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High Gradient (high + gradient)
Selected AbstractsHydrodynamics and geomorphic work of jökulhlaups (glacial outburst floods) from Kverkfjöll volcano, IcelandHYDROLOGICAL PROCESSES, Issue 6 2007Jonathan L. Carrivick Abstract Jökulhlaups (glacial outburst floods) occur frequently within most glaciated regions of the world and cause rapid landscape change, infrastructure damage, and human disturbance. The largest jökulhlaups known to have occurred during the Holocene within Iceland drained from the northern margin of Vatnajökull and along the Jökulsá á Fjöllum. Some of these jökulhlaups originated from Kverkfjöll volcano and were routed through anastomosing, high gradient and hydraulically rough channels. Landforms and sediments preserved within these channels permit palaeoflow reconstructions. Kverkfjöll jökulhlaups were reconstructed using palaeocompetence (point measurements), slope,area (one-dimensional), and depth-averaged two-dimensional (2D) hydrodynamic modelling techniques. The increasing complexity of 2D modelling required a range of assumptions, but produced information on both spatial and temporal variations in jökulhlaup characteristics. The jökulhlaups were volcanically triggered, had a linear-rise hydrograph and a peak discharge of 50 000,100 000 m3 s,1, which attenuated by 50,75% within 25 km. Frontal flow velocities were ,2 m s,1; but, as stage increased, velocities reached 5,15m s,1. Peak instantaneous shear stress and stream power reached 1 × 104 N m,2 and 1 × 105 W m,2 respectively. Hydraulic parameters can be related to landform groups. A hierarchy of landforms is proposed, ranging from the highest energy zones (erosional gorges, scoured bedrock, cataracts, and spillways) to the lowest energy zones (of valley fills, bars, and slackwater deposits). Fluvial erosion of bedrock occurred in Kverkfjallarani above ,3 m flow depth, ,7m s,1 flow velocity, ,1 × 102 N m,2 shear stress, and 3 × 102 W m,2 stream power. Fluvial deposition occurred in Kverkfjallarani below ,8 m flow depth, 11 m s,1 flow velocity, 5 × 102 N m,2 shear stress, and 3 × 103 W m,2 stream power. Hence, erosional and depositional ,envelopes' have considerable overlap, probably due to transitional flow phenomena and the influence of upstream effects, such as hydraulic ponding and topographic constrictions, for example. Holocene Kverkfjöll jökulhlaups achieved geomorphic work comparable to that of other late Pleistocene ,megafloods'. This work was a result of steep channel gradients, topographic channel constrictions, and high hydraulic roughness, rather than to extreme peak discharges. The Kverkfjöll jökulhlaups have implications for landscape evolution in north-central Iceland, for water-sediment inputs into the North Atlantic, and for recognizing jökulhlaups in the rock record. 2D hydrodynamic modelling is likely to be important for hazard mitigation in similar landscapes and upon other glaciated volcanoes, because it only requires an input hydrograph and a digital elevation model to run a model, rather than suites of geomorphological evidence and field-surveyed valley cross-sections, for example. Copyright © 2006 John Wiley & Sons, Ltd. [source] Representing elevation uncertainty in runoff modelling and flowpath mappingHYDROLOGICAL PROCESSES, Issue 12 2001Theodore A. Endreny Abstract Vertical inaccuracies in terrain data propagate through dispersal area subroutines to create uncertainties in runoff flowpath predictions. This study documented how terrain error sensitivities in the D8, Multiple Flow (MF), DEMON, D-Infinity and two hybrid dispersal area algorithms, responded to changes in terrain slope and error magnitude. Runoff dispersal areas were generated from convergent and divergent sections of low, medium, and high gradient 64-ha parcels using a 30 m pixel scale control digital elevation model (DEM) and an ensemble of alternative realizations of the control DEM. The ensemble of alternative DEM realizations was generated randomly to represent root mean square error (RMSE) values ranging from 0·5 to 6 m and spatial correlations of 0 to 0·999 across 180 m lag distances. Dispersal area residuals, derived by differencing output from control and ensemble simulations, were used to quantify the spatial consistency of algorithm dispersal area predictions. A maximum average algorithm consistency of 85% was obtained in steep sloping convergent terrain, and two map analysis techniques are recommended in maintaining high spatial consistencies under less optimum terrain conditions. A stochastic procedure was developed to translate DEM error into dispersal area probability maps, and thereby better represent uncertainties in runoff modelling and management. Two uses for these runoff probability maps include watershed