Continued Channel Research Articles

Near- and mid-IR ultrashort laser pulse filamentation in a molecular atmosphere: a comparative analysis

Self-focusing and filamentation of high-power pulsed laser radiation along a spectrally selective absorbing air path are numerically simulated in three spectral ranges centered at 0.8, 3.9, and 10.6 μm. A detailed comparative analysis of the main pulse parameters in the filamentation area is performed to examine the potential of various laser sources for atmospheric optics. We found that the radiation of mid-IR lasers (3.9 and 10.6 μm) forms the longest filamentation region and the widest supercontinual spectrum in comparison with near-IR radiation. Filamentation of the 3.9 μm laser pulse results in the best spatially continued plasma channel, while the 10.6 μm pulse retains the widest spectral composition under the conditions of strong molecular absorption in the atmosphere.

Numerical Simulation of Equal Channel Angular Pressing for Multi-Pass in Different Routes

In order to investigate the effect of continued equal channel angular pressing (ECAP) with different routes and passes on the homogeneity of structure, the process of continued ECAP for pure aluminum bar was simulated by DEFORM-3D software. It obtains the load-stroke curves for different passes, distribution of effective stress and effective strain in different routes after the sample is extruded eight passes by ECAP in routes A, BC and C. The results show that the uniformity of the sample is improved with the increase of passes. The microstructure of specimen which is extruded in route BC is the most uniform, but it is the worst in route A.

RIVERBED DIGITAL ELEVATION MODELS AS A TOOL FOR HOLISTIC RIVER MANAGEMENT: MOTUEKA RIVER, NELSON, NEW ZEALAND

ABSTRACTRiver management in New Zealand's laterally active gravelly rivers has permitted floodplain development and protection of agricultural resources and infrastructure. Management of these dynamic systems has been hailed as a success for the approaches adopted, namely straightening and confining the river using bank protection and managing riverbed levels by gravel extraction. However, this activity also impacts river morphological/habitat diversity and potential gravel resource, by replacing broad riparian corridors with narrower channels and reducing lateral connectivity with the floodplain. This paper quantifies river behaviour in three laterally confined reaches in the upper Motueka River over a 7‐year period, using annual high‐resolution ground surveys to address the nature of morphological change and associated sediment flux in these reaches with a view to informing management of the gravel resource. Surveys between 2004 and 2010 acquired data to construct digital elevation models (DEMs) of the active riverbed in three ~1‐km‐long reaches. Morphological budgeting based on differencing between successive DEM surfaces reveals complex spatial and temporal patterns of erosion and deposition, demonstrating complex reach dynamics. Overall, volumetric changes suggest these narrowed reaches have been net exporters of sediment, associated with continued channel degradation. This has left bar features, traditionally the focus of gravel extraction in the reaches, relatively isolated from all but extreme flows, limiting replenishment of the gravel resource. The paper demonstrates the utility of riverbed DEMs as a potential tool to frame river character and behaviour at the reach scale in gravel‐bed rivers, thereby providing an important contribution to holistic river management in these systems. Copyright © 2012 John Wiley & Sons, Ltd.

The role of sediment supply in channel instability and stream restoration

This paper examines the interaction between sediment supply and channel instability and implications of system-wide instability for stream restoration. Using examples from southwest Ohio, we show that recent channel instability is largely a consequence of reduced sediment supply over several past decades. The widths of the studied channels narrowed by a factor of three to four between the 1930s and 2000s, and channel beds have incised into glacial sediments. Lateral migration of these channels decreased after 1960, signaling the onset of channel incision. Soil loss rates in 2005 were 4 to 20 times lower than in the 1920s, mainly as a result of the shift from conventional to conservation tillage. The construction of impoundments has further reduced sediment delivery downstream, and climate change may exacerbate this imbalance by increasing sediment transport capacity, including peak flows. Thus the decrease in sediment supply occurred during a time when peak stream flows have been increasing, resulting in an imbalance between sediment supply and sediment transporting power in the stream systems. Because sediment supply and transport are governed by watershed-scale conditions, channel instability will persist in this region for some time to come. Reach-scale restoration will neither improve water quality nor prevent continued system-wide channel instability. We advocate an approach that uses setbacks/buffers that allow streams to develop new, dynamic floodplains with which to reestablish connections while selectively protecting significant sites with stabilization measures.

Optimal pressing route for continued equal channel angular pressing by finite element analysis

In this paper, the deformation distribution of billets after multi-passes by using equal channel angular pressing (ECAP) routes is investigated by 3D finite element models. The simulation results show that the deformation distribution is more homogenous after two passes by using pressing route C. But if the deformation character of different routes was considered, it may be deduced that route B C is the most effective for deformation homogeneity after multi-pressing as long as the total pressing passes are times of four. The above deduction is ultimately proved by the simulation results of four passes of ECAP process. It means that this route is the most favorable for achieving ultra-fine grained material with more homogenous microstructure.

