Similarity of rivers and their systems

Similarity criteria of rivers and river systems are studied. Zonal and azonal indicators of their similarity are established. The role of river system patterns in the changes of hydrographic and hydrologic characteristic of rivers in the passage from river heads to mouths is considered. The efficiency of applying fractal and indication analyses to river systems is examined. The extent and the scale effect of changes in various characteristics of the flow-channel systems are shown to depend on changes in river orders. The fractal dimension and the scale effects of their changes are correlated. A universal relationship between river orders, on the one hand, and river drainage area and river network density, on the other hand, was established. The root-mean-square error of the obtained formula, evaluated based on data on 274 watersheds, was 0.27, enabling it to be used in hydrological calculations under various landscape conditions.     

Cascade processes in the surface layers of pulsars

The influence of the Landau-Pomeranchuk effect on the development of a shower generated by ultrarelativistic particles bombarding the surface of a pulsar is discussed. Because of this effect, the path length of the shower increases while low-energy photon generation is strongly suppressed. In view of this, the mechanism of pair production suggested by Cheng, Ruderman, and Jones for the pulsar magnetosphere, may be essential only for pulsars whose magnetic field intensity at the surface lies in a relatively narrow range of aroundB 10¹² G.     

Reliability analysis of drilled shaft behavior using finite difference method and <Emphasis Type="Italic">Monte Carlo</Emphasis> simulation

Load displacement analysis of drilled shafts can be accomplished by utilizing the “t-z” method, which models soil resistance along the length and tip of the drilled shaft as a series of springs. For non-linear soil springs, the governing differential equation that describes the soil-structure interaction may be discretized into a set of algebraic equations based upon finite difference methods. This system of algebraic equations may be solved to determine the load–displacement behavior of the drilled shaft when subjected to compression or pullout. By combining the finite difference method with Monte Carlo simulation techniques, a probabilistic load–displacement analysis can be conducted. The probabilistic analysis is advantageous compared to standard factor of safety design because uncertainties with the shaft–soil interface and tip properties can be independently quantified. This paper presents a reliability analysis of drilled shaft behavior by combining the finite difference technique for analyzing non-linear load–displacement behavior with Monte Carlo simulation method. As a result we develop probabilistic relationships for drilled shaft design for both total stress (undrained) and effective stress (drained) parameters. The results are presented in the form of factor of safety or resistance factors suitable for serviceability design of drilled shafts.     

Using a Thermal Proxy to Examine Sediment–Water Exchange in Mid-Continental Shelf Sandy Sediments

Fluid exchange across the sediment–water interface in a sandy open continental shelf setting was studied using heat as a tracer. Summertime tidal oscillation of cross-shelf thermal fronts on the South Atlantic Bight provided a sufficient signal at the sediment–water interface to trace the advective and conductive transport of heat into and out of the seabed, indicating rapid flushing of ocean water through the upper 10–40 cm of the sandy seafloor. A newly developed transport model was applied to the in situ temperature data set to estimate the extent to which heat was transported by advection rather than conduction. Heat transported by shallow 3-D porewater flow processes was accounted for in the model by using a dispersion term, the depth and intensity of which reflected the depth and intensity of shallow flushing. Similar to the results of past studies in shallower and more energetic nearshore settings, transport of heat was greater when higher near-bed velocities and shear stresses occurred over a rippled bed. However, boundary layer processes by themselves were insufficient to promote non-conductive heat transport. Advective heat transport only occurred when both larger boundary layer stresses and thermal instabilities within the porespace were present. The latter process is dependent on shelf-scale heating and cooling of bottom water associated with upwelling events that are not coupled to local-scale boundary layer processes.     