management indices that identify the optimal areas for intercepting polluted runoff as well as Monte-Carlo-ready probability distributions that report the cumulative pollution impact of each pixel's downslope dispersal area. Copyright © 2001 John Wiley & Sons, Ltd. [source] Spatial ecology of the white-clawed crayfish in an upland stream and implications for the conservation of this endangered speciesAQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS, Issue 5 2008Damian H. Bubb Abstract 1.Populations of white-clawed crayfish (Austropotamobius pallipes) have undergone substantial declines across Europe. Remaining populations tend to be fragmented and in many catchments they are restricted to upland streams. Information is needed concerning their spatial ecology to assist with conservation and rehabilitation of existing fragmented populations, as well as possible reintroductions. 2.A novel method for the long-term tagging of white-clawed crayfish was used to study the spatial ecology of a white-clawed crayfish population fragment in a small, moderate-gradient upland stream. Internal passive integrated transponder (PIT) tags enabled adult crayfish (carapace length >27 mm) to be permanently tagged, each with a unique identification code and resulted in a high number of recaptures. Of 501 crayfish tagged 413 were subsequently relocated at least once. 3.Crayfish did not make extensive movements, the median annual distance moved was 84.8,m,yr,1, equivalent to annual net movement of 0.233,m,day,1, substantially less than reported in previous studies. The lower levels of movement may reflect the study encompassing all seasons, including winter, when crayfish are relatively inactive. 4.Significantly more crayfish moved downstream compared with upstream and distances moved downstream were significantly greater than those in an upstream direction. This may be linked to the relatively high gradient of the stream and a reduced passability of the abundant riffles to upstream movements of crayfish. 5.A small weir acted as a barrier within the stream, preventing upstream movements of crayfish. Even small barriers may limit the movements of white-clawed crayfish and have the potential to limit connectivity within populations and prevent expansion or recolonization. 6.The results presented suggest that fragmented populations of white-clawed crayfish in upland streams are unlikely to expand rapidly and reconnect to other population fragments, even where habitat is suitable. In establishing stream populations care should be taken to ensure that even small barriers to movement are removed unless these are intended to exclude non-native biota, especially crayfish. Copyright © 2007 John Wiley & Sons, Ltd. [source] Tectono-thermal Evolution in the Bachu Uplift, Tarim Basin, ChinaACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 5 2010QIU Nansheng Abstract: The thermal evolution of source rocks in the Paleozoic has long been a problem to petroleum exploration in the Bachu uplift, Tarim basin, since the thermal history in the Paleozoic could not be rebuilt objectively due to lack of effective thermal indicators in the Lower Paleozoic successions. The apatite and zircon (U-Th)/He thermochronometry can be used as a new kind of technique to study the thermal history and tectonic uplift of sedimentary basins. Based on the measured apatite and zircon (U-Th)/He ages, apatite fission track data and equivalence vitrinite reflectance (%EVRo), the tectonothermal histories in 5 wells of the Bachu uplift were modeled. The modeling results show that there was relatively high gradient at the Early Paleozoic in the Bachu uplift and it decreased gradually during the entire Paleozoic: 33,35°C/km in the Cambrian-Ordovician, 32,33°C/km in the Silurian-Devonian, 30,32°C/km at the end of Carboniferous and 27.5,31°C/km at the end of Permian. Therefore, the thermal history can be modeled by combining multiple thermal indicators of AFT, (U-Th)/He ages and EVRo data. Especially, this provides a new method to rebuild the thermal history for the Low Paleozoic carbonate successions in the Tarim Basin. [source] The fast Gauss transform for non-local integral FE modelsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 6 2006E. Benvenuti Abstract Originally developed for fast solving multi-particle problems, the fast Gauss transform (FGT) is here applied to non-local finite element models of integral type (FEFGT). The focus is on problems requiring fine geometry discretization, as in the case of solutions that exhibit high gradients or boundary layers. As shown by one- and two-dimensional examples, the FEFGT algorithm combines the robustness of the finite element method with the outstanding computational efficiency of the FGT. Copyright © 2005 John Wiley & Sons, Ltd. [source] | ||||||||||