Fluvial response to Late Quaternary climatic and environmental change, Edwards Plateau, Texas

Radiocarbon-controlled late Pleistocene to modern stratigraphic frameworks for fluvial systems that drain the Edwards Plateau, westcentral Texas, coupled with reconstruction of climatic and environmental changes, permit development of a model for the evolution of fluvial landscapes during the past 20,000 years. During this time, large valleys of the Edwards Plateau were characterized by channel aggradation and sediment storage from 20 to 14,000 yrs B.P., deep excavation of bedrock valleys from ca. 14 to 11,000 yr B.P. and deposition of extensive and complex valley fills during the last 11,000 yr. Valley fills contain three distinct unconformity-bounded allostratigraphic units representing episodes of channel aggradation and floodplain construction during the early to middle (ca. 11,000–5000 yr B.P.) and late Holocene (ca. 5000–1000 yr B.P.), and development of the incised channels and associated narrow floodplains of the last 1000 years. Early to middle Holocene alluvial deposits were derived from proximal sources within the respective drainage basins, whereas late Holocene deposits include sediments derived from distal sources. Sediment supply to major valleys axes has been extremely limited during the last 1000 years. Alluvial sequences of the Edwards Plateau record fluvial responses to climatically-driven changes in discharge regimes and the concentration of sediments along valley axes. Allostratigraphic units define time periods when the concentration of sediment exceeded stream power, resulting in sediment storage, whereas unconformities record widespread morphological and sedimentary adjustments. Unconformities between late Pleistocene alluvium and Holocene valley fills record bedrock valley incision in response to decreased sediment loads associated with slope stability in the uplands. By contrast, unconformities within the Holocene valley fill record floodplain abandonment accompanied by continued channel migration and sediment storage, but little additional bedrock valley cutting. Episodes of floodplain abandonment occurred as a result of decreased flood magnitudes following shifts to drier climatic conditions at ca. 5000 and 1000 yr B.P. Fluvial responses to climatic change were conditioned by a progressive degradation of upland soils that caused increases through time in the flashiness of flood events, which in turn led to changes in processes of floodplain construction. Flood events on late Pleistocene and early to middle Holocene rivers were, for the most part, contained within channel perimeters, and floodplains were constructed by lateral migration. By contrast, late Holocene rivers were characterized by deep overbank flooding and floodplain construction by vertical accretion. High magnitude floods were most significant from ca. 2500 to 1000 yr B.P. when large chute channels were cut and filled on floodplain surfaces, and soils developed on previously stable terrace surfaces were buried by up to 2 m of fine sands and muds.

Control of cell function by neuronal calcium-activated nonselective (CAN) cation channels.

Much of the action of the nervous system can be ultimately associated with changes in the potential of individual neurons. Inhibition plays an essential role in neuronal interactions but it is the excitation provided by postsynaptic potentials and action potentials that generally determines neuronal output. Since excitation is usually associated with depolarization, maintained excitatory states of neurons require mechanisms of maintained depolarization. Sodium channels are one common channel type that select for ions whose electrochemical gradient produces an inward depolarizing current. These channels generally have strongly voltage-dependent inactivation mechanisms that prevent continued channel opening during depolarization. While this action prevents prolonged positive feedback action through the Hodgkin cycle, it makes these channels unavailable for maintaining a depolarized state of the channel. Calcium channels, the other large class of channels producing depolarization, exhibit both voltage-dependent and Ca2+-dependent inactivation. One type of calcium channel, the HVA class, is capable of remaining open for hundreds of milliseconds but calcium channels in general are not suited to providing current for maintained depolarization.KeywordsNonselective Cation ChannelOlfactory Receptor NeuronTaste Receptor CellDorsal Root Ganglion CellOpen Time HistogramThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

A multiperipheral model with continued cross channel unitarity

We derive, by using a spectral representation in momentum transfer, t, an integral equation, similar in structure to a multipheral equation, with continued cross channel unitarity, for the absorptive part for a composite particle scattering amplitude from a Bethe-Salpeter equation describing composite particle scattering in the s channel. At high energy in the t channel, the equation becomes homogeneous and has a Reggeized solution. We indicate how this equation may be solved using determinental techniques. We also show how the composite particle amplitude resulting from the original equation may be used to construct production and three body amplitudes. We also infer the possibility of studying, using the amplitude from the cross channel problem, the effect of extra unitarity on Reggeon-Reggeon-particle vertices.

A multiperipheral model with continued cross channel unitarity: Shirley A. Jackson. National Accelerator Laboratory, Batavia, Illinois 60510

A multiperipheral model with continued cross channel unitarity: Shirley A. Jackson. National Accelerator Laboratory, Batavia, Illinois 60510

A Cycle of Sedimentation and Erosion in Urban River Channels

Historical evidence and contemporary measurements indicate in the Piedmont of Maryland that successive changes in land use have been accompanied by changes in sediment yield and in the behavior of river channels. Sediment yields from forested areas in the pre-farming era appear to have been less than 100 tons/sq.mi/year. Yields from agricultural lands in the same region at a later time range from 300 to 800 t/sq.mi. on large drainage areas. Subsequently, on lands marginal to expanding urban centers, a decline in active farming may be accompanied by a decline in sediment yield. In marked contrast, areas exposed during construction can produce sediment loads in excess of 100,000 t/sq.mi./ year. Small channel systems become clogged with sand during this construction period. While sediment deposited in channels during construction is gradually removed by subsequent clearer flows, rates of removal are slow and hampered by deposition of debris. Increased runoff from urban areas coupled with a decline in sediment yields to values on the order of 50 to 100 t/sq.mi. promote continued bank erosion and channel widening. Maximum observed rates of bank erosion were on the order of 1.0 foot per year. Raw banks adjacent to coarse cobble bars and widespread deposits of flotsam and debris attest to the flood regimen of urban rivers. Canalization in concrete does not eliminate such debris nor does it eliminate deposition of sediment as local changes in gradient, excessive channel width, and debris accumulation foster deposition even in canalized reaches.