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Channel Characteristics (channel + characteristic)
Selected AbstractsUnderstanding the temporal dynamics of the wandering Renous River, New Brunswick, CanadaEARTH SURFACE PROCESSES AND LANDFORMS, Issue 10 2005Leif M. Burge Abstract Wandering rivers are composed of individual anabranches surrounding semi-permanent islands, linked by single channel reaches. Wandering rivers are important because they provide habitat complexity for aquatic organisms, including salmonids. An anabranch cycle model was developed from previous literature and field observations to illustrate how anabranches within the wandering pattern change from single to multiple channels and vice versa over a number of decades. The model was used to investigate the temporal dynamics of a wandering river through historical case studies and channel characteristics from field data. The wandering Renous River, New Brunswick, was mapped from aerial photographs (1945, 1965, 1983 and 1999) to determine river pattern statistics and for historical analysis of case studies. Five case studies consisting of a stable single channel, newly formed anabranches, anabranches gaining stability following creation, stable anabranches, and an abandoning anabranch were investigated in detail. Long profiles, hydraulic geometry, channel energy, grain size and sediment mobility variables were calculated for each channel. Within the Renous study area, the frequency of channel formation and abandonment were similar over the 54 years of analysis, indicating that the wandering pattern is being maintained. Eight anabranches were formed through avulsions, five were formed through the emergence of islands from channel bars and 11 anabranches were abandoned. The stable anabranch pair displayed similar hydraulic geometry and channel energy characteristics, while unstable anabranch pairs did not. The anabranch pair that gained stability displayed more similar channel energy characteristics than the anabranch pair that was losing stability (abandoning). It appears that anabranch pairs with similar energy characteristics are more stable than anabranches where these characteristics are out of balance. This is consistent with the hypothesis that anabranch pairs of similar length will be more stable than those with dissimilar lengths. Copyright © 2005 John Wiley & Sons, Ltd. [source] Reach-scale channel geometry of a mountain riverEARTH SURFACE PROCESSES AND LANDFORMS, Issue 8 2004Ellen Wohl Abstract Mountain rivers can be subject to strong constraints imposed by changes in gradient and grain size supplied by processes such as glaciation and rockfall. Nonetheless, adjustments in the channel geometry and hydraulics of mountain rivers at the reach scale can produce discernible patterns analogous to those in fully alluvial rivers. Mountain rivers can differ in that imposed reach-scale gradient is an especially important control on reach-scale channel characteristics, as indicated by examination of North St Vrain Creek in Colorado. North St Vrain Creek drains 250 km2 of the Rocky Mountains. We used 25 study reaches within the basin to examine controls on reach-scale channel geometry. Variables measured included channel geometry, large woody debris, grain size, and mean velocity. Drainage area at the study reaches ranged from 2·2 to 245 km2, and gradient from 0·013 to 0·147 m m,1. We examined correlations among (1) potential reach-scale response variables describing channel bankfull dimension and shape, hydraulics, bedform wavelength and amplitude, grain size, ,ow resistance, standard deviation of hydraulic radius, and volume of large woody debris, and (2) potential control variables that change progressively downstream (drainage area, discharge) or that are likely to re,ect a reach-speci,c control (bed gradient). We tested the hypothesis that response variables correlate most strongly with local bed gradient because of the segmented nature of mountain channels. Results from simple linear regression analyses indicate that most response variables correlate best with gradient, although channel width and width/depth ratio correlate best with discharge. Multiple regression analyses using Mallow's Cp selection criterion and log-transformation of all variables produced similar results in that most response variables correlate strongly with gradient. These results suggest that the hypothesis is partially supported: channel bed gradient is likely to be a good predictor for many reach-scale response variables along mountain rivers, but discharge is also an important predictor for some response variables. Copyright © 2004 John Wiley & Sons, Ltd. [source] River restoration, habitat heterogeneity and biodiversity: a failure of theory or practice?FRESHWATER BIOLOGY, Issue 2010MARGARET A. PALMER Summary 1. Stream ecosystems are increasingly impacted by multiple stressors that lead to a loss of sensitive species and an overall reduction in diversity. A dominant paradigm in ecological restoration is that increasing habitat heterogeneity (HH) promotes restoration of biodiversity. This paradigm is reflected in stream restoration projects through the common practice of re-configuring channels to add meanders and adding physical structures such as boulders and artificial riffles to restore biodiversity by enhancing structural heterogeneity. 2. To evaluate the validity of this paradigm, we completed an extensive evaluation of published studies that have quantitatively examined the reach-scale response of invertebrate species richness to restoration actions that increased channel complexity/HH. We also evaluated studies that used manipulative or correlative approaches to test for a relationship between physical heterogeneity and invertebrate diversity in streams that were not in need of restoration. 3. We found habitat and macroinvertebrate data for 78 independent stream or river restoration projects described by 18 different author groups in which invertebrate taxa richness data in response to the restoration treatment were available. Most projects were successful in enhancing physical HH; however, only two showed statistically significant increases in biodiversity rendering them more similar to reference reaches or sites. 4. Studies manipulating structural complexity in otherwise healthy streams were generally small in scale and less than half showed a significant positive relationship with invertebrate diversity. Only one-third of the studies that attempted to correlate biodiversity to existing levels of in-stream heterogeneity found a positive relationship. 5. Across all the studies we evaluated, there is no evidence that HH was the primary factor controlling stream invertebrate diversity, particularly in a restoration context. The findings indicate that physical heterogeneity should not be the driving force in selecting restoration approaches for most degraded waterways. Evidence suggests that much more must be done to restore streams impacted by multiple stressors than simply re-configuring channels and enhancing structural complexity with meanders, boulders, wood, or other structures. 