Inter-comparison between five devices for determining erodability of intertidal sediments     

J. Widdows  P.L. Friend  A.J. Bale  M.D. Brinsley  N.D. Pope  C.E.L. Thompson 《Continental Shelf Research》2007

Five erosion devices were compared using five intertidal estuarine sites covering a range of sediment stability from newly settled mud to very cohesive mud at the margins of a saltmarsh. The erosion devices use different methods of fluid shearing from horizontal currents/bed shear stresses to vertical water jets, and have different ‘footprint’ areas. The devices included: (1) the annular flumes (AFs—diameter 64 cm; footprint area 0.17 m²) of the Plymouth Marine Laboratory (PML); (2) PML's mini-annular flume (MAF—diameter 19 cm; area 0.026 m²); (3) the annular mini-flume (AMF—diameter 30.5 cm; area 0.032 m²) of the National Oceanography Centre Southampton (NOC); (4) NOC's Cohesive Strength Meter (CSM—diameter 3 cm; area 0.0007 m²); (5) NOC's EROMES (ER—diameter 10 cm; area 0.0079 m²). The quantification of threshold shear stress for bed erosion (τ_e) and sediment erosion rate was complemented by the measurement of physical, chemical and biological properties of the sediment (grain size, bulk density, water content, organic content, chlorophyll a, carbohydrates, macrofauna). The results demonstrated a significant correlation (r²=0.98) between the PML AF (laboratory measurement of undisturbed cored sediment) and PML MAF (in situ) for measurement of erosion thresholds for bed sediment. However, there were no significant correlations between AFs, the CSM and EROMES. There were no consistent correlations with physical or biological sediment properties due to the spatially unrelated sites and the marked differences in benthic assemblages. The sources of differences and the lack of correlations between erosion devices were due to several factors, including operational procedures (e.g., sediment resuspension during filling with water), definition of erosion threshold, the nature of the force applied to the bed, and method of calibration. In contrast to the CSM and EROMES, both types of AFs were able to record significant differences in the erodability of soft sediments from four sites. This indicates that the CSM and EROMES may not be very effective at measuring the differences in erosion thresholds of soft estuarine sediments.  相似文献   

Reaction of the high-latitude lower ionosphere to solar proton events from observations in the ELF range     

O. M. Lebed’  A. V. Larchenko  S. V. Pil’gaev  Yu. V. Fedorenko 《Geomagnetism and Aeronomy》2017,57(1):51-57

The reaction of the lower ionosphere to the solar proton events that occurred in 2011–2012 is studied in this paper based on the results of measurements of the propagation velocity and the E _z /H _τ ratio of the low-frequency electromagnetic pulses (atmospherics) in the ELF range at the high-latitude observatories Lovozero and Barentsburg. With numerical modeling methods, it is shown that horizontal local irregularities of the lower ionosphere conductivity profile could be a cause of the splashes in the E _z /H _τ ratio observed in the experiment during the solar proton event of March 7, 2012, which was a unique event in both the proton flux value and energy.  相似文献   

Effect of magnetic storms in variations in the atmospheric electric field at midlatitudes     

N. G. Kleimenova  O. V. Kozyreva  S. Michnowski  M. Kubicki 《Geomagnetism and Aeronomy》2008,48(5):622-630

The observations of the variations in the vertical component of the atmospheric electric field (E _z ) at Swider midlatitude Poland observatory (geomagnetic latitude 47.8°) under the conditions of fair weather during 14 magnetic storms have been analyzed. The effect of the magnetic storm main phase in the daytime midlatitude variations in E _z in the absence of local geomagnetic disturbances has been detected for the first time. Considerable (～100–300 V m^?1) decreases in the electric field strength (E _z ) at Swider observatory were observed in daytime simultaneously with the substorm onset in the nighttime sector of auroral latitudes (College observatory). The detected effects indicate that an intensification of the interplanetary electric field during the magnetic storm main phase, the development of magnetospheric substorms, and precipitation of energetic electrons into the nighttime auroral ionosphere can result in considerable disturbances in the midlatitude atmospheric electric field.  相似文献   

