Bed Material Size Research Articles

Geologic influences on fluvial hydrology and bedload transport in small mountainous watersheds, Northern New Mexico, U.S.A.

AbstractDischarge characteristics in six adjacent mountainous watersheds in northern New Mexico, U.S.A., vary substantially between basins underlain by different lithologies. Relatively resistant gneisses and granites underlie two basins (drainage areas: 43 and 94 km2) that have high unit discharge (0·010 to 0·14 m3s−1 km−2), high bankfull discharge, and sustained high discharge. Less resistant sandstones and shales underlie four basins (drainage areas: 96 to 215 km2) that have relatively low unit discharge (0·001 to 0·005 m3s−1 km−2), relatively low bankfull discharge, and peak discharges that are not sustained as long as those in the crystalline terrane.Analysis of snowmelt‐runoff water budgets suggests that three factors control hydrologic conditions in the basins. First, area‐elevation distributions appear to control the timing and amounts of water input. These distributions probably reflect the erosional resistance of the different lithologies. Second, lithology appears to control runoff production in areas having minor amounts of storage. Third, glacial deposits in headwater regions control discharge duration and timing via storage and return flow releases. The amount of return flow released by glacial deposits, however, is probably controlled by the permeability of underlying bedrock. Therefore it appears that the duration, timing, and magnitude of discharge events in the study area are controlled both directly and indirectly by lithology.Stream power and shear stress estimates derived from bankfull discharge and bed‐material size data suggest that higher bedload transport rates and larger bedload particle sizes exist in streams draining crystalline rocks than in streams draining sedimentary terrane. It appears that source‐area lithology, by controlling discharge production, also influences stream power, bedload transport capabilities, and therefore total amounts of bedload transport.

Sediment transport and resulting deposition in spawning gravels, north coastal California

Incubating salmonid eggs in streambeds are often threatened by deposition of fine sediment within the gravel. To relate sedimentation of spawning gravel beds to sediment transport, infiltration of fine sediment (<2 mm in diameter) into clean gravel beds, bed material size distributions, scour‐fill depths, and sediment transport during 10 storm flow events were measured in three streams of north coastal California. Although suspended sediment comprised most (75–94%) of the clastic load during storm flows, bed load material (0.25–2 mm) accounted for most (70–78%) of the fine sediment accumulated in experimental gravel implanted in the streambeds. Sand trapped in the interstices of the top several centimeters formed a seal that impeded deeper deposition of very fine sand and finer material. The seal was responsible at least in part for a decrease in the rate of fine‐sediment accumulation with increasing cumulative bed load transport. Areas of the streambeds commonly scoured or filled 0.1 m or more during storm flows, and thus scour and fill commonly created a sandy layer at least as thick as the seal formed by sediment infiltration. Scour could erode eggs laid in the bed and expose deeper levels of the bed to infiltration by fine sediment, but at the same time could allow fine sediment to be winnowed away. Great temporal and spatial variation in sedimentation in these streams suggests that individual storms of moderate size pose a threat to eggs in many but not all areas selected by fish for spawning.

Influence of sand on hydraulics and gravel transport in a braided gravel bed river

Measurements in a braided reach of the proglacial White River, Washington, are used to compare the bed load capacity and competence of flow over the predominant gravel bed and over a narrow ribbon of mobile sand which extended alongside the talweg. Analysis of 144 velocity profiles and 222 bed load samples shows that shear stress and roughness height were significantly lower over sand than gravel, but maximum bed load diameter and total gravel transport rate significantly higher, as were transport rates of given size fractions at similar shear stresses. These differences in competence and capacity are explained by relative size effects on the threshold shear stress for entrainment. The influence on gravel transport of local variation in bed material size may explain the self‐sorting of “bed load sheets” described by other workers. However, in channels like that described here the continuous longitudinal ribbon of sand allows the flow to develop a lower shear stress over the smoother bed, which partly offsets the relative size effects on competence and capacity.

Bed load transport in braided gravel‐bed stream models

AbstractBed load transport rate was measured in ten self‐formed small‐scale gravel braided streams developed in a laboratory flume at several different values of steady discharge and flume gradient. The streams are approximate Froude models of typical prototype braided streams but of no particular river. Slight viscous effects may be present in the models because particle Reynolds numbers are close to 70. Total bed load discharge was measured every fifteen minutes throughout each 60 hour run. In addition, 80 channel cross‐sections were measured in each run to establish the average channel geometry.Total bed load transport rate correlates well with total discharge and total stream power, although at a given stream power bed load discharge is greater when braiding is less intense and the width/depth ratio is lower. Analysis using unit stream power and cross‐section average bed shear stress reveals that the laboratory data conform to existing empirical bed load transport relationships. However, comparison with field data from gravel‐bed rivers shows discrepancies that may be due to differences in bed material size gradation and bed sediment structure.At constant discharge, wide fluctuations in bed load discharge occur with some regularity. Periods range from 2 to 10 hours in the models, which is equivalent to several tens of hours in a prototype. The presence of these long‐period fluctuations compounds the problems of field measurement of bed load in braided streams.

