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1.
The alteration of magnetic minerals taking place during the investigation of the temperature variation of bulk magnetic susceptibility is obvious from different courses of heating and cooling susceptibility vs. temperature curves. A set of indices is introduced to characterize these changes numerically. The A 40 alteration index characterizes the change in susceptibility after executing the whole cycle of heating and cooling. The maximum difference between the heating and cooling curves is characterized by the A max alteration index. The mean or average difference between the heating and cooling curves is characterized by the A m alteration index. The situation whether the heating and cooling curves cross, is characterized by the A cr alteration index. The technique of progressive repeated heating is proposed, together with the above indices, to locate the temperature intervals with weak and strong magnetic mineral changes induced by heating.  相似文献   

2.
Fluvial seed dispersal considers both the transport and deposition of seeds where channel geomorphic structures, hydrology and seed dispersal traits contribute to transport times and depositional locations. This study examines the influence of stream flow patterns on fluvial seed dispersal of buoyant white alder (Alnus rhombifolia) seeds by applying a one‐dimensional transport model. Conceptually, the model separates the stream into two components: (i) the main channel where the seeds are transported downstream; and (ii) the transient storage zone where seeds are temporarily detained or deposited on the river bank. Transport processes are characterized by an advection–dispersion equation which is coupled to a transient storage model using an exponential decay term. The model parameters: longitudinal dispersion (DL), exchange coefficient (α), main channel area (A) and storage zone (As) are estimated based on field experiments conducted in a confined, bedrock‐gravel bed river with pool‐riffle morphology located in coastal northern California. The riparian zone is inhabited by Alnus rhombifolia that disperse buoyant seeds in mid‐spring coinciding with the end of the wet, Mediterranean season. Artificial seeds, with similar traits of buoyancy and density to alder seeds, were used to quantify transport times and depositional locations. Preferential deposition resulted in stream reaches with larger As, high As/A ratios, and faster exchange coefficients corresponding to divergent stream flow (back‐eddies, re‐circulating flow, flow expansions) caused by geomorphic structures such as the ends of bar/riffle features and bends in the stream. The results demonstrate the importance of transient storage for seed transport and depositional processes. Morphological features that increase a channel's complexity create complex flow structures that detain seeds and provide a greater opportunity for deposition to occur. The model provides a simplification of river hydraulics to represent dispersal dynamics and lends itself to further understanding of hydrochory processes and associated population structure. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

3.
The sediment delivery ratio was estimated for two periods (28 years and eight years) following reforestation of seven tributary catchments (0·33 to 0·49 km2) in the headwaters of the Waipaoa River basin, North Island, New Zealand. In these catchments, gully erosion, which largely resulted from clearance of the natural forest between 1880 and 1920, is the main source of sediment to streams. Reforestation commenced in the early 1960s in an attempt to stabilize hillslopes and reduce sediment supply. Efforts have been partially successful and channels are now degrading, though gully erosion continues to supply sediment at accelerated rates in parts of the catchment. Data from the area indicate that the sediment delivery ratio (SDR) can be estimated as a function of two variables, ψ (the product of catchment area and channel slope) and A g (the temporally averaged gully area for the period). Sediment input from gullies was determined from a well defined relationship between sediment yield and gully area. Sediment scoured from channels was estimated from dated terrace remnants and the current channel bed. Terrace remnants represent aggradation during major floods. This technique provides estimates of SDR averaged over periods between large magnitude terrace‐forming events and with the present channel bed. The technique averages out short‐term variability in sediment flux. Comparison of gully area and sediment transport between two periods (1960–1988 and 1988–1996) indicates that the annual rate of sediment yield from gullies for the later period has decreased by 77 per cent, sediment scouring in channels has increased by 124 per cent, and sediment delivered from catchments has decreased by 78 per cent. However, average SDR for the tributaries was found to be not significantly different between these periods. This may reflect the small number of catchments examined. It is also due to the fact that the volume of sediment scoured from channels was very small relative to that produced by gullies. According to the equation for SDR determined for the Waipaoa headwaters, SDR increases with increasing catchment area in the case where A g and channel slope are fixed. This is because the amount of sediment produced from a channel by scouring increases with increasing catchment area. However, this relationship does not hold for the main stem of the study catchments, because sediment delivered from its tributaries still continues to accumulate in the channel. Higher order channels are, in effect, at a different stage in the aggradation/degradation cycle and it will take some time until a main channel reflects the effects of reforestation and its bed adjusts to net degradation. Results demonstrate significant differences among even low order catchments, and such differences will need to be taken into consideration when using SDR to estimate sediment yields. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

