Critical specific stream power in gravel-bed rivers     

F. Petit  F. Gob  G. Houbrechts  A.A. Assani 《Geomorphology》2005,69(1-4):92-101

Experiments with marked pebbles were carried out on different sized rivers of the Belgian Ardenne (catchment areas varying from less than 1 km² to 2700 km²). Specific stream power required to cause bedload movement was evaluated and critical values were obtained. Three types of relationship between critical specific stream power (ω₀) and grain size (D) were established. The values for ω₀ in the largest river (the Ourthe) were the lowest and were close to the values obtained for mountainous rivers carrying large boulders. In medium sized rivers (catchment area between 40 and 500 km²), the critical unit stream power was higher. It is likely that it is due to the bedform's greater resistance. This resistance would use up some of the energy that can cause movement and transport of bedload. The amount of resistance of the bedform can be expressed as bedform shear stress (τ″), determined by the relationship between grain shear stress (τ′—that determines movement and transport of the bedload) and the total shear stress (τ). This ratio varies between 0.4 and 0.5 in the medium sized rivers, compared to 0.7 in the Ourthe. In headwater streams (less than 20 km²), there is greater loss of energy due to bedform resistance (τ′/τ<0.3). Critical specific stream power is higher in this third type of river than in the other two.  相似文献   

Wavy lamination in a mixed sand and gravel foreshore facies of the Pleistocene Hosoya Sandstone, Aichi, central Japan   总被引：1，自引：0，他引：1  

YOSHIHISA HIROKI  TAKASHI TERASAKA 《Sedimentology》2005,52(1):65-75

Although sandy foreshore facies are generally characterized by parallel lamination, wavy lamination is predominant in the mixed sand and gravel foreshore facies of the Pleistocene Hosoya Sandstone, which crops out along the Pacific coast of the Atsumi Peninsula, Aichi, central Japan. The foreshore facies consists of three sedimentary subfacies; interbeds of gravel and parallel laminated sand of the lower foreshore facies, parallel laminated fine to medium sand beds containing scattered pebbles and cobbles of the middle foreshore facies, and wavy laminated fine to medium sand beds containing scattered pebbles and cobbles of the upper foreshore facies. A lack of erosional surfaces in the middle foreshore facies indicates the continuous accumulation of sand in flat beds under upper plane bed flow. The wavy laminated sands of the upper foreshore facies exhibit erosional surfaces indicative of repeated deposition and erosion. The erosional surfaces are undulatory, with depressions (10 cm wide and 3 cm deep) that contain scattered pebbles and cobbles. These depressions reflect backwash erosion of sand around and below the pebbles and cobbles. Sand draping over the undulating erosional surfaces forms the wavy lamination. The wavy laminated sand with scattered pebbles and cobbles is a key facies of an upper foreshore or swash zone, and is a good sea-level marker.  相似文献   

Dewatering effects on the erodibility of newly deposited cohesive beds by unidirectional currents     

EDUARDO A. GÓMEZ  CARL L. AMOS† 《Sedimentology》2005,52(1):183-189

A series of laboratory experiments on cohesive sediments under inorganic conditions was undertaken in order to evaluate the impact of fluid bed shear stress on the build-up of bed resistance to erosion with time. The importance of small pressures due to flowing water to increase bed strength is presented. It is also shown that the susceptibility of a cohesive bed to changes in its erodibility is related to deposited bed thickness due to sediment disturbance caused by dewatering from the consolidating bed. Laboratory experiments that use beds deposited from suspension should thus report the thickness of the bed prior to resuspension.  相似文献   

Contribution de l'hydrogéomorphologie à l'évaluation du risque d'inondation : le cas du Midi méditerranéen français     

Jean-Louis Ballais  Gérald Garry  Marcel Masson 《Comptes Rendus Geoscience》2005,337(13):1120-1130

