Non-submerged Conditions Research Articles

Flow Field and Gas Field Distribution of Non-Submerged Cavitation Water Jet Based on Dual-Nozzle with Concentric Configuration

Cavitation water jet peening is an efficient and green surface treatment technology. The dual-nozzle can realize a cavitation water jet in air (non-submerged condition), which can be used for the surface treatment of large structures. The flow field characteristics of the dual-nozzle determine the cavitation effect. In this paper, the simulation of a cavitation water jet in air is carried out using Fluent software. The flow field characteristics containing velocity distribution, impact pressure, and gas phase volume fraction distribution are studied in detail. Furthermore, the effects of the nozzle structure parameters and incidence pressure on flow field characteristics are discussed. It was found that the structure parameters of the inner nozzle have a great influence on the flow field characteristics. Setting a contraction segment and expansion segment can improve the impact pressure and increase the intensity of the cavitation jet. Increasing the throat diameter and incidence pressure of the internal nozzle is also beneficial to improve the impact pressure and cavitation intensity. In order to assure a good cavitation effect, nozzle optimization should be performed. This study has guiding significance for the design of the dual-nozzle for a non-submerged cavitation water jet.

New formulas addressing flow resistance of floodplain vegetation from emergent to submerged conditions

ABSTRACT Hydraulic modelling of natural floodplain vegetation using leaf area index (LAI) has been applied successfully for non-submerged conditions whereas its suitability for submerged conditions requires further development. This study investigates the vegetative flow resistance at low relative submergences and extends existing LAI-based approaches building upon new flume data and prior experiences from field-scale applications. We provide advanced LAI-based formulas for modelling the flow resistance from emergent to submerged conditions, with water depth up to three times higher than the vegetation height. Such low relative submergences are highly relevant in hydraulic analyses of riverbank and floodplain flows but not adequately represented in existing formulas. The use of the deflected vegetation height as the characteristic height provided the most accurate modelling results, whereas the use of undeflected height resulted in significant errors. As a new development for submerged conditions, we proposed von Kármán scaling factor for improved model predictions. Overall, the results proved that LAI-based modelling is suitable also at low relative submergences for a wide range of vegetation densities (LAI = 1–5) and mean flow velocities (0.05–1.2 m s−1). For both emergent and slightly overtopped vegetation the JAR and VAS approaches outperformed the BAPmod-LAI approach that does not account for reconfiguration. For modellers, we provide a workflow and guidance on the use of the newly developed LAI-based formulas in 1D/2D hydrodynamic models for both emergent and submerged conditions.

Flow characteristics of open channels based on patch distribution of partially discontinuous rigid combined vegetation.

To clarify the flow characteristics of open channels under the combined distribution of vegetation in a patch, this study used the computational fluid dynamics tool FLUENT and the Reynolds stress model to design four combined and four discrete distribution modes under two different inundation states (submerged and non-submerged). The flow characteristics of longitudinally discontinuous rigid vegetation patches occupying half the width of the channel were numerically simulated. The numerical model is verified by indoor open channel flume experiments, and the obtained model data is in good agreement with the measured data. The results showed that: 1) The diameter of vegetation is an important factor affecting the wake structure. Under the submerged condition. 2)The submerged state, distribution pattern and combination form of vegetation are important factors that affect the distribution of flow velocity and change the structure of water flow. That is, the influence of vegetation distribution pattern on flow velocity and turbulence intensity under submerged condition is significantly weaker than that under non-submerged condition, and the flow velocity in non-vegetation area is significantly higher than that in vegetation area. The increase in the combined vegetation comprehensive stem thickness and the discrete degree resulted in an increase in the difference in flow velocity and turbulence intensity. 3) As the water flowed downstream, the flow velocity along the vegetated area continuously decreased, while it increased continuously along the non-vegetated area, and the difference in flow velocity between the two areas became more apparent. 4) The inundation state and combination characteristics of vegetation were important factors affecting the Reynolds stress of the channel location in the patch area.

