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1.
The head velocity of the density current in the convergent and divergent channel is a key parameter for evaluating the extent to which suspended material travels, and for determining the type and distribution of sediment in the water body. This study experimentally evaluated the effects of the reach degree of convergence and divergence on the head velocity of the density current. Experiments were conducted in the flume with 6.0 m long, 0.72 m width and 0.6 m height. The head velocity was measured at three convergent degrees （-8°; -12°; -26°）, at three divergent degrees （8°; 12°; 26°） and two slopes （0.009, 0.016） for various discharges. The measured head velocity of the density current is compared with the head velocity of the density current in the constant cross section channel. Based on non-dimensional and statistical analysis, relations as linear multiple regression are offered for predicting head velocity of the density current in the convergent, divergent and constant cross section channel. Also the results of this research show that for the same slope and discharge, the head velocity of the density current in the convergent and divergent channel are greater and less than the head velocity of the constant cross section, respectively.  相似文献   

2.
Velocity measurements carried out by an acoustic doppler velocimeter (ADV) in a rectangular laboratory ?ume having a gravel bed are presented. The velocity pro?les are measured in six verticals of the channel cross‐section having an increasing distance (from 4 to 38·5 cm) from the ?ume wall. The experimental runs are carried out for ?ve different bed arrangements, characterized by different concentrations of coarser elements, and for the two conditions of small‐ and large‐scale roughness. For both hydraulic conditions, the velocity measurements are ?rst used to test the applicability of the Dean pro?le and of the logarithmic pro?le corrected by a divergence function proposed in this paper. Then, for each value of the depth sediment ratio h/d84, the non‐dimensional friction factor parameter is calculated by integration of the measured velocity distributions in the different verticals of the cross‐section. Finally a semi‐logarithmic ?ow resistance equation is empirically deduced. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

3.
《国际泥沙研究》2020,35(2):134-145
Understanding diffusion and solute transport,is regarded as a main activity in environmental management,sustainable development and sedimentation processes because of their role in diffusion of contaminants.The current paper presents an experimental study which was done in a parabolic channel with permeable and impermeable bed conditions in order to estimate the transverse mixing coefficient(TMC) and the maximum solute mixing length(SML) for different channel bed roughness coefficients and discharges.The cross section of the channel was 0.5 m wide and 0.3 m deep with a parabolic shape(i.e.similar to furrow irrigation).Three levels of bed roughness coefficient of 0.02,0.04,and 0.06,and three discharges of 5,10,and 15 L/s were considered.Bed infiltration channel was applied at a low level and medium level as well as a case without infiltration.In the current experiments,sodium chloride was used as a soluble tracer and was injected into the water at mid depth in the upstream cross section.The tracer concentration profile in the water and velocity profile were measured at eight cross sections in the channel.The results showed that the values of SML ranged between 20 and 46 m in impermeable and permeable beds channel beds respectively.The assessment of usual discharge values,infiltration,and roughness coefficient of a furrow(agricultural field) indicated that the SML of the furrows were less than20 m.In the current research,the average dimensionless mixing coefficient was found to be 0.17 in the nonrectangular channel.Eventually,equations were developed to explain the TMC and SML in a permeable parabolic channel that could be beneficial in prediction of fertilizer transport in furrow irrigatio n.  相似文献   