Application of the Pi theorem to the wear rate of gouge formation in frictional sliding of rocks     

Hiroyuki Nagahama  Norihiro Nakamura 《Pure and Applied Geophysics》1994,142(3-4):795-808

A simple law of wear rate is examined for the process of gouge generation during the frictional sliding of simulated faults in rocks, by use of the Pi theorem method (dimensional analysis) and existing experimental data. The relationship between wear rate (t/d) and the applied stress can be expressed by the power-law relations $$\frac{t}{d} = C_\sigma \sigma ^{m\sigma } ,\frac{t}{d} = C_\tau \tau ^{m\tau }$$ wheret is the thickness of the gouge generated on the frictional surfaces,d is the fault displacement, σ and τ are normal stress and shear stress, respectively, andC _σ,C _τ,m _σ andm _τ are constants. These results indicate that the exponent coefficientsm _σ andm _τ and the coefficientsC _σ andC _τ depend on the material hardness of the frictional surfaces. By using the wear rates of natural faults, these power-law relationships may prove to be an acceptable palaeopiezometer of natural faults and the lithosphere.  相似文献   

The effect of wave-induced turbulence on intertidal mudflats: Impact of boat traffic and wind     

R. Verney  J. Deloffre  J.-C. Brun-Cottan  R. Lafite 《Continental Shelf Research》2007

Semi-diurnal and fortnightly surveys were carried out to quantify the effects of wind- and navigation-induced high-energy events on bed sediments above intertidal mudflats. The mudflats are located in the upper fluvial part (Oissel mudflat) and at the mouth (Vasière Nord mudflat) of the macrotidal Seine estuary. Instantaneous flow velocities and mudflat bed elevation were measured at a high frequency and high resolution with an acoustic doppler velocimeter (ADV) and an ALTUS altimeter, respectively. Suspended particulate matter concentrations were estimated by calibrating the ADV acoustic backscattered intensity with bed sediments collected at the study sites. Turbulent bed shear stress values were estimated by the turbulent kinetic energy method, using velocity variances filtered from the wave contribution. Wave shear stress and maximum wave–current shear stress values were calculated with the wave–current interaction (WCI) model, which is based on the bed roughness length, wave orbital velocities and the wave period (T_S). In the fluvial part of the estuary, boat passages occurred unevenly during the surveys and were characterized by long waves (T_S>50 s) induced by the drawdown effect and by short boat-waves (T_S<10 s). Boat waves generated large bottom shear stress values of 0.5 N m⁻² for 2–5 min periods and, in burst of several seconds, larger bottom shear stress values up to 1 N m⁻². At the mouth of the estuary, west south-west wind events generated short waves (T_S<10 s) of H_S values ranging from 0.1 to 0.3 m. In shallow-water environment (water depth <1.5 m), these waves produced bottom shear stress values between 1 and 2 N m⁻². Wave–current shear stress values are one order of magnitude larger than the current-induced shear stress and indicate that navigation and wind are the dominant hydrodynamic forcing parameters above the two mudflats. Bed elevation and SPM concentration time series showed that these high energy events induced erosion processes of up to several centimetres. Critical erosion shear stress (τ_ce) values were determined from the SPM concentration and bed elevation measurements. Rough τ_ce values were found above 0.2 N m⁻² for the Oissel mudflat and about 1 N m⁻² for the Vasière Nord mudflat.  相似文献   

The impact of inter-flood duration on non-cohesive sediment bed stability     

Annie Ockelford  Stephen Woodcock  Heather Haynes 《地球表面变化过程与地形》2019,44(14):2861-2871