Probability analysis of ex-vessel steam explosion induced pressure load based on analysis code SEINA

In nuclear reactor severe accidents, the interaction of molten core melt and coolant may lead to steam explosion, which seriously threatens the safety of the reactor. The four initial parameters that affect the steam explosion were randomly sampled and calculated multiple times by steam explosion analysis code SEINA, and the probability distributions of steam explosion pressure peak and pressure impulse under uncertain initial conditions are obtained. The results show that the peak pressure of ex-vessel steam explosion is roughly normally distributed, and the pressure peak of 90% of the cases under submerged and non-submerged conditions within the range of 10–60 MPa and 20-80Mpa. respectively. By calculating the four key correlation coefficients between input parameters and output parameters, it is concluded that the influence of melt temperature and breach size on output parameters is greater than that of system pressure and coolant temperature.

Flow field investigation of a straight–swirling integrated jet for radial jet drilling

Radial jet drilling (RJD) technology is applied to low permeability oil-gas reservoirs, coalbed methane, and shale gas. A self-propelled bit is an integral component. The external flow fields of a new-type of straight–swirling integrated jet (SSIJ) were tested under non-submerged conditions and the key parameters of the jet were studied in this paper. A rock-breaking experiment at a specific target distance was conducted to verify the PIV test results. It is indicated that, from the introduction of a central straight jet, the central low-axial-velocity zone can be eliminated, and the maximum radial and tangential velocities are not at the jet axis. Second, the hydraulic parameters do not affect the velocity distribution law, but significantly affected by nozzle structures. The straight-to-swirl ratio and the dip-angle of an impeller slot should be 0.8 and 45°, respectively, to obtain a flow field with both a higher central velocity and larger coverage area. Through the rock-breaking experiment, it was found that when the dip-angle is 45° (swirl number of 0.45), both the borehole depth and diameter are larger, and the bottom is the flattest, simultaneously, the correctness of the PIV test is verified. Finally, since the two jets (straight & swirling) have interacted and mixed after ejected from the nozzle, there is no constant velocity core, the SSIJ has no initial and basic sections compared with the conventional combined jet, and could be divided into a transition section and uniform-mixing section along the axial. By analyzing the swirl numbers at different positions, it was found that the SSIJ has a straight, weak-swirl, strong-swirl, and periphery jet zones along the radial. The results can guide rock-breaking mechanism revelation and the optimal design of an SSIJ bit for the RJD technology.

Waterlogged Conditions Influence the Nitrogen, Phosphorus, Potassium, and Sugar Distribution in Sago Palm (Metroxylon sagu Rottb.) at Seedling Stages.

Sago palm (Metroxylon sagu Rottb.) grows in well-drained mineral soil and in peatland with high groundwater levels until complete submersion. However, the published information on nutrient uptake and carbohydrate content in sago palms growing under waterlogging remains unreported. This experiment observed sago palm growth performance under normal soil conditions (non-submerged conditions) as a control plot and extended waterlogged conditions. Several parameters were analyzed: Plant morphological growth traits, nitrogen, phosphorus, potassium, and sugar concentration in the plant organ, including sucrose, glucose, starch, and non-structural carbohydrate. The analysis found that sago palm morphological growth traits were not significantly affected by extended waterlogging. However, waterlogging reduced carbohydrate levels in the upper part of the sago palm, especially the petiole, and increased sugar levels, especially glucose, in roots. Waterlogging also reduced N concentration in roots and leaflets and P in petioles. The K level was independent of waterlogging as the sago palm maintained a sufficient level in all of the plant organs. Long duration waterlogging may reduce the plant’s economic value as the starch level in the trunk decreases, although sago palm can grow while waterlogged.

Laboratory Pullout Test of Geogrid Reinforcements for Submerged Mechanically Stabilized Earth for Waterfront Applications

The design and evaluation of mechanically stabilized earth (MSE) wall structures for riverbank protection, quays, sea walls and other waterfront structures is critically dependent on a design parameter called the interaction coefficient (Ci) for submerged or fully saturated condition. However, the available research works conducted in this field are limited to non-submerged and, at most, partially saturated condition. This limitation does not closely represent actual conditions where waterfront structures are almost often submerged and fully saturated. Hence, the objective of this study was to determine the Ci in a submerged condition, on top of the Ci in a non-submerged condition for different types of geogrid reinforcement for fine-grained soils and coarsegrained soils used for MSE walls. Test results revealed that the Ci decreased as the effective overburden stress increased. The submerged conditions yielded lower Ci compared with non-submerged conditions, and the reduction wassignificant at a lower effective overburden stress for high density polyethylene uniaxial geogrid than the polyvinyl coated polyester yarn. The design parameters derived from these conditions can be used for the evaluation and design of MSE walls for waterfront applications with a higher level of confidence.