4.
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5.
Due to the temporal decoupling of water and sediment sources in a large river basin, a flood from a sediment source area with high suspended sediment concentration (SSC) may be diluted by flow from a major runoff source area with low SSC. In this paper, this dilution effect is considered for 145 flood events from the Yellow River, China. Two indices (β1 and β2) describing the dilution effect are proposed, based on water and sediment from the clear water source area and the coarse sediment producing area. Regression equations between channel sedimentation (Sdep) and β1 and β2 are established based on flood events and annual data, respectively. The results show that dilution reduces channel sedimentation in the lower reaches by 34?1% and that this is related to a reduced frequency of hyperconcentrated flows in the lower reaches. The Longyangxia Reservoir for hydro‐electric generation has stored huge quantities of clear runoff from the upper Yellow River during high‐flow season since 1985, greatly reducing the dilution of the hyperconcentrated floods and therefore enhancing sedimentation in the lower reaches. For the purpose of reducing sedimentation, changing the operational mode of the Longyangxia Reservoir to restore the dilution effect is suggested. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

6.
The aim of this paper is to illustrate the effects of spatial organization of lake chains and associated storage thresholds upon lake-overflow behaviour, and specifically their impact upon large scale flow connectivity and the flood frequency of lake overflows. The analysis was carried out with the use of a multiple bucket model of the lake chain system, consisting of a network of both lakes and associated catchment areas, which explicitly incorporated within it three storage thresholds: a catchment field capacity threshold that governs catchment subsurface stormflow, a total storage capacity threshold that governs catchment surface runoff, and a lake storage capacity threshold that determines lake overflow. The model is driven by rainfall inputs generated by a stochastic rainfall model that is able to capture rainfall variability at a wide range of time scales. The study is used to gain insights into the process controls of lake-overflow generation, and in particular, to explore the crucial role of factors relating to lake organization, such as the average catchment area to lake area (AC/AL) ratio and the distribution of AC/AL with distance in the downstream direction (increasing or decreasing). The study showed that the average AC/AL value was the most important factor determining the frequency of occurrence and magnitude of floods from a landscape consisting of lake chains. The larger the average AC/AL value the more runoff is generated from catchments thus increasing both the occurrence and magnitude of lake overflows. In this case the flood frequency curve reflects that of the catchment area, and lake organization does not play an important role. When AC/AL is small the landscape is lake dominated, the spatial organization of lakes has a significant impact on lake connectivity, and consequently on flood frequency. One of the aspects of lake organization that may have a significant influence on lake connectivity is the spatial distribution of AC/AL from upstream to downstream (increasing or decreasing). In a landscape in which AC/AL increases downstream, lake overflow will occur more frequently relative to a similar landscape (i.e. identical AC/AL) with a constant value of AC/AL. When AC/AL decreases downstream, however, runoff inputs from the upstream parts will trigger lake overflow in the downstream parts, and consequently, full connectivity may be achieved leading to increased flood frequencies.  相似文献   

7.
Analytical solutions that use diurnal temperature signals to estimate vertical fluxes between groundwater and surface water based on either amplitude ratios (Ar) or phase shifts (Δ?) produce results that rarely agree. Analytical solutions that simultaneously utilize Ar and Δ? within a single solution have more recently been derived, decreasing uncertainty in flux estimates in some applications. Benefits of combined (ArΔ?) methods also include that thermal diffusivity and sensor spacing can be calculated. However, poor identification of either Ar or Δ? from raw temperature signals can lead to erratic parameter estimates from ArΔ? methods. An add‐on program for VFLUX 2 is presented to address this issue. Using thermal diffusivity selected from an ArΔ? method during a reliable time period, fluxes are recalculated using an Ar method. This approach maximizes the benefits of the Ar and ArΔ? methods. Additionally, sensor spacing calculations can be used to identify periods with unreliable flux estimates, or to assess streambed scour. Using synthetic and field examples, the use of these solutions in series was particularly useful for gaining conditions where fluxes exceeded 1 m/d.  相似文献   