Limited field and flume data suggests that both uniform and graded beds appear to progressively stabilize when subjected to inter-flood flows as characterized by the absence of active bedload transport. Previous work has shown that the degree of bed stabilization scales with duration of inter-flood flow, however, the sensitivity of this response to bed surface grain size distribution has not been explored. This article presents the first detailed comparison of the dependence of graded bed stability on inter-flood flow duration. Sixty discrete experiments, including repetitions, were undertaken using three grain size distributions of identical D₅₀ (4.8 mm); near-uniform (σ_g = 1.13), unimodal (σ_g = 1.63) and bimodal (σ_g = 2.08). Each bed was conditioned for between 0 (benchmark) and 960 minutes by an antecedent shear stress below the entrainment threshold of the bed (τ*_c50). The degree of bed stabilization was determined by measuring changes to critical entrainment thresholds and bedload flux characteristics. Results show that (i) increasing inter-flood duration from 0 to 960 minutes increases the average threshold shear stress of the D₅₀ by up to 18%; (ii) bedload transport rates were reduced by up to 90% as inter-flood duration increased from 0 to 960 minutes; (iii) the rate of response to changes in inter-flood duration in both critical shear stress and bedload transport rate is non-linear and is inversely proportional to antecedent duration; (iv) there is a grade dependent response to changes in critical shear stress where the magnitude of response in uniform beds is up to twice that of the graded beds; and (v) there is a grade dependent response to changes in bedload transport rate where the bimodal bed is most responsive in terms of the magnitude of change. These advances underpin the development of more accurate predictions of both entrainment thresholds and bedload flux timing and magnitude, as well as having implications for the management of environmental flow design. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.  相似文献   

Erodibility of cohesive streambeds in the loess area of the midwestern USA   总被引：2，自引：0，他引：2  

G. J. Hanson  A. Simon 《水文研究》2001,15(1):23-38

Excess stress parameters, critical shear stress (τ_c) and erodibility coefficient (k_d), for degrading channels in the loess areas of the midwestern USA are presented based on in situ jet‐testing measurements. Critical shear stress and k_d are used to define the erosion resistance of the streambed. The jet‐testing apparatus applies hydraulic stresses to the bed and the resulting scour due to the impinging jet is related to the excess stress parameters. Streams tested were primarily silt‐bedded in texture with low densities, which is typical of loess soils. Results indicate that there is a wide variation in the erosion resistance of streambeds, spanning six orders of magnitude for τ_c and four orders of magnitude for k_d. Erosion resistance was observed to vary within a streambed, from streambed to streambed, and from region to region. An example of the diversity of materials within a river system is the Yalobusha River Basin in Mississippi. The median value of τ_c for the two primary bed materials, Naheola and Porters Creek Clay Formations, was 1·31 and 256 Pa, respectively. Streambeds composed of the Naheola Formation are readily eroded over the entire range of shear stresses, whereas only the deepest flows generate boundary stresses great enough to erode streambeds composed of the Porters Creek Clay Formation. Therefore, assessing material resistance and location is essential in classifying and modelling streambed erosion processes of these streams.  相似文献   

Dynamics of a spherical viscoelastic shell: Implications to a criterion for fragmentation/expansion of bubbly magma     

Mie Ichihara 《Earth and Planetary Science Letters》2008,265(1-2):18-32

This paper investigates dynamics of a spherical bubble surrounded by a viscoelastic fluid. The purpose of the study is to understand the parameters which control expansion and fragmentation of bubbly magma by decompression. In particular, we focus on which occurs first, fragmentation or expansion. Supposing that rupture of the bubble wall occurs in a critical stress condition, we calculate the change of the bubble radius and tensile stress at the bubble wall for various decompression rates. Conditions in which tensile stress is stored in the shell are represented in terms of dimensionless parameters. The results are interpreted as follows: when magma viscosity is larger than a critical value, and the decompression time is shorter than viscous expansion time, tensile stress is stored before expansion; when magma viscosity is smaller than the critical value, tensile stress is not stored, no matter how rapid the decompression. Although it is a generally accepted theory that fragmentation is effected by stress conditions and decompression time, exactly how decompression time (t₁) effects the fragmentation is not yet fully understood. This study demonstrates that the stress condition is controlled by the length of the decompression time not relative to the viscoelastic relaxation time (t₁ / τ), but relative to the viscous expansion time (t₁ / τ^l_rlx). As suggested by recent experimental studies, the decompression time relative to viscoelastic relaxation time (t₁ / τ) is also significant to the fragmentation process itself. It indicates that the decompression time effects the fragmentation not through the stress condition. However more work must be completed to fully understand the particular relationship between the decompression time and relaxation time in terms of its influence on fragmentation.  相似文献   