Study on the influence of ecological vegetation revetment on river flow and sediment environment

AbstractThis paper studied the influence of vegetation cover on the river water flow environment and on sediment suspended load movement environment, closely combining the development and construction of new ecological water conservancy projects such as ecological embankments and ecological wetlands with ecological vegetation revetment cultivated on river bank slopes. The glass flume experiment was carried out in the vegetation-water flow-sediment environment. In the experiment, natural straight holly in the early growth stage was selected as the experimental vegetation, and erect type vegetation with uniform distribution of the upper and lower branches and leaves was simulated, and a total of 14 experimental groups were set in different combinations with fixed bed, movable bed, submerged and non-submerged conditions. The measured data of flow quantity, flow velocity and suspended load concentration were achieved in the experiment. The experimental results show that: (1) as to the influence of vegetation on the water flow environment, the vegetation canopy density directly affects the turbulent environment of the water flow, and the greater the density, the greater the disturbance to the water flow environment, and the flow velocity near the canopy will be decreased sharply; (2) in the fixed bed, under submerged conditions, the maximum turbulence intensity appears near z/h = 0.6; under non-submerged conditions, the turbulence intensity near the water surface is largest. In the movable bed, under submerged conditions, the maximum turbulence intensity reaches the maximum in the top and upper locations of the top of the plant, and the turbulence intensity reaches the maximum at the top of the plant under non-submerged conditions. (3) In terms of the influence of vegetation on the sediment suspended load movement environment, the concentration of suspended sediment decreases with the increase of plant height, the sediment content in the tail of the plant belt is reduced to some degree. When the relative water depth is z/h > 0.5, almost no suspended sediment exists. And the sediment-retaining effect under non-submerged conditions is better than that under submerged conditions.

Research on erosion characteristics of a novel cavitation nozzle under nonsubmerged condition

When a cavitating jet enters the atmosphere directly, its cavitating effect weakens rapidly, and the erosion energy it produces cannot be fully utilized. Regarding the problem that existing cavitation nozzles are only used in submerged condition, methods to improve the erosion ability of cavitation jets under nonsubmerged condition are studied. The nozzle is visually simulated using Fluent software, and the results show that the dynamic submerged environment at the outlet effectively expands the nearby low-pressure cavitation area. The enhancement effect of the annular cavitation nozzle on the jet cavitation effect in the atmosphere domain is verified by measuring the impact force curve of the jet and through erosion tests on brass surface. Cleaning and derusting tests show that the annular cavitation nozzle has stronger derusting ability than the high-pressure nozzle under nonsubmerged condition and under the same pressure, demonstrating that the cleaning and derusting effect mainly comes from the collapse of cavitation bubbles.

Linking chemical structure of dissolved organic carbon and microbial community composition with submergence-induced soil organic carbon mineralization.

Much research has been devoted to investigating how water-extractable organic carbon (DOC) concentration and microbial activity regulate soil organic carbon (SOC) mineralization when soils are saturated with water. However, the relationships of DOC chemical structure and microbial community composition with SOC mineralization, as well as the relative contributions of microbial decomposers and their substrates on the mineralization rate have rarely been examined. In a laboratory experiment, we incubated two typical cropland soils (an Entisol and a Mollisol) of China for 360 days under submerged and non-submerged conditions, and we evaluated the concentration and chemical structure of soil DOC, soil microbial metabolic potential and community composition by using total C/N analysis, solution-state 1H NMR, Biolog EcoPlates, and 16S rRNA amplicon sequencing, respectively. The results showed that submergence significantly increased DOC concentration (P < 0.01) and microbial activity (P < 0.001) and changed DOC chemical structure in the Entisol (P < 0.01). In the Mollisol, it significantly increased the rate (P < 0.01) and cumulative extent (P < 0.001) of SOC mineralization and DOC concentration (P < 0.01) as well as altering the composition of the microbial community (P < 0.001). Moreover, the SOC mineralization rate was better explained by microbial community composition (Entisol: SPC = -0.71, P < 0.001; Mollisol: SPC = 0.92, P < 0.001) than by DOC concentration (Entisol: SPC = 0.21, P > 0.05; Mollisol: SPC = 0.30, P < 0.05) or DOC chemical structure (Entisol: SPC = 0.12, P > 0.05; Mollisol: SPC = -0.45, P < 0.001). Our study revealed that the bacterial community composition had a close relationship to the rate of submergence-induced SOC mineralization in both soils, but only DOC concentration and chemical structure were effective predictors of mineralization rate in the low-pH Mollisol.

