Measurement Of Sand Transport Research Articles

Long‐Term Evolution of Sand Transport Through a River Network: Relative Influences of a Dam Versus Natural Changes in Grain Size From Sand Waves

AbstractTemporal and spatial nonuniformity in supplies of water and sand in a river network leads to sand transport that is in local disequilibrium with the upstream sand supply. In such river networks, sand is transported downstream as elongating waves in which coupled changes in grain size and transport occur. Depending on the magnitude of each sand‐supplying event and the interval between such events, changes in bed‐sand grain size associated with sand‐wave passage may more strongly regulate sand transport than do changes in water discharge. When sand transport is controlled more by episodic resupply of sand than by discharge, upstream dam construction may exacerbate or mitigate sand‐transport disequilibria, thus leading to complicated and difficult‐to‐predict patterns of deposition and erosion. We analyzed all historical sediment‐transport data and embarked on a 4‐year program of continuous sediment‐transport measurements to describe disequilibrium sand transport in a river network. Results indicate that sand transport in long river segments can evolve over ≥50‐year timescales following rare large sand‐supplying events. These natural changes in sand transport in distal downstream river segments can be larger than those caused by an upstream dam. Because there is no way to know a priori whether sand transport in a river has changed in response to changes in the upstream sand supply, contemporary continuous measurements of sand transport are required for accurate sand loads and budgeting. Analysis of only historical sediment‐transport measurements, as is common in the literature, may lead to incorrect conclusions with respect to current or future sediment‐transport conditions.

Early‐stage aeolian protodunes: Bedform development and sand transport dynamics

AbstractEarly‐stage aeolian bedforms, or protodunes, are elemental in the continuum of dune development and act as essential precursors to mature dunes. Despite this, we know very little about the processes and feedback mechanisms that shape these nascent bedforms. Whilst theory and conceptual models have offered some explanation for protodune existence and development, until now, we have lacked the technical capability to measure such small bedforms in aeolian settings. Here, we employ terrestrial laser scanning to measure morphological change at the high frequency and spatial resolution required to gain new insights into protodune behaviour. On a 0.06 m high protodune, we observe vertical growth of the crest by 0.005 m in two hours. Our direct measurements of sand transport on the protodune account for such growth, with a reduction in time‐averaged sediment flux of 18% observed over the crestal region. Detailed measurements of form also establish key points of morphological change on the protodune. The position on the stoss slope where erosion switches to deposition is found at a point 0.07 m upwind of the crest. This finding supports recent models that explain vertical dune growth through an upwind shift of this switching point. Observations also show characteristic changes in the asymmetric cross‐section of the protodune. Flow‐form feedbacks result in a steepening of the lee slope and a decline in lower stoss slope steepness (by 3°), constituting a reshaping of protodune form towards more mature dune morphology. The approaches and findings applied here, (a) demonstrate an ability to quantify processes at requisite spatial and temporal scales for monitoring early‐stage dune evolution, (b) highlight the crucial role of form‐flow feedbacks in enabling early‐stage bedform growth, alluding to a fluctuation in feedbacks that require better representation in dune models, and (c) provide a new stimulus for advancing understanding of aeolian bedforms. Copyright © 2017 John Wiley & Sons, Ltd.

Sand transport measurements in Chioggia inlet, Venice lagoon: Theory versus observations

This paper presents results of recent measurements of sand transport made in Chioggia inlet as part of an extensive monitoring programme in the Venetian inlets. Measurements were made in order: (1) to define a relationship between sand transport magnitude and tidal flow; (2) to derive the thresholds for sand transport; (3) to identify the dominant modes of transport; (4) to evaluate the concentration profiles of sand within the benthic boundary layer; (5) to compare bedload transport observations with model predictions using existent bedload formulae; and (6) to produce yearly estimates of bedload transport across the inlet. The vertical distribution of sand in the water column was sampled using modified Helley–Smith bedload samplers at three sites. Transport was found to vary according to the flow and bed grain size, with considerable temporal and spatial variability. A difference of up to three orders of magnitude in transport was observed through the inlet, with higher transport rates measured on the seaward part. The dominant mode of transport in the central inlet was suspension, while bedload was dominant in the mouths. The measured profiles of sand concentration varied with the tidal stage and seabed grain size according to the Rouse parameter ( R). R was high at the inlet mouths (1< R<2), indicative of a well-developed bedload layer. The inverse movability number ( W s / U *) was also higher at these sites and appeared to be grain size dependant. Formulae for bedload transport were tested against field data; stochastic methods such as Einstein–Brown, Engelund–Hansen and Van Rijn produce the best fits. The coupled model SHYFEM-Sedtrans05 appears to simulate well observed transport for most conditions of flow. Long-term bedload predictions indicate a dominant export of sand, with a yearly average of 4500 m 3.