Anelastic degradation of acoustic pulses in rock     

M.T. Gladwin  F.D. Stacey 《Physics of the Earth and Planetary Interiors》1974,8(4):332-336

Measurements on acoustic pulses propagating in massive rock lead to a simple empirical relationship between the pulse rise time, τ and the time of propagation of a pulse, t: where τ₀ is the initial rise time (at t = 0), Q is the anelastic parameter which may be expressed in terms of the fractional loss of energy per cycle of a sinusoidal wave, Q = 2π(ΔE/E)^?1, and is assumed to be essentially independent of frequency, and C is a constant whose value we estimate experimentally to be 0.53 ± 0.04. Of the linear theories of seismic pulse attenuation, model 2 of Azimi et al. (1968) is favoured. Pulse shapes computed from equations of Futterman (1962) also give C = 0.5, but the pulse arrives earlier than in a non-attenuating medium with the same elasticity and density. Pulse shapes calculated using Strick's (1967, 1970, 1971) theory give values of C incompatible with our results. The observations suggest that a method of estimating the Q-structure of the earth from seismic pulse rise times may have a particular advantage over the spectral ratio method.  相似文献   

The impact of stress history on bed structure     

Anne‐Marie Ockelford  Heather Haynes 《地球表面变化过程与地形》2013,38(7):717-727

Recent research has started to focus on how prolonged periods of sub‐threshold flows may be capable of imparting structural changes that contribute to increased bed stability. To date, this effect (termed ‘stress history’) has been found to be significant in acting to increase a bed's critical shear stress at entrainment threshold. However, it is supported by only limited, qualitative and often speculative information on the mechanisms of this stabilization process in grade‐specific studies. As such, this paper uses high resolution laser scanning to quantitatively ascertain the granular mechanics underpinning the relationship between stress history and entrainment threshold for beds of a range of grain size distributions. Employing a bed slope of 1/200, three grain size distributions with median grain sizes (D₅₀) of 4·8 mm [uniform (σ_g = (D₈₄/D₁₆)^0.5 = 1·13; bimodal (σ_g = 2·08); and, unimodal (σ_g = 1·63)] were exposed to antecedent stress histories of 60 and 960 minutes duration. Antecedent shear stress magnitude was set at 50% of the critical shear stress for the D₅₀ when no stress history period was employed. Two laser displacement scans of the bed surface (approximate area 100 mm × 117 mm) were taken, one prior to the antecedent period and one after this period, so that changes to surface topography could be quantified (resolution of x = 0·10 mm, y = 0·13 mm and z = 0·24 mm). Rearrangement of bed surface structure is described using statistical analysis and two‐dimensional (2D) semi‐variograms to analyse scaling behaviour. Results reveal vertical settlement, changes to bed roughness and particle repositioning. However, the bed grain size distribution influences the relative importance of each mechanism in determining stress history induced bed stability; this is the focus of discussion in this paper. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Critical erosion profiles in macro-tidal estuary sediments: Implications for the stability of intertidal mud and the slope of mud banks     