Removal of HCl gas from off gases using self-priming venturi scrubber.

Growing concern about the effect of hydrochloric acid gas (HCl) on environment and abatement of it is now a very serious issue. This present paper is focused on developing a realistic model in order to remove the HCl from the off gases using self-priming venturi scrubber. A detailed parametric study of throat gas velocity (36-72 m/s), liquid level in outer cylinder (0.40-0.77 m) and inlet concentration of HCl (100-500 ppm) on HCl removal efficiency have been done with normal water as a scrubbing liquid. Also the removal efficiency was enhanced by using NaOH solution as a scrubbing liquid in submerged and non-submerged conditions. Therefore, the maximum removal efficiency of HCl was obtained as 87.83% with normal water and 92.54% with 0.005N NaOH solution as the scrubbing liquid at inlet HCl concentration of 500 ppm, throat gas velocity of 60 m/s and liquid level of 0.77 m in submerged condition. Experimental results were validated with the developed empirical model and showed excellent agreement with less deviation.

Role of Sulfate-Reducing Bacteria in Mercury Methylation in Soil of the Water-Level-Fluctuating Zone of the Three Gorges Reservoir Area

Soil simulation experiments with or without addition of external mercury and a non-indigenous bacterium were conducted under laboratory conditions, in order to investigate whether sulfate-reducing bacteria (SRB) are the dominant mercury methylators in the water level fluctuation zone (WLFZ) of the Three Gorges Reservoir, which is a typical periodical wet and dry alternating environment, locating in the Shibaozai Xinzhen Village, Zhong County, Chongqing, China (E108°12'3″ and N30°24'36″). The original soil from the WLFZ was firstly sterilized and then inoculated with or without Desulfovibrio africanus (D. africanus DSM-2603, commercially purchased from the Culture Preservation Center of Microorganisms in Germany, DSMZ) as the control (Test A). The sterilized soils were respectively mixed with 0, 1, or 5 mg·kg-1 Hg2+ soil under submerged and non-submerged conditions to simulate changes in the concentrations of total mercury, methylmercury and the growth of D. africanus, and the effects of environmental factor on such changes. The results showed that under the submerged situation the release of total mercury (THg) from soils (Test B) increased with the increase of external Hg2+ addition, and the higher the external Hg2+, the quicker the Hg2+ release was. Meanwhile, D. africanusdisplayed an ecological adaptability to mercury and its colony numbers were significantly correlated with the MeHg concentrations in the soil. In soil with 5 mg·kg-1 of external Hg2+, the total number of D. africanus reached the highest (3.65×104 cfu·g-1) while the highest MeHg concentration was 7.60×104 ng·kg-1. However, on the one hand, the averaged numbers of soil SRB in the test B was only 193 cfu·g-1. On the other hand, in soil with 5 mg·kg-1of external Hg2+, the MeHg concentration was as low as 5.54×103 ng·kg-1, while no significant relationships were observed between SRB and MeHg. As a result, our results indicated that SRB was not a predominant mercury methylator, while other bacterial groups, either aerobic or anaerobic bacteria, might play an important role in mercury methylation in the WLFZ of the Three Gorges Reservoir, China.

The effect of submergence ratio on flow pattern around short T-head spur dike in a mild bend with rigid bed using numerical model

Spur dikes are hydraulic structures frequently applied to protect rivers’ banks. Since flow pattern around a spur dike located in a rivers’ bend is particularly complicated due to the existence of secondary flow and helical flow, the effect of submergence ratio on flow patterns around a short T-head spur dike located in a 90° bend has been monitored numerically in this study. The numerical data are verified with experimental data obtained under non-submerged conditions, which indicate the numerical results are in accordance with experimental data. In this research, the numerical simulation based on various submergence ratios is done using FLOW-3D software to analyze. The results showed an increase in dimensions and the number of vortices associated with an increase in submergence ratio so that for the amount of 50% submergence, two vortices form around outer wall and an area downstream of the wing of spur dike, at bed level. Moreover, the length and width of separation zones were estimated to be 1.6 and 1.5 times larger compared to non-submerged condition, respectively.