8.
Various physical and biological properties affect solute transport patterns in streams. We measured hydraulic characteristics of Payne Creek, a low‐gradient upper Coastal Plain stream, using tracer experiments and parameter estimation with OTIS‐P (one‐dimensional transport with inflow and storage with parameter optimization). The primary objective of this study was to estimate the effects of varying discharge, season, and litter accumulation on hydraulic parameters. Channel area A ranged from 0·081 to 0·371 m2 and transient storage area As ranged from 0·027 to 0·111 m2. Dispersion D ranged from 1·5 to 11·1 m2 min−1 and exchange coefficient α ranged from 0·009 to 0·038 min−1. Channel area and dispersion were positively correlated to discharge Q, whereas storage area and exchange coefficient were not. Relative storage size As/A ranged from 0·17 to 0·59, and was higher during fall than other seasons under a similar Q. The fraction of median travel time due to transient storage ranged from 8·8 to 34·5% and was significantly correlated with Q through a negative power function. Both metrics indicated that transient storage was a significant component affecting solute transport in Payne Creek, especially during the fall. Comparison between the measured channel area Ac and A suggested that surface storage was dominant in Payne Creek. During fall, accumulation of leaf litter resulted in larger A and As and lower velocity and D than during other seasons with similar discharge. Seasonal changes in discharge and organic matter accumulation, and dynamic channel morphology affected the magnitude of transient storage and overall hydraulic characteristics of Payne Creek. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

9.
We discuss the form-drag instability for a quasi-geostrophic channel flow. We first study the characteristics of this instability in a barotropic flow, considering in detail the influence of the meridional scale and discussing which structure of the perturbation zonal flow must be chosen in order to describe properly this instability.We then consider a continuous quasi-geostrophic channel model in which the topography enters only through the bottom boundary condition, and we discuss how in this case the effects of the form-drag are felt by the mean zonal flow through the ageostrophic mean meridional circulation. Because the meridional structure of the perturbation zonal flow cannot simply be extended from the barotropic to the continuous case, we show how to modify it properly.We then study the baroclinic model in the particular case of constant (in the vertical) basic-state zonal flow and show how this case closely resembles the barotropic, demonstrating the barotropic nature of the form-drag instability.Symbols t is the partial derivative with respect tot. - x is the partial derivative with respect tox. - y is the partial derivative with respect toy. - represents the geostrophic stream function. - u is the eastward component of the geostrophic wind. - v is the northward component of the geostrophic wind. - u a is the eastward component of the ageostrophic wind. - v a is the northward component of the ageostrophic wind. - w is the vertical component of the wind. - f is the Coriolis parameter=2 sin f o+y. - f o is the Coriolis parameter evaluated at mid-latitude. - N is the Brunt-Vaisala frequency. - [A] is the zonal (x) average ofA at constantp andy. - <A> is the horizontal (x andy) average ofA at constantp  相似文献   

10.
A series of Hokkaido events, recorded by the FBV Broadband Seismograph System at the KHC Seismic Station, is used to study the structure of the earthquake magnitude scale on the basis of maximum velocity amplitudesA vmax of teleseismicPwaves in different period bands. Amplitude-periodband (APB) diagrams are constructed for each event. According to the shape of the APB diagrams the events investigated can be divided into three types: (a) events with largestA vmax values in the intermediate period range (periods ofA vmax from 2.2 to 23 sec), (b) events with largestA vmax values in the short-period range (periods ofA vmax from 1 to 2 sec), (c) events exhibiting anomalous APB diagrams. Type (a) events seem to represent the process of wave generation that prevails for shallow earthquakes. Type (b) events approach to explosive-like generation of seismic waves. The nature of the exceptionally occurring type (c) events must be clarified in further investigations. The influence of the type of earthquake on the magnitude values estimated on the basis of standard class A and B (short-period and intermediate-band) seismograms is demonstrated. It appears that for estimating correct values of earthquake magnitudes complementary information on the process of seismic wave generation in the focus is necessary. At teleseismic distances this information can be obtained from either APB diagrams or amplitude spectra ofP waves recorded, e.g., by broadband velocity sensing instruments.  相似文献   

