Morphology of riffle–pool sequences in the River Severn,England

Despite the occurrence of riffle–pool sequences in many rivers there are few data concerning riffle–pool unit morphology. Of many criteria proposed to identify riffle–pool units, only three methods can be regarded as objective and robust. These are the ‘zero‐crossing’, the ‘spectral analysis’ and the ‘control‐point’ methods. In this paper statistics are developed using the first two of these methods to describe the streamwise morphology of 275 riffles and 285 pools which form a continuous 32·1 km reach of the bed of the River Severn in Shropshire, England. Yalin's theoretical relationship between the average riffle:pool unit length ( λ _p) and channel width ( W ), λ _p = 3 W , applies to the River Severn. Reach‐average riffle height ( H ) is a constant proportion of bankfull depth ( h ); typically H ≅ 0·16 h . Riffle height is a positive function of riffle length. Pool depth is a positive function of pool length. However, both riffle length and pool length increase more rapidly than the bed‐level amplitude, such that long riffles or pools are relatively ‘flat’. As channel gradient reduces, bedforms flatten and become more asymmetric as riffle stoss sides and the proximal slope of pools lengthen at the expense of riffle lee sides and pool distal slopes. The statistical relationships between riffle steepness (H/L) and water depth are similar to those for equilibrium subaqueous dunes. The Severn data are consistent with Yalin's theoretical analysis relating riffle bedform length (L _r) to water depth, i.e. L _r = α2π h, wherein α ≅ 1 for steep near‐equilibrium bedforms but α ≅ 2 to 3 as the relative depth decreases and riffles become long, low features. Theoretical consideration and turbulence data indicate that the frequency of coherent turbulent‐flow structures associated with the riffle–pool mixing length in the Severn should be of the order of 50 to 100 s. The morphological similarity of the steepest River Severn riffles with dunes raises intriguing questions with respect to self‐similar, convergent organization of periodic alluvial bedforms and to bedform dynamic classification particularly. Copyright © 2000 John Wiley & Sons, Ltd.     

一种新型双磙子FPS隔振系统的动力学模型

本文基于多体动力学理论建立了一种新型双磙子FPS(Friction Pendu lum System)隔振系统动力学模型。在对该系统进行运动学分析的基础上,可将它归结为一个单自由度系统。应用非惯性系中质点系相对动能定理的微分形式可推导出该系统的运动微分方程,它是一个非线性方程。在小角位移的情形下,可近似得到相应的无阻尼线性系统的固有频率,它与观测结果吻合。这一双磙子FPS隔振系统的动力学方程的建立有助于对该系统动力学行为的研究,对方程的直接求解可提高仿真设计中正问题计算的效率,为设计包含该系统的半主动控制奠定了基础。     

Evaluating Natural Attentuation with Spreadsheet Analytical Fate and Transport Models

Superposition techniques can extend the capabilities of relatively simple analytical fate and transport models. Complex source geometries, simple aquifer boundaries, and electron acceptor limited biodegradation can be simulated by using superposition techniques in computer spreadsheets. Spreadsheet models are an easily used tool for interpreting sampling results and for estimating attenuation and degradation rates in relatively homogeneous aquifers. Analytical spreadsheet models are based on the Domenico analytical model and can provide results that are in close agreement with the numerical model UIOPLUMH II.     

Problem of the Relationship between Phosphorites and Evaporites (Critical Comments to the Work by A.S. Sokolov “Paragenetic Interrelations between Phosphoritic and Saliferous Basins,” Lithology and Mineral Resources, 1999, no. 2)

Lithology and Mineral Resources -     

The role of archaeological and cultural-historical records in long-range coastal fisheries resources management strategies and policies in the Pacific Islands

The usefulness of conventional fisheries science for long-term fisheries management and policies in the resource-poor islands of the Pacific is very limited. Fishery managers can, however, make use of such alternative sources of information as archaeological and historical data to develop fishery management initiatives. Most Pacific Islands were settled by humans over the last three millennia, although human settlement in Western Melanesia dates from the late Pleistocene, a period of about 40 millennia. Archaeological studies over the past half-century contain information on the long-term subsistence exploitation of fish and invertebrates from nearshore coral reefs and lagoons. Molluscs appear to have been a very important food source for early human colonists in the Pacific Islands, but declines in abundance through harvest pressure and environmental changes forced a greater reliance on fin-fish capture and on agriculture. There is no firm evidence from the archaeological record to suggest that long-term subsistence exploitation of reef fish populations has had any serious negative effects on abundance or structure of reef fish communities. Sources for more recent historical information are also exemplified and briefly examined for their usefulness in assessing the impacts of commercial fishing. It is argued that fishery managers who adopt a data-less approach to fishery management could profitably use these sources of information to enrich their assessment of the impacts of various management scenarios.     

Distribution and spatial variation of hydrothermal faunal assemblages at Lucky Strike (Mid-Atlantic Ridge) revealed by high-resolution video image analysis

Whilst the fauna inhabiting hydrothermal vent structures in the Atlantic Ocean is reasonably well known, less is understood about the spatial distributions of the fauna in relation to abiotic and biotic factors. In this study, a major active hydrothermal edifice (Eiffel Tower, at 1690 m depth) on the Lucky Strike vent field (Mid-Atlantic Ridge (MAR)) was investigated. Video transects were carried out by ROV Victor 6000 and complete image coverage was acquired. Four distinct assemblages, ranging from dense larger-sized Bathymodiolus mussel beds to smaller-sized mussel clumps and alvinocaridid shrimps, and two types of substrata were defined based on high definition photographs and video imagery. To evaluate spatial variation, faunal distribution was mapped in three dimensions. A high degree of patchiness characterizes this 11 m high sulfide structure. The differences observed in assemblage and substratum distribution were related to habitat characteristics (fluid exits, depth and structure orientation). Gradients in community structure were observed, which coincided with an increasing distance from the fluid exits. A biological zonation model for the Eiffel Tower edifice was created in which faunal composition and distribution can be visually explained by the presence/absence of fluid exits.