APPLICATION TIME OF NITROGEN AND PHOSPHORUS FERTILIZATION MITIGATES THE ADVERSE EFFECT OF SUBMERGENCE IN RICE (ORYZA SATIVA L.)

SUMMARYLarge areas of rainfed lowlands of Asia annually experienced flash flooding during the rice-growing season, which is an important abiotic stress that adversely affect grain yield of rice (Oryza sativa L.) crop. Submergence stress is a common environmental challenge for agriculture sustainability in these areas because lack of high-yielding, flood-tolerant cultivars. In this study, IR64-Sub1 and IR64 were compared for their tolerance to submergence at active tillering (AT), panicle initiation (PI) and heading (H) stages with nitrogen and phosphorus application time. We evaluated the role of cultivars, stage of submergence and N and P application on phenology, leaf senescence (LS), photosynthetic (Pn) rate, yield attributes and yield. Under non-submerged conditions, no difference was observed in phenology, Pn rate and yield of both cultivars. Submergence substantially reduced biomass, Pn rate, yields attributes and yield across cultivars with more drastic reduction in IR64. Submergence at H stage proves to be most detrimental. Nitrogen application after desubmergence with basal P improved the Pn rate resulting in significantly higher yield and yield components. Nitrogen application before submergence resulted in increased LS and ethylene accumulation in shoots leading to drastic reduction in growth, Pn rate and yield. Crop establishment and productivity could therefore be enhanced in areas where untimely flooding is anticipated by avoiding N application before submergence and applying N after desubmergence with basal P (phosphorus).

Changes in proline accumulation, amino acid, sugar and chlorophyll content in leaf and culm of Phourel-amubi, a rice cultivar of Manipur in response to flash flood

Rice (Oryza sativa L.) growth is generally hindered by prolonged exposure to excess of water during its vegetative or reproductive stage. To determine the effect of flood on the plant and to understand the tolerance mechanism of the rice cultivar, variations in the plant height, dry matter, and contents of chlorophyll, total soluble sugar, free amino acid and proline were determined in leaves and culm of Phourel-amubi, a rice cultivar of Manipur under 5, 10 and 15 days of submergence and non-submerged (control) condition. Plant height, dry matter and proline accumulation increased with the increase in days of submergence (highest with 15 days submergence). Total chlorophyll, both chlorophyll a and chlorophyll b content were found to decrease with period of submergence. Likewise concentration of soluble sugar in leaves decreased with submergence but in the case of culm, the sugar concentration gradually increased from control to 15 days submergence. Total free amino acid concentration also decreased with submergence. However, proline accumulation both in leaves and culm gradually increased with submergence, and highest content were observed under 15 days submergence.

Hydrologic influence on the δ13C variation in long chain n-alkanes in the Dajiuhu peatland, central China

Symbiotic methanotrophic bacteria are prevalent on the stems or in the hyaline cells of Sphagnum spp. in peat-moss ecosystems and can contribute a significant amount of isotopically light carbon for moss photosynthesis. Despite this, the influence of the methanotrophs on the δ13C values of Sphagnum-derived lipids is poorly known, especially in peatland lawn or hummock conditions that do not favor the growth of submerged Sphagnum. We have investigated the δ13C variation in individual long chain n-alkanes in samples of both fresh Sphagnum and surface peat collected under Sphagnum-dominated lawns in the Dajiuhu peatland, central China. The δ13C23 values of living Sphagnum negatively correlated with water level, suggesting that the carbon isotope fractionation of Sphagnum is mainly controlled by the diffusion resistance of CO2 in the hyaline cells rather than by the influence of symbiotic methanotrophs in this study area. In contrast, the δ13C values of n-C23 and n-C25 alkanes in the surface peat from two of the three sites correlated positively with water level, indicating the potential influence of symbiotic methanotrophic bacteria. The contrary results between peat and living Sphagnum probably result from surface peat receiving a more important contribution of n-alkanes from the lower parts than the middle and top parts of Sphagnum. In non-submerged conditions, the lower parts of Sphagnum normally have relatively high methane oxidation potential relative to the top parts. Our study reveals that the isotopic signals from symbiotic methanotrophs and the associated hydrological conditions can be preserved in peat deposits through the preferential accumulation of the lower parts of Sphagnum in peat layers.

