首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
A quantitative, three‐dimensional depositional model of gravelly, braided rivers has been developed based largely on the deposits of the Sagavanirktok River in northern Alaska. These deposits were described using cores, wireline logs, trenches and ground‐penetrating radar profiles. The origin of the deposits was inferred from observations of: (1) channel and bar formation and migration and channel filling, interpreted from aerial photographs; (2) water flow during floods; and (3) the topography and texture of the river bed at low‐flow stage. This depositional model quantitatively represents the geometry of the different scales of strataset, the spatial relationships among them and their sediment texture distribution. Porosity and permeability in the model are related to sediment texture. The geometry of a particular type and scale of strataset is related to the geometry and migration of the bedform type (e.g. ripples, dunes, bedload sheets, bars) associated with deposition of the strataset. In particular, the length‐to‐thickness ratio of stratasets is similar to the wavelength‐to‐height ratio of associated bedforms. Furthermore, the wavelength and height of bedforms such as dunes and bars are related to channel depth and width. Therefore, the thickness of a particular scale of strataset (i.e. medium‐scale cross‐sets and large‐scale sets of inclined strata) will vary with river dimensions. These relationships between the dimensions of stratasets, bedforms and channels mean that this depositional model can be applied to other gravelly fluvial deposits. The depositional model can be used to interpret the origin of ancient gravelly fluvial deposits and to aid in the characterization of gravelly fluvial aquifers and hydrocarbon reservoirs.  相似文献   

2.
An experimental study of the preservation of cross-sets during the migration of current ripples under aggrading and non-aggrading conditions was conducted in order to test the modified Paola–Borgman theory for distribution of cross-set thickness as a function of distribution of bed-wave height. In a series of flume experiments, the geometry and migration characteristics of the ripples did not vary systematically with aggradation rate and are comparable to other flume and river data.
Mean cross-set thickness/mean formative bed-wave height is less than 0·4, and mean cross-set thickness/mean bed-wave height is less than 0·53. In the present experiments, the primary control of cross-set thickness is the variability of ripple height. Aggradation rate accounts for only 1–7% of the total cross-set thickness.
A two-parameter gamma density function was fitted to histograms of ripple height to determine the value of parameter a needed for the modified Paola–Borgman model. This model underestimates cross-set thickness because of its assumption that bed-form height spreads evenly above and below the mean bed level, which is not the case in reality. Mean cross-set thickness is predicted quite well if the model constant is increased to 1·3.  相似文献   

3.
Dimensions and plan morphology of current ripples are generally considered to vary with flow velocity and grain size. Recently, however, it has been shown that for sand of D50=0.095 and 0.238 mm the equilibrium dimensions are identical at all velocities within the stability field of ripples and that the plan form of equilibrium ripples is linguoid. On this basis, an empirical unsteady flow model has been developed and tested with flume experiments in order to predict ripple development in natural depositional environments. The model includes the development of washed-out ripples and upper stage plane bed. The unsteady flow model explains the development and preservation of small scale bedforms in various tidal environments more accurately than previous models. Such bedforms can serve, therefore, as indicators of prevailing hydrodynamic conditions.  相似文献   

4.
植物的存在改变了河流水动力特性,造成独特的床面冲淤态势。利用实验室水槽模拟含淹没植物的河道,对床面形态和紊流统计特性参数进行测量,研究不同类型紊流作用下的床面冲淤特征以及床面起伏对流动的影响。结果表明:床面剪切紊流条件下,床面形态为马蹄坑-沙沟/沙脊与沙波复合分布,床面变形加剧了流速沿水深不均匀分布并促进水流动量交换;在自由剪切混合层紊流条件下,床面形态为植物根部马蹄形冲坑及其后方沙沟、沙脊交错分布,床面变形对流动的影响并不显著;“类二重紊流”条件下,床面形态同样表现为马蹄坑-沙沟/沙脊-沙波复合,床面变形促进植物层内部的水流动量交换、抑制紊动清扫,抑制植物层外部的动量交换、促进紊动喷射。  相似文献   