6. Thematic implications: as integrators of all activities on the land, streams are sensitive to a host of stressors including impacts from urbanisation, agriculture, deforestation, invasive species, flow regulation, water extractions and mining. The impacts of these individually or in combination typically lead to a decrease in biodiversity because of reduced water quality, biologically unsuitable flow regimes, dispersal barriers, altered inputs of organic matter or sunlight, degraded habitat, etc. Despite the complexity of these stressors, a large number of stream restoration projects focus primarily on physical channel characteristics. We show that this is not a wise investment if ecological recovery is the goal. Managers should critically diagnose the stressors impacting an impaired stream and invest resources first in repairing those problems most likely to limit restoration. [source] Hyporheic Exchange in Mountain Rivers I: Mechanics and Environmental EffectsGEOGRAPHY COMPASS (ELECTRONIC), Issue 3 2009Daniele Tonina Hyporheic exchange is the mixing of surface and shallow subsurface water through porous sediment surrounding a river and is driven by spatial and temporal variations in channel characteristics (streambed pressure, bed mobility, alluvial volume and hydraulic conductivity). The significance of hyporheic exchange in linking fluvial geomorphology, groundwater, and riverine habitat for aquatic and terrestrial organisms has emerged in recent decades as an important component of conserving, managing, and restoring riverine ecosystems. Here, we review the causes and environmental effects of hyporheic exchange, and provide a simple mathematical framework for examining the mechanics of exchange. A companion paper explores the potential effects of channel morphology on exchange processes and the hyporheic environments that may result in mountain basins (Buffington and Tonina 2009). [source] Transmission on indoor power lines: from a stochastic channel model to the optimization and performance evaluation of multicarrier systemsINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 5 2003Virginie Degardin Abstract This paper deals with optimization and performance evaluation of multicarrier transmissions on indoor power lines, taking the actual channel characteristics into account. From intensive measurements of noise on lines, the most significant parameters of impulsive noise were defined and classified, and then a statistical study was carried out. The other types of noise, i.e. narrow band and coloured noise have also been characterized. In like manner, the transfer functions between two points of a power line network have been measured. From all these data, a stochastic channel model representative of the environment is built and implemented in a software tool simulating the link. Various transmission schemes have been considered to mitigate the effects on one hand of the stationary noise, the channel frequency selectivity and of the change of its state, and on the other, on the impulsive noise. These techniques are optimized for the indoor power line environment and their performances, expressed in terms of bit error rate, are compared. Copyright © 2003 John Wiley & Sons, Ltd. [source] Overview of the DVB-SH specificationsINTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS AND NETWORKING, Issue 4-5 2009Philip Kelley Abstract This article provides an overview of the DVB-SH specifications, and the context within which they have been developed. The main DVB-SH configurations are introduced, with explanations of both the technical and frequency planning considerations that drive choices. Regulatory aspects in the European Union and United States of America are summarized. Then specific DVB-SH issues are being addressed, involving reception conditions and channel characteristics, combining techniques for satellite and terrestrial signals, and synchronization and local content insertion. Some attention is paid to the reuse of DVB link and application layer features. Finally, a couple of likely service introduction scenarios are suggested. It is claimed that DVB-SH fulfills the initial requirements of extending the present UHF-based service offer to other frequencies below 3,GHz with a large area coverage, a reduced total network infrastructure cost and an expansion of the offer in terms of number of TV channels or multimedia services. Copyright © 2009 John Wiley & Sons, Ltd. [source] Propagation considerations and fading countermeasures for mobile multimedia servicesINTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS AND NETWORKING, Issue 3 2001Axel Jahn Abstract This paper presents a summary of land mobile satellite (LMS) channel characteristics derived from several measurement campaigns. Narrowband and wideband measurements allow a broad interpretation of the LMS channels. Channel models and its parameters have been derived from the measured data as well. Furthermore, the impact of the LMS channel on the communication service is investigated and possible countermeasures are discussed. Link margins, power control schemes and satellite diversity can compensate the fading on the transmission links. The benefit of adaptive receivers and the dependence on the signal bandwidth are investigated, too. Copyright © 2001 John Wiley & Sons, Ltd. [source] Properties of ion channels in the protoplasts of the Mediterranean seagrass Posidonia oceanicaPLANT CELL & ENVIRONMENT, Issue 3 2004A. CARPANETO ABSTRACT Posidonia oceanica (L) Delile, a seagrass endemic of the Mediterranean sea, provides food and shelter to marine organisms. As environment contamination and variation in physico-chemical parameters may compromise the survival of the few Posidonia genotypes living in the Mediterranean, comprehending the molecular mechanisms controlling Posidonia growth and development is increasingly important. In the present study the properties of ion channels in P. oceanica plasma membranes studied by the patch-clamp technique in protoplasts obtained from the young non-photosynthetic leaves were investigated. In protoplasts that were presumably originated from sheath cells surrounding the vascular bundles of the leaves, an outward-rectifying time-dependent channel with a single channel conductance of 58 ± 2 pS which did not inactivate, was selective for potassium and impermeable to monovalent cations such as Na+, Li+ and Cs+ was identified. In the same protoplasts, an inward-rectifying channel that has a time-dependent component with single channel conductance of the order of 10 pS, a marked selectivity for potassium and no permeation to sodium was also identified, as was a third type of channel that did not display any ionic selectivity and was reversibly inhibited by tetraethylammonium and lanthanum. A comparison of Posidonia channel characteristics with channels identified in terrestrial plants and other halophytic plants is included. [source] | |||||||||||||