4.
Discharge, especially during flood periods, is among the most important information necessary for flood control, water resources planning and management. Owing to the high flood velocities, flood discharge usually cannot be measured efficiently by conventional methods, which explains why records of flood discharge are scarce or do not exist for the watersheds in Taiwan. A fast method of flood discharge estimation is presented. The greatest advantage of the proposed method is its application to estimate flood discharge that cannot be measured by conventional methods. It has as its basis the regularity of open‐channel flows, i.e. that nature maintains a constant ratio of mean to maximum velocities at a given channel section by adjusting the velocity distribution and the channel geometry. The maximum velocity at a given section can be determined easily over a single vertical profile, which tends to remain invariant with time and discharge, and can be converted to the mean velocity of the entire cross‐section by multying by the constant ratio. Therefore the mean velocity is a common multiple of maximum velocity and the mean/maximum velocity ratio. The channel cross‐sectional area can be determined from the gauge height, the water depth at the y‐axis or the product of the channel width multiplied by the water depth at the y‐axis. Then the most commonly used method, i.e. the velocity–area method, which determines discharge as the product of the cross‐sectional area multiplied by mean velocity, is applied to estimate the flood discharge. Only a few velocity measurements on the y‐axis are necessary to estimate flood discharge. Moreover the location of the y‐axis will not vary with time and water stage. Once the relationship of mean and maximum velocities is established, the flood estimation can be determined efficiently. This method avoids exposure to hazardous environments and sharply reduces the measurement time and cost. The method can be applied in both high and low flows in rivers. Available laboratory flume and stream‐flow data are used to illustrate accuracy and reliability, and results show that this method can quickly and accurately estimate flood discharges. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

5.
We use a hydrodynamic model applied to an idealized fan-shaped basin to explore the morphology and dynamics of radial sand ridges in a convergent coastal system. A positive morphological feedback between channel incision and flow redistribution is responsible for the formation of the channel-ridge pattern. The selection mechanism of bottom wavelength is associated with flow concentration in the deeper part of the channels. Our results are compared to sediment and hydraulic dynamics in the radial sand ridges (RSRs) in China. In a convergent, sloping basin the tangentially averaged tidal velocity peaks at 47 km from the apex. This distance is similar to the arc distance, 62 km, where the RSRs are most incised. An offshore shift in tidal phase results in stronger flows near the north coastline, explaining the presence of asymmetric channel patterns. A numerical stability analysis indicates that small radial oscillations with a wavelength of 10° to 15° maximize the velocity in the troughs. This oscillation wavelength also emerges in the RSRs, which display a peak in spectral energy at a radial wavelength between 25° to 37.5°. High-resolution numerical simulations in the RSRs confirm that flow concentration occurs in the deeper part of the channels, keeping them flushed. We therefore conclude that the RSRs display morphometric characteristics similar to other tidal incisions, like tidal inlets and intertidal channels. This result further supports the dominant role of tidal prism and related peak velocities in incising coastal landscapes. © 2020 John Wiley & Sons, Ltd.  相似文献   

6.
1 INTRODUCTION There are 129 rivers in Taiwan. Most of them are short and steep with small drainage basins and rapid flows. Heavy rains and flood flows usually carry large amount of sediment. The specific peak discharge (peak discharge per unit drainage area) in Taiwan has the highest value in the world. For example, the specific peak discharge of Wu River in central Taiwan is 10.5 cms/km2, which is about 618 times that of Yangtze River in China and 35 times that of Sinno River i…  相似文献   

7.
Ambient noise measurements have been performed at local and small scales in the Neapolitan and surrounding areas (Campania, southern Italy) by employing two broad-band Kinemetrics Q330 stations, equipped with Episensor ES-T three component accelerometers. In both experiments frequency time analysis (FTAN method) has been performed on the vertical and radial components of noise cross correlations to retrieve the Rayleigh wave dispersion (Green??s function). At local scale, over an interstation distance of about 26?km, the group velocity dispersion values have been compared with those obtained from FTAN analysis on recordings of two earthquakes with similar path. At small scale, measurements have been carried out over an interstation distance of about 440?m in the public gardens of Scampia, the northernmost quarter of Naples. The Rayleigh wave group velocity dispersion data obtained from noise cross correlation, have been combined with those from active seismic experiment along the same alignment, but shorter (120?m offset). The non linear inversion of such a dispersion curve has allowed the definition of V S models to depths of 100?m, in agreement with nearby stratigraphy. Moreover, a good agreement has resulted for the resonance frequency among the H/V ratio, the ellipticity of the fundamental mode computed for the chosen V S model, and the average two-dimensional (2D) spectral amplification computed along a cross section representative of the Scampia quarter.  相似文献   

