Threshold Of Sediment Motion Research Articles

Field observations of the threshold of sand motion in a transitional wave boundary layer

Field experiments have been performed to study the threshold of sediment movement by irregular surface swell waves on both rippled and flat sand beds. Measurements of the fluid velocity were made in the free stream flow, and simultaneous observations of the sediment movement were made with an underwater television camera. This information has been used to make deductions about the bed shear stress at the threshold of sediment motion. For the case of the rippled bed, the analysis is based upon a decoupling of the problems of the frictionless flow over the ripple profile and of the flow in the boundary layer itself. For the low wave Reynolds numbers in the experiments, it is argued that the flow in the boundary layer was transitional, not smooth or rough turbulent, and that it is therefore legitimate to treat the boundary layer as an “almost laminar” flow governed by a linear equation. The instants in the measured wave cycles corresponding to the onset of sediment motion are well explained on this basis, and the predicted threshold stress for the mobile coarse sand ( D 50 = 0.14 cm ) is roughly the same as that obtained from Shields curve.

Threshold of sediment motion under unidirectional currents

ABSTRACTCarefully selected data for the threshold of sediment movement under unidirectional flow conditions have been utilized to re‐examine the various empirical curves that are commonly employed to predict this threshold. After a review of the existing data, we employed only that data obtained from open channel flumes with parallel sidewalls where flows were uniform and steady over flattened beds of unigranular, rounded sediments. Without these restrictions, an unmanageable amount of scatter is introduced.This selected data is used to develop a modified Shields‐type threshold diagram that extends the limits of the original diagram by three orders of magnitude in the grain‐Reynolds number. The equally general but more easily employed Yalin diagram for sediment threshold is also examined. Although the Shields and Yalin diagrams are general in that they apply to a wide range of different liquids, in both cases somewhat different curves are obtained for threshold under air than for the liquids.The often used empirical curves of the friction velocity u*, the velocity 100 cm above the bed u100, the bottom stress θt, and Shields’ relative stress θt, all versus the grain diameter D, are limited in their ranges of application to certain combinations of grain density, fluid density, fluid viscosity and gravity. These conditions must be selected before the curves are generated from either the more general Shields or Yalin curves. For example, on the basis of the data selected for use in this paper, empirical threshold relationships for quartz density material in water are image where the velocity u100 measured 100 cm above the sediment bed is given in cm/sec and the grain diameter D is in cm.The limitations on any of the threshold relationships are severe. These limitations should be properly understood so that the empirical curves and relationships are not improperly employed.

Frequency of sediment movement on the Washington continental shelf: A note

Several independent sets of field data have been analyzed in order to estimate the frequency of sediment movement on the continental shelf off Washington over an annual period and to identify the major components of the bottom velocity field causing this motion. Sediment motions resulting from: (1) bottom currents caused by surface wind stress and tides, and (2) wave-induced oscillatory bottom currents have been investigated. Analysis of a 260-day current record from 3 m off the seabed at 80 m depth on the continental shelf and a 205-day open-ocean wave record collected on Cobb Seamount 465 km west of the Washington coast suggest that the threshold of sediment motion was exceeded for approximately 22 days per year as a result of mean currents (20 min time averaged) and approximately 53 days per year from wave-induced oscillatory currents. Substantial variations can be expected from year to year, so these values represent order of magnitude estimates.

On the Comparison Between the Threshold of Sediment Motion Under Waves and Unidirectional Currents with a Discussion of the Practical Evaluation of the Threshold: REPLY

On the Comparison Between the Threshold of Sediment Motion Under Waves and Unidirectional Currents with a Discussion of the Practical Evaluation of the Threshold: REPLY

The Initiation of Oscillatory Ripple Marks and the Development of Plane-bed at High Shear Stresses Under Waves

ABSTRACT Data on the development of oscillatory ripple marks under waves are utilized as a further check on the equations that have been proposed for the threshold of sediment motion. If the threshold curve is correct, the data on ripple occurrence should lie in a stress field above the curve for threshold. Analysis of the data shows this mainly to be true but suggests that the curve for the threshold should be lowered slightly to smaller stress values as a number of ripple occurrences would otherwise fall below the threshold curve. As the stress of the wave orbital motion increases, the ripple heights progressively decrease and ultimately disappear at a critical stress value. Data on this disappearance and plane-bed sheet sand transport development are examined and compared with the data on the high-stress presence of ripples. There is good agreement of the two data sets and also confirmation of the theoretical criterion of Bagnold for the disappearance of ripples.

SEDIMENT THRESHOLD UNDER OSCILLATORY WAVES

As the velocity of the water motion near the bottom under oscillatory waves is increased, there comes a stage when the water exerts a stress on the particles sufficient to cause them to move. This study reviews data on threshold of sediment motion under wave action and compares the results with the established curves for threshold under a unidirectional current.

The Threshold of Sediment Movement Under Oscillatory Water Waves

ABSTRACT As the velocity of the to-and-fro water motion near the bottom under oscillatory waves is increased, there comes a stage when the water exerts a stress on the particles sufficient to cause them to move. This study reviews the analyses and available data on this threshold of sediment motion under wave action. For grain diameters less than about 0.05 cm (medium sands and finer) the threshold is reached while the flow in the boundary layer is still laminar and the threshold is best related by the equation um2/(s-) gD = 0.30 (do/D), where um and do are the near-bottom velocity and orbital diameter of the wave motion, is the density of water, and s and D are respectively the density and diameter of the sediment grains. This relationship is modified after an empirical equation deduced by Bagnold but has a theoretical basis. For grain diameters greater than 0.05 cm (coarse sands and coarser) the threshold occurs after the boundary layer has become turbulent and is best predicted with an empirical curve relating do/D to um/(s - ) g T where T is the wave period. This latter dimensionless number represents the ratio of the acceleration forces to the effective gravity force acting on the grains.

On anemometers

This paper reports on an analysis of instantaneous sediment transport in relation to wind gusts, moisture content and fetch length from a field study carried out at Skallingen, Denmark. Wind speeds were measured with cup anemometers and instantaneous sediment transport with vertical traps coupled to electronic balances. Moisture content was measured gravimetrically using samples taken along a beach profile at intervals of 1 to 2 h. Sediment transport and wind speeds were sampled at 1 Hz and a 5-s running mean was used to smooth the data from the trap because of limitations of the balance resolution.Three measures of the threshold of sediment motion—the intermittency threshold, minimum threshold and maximum threshold—were explored using data from five runs carried out on October 26, 2000. The intermittency threshold and maximum threshold were largely insensitive to wind speed but varied spatially along the fetch distance. The minimum threshold increased significantly with increasing mean wind speed as a result of reduced time for drying of the surface layer. Examination of time series for instantaneous wind speed and sediment transport for a number of runs on October 26 and November 6, 2000 showed close agreement between fluctuations in wind speed and fluctuations in sediment transport. Cross spectral analysis of instantaneous wind speed and sediment transport for one run on each day where there was continuous transport showed high coherence between the two variables over a range of frequencies from 0.2 Hz to <0.01 Hz. Plots of all non-zero instantaneous sediment transport values versus wind speed show considerable scatter. In general, transport rates are highest for traps located further from the upwind non-erodible boundary (i.e. along the fetch). Best fit power relationships show R2 values generally ranging from 0.4 to 0.6 and exponents ranging from 4.5 to 6.94—i.e. much higher than the traditional value of 3 found in most transport equations. Traps located close to the upwind boundary had lower R2 values and exponents <2.5.