5.
  相似文献   

6.
JACO H. BAAS 《Sedimentology》1994,41(2):185-209
An empirical model is constructed for the development and equilibrium dimensions of small scale, unidirectional bedforms in sand with a median grain size of 0·095 mm, based on a series of steady flow experiments in a flume. Current ripples always attain a linguoid plan morphology with constant average height (13·1 mm) and wavelength (115·7 mm), provided that sufficient time is allowed for their formation. The development pattern of these ripples on a flat bed is independent of flow velocity, and involves four stages: (1) incipient ripples; (2) straight and sinuous ripples; (3) non-equilibrium linguoid ripples, and (4) equilibrium linguoid ripples. Straight and sinuous ripples are non-equilibrium bedforms at all flow velocities. The time needed to reach equilibrium dimensions is related to the inverse power of flow velocity and ranges from several minutes to more than hundreds of hours. At flow velocities where washed ripples are stable, the equilibrium wavelength is similar to that of equilibrium linguoid ripples, but the equilibrium height rapidly decreases from 13·1 mm to zero towards upper stage plane bed conditions. The results of the flume experiments correspond reasonably well with those of previous studies, provided that various complicating factors, such as different experimental methods, different sediment characteristics, shallow flow depths and non-equilibrium runs, are accounted for.  相似文献   

7.
An active oolitic sand wave was monitored for a period of 37 days in order to address the relationship between the direction and strength of tidal currents and the resultant geometry, and amount and direction of migration of bedforms in carbonate sands. The study area is situated in a tidal channel near Lee Stocking Island (Exumas, Bahamas) containing an estimated 5.5 to 6 × 105 m3 of mobile oolitic sand. Tidal ranges within the inlet are microtidal and the maximum current velocity at the studied site is 0.6 m s?1. At least 300–400 m3 of mostly oolitic sand are formed within, or brought into, the channel area every year. The tidal inlet is subdivided into an ocean-orientated segment, in which sand waves are shaped by both flood and ebb tides, and a platform-orientated segment, where sand waves are mainly shaped by flood tides. The studied sand wave lies on the platformward flood-tide dominated segment in a water depth of 3.5.4.5 m. During the 37 days of observation, the oolitic and bioclastic sand wave migrated 4 m in the direction of the dominant flood current. The increments of migration were directly related to the strength of the tide. During each tidal cycle, bedforms formed depending on the strength of the tidal current, tidal range and their location on the sand wave. During flood tides, a steep lee and a gentle stoss side formed and current ripples and small dunes developed on the crest of the sand wave, while the trough developed only ripples. The average lee slope of the sand wave is 24.2°, and therefore steeper than typical siliciclastic sand waves. During ebb tides, portions of the crest are eroded creating a convex upward ebb stoss side, covered with climbing cuspate and linguoid ripples and composite dunes. The area between the ebb-lee side and the trough is covered with fan systems, sinuous ripples and dunes. The migration of all bedforms deviated to a variable degree from the main current direction, reflecting complex flow patterns in the tidal inlet. Small bedforms displayed the largest deviation, migrating at an angle of up to 90° and more to the dominant current direction during spring tides.  相似文献   

8.
The catchment basin of the River Hunte (Lower Saxony, NW-German Basin) was studied on a mesoscale (length of ~90 km) to investigate the influence of the geological subground on modern morphology. A Geo Information System (GIS) was used to calculate linear correlation coefficients between the depth of geological strata (Base Zechstein to Base Quaternary) and the height of the modern landscape (Holocene Alluvial Plain, Lower Weichselian Terrace, catchment basin and watershed). High linear correlation coefficients between the Base of Tertiary and the height of the modern topography (catchment basin [r2=0.87], Lower Weichselian Terrace [r2=0.95] and Holocene Alluvial Plain [r2=0.95]) indicate control of the modern topography by the depth of the geological subsurface via tilting of the entire basin. Most likely northward tilting of the NW-German Basin forces the River Hunte to flow in a northerly direction by relative uplift of the hinterland (Wiehengebirge, Rhenish Massif) and subsidence of the North Sea area.  相似文献   

