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91.
Considerable progress has been made recently in characterising the patterns displayed by the anabranches of braided rivers. However, the physical processes of sediment scour, transfer and deposition that govern the generation and evolution of anabranch channels remain largely unexplained. Direct measurement of three-dimensional flow fields and morphological evolution of the anabranches in the braided Brahmaputra–Jamuna River, Bangladesh, were undertaken to investigate the interactions between fluvial processes and anabranch morphology. These data were used to elucidate the circumstances leading to the bifurcation of a single channel, which is a topic of fundamental importance to understanding the physical processes responsible for braiding. Results indicate that division of the velocity field into multiple threads within a single channel precedes a division in the cross-sectional morphology of the channel and appears to be a necessary prerequisite for development of a bifurcation. An empirical relationship was established to discriminate between channels with single and multi-thread velocity fields, based on the depth-to-width ratio and specific energy of the flow at a representative channel cross-section. This function requires further validation, but could be used to predict the conditions under which a single channel is likely to bifurcate to produce two anabranches. 相似文献
92.
93.
The study area is located in the Pampean region of Argentina, in the northwest of Buenos Aires Province, covering about 50,000 km2, within an area of great plain with particular hydrologic characteristics, where vertical water movements predominate over
horizontal. The objective of this work is the analysis of the relations between different hydrologic scenarios (characterized
by the soil water balance and the water table levels) and the yields of soybeans that have been increasing in recent years.
It is concluded that water table fluctuations are strongly related to the elements of the local hydrologic balance. On the
other hand, the geographic distribution of soybean yields corresponds directly with the values of the soil water reserve during
the critical stages of the growth. After dry periods, the lowering of the water table makes access to water from the root
zone difficult, and the crop is susceptible to the effects of drought. However, after wetter periods the extra contribution
from the water table situated near the surface is favorable for growth, often preventing its deterioration in conditions of
drought. Nevertheless, an excessive rise of the water table can produce disadvantages at seedtime and during harvest. The
importance of an evaluation of the different scenarios that may have significant implications for agricultural activity is
discussed. 相似文献
94.
Geomorphic analysis of large alluvial rivers 总被引:1,自引:0,他引:1
Colin R. Thorne 《Geomorphology》2002,44(3-4)
Geomorphic analysis of a large river presents particular challenges and requires a systematic and organised approach because of the spatial scale and system complexity involved. This paper presents a framework and blueprint for geomorphic studies of large rivers developed in the course of basic, strategic and project-related investigations of a number of large rivers. The framework demonstrates the need to begin geomorphic studies early in the pre-feasibility stage of a river project and carry them through to implementation and post-project appraisal. The blueprint breaks down the multi-layered and multi-scaled complexity of a comprehensive geomorphic study into a number of well-defined and semi-independent topics, each of which can be performed separately to produce a clearly defined, deliverable product. Geomorphology increasingly plays a central role in multi-disciplinary river research and the importance of effective quality assurance makes it essential that audit trails and quality checks are hard-wired into study design. The structured approach presented here provides output products and production trails that can be rigorously audited, ensuring that the results of a geomorphic study can stand up to the closest scrutiny. 相似文献
95.
96.
Advances in the Study of Moving Sediments and Evolving Seabeds 总被引:1,自引:0,他引:1
Sands and mud are continually being transported around the world’s coastal seas due to the action of tides, wind and waves.
The transport of these sediments modifies the boundary between the land and the sea, changing and reshaping its form. Sometimes
the nearshore bathymetry evolves slowly over long time periods, at other times more rapidly due to natural episodic events
or the introduction of manmade structures at the shoreline. For over half a century we have been trying to understand the
physics of sediment transport processes and formulate predictive models. Although significant progress has been made, our
capability to forecast the future behaviour of the coastal zone from basic principles is still relatively poor. However, innovative
acoustic techniques for studying the fundamentals of sediment movement experimentally are now providing new insights, and
it is expected that such observations, coupled with developing theoretical works, will allow us to take further steps towards
the goal of predicting the evolution of coastlines and coastal bathymetry. This paper presents an overview of our existing
predictive capabilities, primarily in the field of non-cohesive sediment transport, and highlights how new acoustic techniques
are enabling our modelling efforts to achieve greater sophistication and accuracy. The paper is aimed at coastal scientists
and managers seeking to understand how detailed physical studies can contribute to the improvement of coastal area models
and, hence, inform coastal zone management strategies. 相似文献
97.