STREAM CHANNEL PATTERN: A THRESHOLD MODEL

The focus of this research is the development of a model which explains channel pattern variability in streams. Since channel pattern is commonly regarded as a qualitative phenomenon, the research employs a logistic regression model, which is advocated as an alternative to traditional graphic/discriminant analysis, since the concepts of threshold and instability have very natural expressions in the logistic regression framework. The results demonstrate that channel gradient and mean discharge can effectively explain channel pattern (i.e., whether the channel is single or multithreaded) in an environment where there is a small range of bed material size. Sediment sorting is also shown to be related to channel pattern in the study environment. Models using valley gradient rather than channel gradient are shown to be distinctly inferior, and no advantage is found in using a stream power measure as opposed to separate gradient and discharge measures. [Key Words: fluvial geomorphology, stream channels, channel...

Spouted, fluidized and spout-fluid bed combustion of bituminous coals

Spouted, fluidized and spout-fluid bed combustion of three bituminous coals or coal rejects of differing ash contents was carried out in a 0.3 m internal diameter combustor. Coal feed rates were between 6 and 22 kg h −1, excess air was usually 15–20%, the internal coil coolant was either air or water, and the average bed temperature ranged from 740 to 980 °C. Each of the three contacting modes was able to burn the coal to which it was subjected. Although differences in combustion performance between the three modes were generally small, the experimental results indicated that the spout-fluid bed tended to give somewhat higher combustion efficiencies at the lower temperatures, greater temperature uniformity and improved bed-to-immersed-surface heat transfer compared with the other two modes of operation. For all three modes, combustion efficiencies were over 90%, provided that bed temperatures were higher than 870 °C and that fines captured in the primary cyclone were recycled to the bed. The size and density of inert bed materials played a significant role. Denser and coarser inert particles tended to segregate, leaving burning char particles at or near the bed surface. Bed-to-cooling-coil heat transfer coefficients were of similar magnitude to those measured in earlier studies.

Mutual Adjustments between Process and Form in a Desert Mountain Fluvial System

Abstract The analysis of earth surface systems is complicated by mutual adjustments among geomorphic variables. Most statistical models used by geomorphologists implicitly assume unidirectional causation. Simultaneous-equation models represent an alternative statistical approach by which mutual adjustment mechanisms can be analyzed explicitly. This technique is used to develop a process-response model of a small desert mountain fluvial system in order to examine the system's internal structure and dynamics. In general, results confirm the expected interactions among variables in the model. Discharge, size of bed material, and the type of bank material are the dominant factors directly influencing channel width, suggesting that channel form reflects an interaction among hydrologic conditions, the caliber of the sediment load, and the resistance imposed by the channel perimeter. Results also indicate that local and upstream energy conditions strongly influence the mean grain size and sorting of the streambe...

Stable width and depth of straight gravel rivers with heterogeneous bed materials

Mathematical model for defining stable width and depth of straight gravel rivers is presented, in which it is revealed that the major agency for maintaining stable channel is lateral diffusion of longitudinal fluid momentum due to turbulence. The primary quantities which determine the stable channel geometry are found to be flow discharge, longitudinal free surface slope, the median size of bed material, and gradation of the bed material. Increasing gradation is found to increase the depth and decrease the width. Introduction of one loose and the other rigid bank in laboratory experiments provides good examples to test the model, in which four kinds of sand were used to test the gradation effect. These laboratory data support the mathematical model presented herein. The theory also performs well when it is applied to natural gravel rivers.

Temperature profiles in a lignite flotsam-rich fluidized bed

Temperature measurements along the vertical axis of an 11 cm i.d. fluidized bed were successfully used to interpolate the segregation patterns for Can lignite burning in quartz sand. Using differential temperature data the lignite flotsam-rich type segregation patterns were investigated for changes in lignite type, bed materials, particle size, fluidization velocity and static dynamic bed heights. It is concluded that the quality of mixing deteriorates when the lignite particle size, the fluidization velocity and the length of static and dynamic bed heights are each decreased and ash content, density and particle size of bed material increased. These were in accordance with the behaviour of a flotsam-rich type system in which lignite was the flotsam component. In this study the effect of volatile matter content on freeboard combustion was found to be unimportant compared with the effect of changes in the segregation patterns.