8.
Abstract

The behaviour of the shear velocity along a gravel-bed channel is investigated experimentally in the presence of a negative pressure gradient (accelerating flow). Different methods of estimation of the shear velocity, derived from vertical profiles of the mean longitudinal point velocity, are examined and a new method is proposed. Results show that the proposed method of estimation is comparable to the St Venant and Clauser's methods. At a specific cross section, for constant bottom slope and relative roughness, shear velocity increases with discharge.  相似文献   

9.
Field experiments at Tiramoana station 30 km north of Christchurch, New Zealand using an erosion plot 16·5 m long, 0·6 m wide, and with a slope of 14–14·5° on rendzina soil aimed to measure the variability of flow velocity and of soil aggregates transport rate in shallow overland flow. Discharge/cross‐section area ratio was used to estimate mean velocity, and high‐speed digital video camera and image analysis provided information about flow and sediment transport variability. Six flow runs with 0·5–3·0 L s?1 discharges were supercritical with Froude numbers close to or more than 1. Mean flow velocity followed Poiseuille law, float numbers were more than 1·5 and hydraulic resistance was an inverse proportional function of the Reynolds number, which is typical for laminar flows. Hence actual velocity varied through time significantly and the power spectrum was of ‘red‐noise’, which is typical for turbulent flow. Sediment transport rates had even higher variability, and soil aggregates transport was a compound Poisson process. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

10.
Large rivers have been previously shown to be vertically heterogeneous in terms of suspended particulate matter (SPM) concentration, as a result of sorting of suspended solids. Therefore, the spatial distribution of suspended sediments within the river section has to be known to assess the riverine sedimentary flux. Numerous studies have focused on the vertical distribution of SPM in a river channel from a theoretical or experimental perspective, but only a few were conducted so far on very large rivers. Moreover, a technique for the prediction of depth‐integrated suspended sediment fluxes in very large rivers based on sediment transport dynamics has not yet been proposed. We sampled river water along depth following several vertical profiles, at four locations on the Amazon River and its main tributaries and at two distinct water stages. Depending on the vertical profile, a one‐ to fivefold increase in SPM concentration is observed from river channel surface to bottom, which has a significant impact on the ‘depth‐averaged’ SPM concentration. For each cross section, a so‐called Rouse profile quantitatively accounts for the trend of SPM concentration increase with depth, and a representative Rouse number can be measured for each cross section. However, the prediction of this Rouse number would require the knowledge of the settling velocity of particles, which is dependent on the state of aggregation affecting particles within the river. We demonstrate that in the Amazon River, particle aggregation significantly influences the Rouse number and renders its determination impossible from grain‐size distribution data obtained in the lab. However, in each cross section, the Rouse profile obtained from the fit of the data can serve as a basis to model, at first order, the SPM concentration at any position in the river cross section. This approach, combined with acoustic Doppler current profiler (ADCP) water velocity transects, allows us to accurately estimate the depth‐integrated instantaneous sediment flux. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

11.
Vessel-mounted ADCP measurements were conducted to describe the transverse structure of flow between the two headland tips in Khuran Channel, south of Iran (26° 45′ N), where the highest tidal velocities in spring tides were ~?1.8 m/s. Current profiles were obtained using a 614.4 kHz TRDI WorkHorse Broadband ADCP over nine repetitions of three cross-channel transects during one semidiurnal tidal cycle. The 2.2-km-long transects ran north/south across the channel. A least-square fit to semidiurnal, quarter-diurnal, and sixth diurnal harmonics was used to separate the tidal signals from the observed flow. Spatial gradients showed that the greatest lateral shears and convergences were found over the northern channel and near the northern headland tip due to very sharp bathymetric changes in this area. Contrary to the historical assumption, the across-channel momentum balance in the Khuran Channel was ageostrophic. The current study represents one of the few examples reported where the lateral friction influences the across-channel momentum balance.  相似文献   

