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Downstream Distance (downstream + distance)
Selected AbstractsRegional analysis of bedrock stream long profiles: evaluation of Hack's SL form, and formulation and assessment of an alternative (the DS form)EARTH SURFACE PROCESSES AND LANDFORMS, Issue 5 2007Geoff Goldrick Abstract The equilibrium form of the fluvial long profile has been used to elucidate a wide range of aspects of landscape history including tectonic activity in tectonic collision zones, and in continental margin and other intraplate settings, as well as other base-level changes such as due to sealevel fluctuations. The Hack SL form of the long profile, which describes a straight line on a log,normal plot of elevation (normal) versus distance (logarithmic), is the equilibrium long profile form that has been most widely used in such studies; slope,area analysis has also been used in recent years. We show that the SL form is a special case of a more general form of the equilibrium long profile (here called the DS form) that can be derived from the power relationship between stream discharge and downstream distance, and the dependence of stream incision on stream power. The DS form provides a better fit than the SL form to river long profiles in an intraplate setting in southeastern Australia experiencing low rates of denudation and mild surface uplift. We conclude that, if an a priori form of the long profile is to be used for investigations of regional landscape history, the DS form is preferable. In particular, the DS form in principle enables equilibrium steepening due to an increase in channel substrate lithological resistance (parallel shift in the DS plot) to be distinguished from disequilibrium steepening due to long profile rejuvenation (disordered outliers on the DS plot). Slope,area analysis and the slope,distance (DS) approach outlined here are complementary approaches, reflecting the close relationship between downstream distance and downstream catchment area. Copyright © 2006 John Wiley & Sons, Ltd. [source] Turbulence decay behind expanded metal screensTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 6 2000Lanre Oshinowo Abstract An experiment study of the turbulent flow behind expanded metal screens has been carried out in a low-turbulence wind tunnel using an X-probe hot-wire anemometer system. The expanded metal screens turn flow due to a complex array of vaned elements. The flow turning was found to vary accordingly with the dimensions of the strands that make up the screen. The turbulence generated by the screens decays at a rate proportional to the downstream distance to the power - 5/7, consistent with studies in the literature of conventional screens types, and was found to scale with the thickness of the screen strands. The mean velocity, pressure drop and turbulence characteristics of expanded metal screens are presented. Une étude expérimentale de l'écoulement turbulent derrière des écrans de metal expansé a été réalisée dans un tunnel de faible turbulence avec un système anémométrique à film chaud couplé à une sonde à rayons X. Les écrans de métal expansé détournent l'écoulement du fait d'un arrangement complexe des éléments déflecteurs. La déviation de l'éecoulement varie en fonction des dimensions des torons qui composent l'écran. La turbulence engendrée par les écrans décro,t à un rythme proportionnel à la distance de l'écoulement descendant à la puissance ,5/7, ce qui concorde avec les études publiées antérieurement pour des écrans de type traditionnel, et on a trouvé qu'elle change d'échelle avec l'épaisseur des torons des écrans. Les propriétés de vitesse, de perte de charge et de turbulence moyennes des écrans de métal expansé sont présentées. [source] Investigating the surface process response to fault interaction and linkage using a numerical modelling approachBASIN RESEARCH, Issue 3 2006P.A. Cowie ABSTRACT In order to better understand the evolution of rift-related topography and sedimentation, we present the results of a numerical modelling study in which elevation changes generated by extensional fault propagation, interaction and linkage are used to drive a landscape evolution model. Drainage network development, landsliding and sediment accumulation in response to faulting are calculated using CASCADE, a numerical model developed by Braun and Sambridge, and the results are compared with field examples. We first show theoretically how the ,fluvial length scale', Lf, in the fluvial incision algorithm can be related to the erodibility of the substrate and can be varied to mimic a range of river behaviour between detachment-limited (DL) and transport-limited (TL) end-member models for river incision. We also present new hydraulic geometry data from an extensional setting which show that channel width does not scale with drainage area where a channel incises through an area of active footwall uplift. We include this information in the coupled model, initially for a single value of Lf, and use it to demonstrate how fault interaction controls the location of the main drainage divide and thus the size of the footwall catchments that develop along an evolving basin-bounding normal fault. We show how erosion by landsliding and fluvial incision varies as the footwall area grows and quantify the volume, source area, and timing of sediment input to the hanging-wall basin through time. We also demonstrate how fault growth imposes a geometrical control on the scaling of river discharge with downstream distance within the footwall catchments, thus influencing the incision rate of rivers that drain into the hanging-wall basin. Whether these rivers continue to flow into the basin after the basin-bounding fault becomes fully linked strongly depends on the value of Lf. We show that such rivers are more likely to maintain their course if they are close to the TL end member (small Lf); as a river becomes progressively more under supplied, i.e. the DL end member (large Lf), it is more likely to be deflected or dammed by the growing fault. These model results are compared quantitatively with real drainage networks from mainland Greece, the Italian Apennines and eastern California. Finally, we infer the calibre of sediments entering the hanging-wall basin by integrating measurements of erosion rate