Friction Factors and Bed Forms in Fluvial Channels

Squarer, D.

doi:10.1061/jyceaj.0002478

Cited by 7 publications

(1 citation statement)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Individual bedforms, however, change in size, shape and relative position as the local sand-transporting currents remodel them, and they, simultaneously, affect the local pattern and strength of flow over the bed. This stochastic behaviour is well established for both current ripples (Ashida & Tanaka 1967;O'Loughlin & Squarer 1967;Allen 1968Allen , 1973Squarer 1970;Taylor 1971;Nordin 1971;P ratt & Smith 1972;Jain & Kennedy 1974) and dunes (Nordin & Algert 1966;Ashida & Tanaka 1967;Fukuoka 1968;Hino 1968;Crickmore 1970;Taylor 1971;Nordin 1971;Annambhotla et al 1972;P ratt & Smith 1972;Shen & Cheong 1976). Not only do individual bedforms change once established, but new individuals arise to replace those being destroyed, in what amounts to a birth-death process (see, for example Allen 1973;Gabel 1993).…”

Section: (/) Comparison With Other Recordsmentioning

confidence: 81%

Morphodynamics of intertidal dunes: a year-long study at Lifeboat Station Bank, Wells-next-the-Sea, Eastern England

Allen¹,

Friend

Lloyd³

et al. 1994

Phil. Trans. R. Soc. Lond. A

View full text Add to dashboard Cite

Sand is driven clockwise by tidal currents round a diamond-shaped sand shoal in the entrance to the harbour at Wells-next-the-Sea. A population of dune bedforms occupying the ebb-dominated channel on the western side of the shoal was monitored for a year (October 1975 to October 1976) in terms of shape (height, wavelength, superimposed immature dunes), movement and bedform composition. Individual dunes and the population of dunes as a whole changed in response to sediment transport on astronomical tidal scales varying from the semi-diurnal to the equinoctial and in response to seasonal meteorological forcing. Most change occurred during the autumn and winter, when strong winds and gales created surges which, in some cases very significantly, enhanced the sediment transport ue purely to the astronomical tide. Dune height was on average greatest during the winter, when the sea temperature was low, and least during the period of summer warmth. Dune height also varied substantially on a spring-neap tidal scale, an increase in height with the onset of many of the springs being followed by a gradual lowering. Dune wavelength showed little response to the springneap variation of sediment transport, but decreased significantly between winter and summer. Varying degrees of time-lag accompanied all changes in bedform characteristics in response to hydraulic change. Although individual dunes had surprisingly large lifespans and ebb-directed excursions, some change was noted m the composition and statistical attributes of the bedform population as the result of appearances and disappearances. The more vigorous episodes of sediment transport created immature (nascent) dunes, some of which grew large enough to become incorporated as new members into the population of mature forms Internally, the dunes were dominated by ebb-oriented cross-bedding, complicated by a variety of intra-set discontinuities commonly associated with mud drapes Some recorded the smoothing of crests during tidal reversal and others the immobility of the bedform over a number of tidal cycles (occasionally many). Other discontinuities expressed the ‘capture’ of immature dunes by the main bedform within the duration of a single ebb-tide. Because of the frequent perturbation of the (astronomical) tidal sediment transport by meterological events, little order to the horizontal arrangement of discontinuities within the sets was detected in contrast to other reported cases.

show abstract

Section: (/) Comparison With Other Recordsmentioning

confidence: 81%

Morphodynamics of intertidal dunes: a year-long study at Lifeboat Station Bank, Wells-next-the-Sea, Eastern England

Allen¹,

Friend

Lloyd³

et al. 1994

Phil. Trans. R. Soc. Lond. A

View full text Add to dashboard Cite

show abstract

Riffle‐pool morphometry and stage‐dependant morphodynamics of a large floodplain river (Vereinigte Mulde, Sachsen‐Anhalt, Germany)