12.
Finite‐difference P‐SV simulations of seismic scattering characteristics of faulted coal‐seam models have been undertaken for near‐surface P‐ and S‐wave sources in an attempt to understand the efficiency of body‐wave to channel‐wave mode conversion and how it depends on the elastic parameters of the structure. The synthetic seismograms clearly show the groups of channel waves generated at the fault: one by the downgoing P‐wave and the other by the downgoing S‐wave. These modes travel horizontally in the seam at velocities less than the S‐wavespeed of the rock. A strong Airy phase is generated for the fundamental mode. The velocity contrast between the coal and the host rock is a more important parameter than the density contrast in controlling the amplitude of the channel waves. The optimal coupling from body‐wave energy to channel‐wave energy occurs at a velocity contrast of 1.5. Strong guided waves are produced by the incident S‐sources for source angles of 75° to 90° (close to the near‐side face of the fault). As the fault throw increases, the amplitude of the channel wave also increases. The presence of a lower‐velocity clay layer within the coal‐seam sequence affects the waveguiding characteristics. The displacement amplitude distribution is shifted more towards the lower‐wavespeed layer. The presence of a ‘washout’ zone or a brecciated zone surrounding the fault also results in greater forward scattering and channel‐wave capture by the coal seam.  相似文献   

13.
Landform and landscape evolution may be convergent, whereby initial differences and irregularities are (on average) reduced and smoothed, or divergent, with increasing variation and irregularity. Convergent and divergent evolution are directly related to dynamical (in)stability. Unstable interactions among geomorphic system components tend to dominate in earlier stages of development, while stable limits often become dominant in later stages. This results in mode switching, from unstable, divergent to stable, convergent development. Divergent‐to‐convergent mode switches emerge from a common structure in many geomorphic systems: mutually reinforcing or competitive interrelationships among system components, and negative self‐effects limiting individual components. When the interactions between components are dominant, divergent evolution occurs. As threshold limits to divergent development are approached, self‐limiting effects become more important, triggering a switch to convergence. The mode shift is an emergent phenomenon, arising from basic principles of threshold modulation and gradient selection. As an example, the relationships among flow concentration, erosive force, and channel incision in fluvial systems are examined in the context of mode switching and thresholds. The commonly observed divergence in channel growth and fluvial dissection and network development, eventually transitioning to a stable, convergent configuration, is an emergent outcome of gradient selection and threshold modification, and does not imply any goal functions of balancing mass fluxes or limiting change. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

14.
In the Izu Peninsula (Japan), the Pliocene pumice-rich Dogashima Formation (4.55?±?0.87 Ma) displays exceptional preservation of volcaniclastic facies that were erupted and deposited in a below wave-base marine setting. It includes high-concentration density current deposits that contain clasts that were emplaced hot, indicating an eruption-fed origin. The lower part of the Dogashima 2 unit consists of a very thick sequence (<12 m) of massive grey andesite breccia restricted to the base of a submarine channel, gradationally overlain by pumice breccia, which is widespread but much thinner and finer in the overbank setting. These two breccias share similar mineralogy and crystal composition and are considered to be co-magmatic and derived from the destruction of a submarine dome by an explosive, pumice-forming eruption. The two breccias were deposited from a single, explosive eruption-fed, sustained, sea floor-hugging, water-supported, high-concentration density current in which the clasts were sorted according to their density. At the rim of the channel, localised good hydraulic sorting of clasts and stratification in the pumice breccia are interpreted to reflect local current expansion and unsteadiness rather than to be the result of hydraulic sorting of clasts during fall from a submarine eruption column and/or umbrella plume. A bimodal coarse (>1 m) pumice- and ash-rich bed overlying the breccias may be derived from delayed settling of pyroclasts from suspension. In Dogashima 1 and 2, thick cross- and planar-bedded facies composed of sub-rounded pumice clasts are intercalated with eruption-fed facies, implying inter-eruptive mass-wasting on the flank of a submarine volcano, and reworking and resedimentation by high-energy tractional currents in a below wave-base environment.  相似文献   