9.
Current understanding of bedform dynamics is largely based on field and laboratory observations of bedforms in steady flow environments. There are relatively few investigations of bedforms in flows dominated by unsteadiness associated with rapidly changing flows or tides. As a consequence, the ability to predict bedform response to variable flow is rudimentary. Using high‐resolution multibeam bathymetric data, this study explores the dynamics of a dune field developed by tidally modulated, fluvially dominated flow in the Fraser River Estuary, British Columbia, Canada. The dunes were dominantly low lee angle features characteristic of large, deep river channels. Data were collected over a field ca 1·0 km long and 0·5 km wide through a complete diurnal tidal cycle during the rising limb of the hydrograph immediately prior to peak freshet, yielding the most comprehensive characterization of low‐angle dunes ever reported. The data show that bedform height and lee angle slope respond to variable flow by declining as the tide ebbs, then increasing as the tide rises and the flow velocities decrease. Bedform lengths do not appear to respond to the changes in velocity caused by the tides. Changes in the bedform height and lee angle have a counterclockwise hysteresis with mean flow velocity, indicating that changes in the bedform geometry lag changes in the flow. The data reveal that lee angle slope responds directly to suspended sediment concentration, supporting previous speculation that low‐angle dune morphology is maintained by erosion of the dune stoss and crest at high flow, and deposition of that material in the dune trough.  相似文献   

10.
Granule ripples are found mainly in four regions of the Kumtagh Desert in China; they are characterized by an asymmetrical shape, with gentle lower slopes on both sides and abrupt crests. The ripples tend to be oriented perpendicular to the prevailing winds, except when they form near obstacles such as yardangs. The wavelengths (λ) range between 0·31 m and 26 m and heights (h) range from 0·015 m to 1 m. The relationship between wavelength and height can be described by a simple linear function, and the mean ripple index (λ/h) is about 20·4 for the study sites. The sediments are poorly sorted, with negative to very negative skewness at lee and stoss slopes and between‐ripple troughs, which confirms the ‘poured in’ and ‘shadow’ appearance described by previous researchers. The bimodal or trimodal distributions of grains (with modes of ?1·16φ, ?0·5φ and 3·16φ) and the enrichment of coarse particles at the ripple surface (with coarse granule contents ranging between 5·2% and 62·1%) indicate that the underlying layer is the original sediment source and that the granule ripples resist erosional processes. Although the impact of saltating particles and, consequently, the creep and reptation of coarse grains are responsible for granule ripple initiation at a micro‐scale, however, the characteristics of local sediments, wind regimes and topographical obstacles, as well as the feedbacks among bedform and airflow, more strongly affect the development and alignment of granule ripples at a macro‐scale.  相似文献   

11.
  相似文献   

12.
Flume experiments show that current ripples on very fine sand surfaces always develop towards a linguoid shape with constant height and wavelength provided that sufficient time is allowed for their formation. Straight and sinuous current ripples only reflect intermediate stages in ripple development and may be regarded as non-equilibrium bedforms. The time period which current ripples require to reach linguoid equilibrium morphology is related to an inverse power of flow velocity. In the transitional stage from current ripples to upper stage plane bed (i.e. washed-out ripple stage) only the equilibrium wavelength remains constant, whereas equilibrium height rapidly decreases to zero. Our observations imply that bed-roughness parameters in sediment transport calculations can be simplified when equilibrium conditions are attained, and that inferences about flow energy from the dimensions of current ripples in very fine sand need to be regarded with caution.  相似文献   

13.
A grid of seismic reflection lines has been used to image basal topography and infer basal conditions and flow processes beneath ～140 km2 of Rutford Ice Stream, West Antarctica. The subglacial topography in this region consists of two troughs flanking a central high and the bed is composed of water-saturated sediments. The two troughs are filled with deforming sediment, whereas the bed in the central region appears to undergo a transition from largely deforming conditions upstream to basal sliding downstream. The deforming bed is very flat along flow, but undulates across flow. Sliding areas show rougher bed topography. Cross-stream bed topography is characterised by streamlined mounds of deforming sediment aligned in the ice flow direction. These bedforms occur superimposed on the bed in regions of both basal sliding and sediment deformation. In places, they form finger-like mounds of material, which extend into the sliding region further downstream. Mean bedform height is 22 m, mean width is 267 m, and many of them extend for at least 1–2 km along flow. We interpret most of these bedforms as drumlins and one as a mega-scale glacial lineation. The juxtaposition of different basal conditions is consistent with models proposed from terrestrial studies in which the glacier bed is a mosaic of stable and deforming bed areas, variable both spatially and temporally. Any theory of subglacial sediment rheology must also be able to account for our conclusion that, at any given time, pervasive deformation extends at least a few metres into the bed and can persist over a considerable area (many km2). Bedform geometry and basal conditions concur with interpretations of former ice streams, with evidence for increasing elongation ratio with distance downstream. However, those studies also identified bedrock cropping out at the ice-bed interface, for which there is no evidence on Rutford Ice Stream.  相似文献   