Fracture Mechanism of Radial Tyre Rubber under Ultra-high Pressure Water Jet Impact

A new method to recycle radial tyre is studied. The ultra-high pressure water jet technology is applied to achieve the material recycle of radial tyre rubber, the experiments of ultra-high water jet impinging on tread rubber under non-submerged condition are performed, rubber failure surfaces and rubber particles by jet cutting under different actuating pressures are obtained, rubber particle size analysis and low field nuclear magnetic resonance(NMR) analysis are performed to evaluate the recycle performance of this new method. The stress distribution of rubber material interior under the impact of water jet is computed by establishing the finite element model of tread rubber using fluid-structure interaction(FSI) theory, and the fracture behavior of the tread rubber is studied by using damage criterion. The simulation analysis agrees well with the experiment results. Micro-failure mechanism analysis with scanning electron microscope(SEM) for rubber failure surfaces and rubber particles is performed, the results show that the micro-mechanism of rubber failure due to water jet impingement is brittleness tendency ductile fracture in condition of tensile and shearing stress, intergranular damage and transgranular fracture are the main microscopic failure modes.

IODINE REMOVAL EFFICIENCY IN NON-SUBMERGED AND SUBMERGED SELF-PRIMING VENTURI SCRUBBER

The objective of this conducted research is to study the iodine removal efficiency in a self-priming venturi scrubber for submerged and non-submerged operating conditions experimentally and theoretically. The alkaline solution is used as an absorbent, which is prepared by dissolving sodium hydroxide (NaOH) and sodium thiosulphate (Na2S2O3) in water to remove the gaseous iodine (I2) from the gas. Iodine removal efficiency is examined at various gas flow rates and inlet concentrations of iodine for submerged and non-submerged operating conditions. In the non-submerged venturi scrubber, only the droplets take part in iodine removal efficiency. However, in a submerged venturi scrubber condition, the iodine gas is absorbed from gas to droplets inside the venturi scrubber and from bubbles to surrounding liquid at the outlet of a venturi scrubber. Experimentally, it is observed that the iodine removal efficiency is greater in the submerged venturi scrubber as compare to a nonsubmerged venturi scrubber condition. The highest iodine removal efficiency of 0.99±0.001 has been achieved in a submerged self-priming venturi scrubber condition. A mathematical correlation is used to predict the theoretical iodine removal efficiency in submerged and non-submerged conditions, and it is compared against the experimental results. The Wilkinson et al. correlation is used to predict the bubble diameter theoretically whereas the Nukiyama and Tanasawa correlation is used for droplet diameter. The mass transfer coefficient for the gas phase is calculated from the Steinberger and Treybal correlation. The calculated results for a submerged venturi scrubber agree well with experimental results but underpredicts in the case of the nonsubmerged venturi scrubber.

Supercritical Flow over a Dentated Sill

Experimental analysis concerning supercritical flow over a dentated sill in nonsubmerged conditions and the local scour downstream of this sill is presented. In this case, the energy dissipation is incomplete so that a scour control downstream of the hydraulic structure could be required. The use of this type of dissipation device could be needed in cases where the maximum flow depth upstream of the sill must be limited to satisfy physical or structural conditions. This could happen, for instance, in the restoration of stilling basin in gravity dams and at the outlet of diversion systems. Experimental and theoretical results, useful in the design of this type of stilling basin and scour control, are presented in the paper, with an application to the case of the Molato Dam in Italy.

STUDY ON LOCAL SCOUR AND VARIATION OF BED COMPOSITION AROUND NON-SUBMERGED SPUR DYKE

Several studies have been made on local bed scour by movable bed experiments. Researches taking into account non-uniform sediment transport are still very few, in particular in case of the involvement of spur dykes. In this paper, we carried out movable bed experiments around an impermeable spur dyke under non-submerged condition. Both uniform and non-uniform sediment beds with similar mean diameter used and the changes of the bed configuration and bed surface composition have been considered. In addition, we used color sediment which is divided in three ranges of diameter with different colors in order to clarify the spatial differences of bed surface composition through pictures and visual observation. We focused on the relation between initial grain-size distribution and the final scour hole depth and the influence of vortices around a spur dyke on bed surface composition.