The measurement of sand transport in two inlets of Venice lagoon, Italy

Sand transport in Lido and Chioggia inlets was measured using modified Helley–Smith sand traps equipped with 60-micron nets. The traps had an efficiency of about 4% only but provided enough material for analysis. Very fine sand (0.07 < d < 0.11 mm) only was collected in the traps. Transport of sand was greatest in the bottom 10% of the water column and followed a Rouse profile. Sand extended to a height of about 4 m above the bed during peak flows corresponding to the estimated thickness of the boundary layer; and observed in synoptic ADCP profiles. The sand in the benthic boundary layer was largely inorganic (>95%); above this layer, organic content varied widely and was greatest near the surface. The movability number W s / U ∗ showed a linear relationship to dimensionless grain diameter ( D *): ( W s / U ∗ ) = ( D ∗ / 10 ) ; D * < 10. Sand concentration in suspension was simulated by a mean Rouse parameter of −2.01 ± 0.66 (Lido inlet) and −0.82 ± 0.27 (Chioggia inlet). The β parameter ( Hill et al., 1988) was correlated with D * and movability number in the form: β = 2.07 − 2.03 D ∗ + 59 ( W s / U ∗ ) 2 ( r 2 = 0.42). Von Karman's constant was back-calculated from a Law of the Wall relationship as a test on the accuracy of U * estimates; a mean value of 0.37 ± 0.1 (compared to the accepted value of 0.41) suggest U * was accurate to within 10%. The constant of proportionality ( γ = 3.54 × 10 −4) between reference concentration ( C a) and normalized excess bed shear stress was in line with the published literature.

Wind tunnel modeling and measurements of the flux of wind-blown sand

This paper presents a way to empirically fit experimental data for the horizontal flux of various sizes of wind-blown dry sand using data from wind tunnel experiments. We measured vertical wind profiles to derive threshold shear velocity and estimated shear velocity and the flux of sand mass as a function of the height for nine different grain sizes. We propose a fitting model based on the experimental data and a least-squares method and derive an explicit form of sand flux as a function of height and shear velocity for these grain sizes. We also obtained an explicit form of the empirical equation for the measurement of sand transport per unit width and unit time by integrating the empirical equation as a function of height. Finally, we compared the effectiveness of Bagnold's equation, Kawamura's expression and Lettau and Lettau's equation, for predicting sand transport with the results of our empirical equation. The results show that the transport predicted by all of the equations were always lower than the measured results from the empirical equation for all grain sizes and shear velocities. However, the empirical equation matched Bagnold's equation, Kawamura's equation, and Lettau and Lettau's equation if the coefficients in these equations were adjusted instead of using their original coefficients. The empirical equation for sand transport in the present study contradicts previous conclusions generated by Bagnold's equation, which predict that for a given wind drag, the transport of a coarse sand is greater than that of a fine sand.

Measurement of sand transport by spatial method with fluorescent sand tracer along the Damietta promontory, Egypt

Measurement of sand transport by spatial method with fluorescent sand tracer along the Damietta promontory, Egypt

Desert dune dynamics and the evaluation of shear velocity: an integrated approach

Abstract Few empirical measurements have been undertaken of shear velocity, sand transport or dune morphology in the upwind basal regions of dunes. This study compares field measurements in this region from a barchan dune in Oman with calculations from a complex flow model using linearised equations (FLOWSTAR) and measurements in a wind tunnel. The calculations of shear velocity from wind velocity observations in the field and those predicted by the FLOWSTAR model reflect the observations of previous studies in exhibiting the widely reported upwind reduction in shear velocity. Despite these observations, measurements of sand transport did not reduce at the base of the dune. The implications of these findings for the predictive power of typical saltation flux equations and the dynamics of the dune itself are discussed. Wind tunnel modelling using a pulse wire probe suggests that the conventional methods of shear velocity derivation are inadequate. The pulse wire measurements exhibited no reduction in shear at the toe. An argument is presented to explain the maintenance of shear velocity in this position. It is suggested that the reduction in shear stress due to the fall in wind velocity is counteracted by an increase in shear stress due to streamline curvature. The adequacy of field methods for determining shear velocity in this respect is considered.

Sand Transport in Texas Tidal Inlet

Rollover Pass is a manmade, artificially‐stabilized tidal inlet on the upper Gulf Coast of Texas. The magnitude of the net annual sand transport through the inlet was estimated: (1) As a percentage of the net longshore transport; (2) from excess beach erosion downdrift of the Pass; and (3) from dredge‐material volumes for the Intracoastal Waterway landward of the inlet. Limited direct measurements of sand transport were also made. The excess beach erosion downdrift of the Pass was between about 9,000 and 26,000 cu yd/yr (6,700 and 20,000m3/yr), which was in general agreement with the estimation of net annual sand transport calculated as a percentage of longshore transport. Net annual transport through the inlet estimated from extensive data on dredging rates was nearly an order of magnitude greater than estimates based on longshore transport and beach erosion.

