The dynamics of turbulent, transitional and laminar clay-laden flow over a fixed current ripple

Most aqueous sedimentary environments contain varying concentrations of fine‐grained, often clay‐rich, sediment that is transported in suspension and may modify the properties of the flow and underlying mobile bed. This paper presents results from a series of laboratory experiments examining the mean and turbulent properties of clay‐laden (kaolinite) flows, of various volumetric sediment concentrations between 0·046% and 12·7%, moving over a fixed, idealized current ripple. As the kaolinite concentration was raised, with flow velocity and depth constant, four flow types were observed to occur: (i) turbulent flow, in which flow separation is dominant in the leeside of the ripple; (ii) turbulence‐enhanced transitional flow, in which turbulence in the leeside separation zone region is enhanced; (iii) turbulence‐attenuated transitional flow, in which turbulence along the separation zone shear layer and in the free flow above it becomes damped, eventually leading to a reduction in the size of the separation zone wake region; and (iv) laminar plug flow, in which turbulence is damped and flow is almost stagnant in the lee of the ripple. Such modulation of turbulence by increasing clay concentrations suggests that many paradigms of flow and bedform dynamics, which have been based on extensive past work in clear water flows, require revision. The present results highlight a need to fully characterize the boundary conditions for turbulence modulation as a function of clay type and applied flow conditions, and the effects of such flows on fully mobile cohesionless beds.     

Transport rate of calcareous sand in unidirectional flow

This paper examines the transport of calcareous sand in unidirectional flow and its prediction through existing sediment transport models. A flume experiment of four sand samples collected on Oahu, Hawaii, provides 29 sets of sediment transport data in the bed-form and suspended transport stages. The measured transport data are compared with direct predictions from four energy-based transport models developed for siliceous particles. Corrections for the grain-size, fall velocity, and critical velocity of calcareous sand based on recent research are applied to the models and the results are compared with the direct calculations and measured data. The comparison illustrates the important role particle shape plays in the transport of calcareous sand. All four sediment transport models give consistent predictions and good agreement with the majority of the measured data. Two of the models respond positively to the corrections in both the bed-form and suspended transport stages indicating that such an approach may provide an interim solution for the transport of calcareous sand.     

Numerical study of sediment transport above rippled beds under the action of combined waves and currents

To study sediment suspension above ripples under the combined action of waves and currents, a three‐dimensional numerical model has been developed based on the use of FLUENT software, and an external sediment transport model. The computer model has been tested against laboratory measurements involving oscillatory wave motion, as well as cases of co‐linear waves with following and opposing currents, with satisfactory results. Compared with the situation in which only waves are present (called waves‐alone cases), the effects from the steady current on both vortex shedding and sediment suspension above ripples have clearly been revealed by the model results. In particular, the vortices generated in combined waves and currents tend to stay low in the trough area of the ripple and are ejected earlier than those in the waves‐alone case at both the ripple crest and trough, which leads to concentration peaks at different phases and with different magnitudes. The model was also applied to a field case from a multi‐barred, dissipative beach at Egmond‐an‐Zee, in the Netherlands, to investigate the influences of a long‐shore current on cross‐shore sediment transport. The model results show reasonable overall agreement with the field measurements, as well as the important effects of the three dimensional flow structure on the sediment entrainment process close to the ripple surface, which is very difficult to observe in such detail in field studies.     

Regulation of sand transport in the Colorado River by changes in the surface grain size of eddy sandbars over multi-year timescales

In settings where the transport of sand is partially or fully supply limited, changes in the upstream supply of sand are coupled to changes in the grain size of sand on the bed. In this manner, the transport of sand under the supply-limited case is ‘grain-size regulated’. Since the closure of Glen Canyon Dam in 1963, the downstream reach of the Colorado River in Marble and Grand Canyons has exhibited evidence of sand-supply limitation. Sand transport in the river is now approximately equally regulated by changes in the discharge of water and changes in the grain sizes of sand on the channel bed and eddy sandbars. Previous work has shown that changes in the grain size of sand on the bed of the channel (driven by changes in the upstream supply of sand owing to both tributary floods and high dam releases) are important in regulating sand transport over timescales of days to months. In this study, suspended-sand data are analysed in conjunction with bed grain-size data to determine whether changes in the grain size of sand on the bed of the channel or changes in the grain size of sand on the surface of eddy sandbars have been more important in regulating sand transport in the post-dam Colorado River over longer, multi-year timescales. The results of this study show that this combined theory- and field-based approach can be used to deduce which environments in a complicated setting are the most important environments for regulating sediment transport. In the case of the regulated Colorado River in Marble and Upper Grand Canyons, suspended-sand transport has been regulated mostly by changes in the surface grain size of eddy sandbars.     

Wind tunnel experiments of air flow patterns over nabkhas modeled after those from the Hotan River basin, Xinjiang, China (I): non-vegetated

A nabkha is a vegetated sand mound, which is typical of the aeolian landforms found in the Hotan River basin in Xinjiang, China. This paper compares the results of a series of wind tunnel experiments with an on-site field survey of nabkhas in the Hotan River basin of Xinjiang. Wind tunnel experiments were conducted on semi-spherical and conical sand mounds without vegetation or shadow dunes. Field mounds were 40 times as large as the size of the wind tunnel models. In the wind tunnel experiments, five different velocities from 6 to 14 m/s were selected and used to model the wind flow pattern over individual sand mound using clean air without additional sand. Changes in the flow pattern at different wind speeds resulted in changes to the characteristic structure of the nabkha surface. The results of the experiments for the semi-spherical sand mound at all wind velocities show the formation of a vortex at the bottom of the upwind side of the mound that resulted in scouring and deposition of a crescentic dune upwind of the main mound. The top part of the sand mound is strongly eroded. In the field, these dunes exhibited the same scouring and crescentic dune formation and the eroded upper surface was often topped by a layer of peat within the mound suggesting destroyed vegetation due to river channel migration or by possible anthropogenic forces such as fuel gathering, etc. Experiments for the conical mounds exhibit only a small increase in velocity on the upwind side of the mound and no formation of a vortex at the bottom of the upwind side. Instead, a vortex formed on the leeward side of the mound and overall, no change occurred in the shape of the conical mound. In the field, conical mounds have no crescentic dunes on the upwind side and no erosion at the top exposed below peat beds. Therefore, the field and laboratory experiments show that semi-spherical and conical sand mounds respond differently to similar wind conditions with different surface configuration and development of crescent-shaped upwind deposits when using air devoid of additional sediment. __________ Translated from Journal of Desert Research, 2007, 27(1): 9–14 [译自:中国沙漠]