Laurent Bonnifait Guy Delrieu Matthieu Le Lay Brice Boudevillain Arielle Masson Philippe Belleudy Eric Gaume Georges-Marie Saulnier 《Advances in water resources》2009
On 8–9 September 2002, an extreme rainfall event caused by a stationary mesoscale convective system (MCS) occurred in the Gard region, France. Distributed hydrologic and hydraulic modelling has been carried out to assess and compare the various sources of data collected operationally and during the post-event field surveys. Distributed hydrological modelling was performed with n-TOPMODELs and assessed for ungauged basins with the discharge estimates of the post-event surveys. A careful examination of the occurrence in time and space of the flash floods over the head watersheds indicates that flooding was controlled by the trajectory of the convective part of the MCS. Stationarity of the MCS over the Gardon watershed (1858 km2 at Remoulins) for 28 h was responsible for the exceptional magnitude of the flood at this scale. The flood dynamics were characterized by an extensive inundation of the Gardonnenque plain upstream of the Gardon Gorges resulting in a significant peak flow reduction downstream. One-dimensional unsteady-flow hydraulic modelling was found to be required to reproduce these dynamics. Hydraulic modelling also proved to be potentially useful for the critical analysis and extrapolation of operational discharge rating curves. 相似文献
98.
The effects of variability in bank material properties on riverbank stability: Goodwin Creek, Mississippi 总被引:1,自引:1,他引:0
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. 相似文献
99.
G. V. Last V. J. Rohay F. J. Schelling A. L. Bunn M. A. Delamare R. L. Dirkes R. D. Hildebr J. G. Morse B. A. Napier R. G. Riley L. Soler P. D. Thorne 《Stochastic Environmental Research and Risk Assessment (SERRA)》2004,18(2):109-116
The U. S. Department of Energys Richland Operations Office has initiated efforts to adapt and implement the features, events, and processes (FEP) methodology used in scenario development for nuclear waste disposal programs to the environmental management and remediation problems facing the Hanford site. These efforts have shown that modification of the FEPs methodology to incorporate the use of process relationship diagrams (PRD) is effective in facilitating the development of conceptual models and selection of potentially relevant factors (i.e., FEPs) to be incorporated into a specific environmental assessment. In adopting this methodology for Hanford, a master PRD was created to provide a structure to identify these factors and to illustrate the relationships among them. The organizational framework of the master PRD was developed to match the organization of current Hanford site-wide environmental assessment activities and to facilitate screening of the FEPs relevant to the specific assessments needed for the site. 相似文献
100.
Kathleen M. Swanson Judith Z. Drexler David H. Schoellhamer Karen M. Thorne Mike L. Casazza Cory T. Overton John C. Callaway John Y. Takekawa 《Estuaries and Coasts》2014,37(2):476-492
Salt marsh faunas are constrained by specific habitat requirements for marsh elevation relative to sea level and tidal range. As sea level rises, changes in relative elevation of the marsh plain will have differing impacts on the availability of habitat for marsh obligate species. The Wetland Accretion Rate Model for Ecosystem Resilience (WARMER) is a 1-D model of elevation that incorporates both biological and physical processes of vertical marsh accretion. Here, we use WARMER to evaluate changes in marsh surface elevation and the impact of these elevation changes on marsh habitat for specific species of concern. Model results were compared to elevation-based habitat criteria developed for marsh vegetation, the endangered California clapper rail (Rallus longirostris obsoletus), and the endangered salt marsh harvest mouse (Reithrodontomys raviventris) to determine the response of marsh habitat for each species to predicted >1-m sea-level rise by 2100. Feedback between vertical accretion mechanisms and elevation reduced the effect of initial elevation in the modeled scenarios. Elevation decreased nonlinearly with larger changes in elevation during the latter half of the century when the rate of sea-level rise increased. Model scenarios indicated that changes in elevation will degrade habitat quality within salt marshes in the San Francisco Estuary, and degradation will accelerate in the latter half of the century as the rate of sea-level rise accelerates. A sensitivity analysis of the model results showed that inorganic sediment accumulation and the rate of sea-level rise had the greatest influence over salt marsh sustainability. 相似文献