11.
The structure and dynamics of vegetation in valley bottoms are both strongly associated with fluvial processes and landform dynamics. All of these associations are disrupted by the installation of engineering control works. We use survey and analysis methods developed previously to investigate the impact of the installation of check‐dams within the confined headwaters of steep seasonally‐flowing streams (fiumaras) in Calabria, southern Italy, on active channel form, sediment calibre, and the richness, cover and development of riparian vegetation. Based on detailed field measurements along transects across the active channel, estimates of indices of vegetation extent (GCC), development (WCH) and their cross‐sectional variability (coefficients of variation of both indices at each survey site CVGCC, CVWCH), the number of species present (Ns), channel shape (w/d – the width/depth ratio), cross‐sectional area (CSA), downstream gradient (slope), surface bed sediment calibre (D50) and subsurface fine sediment content (percentage less than 250 µm by weight) were obtained for 60 transects located immediately upstream (U), downstream (D) and at intermediate sites (I) around 20 check‐dams located in four different headwater catchments. Analysis of this data set suggests that statistically significant changes in channel form and sediment calibre upstream of check‐dams are associated with more consistent vegetation development across the active channel, including an increase in species richness relative to other transects, but notable increases in vegetation cover and development only arise where the physical characteristics of the channel are notably different from intermediate and downstream channels. Because of the naturally steep profile of the study torrents, intermediate sections between check‐dams tend to be more similar in form to channels located immediately downstream of check‐dams than those located upstream, leading to similar structural properties in the riparian vegetation. The intermediate transects support considerably more species than downstream reaches, but the conditions upstream of the check‐dams appear to be so favourable for riparian vegetation development that species richness exceeds that found in intermediate reaches. Despite the confined headwater locations, these contrasts in form, sediment and vegetation development around check‐dams are strong and consistent across the study catchments, over‐riding more subtle contrasts in species richness and sediment calibre between catchments. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

12.
The thermal diffusivity is the key parameter that controls near‐surface temperature where periodic temperature variation is progressively attenuated and delayed with depth. This article presents the results of apparent thermal diffusivity using temperatures recorded by a bedrock temperature measurement network in the fault zones of western Sichuan. High sensitivity temperature sensors (10?4 K) were installed at a maximum depth reaching 30 m. The apparent thermal diffusivities were deduced from both amplitude damping and phase shifting of annual temperature variations between two different depths. Under pure conduction, the thermal diffusivity determined through the phase method (αΦ) should be equivalent to that determined through the amplitude method (αA), whereas effects of the upward (downward) water flow are evidently reflected in the amplitude decay to make αΦ larger (lesser) than αA. The discrepancy between αΦ and αA can thus be a tracer of water movement or convective heat transfer. The calculated αΦ of the measurement stations varies from 1.22 × 10?6 to 3.00 × 10?6 m2/s, and the estimated αA ranges from 0.93 × 10?6 to 2.41 × 10?6 m2/s. Two regimes of heat transfer underground were suggested from the results. Conductive heat transport prevails over the nonconductive processes at five stations, which is characterized by αΦ coincident with αA for the same depth pair. On the contrary, the values of αΦ differ from αA at six stations in the intersection area of the Y‐shaped fault system, implying that convective heat transfer also plays a comparably important role. This finding is consistent with the hot springs distribution of the area. The results also indicate that water moves upward with an average Darcy velocity of approximately ?1 × 10?7 m/s in this region. Our research provides new evidence for the hydrothermal activity in the fault zones at the eastern margin of the Tibetan Plateau.  相似文献   

13.
Various models that describe the dependence of pulsation amplitude A on the solar wind velocity V are discussed. Phenomenological considerations and experimental data count in favour of a model of the form A/A0=(V/V0) exp (-V0/V). The linear model A=a+bV which is common in the literature gives negative values of amplitude as V->0, which is strange from the physical point of view. The possibility of modifying the linear model by including critical phenomena is discussed.  相似文献   

14.
Quantification of the transient storage zone (As) has become critical in stream biogeochemical studies addressed to examine factors controlling nutrient uptake. It is expected that higher As may enhance the interaction between nutrients and biota and thus, increase nutrient uptake. However, results from the literature are controversial. We hypothesized that besides of the size of As, the intrinsic physical and biological characteristics of stream structures that generate As are also relevant for nutrient uptake. We performed 24 additions of phosphate, ammonium, and chloride in four reaches of a man-made channel where we introduced three types of naturally colonized substrata packs (mud, sand and cobbles) to modify As. We estimated ammonium and phosphate uptake coefficients in both the main channel and As using a solute transport model (OTIS-P) and compared the results among reaches with different substrata types. The introduction of substrata packs decreased water velocity and increased As similarly among treatments. Nutrient uptake coefficients in the main channel were similar among reaches with different type substrata packs; however, nutrient uptake coefficients measured in As differed among them as well as the ratio between ammonium and phosphorus uptake coefficients in As, which were 1.6 in reaches with mud packs and 0.02 in reaches with sand or cobble packs. Results obtained in this study suggest that the contribution of As in nutrient uptake not only depends on the size of As but on the type of materials used to increase As, and thus, have biogeochemical implications on restoration projects aimed to modify channel morphology.  相似文献   