15.
This paper focuses on a topographic methodology to characterize the amount of sediment stored in channels and the use of historical photographs for aerial survey by stereophotogrammetry, as part of wider research on debris‐flow magnitude prediction. The topographic methodology uses equidistant four‐point cross‐sections along the long profile of the channel. Each cross‐section is representative of a 50‐m reach of the channel. To calculate the volume of each reach, the difference is calculated between a reference level and the topographic surface. The reference level is the lowest level where the debris flow can erode, and in the current method this level is estimated from fixed points along the long profile of the channel. The accuracy of the method has been estimated by comparing results of a detailed topographic survey, with a standard deviation corresponding to about 6 per cent of the total calculated sediment volume. This topographic methodology has been used on aerial photographs by photogrammetry. This tool was applied to photographs taken on 12 past dates. The scales of the archive photographs used range from 1:3000 to 1:30 000, but results are consistent and permit us to calculate sediment states of the channel for different past dates with an uncertainty of about 6 per cent of the total volume. The application of the technique to the Manival debris‐flow torrent has permitted us to propose some partial sediment budgets and erosion‐rate estimates. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

16.
Many rivers worldwide show converging sections where a characteristic limiting front for vegetation establishment on gravel bars is observed. An important conceptual model was advanced in 2006 by Gurnell and Petts, who demonstrated that for the convergent section of the Tagliamento River the downstream front of vegetation establishment can be explained by unit stream power. We introduce a theoretical framework based on 1D ecomorphodynamic equations modified to account for the biological dynamics of vegetation. We obtain the first analytical result explaining the position and river width where vegetation density is expected to vanish in relation to a characteristic streamflow magnitude and both hydraulic and biological parameters. We apply our model to a controlled experiment within a convergent flume channel with growing seedlings perturbed by periodic floods. For a range of timescales where hydrological and biological processes interact, we observe the formation of a front in the convergent section beyond which vegetation cannot survive, the location of which is explained by flow magnitude. This experiment confirms that the timescales of the involved processes and the unit stream power determine the existence and the position of the front within convergent river reaches, respectively. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

17.
A short length of channel on Pico Partido volcano, Lanzarote, provides us the opportunity to examine the dynamics of lava flowing in a channel that extends over a sudden break in slope. The 1–2-m-wide, 0.5–2-m-deep channel was built during the 1730–1736 eruptions on Lanzarote and exhibits a sinuous, well-formed channel over a steep (11° slope) 100-m-long proximal section. Over-flow units comprising smooth pahoehoe sheet flow, as well as evidence on the inner channel walls for multiple (at least 11) flow levels, attest to unsteady flow in the channel. In addition, superelevation is apparent at each of the six bends along the proximal channel section. Superelevation results from banking of the lava as it moves around the bend thus causing preferential construction of the outer bank. As a result, the channel profile at each bend is asymmetric with an outer bank that is higher than the inner bank. Analysis of superelevation indicates flow velocities of ~8 m s–1. Our analysis of the superelevation features is based on an inertia-gravity balance, which we show is appropriate, even though the down-channel flow is in laminar flow. We use a viscosity-gravity balance model, together with the velocities calculated from superelevation, to obtain viscosities in the range 25–60 Pa s (assuming that the lava behaved as a Newtonian liquid). Estimated volume fluxes are in the range 7–12 m3 s–1. An apparent down-flow increase in derived volume flux may have resulted from variable supply or bulking up of the flow due to vesiculation. Where the channel moves over a sharp break in slope and onto slopes of ~6°, the channel becomes less well defined and widens considerably. At the break of slope, an elongate ridge extends across the channel. We speculate that this ridge was formed as a result of a reduction in velocity immediately below the break of slope to allow deposition of entrained material or accretion of lava to the channel bed as a result of a change in flow regime or depth.  相似文献   