14.
Open‐framework gravel (OFG) in river deposits is important because of its exceptionally high permeability, resulting from the lack of sediment in the pore spaces between the gravel grains. Fluvial OFG occurs as planar strata and cross strata of varying scale, and is interbedded with sand and sandy gravel. The origin of OFG has been related to: (1) proportion of sand available relative to gravel; (2) separation of sand from gravel during a specific flow stage and sediment transport rate (either high, falling or low); (3) separation of sand from gravel in bedforms superimposed on the backs of larger bedforms; (4) flow separation in the lee of dunes or unit bars. Laboratory flume experiments were undertaken to test and develop these theories for the origin of OFG. Bed sediment size distribution (sandy gravel with a mean diameter of 1·5 mm) was kept constant, but flow depth, flow velocity and aggradation rate were varied. Bedforms produced under these flow conditions were bedload sheets, dunes and unit bars. The fundamental cause of OFG is the sorting of sand from gravel associated with flow separation at the crest of bedforms, and further segregation of grain sizes during avalanching on the steep lee side. Sand in transport near the bed is deposited in the trough of the bedform, whereas bed‐load gravel avalanches down the leeside and overruns the sand in the trough. The effectiveness of this sorting mechanism increases as the height of the bedform increases. Infiltration of sand into the gravel framework is of minor importance in these experiments, and occurs mainly in bedform troughs. The geometry and proportion of OFG in fluvial deposits are influenced by variation in height of bedforms as they migrate, superposition of small bedforms on the backs of larger bedforms, aggradation rate, and changes in sediment supply. If the height of a bedform increases as it migrates downstream, so does the amount of OFG. Changes in the character of OFG on the lee‐side of unit bars depend on grain‐size sorting in the superimposed bedforms (dunes and bedload sheets). Thick deposits of cross‐stratified OFG require high bedforms (dunes, unit bars) and large amounts of aggradation. These conditions might be expected to occur during high falling stages in the deeper parts of river channels adjacent to compound‐bar tails and downstream of confluence scours. Increase in the amount of sand supplied relative to gravel reduces the development of OFG. Such increases in sand supply may be related to falling flow stage and/or upstream erosion of sandy deposits.  相似文献   

15.
Gravel antidunes in the tropical Burdekin River, Queensland, Australia   总被引：4，自引：0，他引：4  
The geological record is punctuated by the deposits of extreme event phenomena, the identification and interpretation of which are hindered by a lack of data on contemporary examples. It is impossible to directly observe sedimentary bedforms and grain fabrics forming under natural particle-transporting, high-velocity currents, and therefore, their characteristics are poorly documented. The deposits of such flows are exposed however, in the dry bed of the Burdekin River, Queensland, Australia following tropical cyclone-induced floods. Long wave-length (up to 19 m) gravel antidunes develop during short (days) high-discharge flows in the upper Burdekin River (maximum recorded discharge near the study reach over 25 600 m3 s?1 in February 1927). Flood water levels fall quickly (metres in a day) and flow is diverted away from raised areas of the river bed into subchannels, exposing many of the high-stage bedforms with little reworking by falling-stage currents. Gravel bedforms were observed on the dry river bed after the moderate flows of February 1994 (max. 7700 m3 s?1) and January 1996 (max. 3200 m3 s?1). The bedforms had wave-lengths in the range 8–19 m, amplitudes of up to 1 m with steeper stoss than lee faces and crest lines generally transverse to local peak-discharge flow direction. The gravel fabric and size sorting change systematically up the stoss and down the lee faces. The antidune deposits form erosive based lenses of sandy gravel with low-angle downstream dipping lamination and generally steep upstream dipping a-b planes. The internal form and fabric of the antidune gravel lenses are distinctly different from those of dune lee gravel lenses. The erosive based lenses of low-angle cross-bedded gravel with steep upstream dipping a-b planes are relatively easy to recognize and may be diagnostic of downstream migrating antidunes. The antidune gravel lenses are associated with thick (to 1 m) high-angle cross bed sets. Ancient antidune gravel lenses may be diagnostic of episodic high-discharge conditions and particularly when they are associated with high-angle cross-bedded gravelly sand they may be useful for palaeoenvironmental interpretation.  相似文献   