Bed Topography and Sorting in Bends

A mathematical model for defining the bed topography and the bed material size distribution in uniformly curved bends of alluvial rivers is developed, in which the coarsening of sediment size toward the outside bank is predicted and the bed topography is calculated coupling with the lateral sorting. The major agency for the sorting is that coarser grains feel a larger ratio of transverse gravitational force to fluid force than finer grains. It is revealed that the sorting reduces the depth of scour considerably in the outer area of bends. The model is tested with a laboratory test, and it is applied to an actual river, both of which support the current theory.

Changing market realities enhance the appeal of ASICs : Iwao Sagara. J. Electron. Engng, 28 (October 1986)

Anti-wear beams installed on water walls of circulating fluidized bed (CFB) boilers are one method to protect against water wall wear. Beam effects from superficial gas velocity, solid suspension density, bed material size and beam size were investigated on water wall wear in a CFB test rig experimentally using a measurement system consisting of thin-film resistor sensors and high-precision resistance probers. Multiple sensors were set up around an anti-wear beam vertically to obtain the axial profile of the wear rate of water wall.Results show that installed anti-wear beams may reduce the wear rate of water wall. Along the water wall around the beam vertically, the maximum wear rate occurs at the upper surface of the beam while the minimum rate occurs at the bottom surface. In general, the wear rate of the water wall around an anti-wear beam increased with the superficial gas velocity, initial bed material (static bed height) and particle size, while the wear rate just below the beam was not notably influenced by those parameters. An optimum anti-beam width in the range of 6–9 mm is recommended under the test condition.

Interrelationships of Channel Processes, Changes and Sediments in a Proglacial Braided River

Spatial and temporal variations in channel morphology, velocity and shear stress, bedload transport rate, and bedload and bed material size distribution were measured over five summer weeks in the braided Lyngsdalselva in Arctic Norway. Velocities and shear stresses reflected streamwise convergence and divergence of flow, and in turn influenced bedload transport rates although these depended also on the availability at the surface of appropriate-sized sediment. Large and small particles had almost equal mobility, so transport rate increased rapidly with shear stress as coarse sediment began to move and in so doing exposed finer particles. Size-selective transport is nevertheless indicated by down-reach and down-bar fining. The spatial patterns of shear stress and bedload transport rate within one divided-single-divided channel cycle changed with discharge, as did the resulting channel changes. In high meltwater flows, with a peak transport rate of 0.3 kg m-1 s-1, erosion was localised at two zones of flow convergence and tended to maintain the non-uniform channel geometry. During a higher rain-flood, with transport rates up to 3 kg m-1 s-1 a downstream fall in shear stress in the main distributary led to medial deposition and consequent lateral erosion, creating a wider, shallower, more uniform channel. Grain size distributions of bedload at high and low shear stresses match fairly well those of floodplain gravels and falling stage flood deposits respectively.

STABLE BED TOPOGRAPHY AND SORTING IN UNIFORMLY-CURVED CHANNELS WITH HETEROGENEOUS BED MATERIALS

A mathematical model for defining the bed topography and the bed material size distribution in uniformly-curved bends is developed, in which the coarsening of sediment size toward the outside is predicted and the bed topography is calculated coupling with the lateral sorting. The major agency for sorting is that coarser grains feel a larger ratio of transverse gravitational force to fluid force than finer grains. It is revealed that the sorting reduces the depth of scour considerably in the outer area of bends. The model is tested with a laboratory test, and it is applied to an actual river, both of which support the present theory.

Stable Channels with Mobile Gravel Beds

Regime type equations for mobile gravel-bed rivers are presented based on data obtained from 62 stable gravel-bed river reaches in the United Kingdom. Multiple regression techniques are applied to derive equations relating reach average and riffle values of width, mean and maximum depth, slope, velocity, sinuosity and riffle spacing to bankfull discharge, bed and bank material characteristics, valley slope, bank vegetation type and an independent estimate of bankfull bed load transport rates. Although discharge has a dominant control on channel geometry, these equations indicate that bed load discharge also has a significant influence, particularly with regard to channel slope. Bank vegetation has a major control on width and velocity while depth, velocity and slope are storngly affected by bed material size. Reasons for these results are considered in terms of the physical processes controlling channel adjustment. The application of these regime equations is discussed, and particular consideration is given to the design of sinuous channels with a pool-riffle bed topograph.

