Entrainment Of Bed Material Research Articles

ASSESSING STREAM CHANNEL STABILITY THRESHOLDS USING FLOW COMPETENCE ESTIMATES AT BANKFULL STAGE1

ABSTRACT: Stream channel stability is affected by peak flows rather than average annual water yield. Timber harvesting and other land management activities that contribute to soil compaction, vegetation removal, or increased drainage density can increase peak discharges and decrease the recurrence interval of bankfull discharges. Increased peak discharges can cause more frequent movement of large streambed materials, leading to more rapid stream channel change or instability. This study proposes a relationship between increased discharge and channel stability, and presents a methodology that can be used to evaluate stream channel stability thresholds on a stream reach basis. Detailed surveys of the channel cross section, water surface slope, streambed particle size distribution, and field identification of bankfull stage are used to estimate existing bankfull flow conditions. These site specific stream channel characteristics are used in bed load movement formulae to predict critical flow conditions for entrainment of coarse bed material (D84 size fraction). The “relative bed stability” index, defined as the ratio of critical flow condition to the existing condition at bankfull discharge, can predict whether increased peak discharges will exceed stream channel thresholds.

Size‐selective entrainment of bed load in gravel bed streams

Entrainment of mixed size gravel bed material was studied in nine reaches of three high‐power streams in Scotland and Norway. Paired measurements of at‐a‐point shear stress (estimated from velocity profiles) and bed load transport (by hand‐held sampler) were made. They extended to very high values (400 N m−2, 3.5 kg m−1 s−1). Analyses of maximum bed load diameter, mean bed load diameter, transport rates of individual size fractions, and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size, despite strong relative size effects. Precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates. Size‐selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant.

The concept of dominant discharge applied to two gravel‐bed streams in relation to channel stability thresholds

AbstractDominant discharge may be defined as that discharge which transports most bed sediment in a stream that is close to steady‐state conditions. The concept is examined in relation to two single thread gravel‐bedded streams. One stream is alluvial and free to adjust its geometry whilst in the other, channel capacity and form are partially constrained by cohesive till‐banks and a heavily compacted bed.The total quantity of bedload and its calibre were measured for every flood over a six year period, so that the relationship between the grain‐size of bedload and the most effective discharge could be examined in the context of thresholds for channel change.The dominant discharge concept was applicable to the alluvial stream in that the bankfull value is an effective discharge for maintaining channel capacity. The concept applied less well to the ‘non‐alluvial’ stream. Although in both streams the bankfull value was exceeded for less than 0.34 per cent of the time, overbank flows are important in instigating channel change. It is only during overbank flows that the largest bed material is entrained in quantity. For within‐channel flows a threshold separates flows which winnow fine matrix from those which entrain the finer bed gravels. This threshold occurred at 60 per cent bankfull.It was concluded that the dominant discharge concept can be applied to streams close to steady‐state which are alluvial, competent, and free to adjust their boundaries. An important proviso is that two channel‐stability domains can be recognized. These domains represent channel maintenance and channel adjustment and are defined by intrinsic thresholds in the bed material entrainment function.

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.

Simple models for the shape and symmetry of tidal sand waves: (3) dynamically stable asymmetrical equilibrium forms without flow separation

The complex interaction between the local steepness of a sand bed and the tidally induced bedload transport over it is theoretically such that dynamically stable sand waves of asymmetrical cross-section can exist beneath a tidal current in which one stream (dominant) exceeds in peak speed the other (subordinate). Calculations based on a mathematical model embodying these effects show that, as with naturally occurring sand waves, the degree of sand-wave morphological asymmetry increases with growing hydraulic asymmetry, it being supposed as a compromise and for simplicity that the streams are of equal duration and simple-harmonic in variation but of different peak speeds. The calculated sand waves are translatory and their steeper side faces in the direction of the dominant tidal stream and the net bedload transport, as would appear to be the case for naturally occurring forms. However, the precise form of the increase of morphological asymmetry with hydraulic asymmetry, together with the bounds on the existence field for sand waves, is influenced by the magnitude of the bed-material entrainment threshold relative to the peak speeds of the tidal streams. The main limitations on the model are its severely simplified treatment of flow and bedload transport over sand waves, and the lack of empirical data by which to test the proposed bed-steepness effects.

Simple models for the shape and symmetry of tidal sand waves: (2) Dynamically stable symmetrical equilibrium forms

The interaction between the local steepness of a sand bed and the tidally-induced bedload transport over it is such that dynamically stable sand beds of two alternative slopes can exist. When as in the simplest conceivable model the bedload transport is described kinematically, and the tidal current is treated as simple-harmonic (symmetrical), the nontrivial case is found to represent symmetrical sand waves of broadly saw-tooth cross-sectional profile. Their sloping sides can be either concave-up or straight, as is found on naturally occurring sand waves of approximately symmetrical form, and of a degree of steepness increasing as the peak tidal streams weaken relative to the bed-material entrainment threshold, as also seems true of sand waves beneath natural tidal currents. Bed-material calibre has little effect on sand wave profiles, at least within the constraints of the model.

Simple models for the shape and symmetry of tidal sand waves: (1) Statically stable equilibrium forms

Tidal sand waves are divisible between dynamically and statically stable equilibrium types. Statically stable equilibrium forms take that cross-sectional shape in relation to prevailing tidal currents such that bedload transport just fails to occur at every point on the surface. Given that the bed-material entrainment threshold is a function of bed slope, this shape is calculated to be: (1) symmetrical and convex-up under a symmetrical tidal current; and (2) asymmetrical and locally concave-up under an asymmetrical flow, the longer side of the bedform facing in the direction of the residual current. Sand waves similar to theoretical statically stable forms occur near the edge of the continental shelf off the eastern U.S.A. (Knebel and Folger, 1976). The bedforms are today found in a substantial water depth and are being affected by only weak currents. The structures, containing sediment dating from the early Flandrian, appear to be relict and could represent a modification of originally dynamically stable forms brought about as the sea deepened and currents weakened during the post-glacial sea-level rise.