Geometrical spreading factor of head waves in cased and open boreholes

Factors (coefficients) of geometrical spreading of compressional and shear head waves are calculated for an impulse multipole source of elastic oscillations in boreholes. It is shown that the length of the logging tool (i.e., the distance between the source and the nearest receiver) used for sonic measurements and the velocities of elastic waves in the medium both contribute to the factor of geometrical spreading. For a high-velocity formation (the shear wave velocity in the rock is higher than the compressional wave velocity in the fluid that fills the borehole) and a sufficiently long sonic tool with a monopole source, the coefficient of geometrical spreading is approximated by asymptotic formula 1/Z [Roever et al., 1974; Krauklis and Krauklis, 1976], where Z is the length of the tool; i.e., the amplitude of the compressional head wave decreases proportionally to the distance between the source and the receiver. In acoustically soft formations, this approximation is inapplicable even for long tools with length Z > 4 m. Waveforms in cased boreholes have a significant frequency dispersion even in case of good-quality cementing, and the factor of geometrical spreading there depends considerably on the length of the tool and the elastic properties of the rocks.     

Radio-induced activity in galaxy pairs

The close proximity of a radio-loud active galactic nucleus (AGN) can trigger star formation in nearby objects containing gas if they are hit by a radio jet emerging from the active nucleus (as is observed, for example, in the case of Minkowski’s Object). The predicted frequency of such events is modest for close pairs of galaxies—of the order of several percent of all close (with separations of the order of several tens of parsec) pairs containing a radio AGN. A statistical study of this effect is carried out using the SDSS and FIRST surveys, by searching for spatially close pairs (projected separations <150 kpc, relative radial velocities <600 km/s) containing AGNs with radio jets. The frequency of galaxies with bursts of star formation, f _SF, and active nuclei f _AGN, in pairs either containing or not containing an AGN with radio jets are evaluated as functions of the separation of the galaxies in the pair. It is concluded that (1) the predicted effect should be of the order of 5%, falling off with increasing separation between the galaxies in the pair; (2) the observed values of f _SF and f _AGN and their dependences on the galaxy separation are consistent withmodel predictions, but the large uncertainties associated with the limited size of the studied sample hinders firm conclusions about the existence of radio-induced activity in close galaxy pairs; (3) further investigations using a larger volume of observational material are required, for example, using only photometric redshifts.     

Dynamical evolution properties of debris flows controlled by different mesh-sized flexible net barriers

Because the flexible net barrier is a gradually developed open-type debris-flow counter-measure, there are still uncertainties in its design criterion. By using several small-scale experimental flume model tests, the dynamical evolution properties of debris flows controlled by large and small mesh-sized (equal to D₉₀ and D₅₀, respectively) flexible net barriers are studied, including the debris flow behaviors, segregation, and permeability of sediments, as well as the energy absorption rates and potential overtopping occurring when debris flows impact the small mesh-sized one. Experimental results reveal that (a) two sediment deposition patterns are observed depending on variations in debris flow textures and mesh sizes; (b) the aggregation against flexible net barriers is dominated by flow dynamics; (c) the segregation and permeable functions of the barrier are determined by the mesh size, concentration, and flow dynamics; and (d) the smaller mesh-sized flexible net barrier tends to be more efficient in restraining more turbulent debris flows and can absorb greater rate of kinematic energy, and finally, the great kinematic energy dissipation that occurs when secondary debris flows interact with the post-deposits in front of the small mesh-sized flexible net barrier is believed to cause the failure of overtopping phenomenon. The mesh size is concluded to be the decisive parameter that should be associated with debris flow textures to design the control functions of flexible net barriers.     

Thin accretion disc with a corona in a central magnetic field

We study the steady-state structure of an accretion disc with a corona surrounding a central, rotating, magnetized star. We assume that the magneto-rotational instability is the dominant mechanism of angular momentum transport inside the disc and is responsible for producing magnetic tubes above the disc. In our model, a fraction of the dissipated energy inside the disc is transported to the corona via these magnetic tubes. This energy exchange from the disc to the corona which depends on the disc physical properties is modified because of the magnetic interaction between the stellar magnetic field and the accretion disc. According to our fully analytical solutions for such a system, the existence of a corona not only increases the surface density but reduces the temperature of the accretion disc. Also, the presence of a corona enhances the ratio of gas pressure to the total pressure. Our solutions show that when the strength of the magnetic field of the central neutron star is large or the star is rotating fast enough, profiles of the physical variables of the disc significantly modify due to the existence of a corona.