Interrelationships of Channel Processes, Changes and Sediments in a Proglacial Braided River

Spatial and temporal variations in channel morphology, velocity and shear stress, bedload transport rate, and bedload and bed material size distribution were measured over five summer weeks in the braided Lyngsdalselva in Arctic Norway. Velocities and shear stresses reflected streamwise convergence and divergence of flow, and in turn influenced bedload transport rates although these depended also on the availability at the surface of appropriate-sized sediment. Large and small particles had almost equal mobility, so transport rate increased rapidly with shear stress as coarse sediment began to move and in so doing exposed finer particles. Size-selective transport is nevertheless indicated by down-reach and down-bar fining. The spatial patterns of shear stress and bedload transport rate within one divided-single-divided channel cycle changed with discharge, as did the resulting channel changes. In high meltwater flows, with a peak transport rate of 0.3 kg m-1 s-1, erosion was localised at two zones of flow convergence and tended to maintain the non-uniform channel geometry. During a higher rain-flood, with transport rates up to 3 kg m-1 s-1 a downstream fall in shear stress in the main distributary led to medial deposition and consequent lateral erosion, creating a wider, shallower, more uniform channel. Grain size distributions of bedload at high and low shear stresses match fairly well those of floodplain gravels and falling stage flood deposits respectively.

FLOW RESISTANCE AND SEDIMENT DISCHARGE IN OPEN CHANNELS WITH NEARLY UNIFORM MOVEABLE BED MATERIALS

The main purpose of this paper is to describe the relation between sediment discharge, resistance to flow and bed configulation in channels with nearly uniform bed materials by appling the procedures of dimensional analysis to a vast number of flume and river channel data, and to present a estimation method of dediment discharge and resistance to flow from a set of water depth, bed material size and slope.

Estimating Mean Velocity in Mountain Rivers

It is often necessary to estimate the mean velocity in ungaged mountain rivers, but the flow resistance equations available for this purpose require further testing and development. Such rivers are characterized by coarse bed materials, steep slopes, and low depths. For these conditions, boulders protrude well into or completely through the flow, and bed roughness is said to be large‐scale. Recently, three equations specifically intended for large‐scale roughness have been developed to address this problem. Data from a mountain river were used to test the equations. All were prone to errors of the order of 30%. The errors were systematic, all the equations tending to overestimate mean velocity compared to observed values. An investigation of the possible sources of error suggested that sampling error in the knowledge of bed material size was a major source of uncertainty in the predicted velocities, but that this and discharge errors could not wholly account for the observed discrepancies. Boulder shape is not represented in the equations, but this was discounted as an important parameter on the basis of a flume study. It was concluded that further research is required to produce a reliable, process based, flow resistance equation for mountain rivers.

Grain‐Size Distribution of River‐Bed Armor Layers

There appears to be both experimental and theoretical support for the selection of a normal curve for the description of the armor-layer grain size distribution. The findings of this study indicate that if a sufficiently wide range of grain sizes is available in the bed-sediment material, the normal curve will be similar to that describing the statistical variation of the bed-shear stress, and its parameters (mean and standard deviation) will not depend on the original bed-material size distribution. However, it should be stressed that this conclusion is only tentative, as there obviously is a need for more research in this area, in particular with regard to the sampling procedure for armor layers.

Causes of river bed degradation

River bed degradation can proceed downstream as well as upstream depending upon the cause of degradation. The causes of downstream progressing degradation are primarily related to changes in independent river channel variables, such as increase in water discharge, decrease in size of bed material, and decrease in bed material discharge. The causes of upstream progressing degradation are all related to an imposed increase in river slope which can occur as a result of natural river behavior or by man‐made changes. Study of various case histories indicates that river slopes are increased by lowering a base level, by decreasing the length of a river, or by removal of a control point. Case histories also show that downstream and upstream progressing degradation can act in combination along a river system: downstream progressing degradation along the main stream of a river system can initiate upstream progressing degradation on a tributary.

Closure to “ Bed Material Sorting in Pools and Riffles ” by Nani G. Bhowmik and Misganaw Demissie (October, 1982)

When rivers flow in a series of riffles and pools during periods of low flow, bed material sizes change from coarser to finer in the riffle-pool sequence. The Froude Number also changes from high to low from riffle to pool. Larger particle sizes are therefore associated with higher Froude Number and vice versa. The sorting of bed materials, with coarser material at the riffles and finer materials at the pools, is mainly due to the high bed shear stress at the riffles, where the fine materials unable to withstand the high shear stress are washed away and deposited in the pools downstream where the bed shear stress is lower. The aforementioned process of sorting takes place during low discharges when the difference in velocity between that of a riffle and a pool are the greatest.