摘要:

Water-surface slope is usually assumed to be constant when predicting bedload sediment transport in rivers despite its significance as a determinant of shear stress and the impact that variability would have on calculated sediment flux. This is pragmatic. It recognises that confirmatory data are unlikely to be available, especially during flood flows, and it is an appropriate assumption where discharge is steady. Where discharge is unsteady, water-surface slope varies and an expected pattern of hysteresis in the relation between watersurface slope and flow depth emerges from datasets collected in four gravel-bed streams, two ephemeral, one seasonal and one perennial. When water-surface slope is treated as a variable in applying a bedload equation, it is shown that flood bedload yields are about 8 percent higher than those derived with the same equation but with water-surface slope held constant and approximating the slope of the channel bed. It is concluded that, in engineering design, accounting for the variation in water-surface slope in arid-zone ephemeral streams, where bedload yield is high, is more significant than in perennial streams, where event frequency may be high but transport rates are low and highly variable.

ISSN:0022-1686    

DOI:10.1080/00221689809498630

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Many rivers have an abrupt transition from gravel-bed to sand-bed morphology. In many cases the point of transition is neither prograding nor retrograding, but is rather arrested in place. Two mechanisms are hypothesized as responsible for stabilizing the gravel-sand transition, basin subsidence (or alternatively base level rise) and abrasion of gravel. The companion paper offers a simplified analytical solution for the long profile of a river with such a transition. This treatment allows for direct insight into the relation between the morphology and the controlling mechanisms at the expense of several gross approximations. The present paper offers a rigorous complete numerical solution which takes such facts as the streamwise sorting of heterogeneous gravel into consideration.

ISSN:0022-1686    

DOI:10.1080/00221689809498631

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Results of recent laboratory investigations of local scour at bridge piers and abutments are presented. Similarities between the principal vortex occurring in local scour holes at abutments and the horseshoe vortex and downflow at piers are highlighted. Pier and abutment laboratory data, collected near the threshold conditions for sediment movement, display similar trends in the variation of scour depth with length and flow depth. This variation is best described in a three dimensional plot. A simple equation, that can be used to predict the maximum local scour depth at either piers or abutments aligned perpendicular to the flow, is presented and compared with field data.

ISSN:0022-1686    

DOI:10.1080/00221689809498632

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Investigations have been conducted ofin situ(unbounded) fluidisation produced by a vertical jet acting internally within saturated sands. This produces a sharply definedin situfluidised zone, which changes with increasing jet depth from an open, axisymmetric, approximately ellipsoidal form to an asymmetric, spouted profile, and thence to a submerged fluidised cavity. Measurement and dimensional analysis of fluidised zone geometries indicatein situfluidisation to be controlled by two mechanisms: (i) scour below the jet tip, which follows the linear velocity decrease of a submerged jet; and (ii) the ability of the flow to maintain fluidisation, which controls the zone diameter. The depth of fluidised cavity formation is shown to be a function of the ratio of the flow rate to that required for minimum (turbulent) fluidisation. The observations are justified in terms of jet diffusion, sediment transport, fluidisation and slope stability theory.

ISSN:0022-1686    

DOI:10.1080/00221689809498633

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The results of a laboratory investigation on the behaviour of plane turbulent jets discharging vertically upwards into a stagnant shallow water body have shown that for any depthHin terms of the half widthb0of the nozzle, the jet could be in an oscillating mode if the velocity at the nozzleU0is greater than a critical value of [0.009(Hlb0) + 0.357] √gHwhere g is the acceleration due to gravity. When the jet is oscillating, the frequency of oscillation ƒ is equal to 0.18 √g/H. An earlier study on vertical circular jets in shallow water did not find such an oscillating flow pattern.

ISSN:0022-1686    

DOI:10.1080/00221689809498634

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

This paper reports different aspects of turbulent flow structure using the ship-board time-series velocity data collected by an Acoustic Doppler Current Profiler (ADCP) in the large multi-thread Jamuna River in Bangladesh. The measurements were made during different river stages in various bed-form environments ranging from mega-ripples to dunes. Based on the analysis of the downstream flow velocity obtained from the three subdivisions of the water column, different turbulence parameters such as frequency spectra and intensities were found. Spectral analysis indicates a mixed picture of random turbulent motion and periodicity. The maximum period observed was about 18 min and the minimum was about 11.5 s. Analysis of the frequency of occurrence of all the grouped events shows that about 70% of the turbulence have their origins in the river-bed, the rest are horizontal eddies. It is shown that a 15 min averaging time is required to estimate the higher order moments such as the turbulence intensity. The vertical distribution of' the ratio of turbulence intensity and bed friction velocity indicates the presence of two segments separated by a peak: a growing segment limited within a height of 5 to 10% above the bed and an exponentially decaying segment above. The turbulence intensity is 7 to 10% of the local downstream velocity in the free-stream region but increases to about 11 to 23% in the wall region.

ISSN:0022-1686    

DOI:10.1080/00221689809498635

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The entrainment rate across a density interface formed inside a cylindrical tank by a fresh water layer floating on top of sea water is investigated. The tank has a concentric opening at its base, and floats in a body of sea water. Mixing across the interface is induced by a uni-directional current flow below the tank. The study was carried out to investigate the feasibility of storing freshwater in a bottomless tank in the sea. From dimensional and regression analyses of laboratory measurements over a range of current speeds and sizes of the tank and base opening, empirical equations for estimating the mixing rate in such a tank system are derived. The results are compared with those for a rectangular compartment from a previous study by Foo (1995). An approximate unified equation for estimating the mixing rate in both circular and rectangular tanks is derived. Preliminary field measurements in a pilot storage tank show much higher mixing rates than the laboratory results. This is attributed to the different dynamic conditions experienced by the freely floating field tank, as it is subjected to constant impacts of wind, tides, currents and waves.

ISSN:0022-1686    

DOI:10.1080/00221689809498636

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Experimental results of longitudinal dispersion in rivers are discussed in the light of new findings with respect to the “dead-zone” theory. Both the experiments made by authors as well as some literature results are taken into consideration. The dead-zone model parameters, namely the relative dead-zone volume, penetration time of the tracer into dead-zones, constant mean flow velocity and dispersion coefficient, are obtained with the use of the nonlinear least square technique applied to the image functions of concentration time-distributions. The statistical moments of the concentration time distributions as functions of distance are also analyzed. Expressions for statistical moments, recently obtained by authors, are found to agree well with both experimental and computational results.

ISSN:0022-1686    

DOI:10.1080/00221689809498637

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

ISSN:0022-1686    

DOI:10.1080/00221689809498638

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

ISSN:0022-1686    

DOI:10.1080/00221689809498639

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor