimentological settings: a cobble and boulder bed channel, a gravel bed channel, or a sand bed channel. Three physical factors (relief, lithology, and runoff) are selected as state factors that control all other interacting parameters associated with channel form. When these factors are integrated across the conterminous United States, seven distinct stream regions are evident, each representing a most probable succession of channel forms downstream from the headwaters to the mouth. Coupling these different channel profiles with typical biotic community structures usually associated with each of the channel types should result in considerable refinement of the applicability of the River Continuum Concept and other holistic ecosystem models by realizing the nonrandomness of the effects of geo‐morphology on stream ecosystems. Thus, this regional perspective of streams should serve to make persons concerned with water resources more aware of the geographical considerations that affect their study areas.

Bed-material entrainment and hydraulic geometry of gravel-bed rivers in Colorado

Research Article| March 01, 1984 Bed-material entrainment and hydraulic geometry of gravel-bed rivers in Colorado E. D. ANDREWS E. D. ANDREWS 1U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Author and Article Information E. D. ANDREWS 1U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1984) 95 (3): 371–378. https://doi.org/10.1130/0016-7606(1984)95<371:BEAHGO>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation E. D. ANDREWS; Bed-material entrainment and hydraulic geometry of gravel-bed rivers in Colorado. GSA Bulletin 1984;; 95 (3): 371–378. doi: https://doi.org/10.1130/0016-7606(1984)95<371:BEAHGO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Twenty-four gravel-bed rivers in the Rocky Mountain region of Colorado were selected for a detailed investigation of bed-material mobility and hydraulic geometry. The criteria for choosing the study reaches were: a nonbraided channel with self-formed bed and banks, evidence of quasi-equilibrium, minimal flow regulation, and a streamflow-gaging station record of at least ten years. In each river reach, several cross sections and longitudinal profiles of the riverbed, water surface, and bankfull elevation were surveyed. Samples of bed surface and subsurface material were collected for size analysis. The median diameter of the bed-material surface ranged from 23 mm to 120 mm. Bankfull discharges ranged from 0.70 m3 /sec to 255 m3/sec. Computed values of the dimensionless shear stress (τ*) showed that the threshold of particle motion was exceeded at flows slightly less than bankfull. The average value of τ* at bankfull was 0.046. Bankfull discharges were equaled or exceeded on an average of 8.1 days per year. The averaged computed value of τ* for the 100-yr flood discharges was only 0.068. Consequently, transport of bed-material particles is a relatively frequent occurrence. Large bed-material transport rates, however, are extremely rare. Empirical evidence obtained at these and other streams indicates that a stable, self-formed gravel-bed channel cannot be maintained at a dimensionless shear stress greater than about 0.080.The 24 river reaches were divided into 2 groups, depending upon bank stability as indicated by bank vegetation. Separately determined bankfull hydraulic-geometry equations for each group were found to have the same exponents, but slightly different coefficients. An analysis of variance determined that 67% of the observations were within ± 15% to 20% of the predicted values for most hydraulic variables. The bankfull hydraulic-geometry equations for the Colorado rivers studied were compared with those of British gravel-bed rivers that had been similarly divided according to the extent of bank vegetation. No significant difference between the hydraulic geometries of Colorado and British gravel-bed rivers with thick bank vegetation was found. The comparison of hydraulic-geometry equations for those rivers with thin bank vegetation determined that there was no difference in the width-versus-discharge relations. No significant difference was found for the exponents of the depth, velocity, and slope equations; however, the coefficient values were slightly different. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Dynamic process-response model of river channel development

Feedback mechanisms, which operate upstream through drawdown and backwater effects and downstream through sediment discharge are responsible for channel evolution. By combining these mechanisms with channel processes it euables a dynamic process-response model to be developed to simulate the initial evolution of straight gravel-bed channels. When erosion commences on a land surface, sediment entrained in the headwater reach by hydraulic action is selectively transported, deposited and reworked. This produces a damped oscillation between degradation and aggradation as the channel and valley respond to spatial and temporal variations in sediment calibre and hydraulic conditions. The initial cut and fill phases are responsible for valley incision and floodplain development while secondary and subsequent activity can produce river terraces. Eventually sediment entrainment in the headwaters declines as slopes are reduced. Subsequent channel evolution is relatively insignificant because it is dependent on local weathering activity producing material that can be transported on declining slopes. Therefore landforms produced during the initial phase of development, when local weathering was non-limiting, dominate the landscape.