15.
Only comparatively few experimental studies have been carried out to investigate the performance of the HEC-6 river morphological model. The model was developed by the Hydrologic Engineering Center of the US Army Corps of Engineers. In this study, experiments were carried out in a 20 m long concrete flume 0.6 m wide with varying rectangular cross-sections. The channel bed is paved with uniform sand of D50 = 0.9 mm and D90 = 1.2 mm within the test reach of 12 m. Two types of experiments were carried out with sediment transport, one under steady uniform flow and another under steady non-uniform flow conditions. Nine steady uniform flow experiments were carried out to compare the measured equilibrium relationship of flow and sediment transport rate with two bedload formulae, namely, Du Boys and Meyer–Peter and Muller, and with three total load formulae, namely, Toffaleti, Laursen and Yang. It was found that even though the sediment transport consists of a certain portion of bedload, the total load formulae give satisfactory results and better agreement than the two bedload formulae. Five steady non-uniform flow experiments were carried out under various conditions of varying bed profile and channel width and also with sediment addition and withdrawal. The measured transient water surface and bed profiles are compared with the computed results from the HEC-6 model. It was found that the Toffaleti and Yang total load formulae used in the HEC-6 model give the most satisfactory prediction of actual bed profiles under various conditions of non-uniform flow and sediment transport. The effects of Manning's n, variations of sediment inflow, various sediment transport formulae, sediment grain size and the model numerical parameters, i.e. distance interval Δx and numerical weighting factor, on the computed water surface and bed profiles were determined. It was found that the selection of the sediment transport formulae has the most significant effect on the computed results. It can be concluded that the HEC-6 model can predict satisfactorily a long-term average pattern of local scour and deposition along a channel with either a small abrupt change in geometry or gradually varying cross-sections. However, the accuracy of the model prediction is reduced in the regions where highly non-uniform flow occurs.  相似文献   

16.
We consider the problem of determining and predicting how the wave speeds in particular directions for a transversely isotropic (TI) medium depend on particular combinations of the density-normalized moduli Aij. The expressions for the qP and qSV velocities are known to depend on four moduli. Normally, we can only determine three independent parameters from qP data, or two from qSZ data, as the others have much lower sensitivity. The resolvable parameters are conveniently described by axial and off-axis parameters: for qP rays, P0°= A11, P90°= A33 and P45°=(A11+ A33)/4 + (A13+2A55)/2; and for qSV rays, S0°= S90°=A55 and S 45°= (A11+ A 33)/4- A13/2. These parameters control the magnitude of the squared-velocities on the axes and at approximately 45°. For an arbitrary TI medium, if the medium is perturbed in a way that preserves a particular parameter, then slowness points in the associated direction and mode witl be approximately preserved in the new medium. we refer to these parameters as ‘push-pins’, i.e. if a parameter is fixed, the associated part of the slowness surface is pinned in place. Because, these five push-pins only contain four independent moduli, we can only fix at most three push-pins. Perturbing one of the other parameters inevitably perturbs the other. Numerical results illustrating the linkage between two push-pins, when three are fixed, are presented. So-called anomalous TI media occur when the roles of the qP and qSV waves are reversed: in some directions the faster ray has transverse polarization. That, in turn, requires anomalous velocities at the push-pins, i.e. S0° > P0°, S45° > P45° and/or S90° > P90° (equivalent to the usual anomalous conditions A11 < A55, < 0 and/or A33 < A55). In the Appendix, we confirm that anomalous sensitivities of the velocities at the five push-pins only occur in such media, although the push-pins still apply if interpreted appropriately. Truly anomalous sensitivities, in which push-pins play no role, only occur in media near the boundary between normal and anomalous.  相似文献   

17.
The Alaskar pipeline is a highly conducting anomaly extending 800 miles (1300 km) from about 62° to 69° geomagnetic latitude beneath the most active regions of the ionospheric electrojet current. The spectral behavior of the magnetic field from this current was analyzed using data from standard geomagnetic observatories to establish the predictable patterns of temporal and spatial changes for field pulsation periods between 5 min and 4 hr. Such behavior is presented in a series of tables, graphs and formulae. Using 2- and 3-layer models of the conducting earth, the induced electric fields associated with the geomagnetic changes were established. From the direct relationship of the current to the geomagnetic field variation patterns one can infer counterpart temporal and spatial characteristics of the pipeline current. The relationship of the field amplitudes to geomagnetic activity indices,A p, and the established occurrence of various levels ofA p over several solar cycles were employed to show that about half of the time the induced currents in the pipe would be under 1 A for the maximum response oscillatory periods near 1 hr. Such currents should be of minimal consequence in corrosion effects for even a section of the pipeline unprotected by sacrificial electrodes. Of greater interest was the result that the extreme surges of current should reach over one-hundred amperes in the pipeline during high activity.  相似文献   

