The effects of variability in bank material properties on riverbank stability: Goodwin Creek, Mississippi

Bank retreat is an important area of research within fluvial geomorphology and is a land management problem of global significance. The Yazoo River Basin in Mississippi is one example of a system which is experiencing excessive erosion and bank instability. The properties of bank materials are important in controlling the stability of stream banks and past studies have found that these properties are often variable spatially. Through an investigation of bank material properties on a stretch of Goodwin Creek in the Yazoo Basin, Mississippi, this study focuses on: i) how and why effective bank material properties vary through different scales; ii) how this variation impacts on the outputs from a bank stability model; and iii) how best to appropriately represent this variability within a bank stability model.The study demonstrates the importance that the variability of effective bank material properties has on bank stability: at both the micro-scale within a site, and at the meso-scale between sites in a reach. This variability was shown to have important implications for the usage of the Bank Stability and Toe Erosion Model (BSTEM), a deterministic bank stability model that currently uses a single value to describe each bank material property. As a result, a probabilistic representation of effective bank material strength parameters is recommended as a potential solution for any bank stability model that wishes to account for the important influence of the inherent variability of soil properties.     

AVIAN NESTING HABITS AND VEGETATION STRUCTURE

Discriminant analysis is used to identify relationships between vegetation structure and nesting behavior in 78 breeding bird communities in North America. Results indicated that bird communities associated with structurally similar vegetation exhibited similar arrays of nesting habits despite their geographical separation. Tree-cavity, intermediate-cup, and high-cup nesters dominated forests and woodlands; ground-cup and ground-scrape nesters dominated grasslands; and low-cup nesters dominated desert, scrub, and some woodland sites. These functional relationships are distinct from previously reported continental scale relationships between avian feeding habits and gross vegetation structure. Links between nesting behavior and vegetation structure provide us, as geographers, with a means for placing birds into a physical landscape perspective.     

The nature of the sunspot phenomenon

Heat transport in the Sun is describable by a Fokker-Planck, or diffusion, transfer equation. A study of the general character of the solutions of the transfer equation shows that the inhibition of convective transport beneath the photosphere produces a photospheric dark ring surrounded by a bright ring, or at best, a dark area surrounded by a bright ring. The mean temperature beneath the sunspot is unavoidably above normal, so that the enhanced gas pressure would disperse, rather than concentrate, the magnetic field. Hence we conclude that the inhibition of convection cannot be the cause of a sunspot.We suggest, instead, that a sunspot is a region of enhanced, rather than inhibited, energy transport and emissivity. The magnetic field of the sunspot causes a dynamical overstability in the outer thousand km of the convective zone, generating copious fluxes of hydromagnetic waves, which propagate rapidly out of the region along the magnetic field. We suggest that this heat engine is so efficient as to convert at least three fourths of the heat flux into waves. Solutions of the heat transport equation in the presence of a heat sink automatically resemble the observed sunspot, including a dark interior, a sharp transition at the edge of the umbra, and an extended grey area around the outside, the penumbra. The mean temperature is reduced, causing the observed concentration of the magnetic field.The enhanced radiation is in the form of hydromagnetic waves, which do not appear in ordinary photographs, but which light up the sky over the sunspot in a manner conspicuous in any UV or X-ray picture. In this respect, then, a sunspot is effectively a hole in the Sun, extending down to temperatures of 2 × 10⁴ K or more.This work was supported in part by the National Aeronautics and Space Administration under Grant NGL 14-001-001.     

BIOGEOGRAPHY: RESEARCH OPPORTUNITIES FOR GEOGRAPHERS

The authors of this paper suggest several themes in biogeography worthy of increased research attention by geographers: human impacts on biota, disturbance ecology, and certain aspects of biota-environment interactions. Researchers must gain a firmer command of biological as well as geographical concepts and approaches if biogeography is to join geomorphology and climatology in developing a true geography of environments.     

The existence of a region inaccessible to magneto-telluric sounding

Summary. The exponential attenuation of fluctuating electromagnetic fields suggests that practical magneto telluric measurements may be uninformative about the electrical conductivity at sufficiently great depths. This notion can be made precise for one-dimensional systems. Below a critical depth the conductivity function may be chosen freely without affecting the consistency of the model with the data. This depth is readily computable with quadratic or linear programming techniques and does not rely upon linearization of the equations.