across the growing footwall with the variation in surface processes in space and time. Combining this information with the observed structural control of sediment entry points into individual hanging-wall depocentres we develop a greater understanding of facies changes associated with the rift-initiation to rift-climax transition previously recognised in syn-rift stratigraphy. [source] A conceptual model for the longitudinal distribution of wood in mountain streamsEARTH SURFACE PROCESSES AND LANDFORMS, Issue 3 2009Ellen Wohl Abstract Wood load, channel parameters and valley parameters were surveyed in 50 contiguous stream segments each 25 m in length along 12 streams in the Colorado Front Range. Length and diameter of each piece of wood were measured, and the orientation of each piece was tallied as a ramp, buried, bridge or unattached. These data were then used to evaluate longitudinal patterns of wood distribution in forested headwater streams of the Colorado Front Range, and potential channel-, valley- and watershed-scale controls on these patterns. We hypothesized that (i) wood load decreases downstream, (ii) wood is non-randomly distributed at channel lengths of tens to hundreds of meters as a result of the presence of wood jams and (iii) the proportion of wood clustered into jams increases with drainage area as a result of downstream increases in relative capacity of a stream to transport wood introduced from the adjacent riparian zone and valley bottom. Results indicate a progressive downstream decrease in wood load within channels, and correlations between wood load and drainage area, elevation, channel width, bed gradient and total stream power. Results support the first and second hypotheses, but are inconclusive with respect to the third hypothesis. Wood is non-randomly distributed at lengths of tens to hundreds of meters, but the proportion of pieces in jams reaches a maximum at intermediate downstream distances within the study area. We use these results to propose a conceptual model illustrating downstream trends in wood within streams of the Colorado Front Range. Copyright © 2009 John Wiley & Sons, Ltd. [source] A comparative study of the dispersal of 10 species of stream invertebratesFRESHWATER BIOLOGY, Issue 9 2003J. M. Elliott Summary 1. Apart from downstream dispersal through invertebrate drift, few quantitative data are available to model the dispersal of stream invertebrates, i.e. the outward spreading of animals from their point of origin or release. The present study provides comparative data for 10 species, using two independent methods: unmarked animals in six stream channels built over a stony stream and marked animals in the natural stream. Experiments were performed in April and June 1973 and 1974, with initial numbers of each species varying from 20 to 80 in the stream channels and 20 to 60 for marked animals. 2. Results were the same for marked invertebrates and those in the channels. Dispersal was not density-dependent; the number of dispersing animals was a constant proportion of the initial number for each species. The relationship between upstream or downstream dispersal distance and the number of animals travelling that distance was well described by an inverse power function for all species (exponential and log models were poorer fits). Results varied between species but were similar within species for the 4 months, and therefore were unaffected by variations in mean water velocity (range 0.04,0.35 m s,1) or water temperature (range 6.7,8.9 °C in April, 12.1,14.8 °C in June). 3. Species were arranged in order, according to their dispersal abilities. Three carnivores (Perlodes, Rhyacophila, Isoperla) dispersed most rapidly (70,91% in 24 h, maximum distances 9.5,13.5 m per day), followed by two species (Protonemura, Rhithrogena) in which about half their initial numbers dispersed (50,51% in 24 h, 7.5,8 m per day), and four species (Ecdyonurus, Hydropsyche, Gammarus, Baetis) in which less than half dispersed (33,40% in 24 h, 5.5,7 m per day). Dispersal was predominantly upstream for all nine species. Few larvae (20%) of Potamophylax dispersed, with similar maximum upstream and downstream distances of 3.5 m per day. The mean time spent drifting downstream was known for seven species from previous studies, and correlated positively with their dispersal distances. Therefore, the species formed a continuum from rapid to very slow dispersers. These interspecific differences should be considered when evaluating the role of dispersal in the maintenance of genetic diversity in stream invertebrates, and in their ability to colonise or re-colonise habitats. [source] Scale-dependence of movement rates in stream invertebratesOIKOS, Issue 1 2004Göran Englund We used analytical models and random walk simulations in a one-dimensional habitat to study the scale-dependence of migration rates in stream invertebrates. Our models predict that per capita migration rate is inversely proportional to patch length when patches are large compared to the scale of movements. When patches are small the scale-dependence is weaker and primarily determined by the length of individual movements (steps) relative to patch size. Laboratory experiments using isopods (Asellus aquaticus L.) and mayfly nymphs (Baetis sp.) confirmed that the strength of the scale-dependence decreased with increasing step length. For the case when step length distributions follow an exponential probability distribution, which is often the case for stream organisms, we provide a simple model that allows the scale-dependence to be predicted from the mean step length. We fitted this model to published field data on drift densities at different downstream distances from a net that blocks the drift from upstream areas. Agreement between model and data was excellent in most cases. We then used already published data on the length of induced drift movements to predict the scale-dependence that was observed in block experiments performed in the same system. Predicted and observed scale-dependence showed very close agreement. We conclude that our models and published data on drift distances can be used to calculate the expected scale-dependence of per capita emigration rates for a large number of taxa under a wide range of environmental conditions. [source] | |||||||||||||