A.J. Bale  J.A. StephensC.B. Harris 《Continental Shelf Research》2007

Vertical profiles of the critical erosion threshold (τ_crit) in sediment have been measured at 11 stations along the axis of the Tamar Estuary and at a single station in a tributary of the Tamar at St. John's Ford. The τ_crit of surface sediment increased from 0.04 Pa in the upper, brackish estuary to 0.09 Pa in the lower estuary. In the upper estuary τ_crit only increased slightly with depth whereas in the marine estuary τ_crit increased rapidly from 0.09 Pa at the surface to 0.25 Pa at 15 cm below the sediment surface. The results showed that the relationship between τ_crit and bulk density (ρ_b) obtained previously for surface sediment was also applicable to sediments from depths of 10–15 cm and probably deeper. Profiles of ρ_b were measured to depths of 70 cm using a corer. In the lower (marine) estuary ρ_b increased with depth in the sediment from 1580 kg m⁻³ at the surface to 1720 kg m⁻³ at 70 cm. In the upper estuary ρ_b values were lower at 1170–1200 kg m⁻³ and profiles were almost homogeneous indicating that consolidation was not occurring. The mid-estuary was transitional between these two situations. These results are consistent with the seasonal accumulation and loss of ‘mobile’ sediment observed previously in the upper estuary with changes in river flow, and with the apparent stability of intertidal mud in the lower marine estuary deduced from historical bathymetric survey records. The slopes of the intertidal mud banks ranged from 1–2% in the lower estuary to 20–25% in mid-estuary but, instead of continuing to increase in steepness towards the head as the estuary became narrower, the measured slopes actually decreased. It is speculated that the lack of consolidation through continual mobilisation and settlement cycles combined with an increase in silt content in the upper estuary resulted in sediment that lacked the mechanical strength to maintain steep slopes.  相似文献   

Variations in global cloud cover and the fair-weather vertical electric field     

《Journal of Atmospheric and Solar》2008,70(13):1633-1642

Statistically significant (at the 95% significance level) changes in daily cloud cover are found to occur globally over land coincident with extreme increases in ‘fair-weather’ measurements of vertical electric field (E_z) measured at Vostok, Antarctica. Using global cloud products from the International Satellite Cloud Climatology Project (ISCCP) D1 data series, superposed epoch analyses were made of both increases and decreases in E_z. Field significance testing revealed that, both before and after extreme increases in E_z, significant absolute cloud cover changes (of 13–15%) occur in the tropics and high latitudes. While the linkages in the tropics may reflect changes in the main convective cloud generators of current flow in the global circuit, the linkages at high latitudes appear to represent responses of clouds to the current flow. This linkage offers a possible explanation of a possible solar–terrestrial climate amplification mechanism.  相似文献   

Hydrologic and geomorphic flow thresholds in the Lower Brazos River,Texas, USA     

Jonathan D. Phillips 《水文科学杂志》2013,58(9):1631-1648

Abstract

River science and management often require a design or reference discharge. The common (and sometimes unavoidable) use of such discharges may, however, obscure the fact that the magnitude and frequency of critical flows can differ due to various hydrological, geomorphological, and ecological criteria. Threshold stages and discharges were identified for six lower Brazos River, Texas gaging stations corresponding to thalweg connectivity, bed inundation, high sub-banktop flows, channel–floodplain connectivity (CFC), and overbank flooding. Critical flows were also identified for estimated thresholds for sandy bedform and medium gravel mobility, critical specific stream power for potential channel modifications, and cohesive-bank channel erosion. These thresholds have variable relationships to mean, median, and maximum flows. For four of the six stations, daily recurrence probabilities for all but flood flows are at least 1%, and as high as 11%. All stations achieve channel–floodplain connectivity at stages less than banktop. Estimated threshold flows for sediment mobility and channel erosion occur relatively frequently, with daily probabilities of 2–77%. Critical flows for bank erosion occur least often, and for sandy bedform and gravel mobility most often. Thalweg connectivity is always maintained at all sites, while bed inundation flows have a daily probability of about 80% or more. Overall, results suggest that no single flow level is dominant in hydrological or geomorphic dynamics, and that the frequency of a given threshold varies considerably even along a single river. The results support the idea that multiple flow levels and ranges are necessary to create and maintain the hydrological, geomorphological, and ecological characteristics of rivers, and that no single flow level is a reliable determinant of fluvial state.