18.
The Bay of Bengal (BoB) is a distinct oceanic region for mesoscale oceanic eddies. The sea level anomaly from the Archiving, Validation, and Interpretation of Satellite Oceanographic (AVISO) help to identify an unusual anti-cyclonic eddy (ACE) over head BoB during May–July 2014. Two Indian moored buoys (BD08 and BD09) located over this region aided to study the subsurface thermohaline structures of the ACE. Compared to no-eddy environment, the temperature and salinity showed an increment of ~?3–4 °C and ~?1–2 PSU, respectively, during the ACE life period. The temperature and depth of the isothermal layer at genesis (peak) stages are increased to ~?30 °C (~?30.7 °C) and ~?20 m (30 m) when compared with no-eddy conditions (28.2 °C and 10 m). The thermocline depth is deepened to 75 m at the peak stage, while it is 50 m in no-eddy condition. A temperature difference of 3 °C between no-eddy and peak stages of ACE is observed up to 50 m. The ocean heat content (OHC) at BD08 (BD09) during genesis and peak stages has increased by ~?72% (~?50%) and ~?247% (~?181%), respectively, when compared with no-eddy conditions. Moreover, the MOHC also shows a similar increment of ~?125% (~?123%) and ~?258% (~?284%), respectively. A noticeable influence is seen in turbulent fluxes and lower atmospheric variables during eddy life. This study highlights the capability of moored buoys in understanding the subsurface thermohaline features of the eddies over northern BoB.  相似文献   

19.
The influence of emergent and submerged macrophytes on flow velocity and turbulence production is demonstrated in a 140 m reach of the River Blackwater in Farnborough, Hampshire, UK. Macrophyte growth occurs in patches and is dominated by Sparganium erectum and Sparganium emersum. In May 2001, patches of S. erectum were already established and occupied 18% of the channel area. The flow adjusted to these (predominantly lateral) patches by being channelled through a narrower cross‐section. The measured velocity profiles showed a logarithmic form, with deviations attributable to topographic control. The channel bed was the main source of turbulence. In September 2001, in‐stream macrophytes occupied 27% of the channel, and overhanging bank vegetation affected 32% of the area. Overall flow resistance, described by Manning's n, showed a threefold increase that could be attributed to the growth of S. emersum in the middle of the channel. Velocity profiles showed different characteristic forms depending on their position relative to plant stems and leaves. The overall velocity field had a three‐dimensional structure. Turbulence intensities were generally higher and turbulence profiles tended to mirror the velocity profiles. Evidence for the generation of coherent eddies was provided by ratios of the root mean square velocities. Spectral analysis identified deviations from the Kolmogorov ?5/3 power law and provided statistical evidence for a spectral short‐cut, indicative of additional turbulence production. This was most marked for the submerged vegetation and, in some instances, the overhanging bank vegetation. The long strap‐like leaves of S. emersum being aligned approximately parallel to the flow and the highly variable velocity field created by the patch arrangement of macrophytes suggest that the dominant mechanism for turbulence production is vortex shedding along shear zones. Wake production around individual stems of S. emersum close to the bed may also be important locally. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

20.
The in?uence of pool length on the strength of turbulence generated by vortex shedding was investigated in a 6 m long recirculating ?ume. The experiment utilized a 38% constriction of ?ow and an average channel‐bed slope of 0·007. The base geometry for the intermediate‐length pool experiment originated from a highly simpli?ed, 0·10 scale model of a forced pool from North Saint Vrain Creek, Colorado. Discharge in the ?ume was 31·6 l/s, which corresponds to a discharge in the prototype channel of 10 m3/s. Three shorter and four longer pool lengths also were created with a ?xed bed to determine changes in turbulence intensities and energy slope with pool elongation. Three‐dimensional velocities were measured with an acoustic Doppler velocimeter at 31–40 different 0·6‐depth and near‐bed locations downstream of the rectangular constriction. The average velocity and root mean square (RMS) of the absolute magnitude of velocity at both depths are signi?cantly related to the distance from the constriction in most pool locations downstream of the constriction. In many locations, pool elongation results in a non‐linear change in turbulence intensities and average velocity. Based on the overall ?ow pattern, the strongest turbulence occurs in the center of the pool along the shear zone between the jet and recirculating eddy. The lateral location of this shear zone is sensitive to changes in pool length. Energy slope also was sensitive to pool length due to a combination of greater length of the pool and greater head loss with shorter pools. The results indicate some form of hydraulic optimization is possible with pools adjusting their length to adjust the location and strength of turbulent intensities in the center of pools, and lower their rate of energy dissipation. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

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