16.
The Burdekin River is an example of a class of tropical streams which experience two to four orders of magnitude variation in discharge, in response to seasonal but erratic monsoonal rainfall. Floods of the Burdekin rise abruptly, reaching peak discharges of up to 40,000 m3 s-1 in less than 24 h; maintain peak flow for up to a few days, and recede exponentially. The geomorphology and deposits of these rivers reflect the extreme discharge fluctuations, and have not previously been described. A stretch of the upper Burdekin River comprising four bends and one straight reach was examined near the town of Charters Towers. The river bed is largely exposed for most of any year, with a small, misfit perennial channel carrying low stage flow. Major geomorphic elements of bends include point bars with ridge-and-swale topography, three distinct types of chute channels, avalanche slipfaces up to 5 m or more high around the downstream edges of bars, and on the outer part of one point bar an elevated, vegetated ridge. Straight reaches are flat or gently inclined, sand- and gravel-covered surfaces. Much of the river bed is covered by well sorted, in places gravelly, coarse to very coarse-grained sand with local accumulations of pebble to boulder gravel. Lower parts of the river bed are periodically draped by mud which is desiccated on exposure. Dunes and plane beds are the most commonly occurring bedforms, with local development of gravelly antidunes. Most bank tops and upper, vegetated bars are covered by silt and fine-grained sand. The river bed also hosts a low-diversity but locally high-abundance, flood-tolerant flora dominated by the paperbark tree Melaleuca argentea, which plays an important role in controlling the distribution of sediment. The gross geomorphology of the river bed and most of the sedimentary features are interpreted as having formed during major (bankfull or near bankfull) flows, which have a recurrence of about 18 years (based on 65 years hydrographic data). The initial rapid drop in discharge following flood peaks appears to preserve flood peak features on upper bars more or less intact, whereas lower areas are subjected to variable degrees of modification during falling stage and by more frequent, non-bankfull discharge events.  相似文献   

17.
Stratification in channel belts is the key to reconstructing formative channel dimensions and palaeoflow conditions; this requires an understanding of the relation between river morphodynamics and set thickness. So far, theories for reconstruction of the original morphology from preserved stratification have not been tested for meandering river channels due to the lack of detailed bathymetry. This paper reports the results of an experiment that reproduced a dynamic meandering gravel‐bed river with the objectives to: (i) test the prediction of set thickness as a function of the morphology formed by a meandering river channel; and (ii) explore and explain spatial and temporal set thickness variations in the resulting channel belt. High‐resolution measurements of time‐dependent surface elevation were used to quantitatively relate the preserved stratification to the meandering river morphology. Mean set thickness agrees well with the theoretical prediction from channel morphology. The mean preserved set thickness was 30% of the mean channel depth. Due to the absence of aggradation during the experiment, this provides a lower limit for the preserved mean set thickness which is also to be expected for aggrading systems, because reworking is some orders of magnitude faster than aggradation. Furthermore, the time required to mature a channel belt and its set thickness distribution was about the same as the time required to develop and propagate bends that fill the channel belt surface. Finally, there was much systematic spatial variation in set thickness related to repetitive point bar growth and chute cut‐off. Undisturbed and thick sets occurred close to channel belt margins and more irregular stratification with stacked thinner sets was observed in the centre of the channel belt.  相似文献   