18.
Summary Radioactive heat productionA is a scalar and isotropic petrophysical property independent of in situ temperature and pressure. Its value is usually expressed in HGU units (1 HGU=10–13 cal/cm3 sec) and depends on the amounts of uranium, thorium and potassium.A varies with rock type over several orders of magnitude and reflects the geochemical conditions during rock formation (magmatic differentiation, sedimentation or metamorphism).In order to assign realistic thermal parameters to deeper-seated rocks correlations with seismic velocity (which can be determined from the surface) have been looked for. In the range characteristic for crystalline rocks of the crust (5–8 km/sec)A is strongly correlated with density and compressional wave velocityv p:A decreases with increasingv p orp. From this relationship it is now possible to estimate heat production values for any particular layer of a crustal section from measured seismic velocities. Contrary to earlier belief there is, as shown by experimental determinations, no correlation between heat productionA and thermal conductivityK in igneous and metamorphic rocks. In sediments however, especially in sand/shale sequences, a correlation betweenK andA is most likely: increasing clay mineral content, characterized by increasingA, causes the decrease ofK in these rocks.Contribution No. 111, Institute of Geophysics, Swiss Federal Institute of Technology, Zurich, Switzerland.  相似文献   

19.
The relationship between the auroral electrojet indices (AE) and the ring current magnetic field (DR) was investigated by observations obtained during the magnetic storm on 1–3 April 1973. During the storm main phase the DR development is accompanied by a shift of the auroral electrojets toward the equator. As a result, the standard AE indices calculated on the basis of data from auroral observatories was substantially lower than the real values (AE’). To determine AE’ during the course of a storm main phase data from subauroral magnetic observatories should be used. It is shown that the intensity of the indices (AE’) which take into account the shift of the electrojets is increased substantially relative to the standard indices during the storm main phase. AE’ values are closely correlated with geoeffective solar wind parameters. A high correlation was obtained between AE’ and the energy flux into the ring current during the storm main phase. Analysis of magnetic field variations during intervals with intense southward IMF components demonstrates a decrease of the saturation effect of auroral electrojet currents if subauroral stations magnetic field variations are taken into account. This applies both to case studies and statistical data. The dynamics of the electrojets in connection with the development of the ring current and of magnetospheric substorms can be described by the presence (absence) of saturation for minimum (maximum) AE index values during a 1-h interval. The ring current magnetic field asymmetry (ASY) was calculated as the difference between the maximum and minimum field values along a parallel of latitude at low latitudes. The ASY value is closely correlated with geoeffective solar wind parameters and simultaneously is a more sensitive indicator of IMF Bz variations than the symmetric ring current. ASY increases (decreases) faster during the main phase (the recovery phase) than DR. The magnetic field decay at low latitudes in the recovery phase occurs faster in the afternoon sector than at dusk.  相似文献   

20.
A five-stage model of the development of cross-sectional asymmetry is proposed in which alternating bar deposition and bed scour are important elements. Quasi-periodic reversals in the sense of asymmetry and a progressive increase in the magnitude of asymmetry at sections which develop an asymmetry initially are major features of the model. Application of Bridge's (1977) model confirms several features of the model and suggests that the degree of asymmetry is directly related to bend curvature. After the initial stages, subsequent changes to the cross-sectional and planimetric form of the channel may be intensified by the developing asymmetry through a system of positive feedbacks in which the levels of form and flow distortion are interrelated. Asymmetry appears to provide an important link between cross-sectional and plan form adjustment. Spatial series of asymmetry obtained for three lengths along a mountain stream contain features predicted by the model. In particular, the dominant peak in spectral density functions occurs at a frequency with a corresponding wavelength close to 4πw which has significance in terms of the meander wavelength-width relationship (Richards, 1976a). This consistency is remarkable considering local differences. In addition, sections in curved reaches tend to be more asymmetric, especially when sited at pools. However, contrary to expectation, channel width is not significantly correlated with asymmetry.  相似文献   

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