Measurement of Sand Transport in a Cross Unidirectional-Oscillatory Flow Tank

Measurement of Sand Transport in a Cross Unidirectional-Oscillatory Flow Tank

Errors in measuring uniform aeolian sand flow by means of an adjustable trap

ABSTRACTA vertical‐slot sand trap was constructed incorporating a device to adjust the freedom of flow through the trap. Using a dune‐sand of equivalent diameter 0.25 mm, measurements of sand transport were made with this flow so adjusted that no change of sand surface geometry detectable by eye occurred upstream of the trap; this was considered to be a correct procedure. Further measurements were taken with the flow through the trap deliberately set to be (a) more vigorous and (b) less vigorous than the correct setting. In the latter case very significant differences were observed in measured transport rates of sand compared with the ‘correct’measurements. The disparities are of interest because it is commonly assumed that sand grains have enough ‘penetration’to enter a slot trap freely despite a degree of stagnation upstream of it. This does not appear to be a prudent assumption and substantial errors may result from lack of attention to the provision of free air flow through the trap.

Measurements of sand transport by wind on a natural beach

ABSTRACTBagnold's (1954) and Kawamura's (1951) formulae may be used for the calculation of the sand movement on a natural beach, provided the shear stress velocity U* &gt; 0·D4 m/s. Great discrepancies have been found between calculated and measured sand transport rates for U* &lt; 0·D4 m/s, mainly because of the capillary forces acting on a wet beach.The measured critical shear velocity U*c at the beginning of sand movement on a clean dry beach agrees very well with that predicted by Bagnold's formula. On a dry beach where the sand grains are stuck together, U*c was found to be about 10% higher. On a wet beach U*c appeared to depend on the moisture content of the surface layer. Grain size is a determining parameter in the U*c‐moisture content relation.When the angle a between the wind direction at sea and the dune face is between 15° and 85° the streamlines of the wind will bend in the vicinity of the dune face. In consequence this may influence the direction of sediment movement.

The measurement of sand transport by the time-integration method with radioactive tracers : Crickmore, M. J. and G. H. Lean 1962 Proc. roy. soc. ser. A 270(1340): 27–47

The measurement of sand transport by the time-integration method with radioactive tracers : Crickmore, M. J. and G. H. Lean 1962 Proc. roy. soc. ser. A 270(1340): 27–47

The measurement of sand transport by the time-integration method with radioactive tracers

A method of measuring the transport of sand in a laboratory flume is described in which a small quantity of radioactive sand is introduced into the flow. The method is similar in principle to the time-integration method used for measuring the discharge of liquids. The mean velocity of the sand particles can be deduced directly from the measurements. The volume rate of transport can be inferred either by an independent measurement of the thickness of movement of the bed ripples, or by mixing the active sand uniformly with a strip of the bed at the start and measuring the loss of active sand during the experiment. An adequate length must be left between the point of injection and the section at which the passage of tracer is measured. This length depends on the hop lengths of the particles and not on the frequency with which hops are performed.

The measurement of sand transport by means of radioactive tracers

Experiments are described which have for their object the development of a method of measuring the sand transport in rivers by means of radioactive tracers. The method used is analogous to that which has been used for the measurement of water discharge in which the mean velocity is determined from the rate of displacement of the centroid of a marked cloud of fluid. In these experiments the spread of radioactive sand has been measured from a transverse strip of the bed of a flume in which a steady flow of water and sand is maintained. The distribution of active grains differs from that of a cloud of fluid, however, in that the time scale of the motion is longer due to the slower drift velocity of the particles. Also the particles at low levels in the ripples are at rest for a very much longer time than those in the upper levels so that the spread of the sand particles is much larger than is the case for fluid flow of the same mean velocity and depth. If the motion were allowed to continue for a sufficiently long time it is probable that the distribution would become normal and centred about a point moving with the centroid, but the time for such development would be impracticably long and the transport rate must be deduced from the distributions in the early stages of the motion when the dilution is not very great. In this stage an attempt has been made to explain the shape of the distribution curves in terms of a simple model of the mechanism of transport in which it has been assumed that every particle has an equal chance of moving a certain hop length. Whilst the distributions given by such a model have a general resemblance to those obtained, the similarity is not quantitatively affirmed. A more complicated model which takes into account both the statistical variation of the hop length of the particles and the times that particles spend at different levels in the ripples appears to be necessary. The experiments have shown that the transport rate can be deduced from the mass distribution of tracer particles initially laid to a depth sufficient to cover all levels of movement of the bed ripples, or by measuring the velocity of the centroid of the activity distribution together with an independent measurement of the ripple movements at a point.