Vetter¹

2011

Earth Surf Processes Landf

View full text Add to dashboard Cite

Riffle-pool sequences are a common feature of gravel-bed rivers. However, mechanisms of their generation and maintenance are still not fully understood. In this study a monitoring approach is employed that focuses on analysing crosssectional and longitudinal channel geometry of a large floodplain river (Vereinigte Mulde, Sachsen-Anhalt, Germany) with a high temporal and spatial resolution, in order to conclude from stage-dependant morphometric changes to riffle and pool maintaining processes.In accordance with previous authors, pool cross-sections of the Mulde River are narrow and riffle cross-sections are wide suggesting that they should rather be addressed as two general types of channel cross-sections than solely as bedforms. At high flows, riffles and pools in the study reaches changed in length and height but not in position. Pools were scoured and riffles aggraded, a development which was reversed during receding flows below the threshold of 0·4Q bf (40% bankfull discharge). An index for the longitudinal amplitude of riffle-pool sequences, the bed undulation intensity or bedform amplitude, is introduced and proved to be highly significant as a form parameter, its first derivative as a process parameter. The process of pool scour and riffle fill is addressed as bedform maintenance or bedform accentuation. It is indicated by increasing longitudinal bed amplitudes.According to the observed dynamics of bed amplitudes, maintenance of riffle-pool sequences lags behind discharge peaks. Maximum bed amplitudes may be reached with a delay of several days after peak discharges. Increasing bed undulation intensity is interpreted to indicate bed mobility. Post-flood decrease of the bed undulation intensity indicates a retrograde phase when transport from pools to riffles has ceased and bed mobility is restricted to riffle tails and heads of pools. This type of transport behaviour is referred to as disconnected mobility.The comparison of two river reaches, one with undisturbed sediment supply, the other with sediment deficit, suggests that high bed undulation intensity values at low flows indicate sediment deficit and potentially channel degrading conditions. It is more generally hypothesized that channel bed undulations constitute a major component of form roughness and that increased bed amplitudes are an important feature of channel bed adjustment to sediment deficit be it temporally during late floods or permanently due to a supply limitation of bedload.

show abstract

On the modelling of sand bedforms using the semivariogram

Robert

Richards

1988

Earth Surf Processes Landf

View full text Add to dashboard Cite

This study shows the usefulness of the semivariogram for modelling sand ripples created by water flows of varied flow intensity. A combination of two mathematical functions is fitted to each sample semivariogram, that is an exponential (or stochastic) component and a periodic component. The parameters of each of these components have direct physical meaning. A non-dimensional ratio combining the two parameters of the exponential model is interpreted as a regularity index (which increases with the degree of regularity of bedform arrangement). This regularity index is inversely related to the Froude number of the flow. The non-dimensional wavelength, estimated from the dominant periodic function, is also inversely and closely related to the Froude number. The wave height, accurately estimated from properties of the two fitted components, is a direct function of flow velocity and is also proportional to the standard deviation of bed elevations. The bedform shape introduces a considerable discrepancy between the generally assumed normal frequency distribution and the empirical distributions of bed height. The series of bed elevations are generally characterized by a mixture of normal distributions having the same variance but different means. The calculation of a covariance assuming a constant and single mean (as in spectral analysis) can therefore be misleading and the problem may be avoided by using the semivariogram.

show abstract

Friction Factors and Bed Forms in Fluvial Channels

Cited by 7 publications

References 0 publications

Morphodynamics of intertidal dunes: a year-long study at Lifeboat Station Bank, Wells-next-the-Sea, Eastern England

Morphodynamics of intertidal dunes: a year-long study at Lifeboat Station Bank, Wells-next-the-Sea, Eastern England

Riffle‐pool morphometry and stage‐dependant morphodynamics of a large floodplain river (Vereinigte Mulde, Sachsen‐Anhalt, Germany)

On the modelling of sand bedforms using the semivariogram

Contact Info

Product

Resources

About