The hydrogeomorphological method for delimiting flood risk zones in France was developed some twenty years ago. It is based on a simple principle: the outer limits of a stream's flood plain constitute the outer envelope of past floods. These limits are determined with the use of aerial photographs and field surveys of micro-topography as well as analyses of deposit granulometry and colour. Indications of present or past land use (fields, location and distribution of archaeological sites, houses and farm buildings, roads) are also useful. This field-based method long remained ignored, but being reliable, easy to use and inexpensive, it has now been incorporated into the package of methods recommended by French risk-prevention plans (PPRI). The many recent catastrophes that have occurred over the past fifteen years in the Mediterranean regions of southern France demonstrated both the inadequacy of the hydrological–hydraulic method and the reliability of the hydrogeomorphological method, which can, however, be improved by setting observations for the present period against information on the more ancient Holocene evolution of flood plains. To cite this article: J.-L. Ballais et al., C. R. Geoscience 337 (2005).  相似文献   

用反射共偏移法探测煤层厚度          下载免费PDF全文

丁宽 《甘肃地质》2007,(Z1):93-97,70

地震波在煤、岩层中传播,当遇到不同岩性之间的界面时,就会发生反射。根据这一原理,对四台矿8307工作面12#煤层利用KDZ1114-3型便携式矿井地质探测仪,接收煤、岩层中反射波信息,解析得到反射界面的位置,进而确定煤、岩层的厚度。  相似文献   

Numerical Analysis of Multi Layer Geosynthetic-Reinforced Granular Bed over Soft Fill     

Kousik Deb  N. Sivakugan  Sarvesh Chandra  P. K. Basudhar 《Geotechnical and Geological Engineering》2007,25(6):639-646

In this paper, considering the plain strain conditions, a numerical study has been conducted to investigate the behavior of multi layer geosynthetic-reinforced granular bed overlying a soft soil using the Fast Lagrangian Analysis of Continua (FLAC) program. The granular fill, soft soil, and geosynthetic reinforcements are considered as linear elastic materials. The geosynthetic reinforcements are modeled as cable elements fully bonded with the surrounding soil, thus neglecting any slip. The results obtained from the present investigation showed very close agreement when compared with the results of finite element analysis and lumped parameter modeling. The distribution of vertical, lateral and shear stresses in the soil are greatly affected as the number of reinforcement layers is increased. If the tensile stiffness of geosynthetic layers increases and its value is no more than 4,000–5,000 kN/m, the settlement of the reinforced foundation decreases significantly. The reduction in settlement is insignificant when the tensile strength of the geosynthetics exceed the above value.  相似文献   

Coarse bed load transport in an alluvial gravel bed stream,Dupuyer Creek,Montana     

Andrew C. Whitaker  Donald F. Potts 《地球表面变化过程与地形》2007,32(13):1984-2004

Coarse bed load was sampled in a gravel/cobble bed stream during two major floods in the snowmelt runoff season. The channel is characterized by high rates of bank erosion and, therefore, high rates of sediment supply and bed load flux. Peak discharge reached four times bank‐full, and bed load was sampled at flows 0·7–1·7 times bank‐full. A large aperture bed load sampler (1 m by 0·45 m) captured the largest particles in motion, and specifically targeted the coarse bed load size distribution by using a relatively large mesh (32 mm or D₂₅ of streambed surface size distribution). Bed load flux was highly variable, with a peak value of 0·85 kg/s/m for the coarse fraction above 38 mm. Bed load size distribution and maximum particle size was related to flow strength. Entrainment was size selective for particles D₇₀ and larger (88–155 mm), while particles in the range D₃₀–D₇₀ (35–88 mm) ceased to move at essentially the same flow. Bed load flux was size selective in that coarse fractions of the streambed surface were under‐represented in or absent from the bed load. Painted tracer particles revealed that the streambed surface in the riffles could remain stable even during high rates of bed load transport. These observations suggest that a large proportion of bed load sediments was sourced from outside the riffles. Repeat surveys confirmed major scour and fill in pools (up to 0·75 m), and bank erosion (>2 m), which together contributed large volumes of sediment to the bed load. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