18.
Bedform climbing in theory and nature   总被引：7，自引：0，他引：7  
Where bedforms migrate during deposition, they move upward (climb) with respect to the generalized sediment surface. Sediment deposited on each lee slope and not eroded during the passage of a following trough is left behind as a cross-stratified bed. Because sediment is thus transferred from bedforms to underlying strata, bedforms must decrease in cross-sectional area or in number, or both, unless sediment lost from bedforms during deposition is replaced with sediment transported from outside the depositional area. Where sediment is transported solely by downcurrent migration of two-dimensional bedforms, the mean thickness of cross-stratified beds is equal to the decrease in bedform cross-sectional area divided by the migration distance over which that size decrease occurs; where bedforms migrate more than one spacing while depositing cross-strata, bed thickness is only a fraction of bedform height. Equations that describe this depositional process explain the downcurrent decrease in size of tidal sand waves in St Andrew Bay, Florida, and the downwind decrease in size of transverse aeolian dunes on the Oregon coast. Using the same concepts, dunes that deposited the Navajo, De Chelly, and Entrada Sandstones are calculated to have had mean heights between several tens and several hundreds of metres.  相似文献   

19.
The development of bedforms under unidirectional, oscillatory and combined‐flows results from temporal changes in sediment transport, flow and morphological response. In such flows, the bedform characteristics (for example, height, wavelength and shape) change over time, from their initiation to equilibrium with the imposed conditions, even if the flow conditions remain unchanged. These variations in bedform morphology during development are reflected in the sedimentary structures preserved in the rock record. Hence, understanding the time and morphological development in which bedforms evolve to an equilibrium stage is critical for informed reconstruction of the ancient sedimentary record. This article presents results from a laboratory flume study on bedform development and equilibrium development time conducted under purely unidirectional, purely oscillatory and combined‐flow conditions, which aimed to test and extend an empirical model developed in past work solely for unidirectional ripples. The present results yield a unified model for bedform development and equilibrium under unidirectional, oscillatory and combined‐flows. The experimental results show that the processes of bedform genesis and growth are common to all types of flows, and can be characterized into four stages: (i) incipient bedforms; (ii) growing bedforms; (iii) stabilizing bedforms; and (iv) fully developed bedforms. Furthermore, the development path of bedform; growth exhibits the same general trend for different flow types (for example, unidirectional, oscillatory and combined‐flows), bedform size (for example, small versus large ripples), bedform shape (for example, symmetrical or rounded), bedform planform geometry (for example, two‐dimensional versus three‐dimensional), flow velocities and sediment grain sizes. The equilibrium time for a wide range of bed configurations was determined and found to be inversely proportional to the sediment transport flux occurring for that flow condition.  相似文献   

20.
ABSTRACT There are very few field measurements of nearshore bedforms and grain‐size distribution on low‐energy microtidal beaches that experience low‐amplitude, long‐period waves. Field observations are needed to determine grain‐size distribution over nearshore bedforms, which may be important for understanding the mechanisms responsible for ripple development and migration. Additional nearshore field observations of ripple geometry are needed to test predictive models of ripple geometry. Ripple height, length and sediment composition were measured in the nearshore of several low‐energy beaches with concurrent measurements of incident waves. The distribution of sediment sizes over individual ripples was investigated, and the performance of several models of ripple geometry prediction was tested both spatially and temporally. Sediment samples were collected from the crest and trough of 164 ripples. The sand‐sized sediment was separated from the small amount (generally <3%) of coarser material (>2 mm) that was present. Within the sand‐sized fraction, the ripple crests were found to be significantly coarser, better sorted and more positively skewed than the troughs. Overall, the troughs were finer than the crests but contained a greater proportion of the small fraction of sediment larger than 2 mm. The field model of Nielsen (1981 ) and the model of Wiberg & Harris (1994 ) were found to be the most accurate models for predicting the wavelength of parallel ripples in the nearshore of the low‐energy microtidal environments surveyed. The Wiberg & Harris (1994 ) model was also the most accurate model for predicting ripple height. Temporal changes in ripple wavelength appear to be dependent on the morphological history of the bed.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号