Bed stability variations after check dam construction in torrential channels (South‐East Spain)     

C. Conesa‐García  F. López‐Bermúdez  R. García‐Lorenzo 《地球表面变化过程与地形》2007,32(14):2165-2184

The effects of check dams on the bed stability of torrential channels have been analysed in several tributary basins of the Segura and Guadalentín rivers (South‐East Spain). In order to illustrate the large variability in channel bed‐forms and bed sediment sizes along the stream, 52 reaches of 150 m in length were surveyed. This variability is due to the behaviour of check dams, which depends on bedrock control, bed slope, channel roughness, lateral sediment input and a highly variable sediment transport capacity. Though the purpose of check dams is to diminish the boundary shear stress, reducing the longitudinal slope, and to stabilize the channel bed, downstream they reduce the volume of channel‐stored material, favouring local scour processes, and upstream they can destabilize the sidewalls. The results enable us to evaluate the impact of every check dam on the bed morphology, distinguishing the structures installed in limy marl areas (e.g. catchment of the Cárcavo rambla, Cieza) and in schist and slate terrains (e.g. catchment of the Torrecilla rambla, close to Lorca). In the first type, bedrock and moderately thick granular beds predominate downstream from the check dams, so that the length of bedrock reaches and increase of roughness due to scour processes are the best indicators to verify its geomorphological effectiveness. On the other hand, the metamorphic areas drained by ramblas and gullies produce great quantities of gravel that are retained by check dams, creating more uniform and permeable beds, where the balance between sedimentation and scouring, and the ratio τ_c84/τ₀ (RBS), appear to be the parameters most frequently adopted to estimate the bed stability. Analysis of slope adjustments and the application of other indices to estimate the bed substrate stability (LRBS, SRI) and the structural influence of the dams (SIBS) corroborate the differences in bed stability found in the corrected reaches in each catchment. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

Interactions between sediment delivery,channel change,climate change and flood risk in a temperate upland environment   总被引：1，自引：0，他引：1  

S. N. Lane  V. Tayefi  S. C. Reid  D. Yu  R. J. Hardy 《地球表面变化过程与地形》2007,32(3):429-446

This paper uses numerical simulation of flood inundation based on a coupled one‐dimensional–two‐dimensional treatment to explore the impacts upon flood extent of both long‐term climate changes, predicted to the 2050s and 2080s, and short‐term river channel changes in response to sediment delivery, for a temperate upland gravel‐bed river. Results show that 16 months of measured in‐channel sedimentation in an upland gravel‐bed river cause about half of the increase in inundation extent that was simulated to arise from climate change. Consideration of the joint impacts of climate change and sedimentation emphasized the non‐linear nature of system response, and the possibly severe and synergistic effects that come from combined direct effects of climate change and sediment delivery. Such effects are likely to be exacerbated further as a result of the impacts of climate change upon coarse sediment delivery. In generic terms, these processes are commonly overlooked in flood risk mapping exercises and are likely to be important in any river system where there are high rates of sediment delivery and long‐term transfer of sediment to floodplain storage (i.e. alluviation involving active channel aggradation and migration). Similarly, attempts to reduce channel migration through river bank stabilization are likely to exacerbate this process as without bank erosion, channel capacity cannot be maintained. Finally, many flood risk mapping studies rely upon calibration based upon combining contemporary bed surveys with historical flood outlines, and this will lead to underestimation of the magnitude and frequency of floodplain inundation in an aggrading system for a flood of a given magnitude. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Formation and preservation of open-framework gravel strata in unidirectional flows   总被引：1，自引：0，他引：1  

I. A. LUNT  J. S. BRIDGE 《Sedimentology》2007,54(1):71-87

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