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1.
Detlev Ingendahl Dietrich Borchardt Nicole Saenger Peter Reichert 《Aquatic Sciences - Research Across Boundaries》2009,71(4):399-410
To quantify the contribution of hyporheic community respiration to whole running-water ecosystem respiration in a cultural
landscape setting, we studied the vertical hydraulic exchange in riffle–pool sequences of the River Lahn (Germany). We used
flow through curves from four tracer experiments to estimate flow velocities in the surface and subsurface water. Generally,
vertical exchange velocities were higher in riffle sections and a high temporal variability was observed (range of values
0.11–1.08 m day−1). We then used (1) the exchange velocities and (2) time series of dissolved oxygen concentration in surface and subsurface
water to calculate hyporheic respiration. Hyporheic respiration was estimated in a range of 10–50 mg O2 m−3 day−1 for the upper sediment layer (first 20 cm). It was much lower in the deeper sediment layer (20–40 cm), ranging from 0 to
10 mg O2 m−3 day−1 (volumes are volumes of interstitial water; the average porosity was 20%). We determined primary production and respiration
of the biofilm growing on the sediment by modelling dissolved oxygen concentration time series for a 2,450 m long stream reach
(dissolved oxygen concentrations with diurnal variations from 8 to 16 mg L−1). Modelled respiration rates ranged from 2 to 21 g O2 m2 day−1. All information was integrated in a system analysis with numerical simulations of respiration with and without sediments.
Results indicated that hyporheic respiration accounted for 6 to 14% of whole ecosystem respiration. These values are much
lower than in other whole system respiration studies on more oligotrophic river systems. 相似文献
2.
In June/July 2000, a hydraulic stimulation experiment took place at the geothermal EGS site of Soultz-sous-Forêts (Alsace,
France) in order to enhance the permeability of the fractured granitic massif at 5 km depth. As it is well known that fluid
injections tend to induce microseismic events, a downhole and a surface seismological network have been installed to monitor
the seismic activity during the stimulation test. 23400 m3 of fluid have been injected in the rock volume through the open-hole section (4400 m–5000 m) of the well GPK2 at increasing
rates of 30 l.s−1, 40 l.s−1 and then 50 l.s−1. More than 7200 microseismic events in the magnitude range –0.9 to 2.6 have been precisely located through a simultaneous
inversion of the seismic velocity structure and location parameters. The analysis of the behavior of the seismicity relative
to the hydraulic parameters gives important information about the geothermal reservoir. It appears that the evolution of the
seismicity strongly depends on the variations of the injection rate: An increase or a decrease leads to changes of the velocity
structure, the number and magnitude of microseismic events. This involves different hydro-mechanical processes between the
fluid flow and the fracture planes, which will control the final shape of the microseismic cloud. Moreover, the study of the
variations of the b-value with time suggests that the stimulation experiment produces a large proportion of small earthquakes, but records of
events of magnitude higher than 2 indicate that fluid injection could reactivate structures whose dimensions allow the generation
of such earthquakes. 相似文献
3.
《Limnologica》2016
Freshwater snails are widely distributed in running water systems. Most of them are feeding on biofilms attached to substrata. We used gastropods with different morphology and potentially different traits to analyse the effect of water currents and substratum roughness on the interactions between grazers and biofilms. The gastropods were exposed to naturally grown biofilms from the River Rhine that differed in age and in their artificial substratum roughness (mimicked by abrasive paper). The experiments were conducted in endless channels with paddle wheels where the current velocity could be set steplessly. The freshwater pulmonates Ancylus fluviatilis and Physella acuta as well as the freshwater prosobranchs Bithynia tentaculata and Potamopyrgus antipodarum were used as model organisms in this investigation. The time the snails remained on the test area related to the substratum roughness, and current velocity was used to compare the performance of the four snail species on different aged biofilms. In one experiment, the locomotive activity of snails was traced.Gastropods left substrata without biofilms and did not show different responses to biofilms of diverse ages with significant differences in their biochemical contents. Grazing by snails altered the biofilms in their chlorophyll-a content, ash-free dry mass, and ash mass. A current velocity above 0.12 m s−1 drove B. tentaculata, P. acuta, and P. antipodarum off the test areas, only A. fluviatilis remained unaffected by the applied current velocities (up to 0.23 m s−1). The substratum roughness had no direct influence on the retention time of snails and their locomotive activity on the substrata, whereas the chlorophyll-a content of the biofilm strongly influenced snail activity. Substratum roughness had only an indirect influence on the behaviour of snails contrary to studies on marine snail species. 相似文献
4.
5.
Hydro carbon fields beyond the shelf break are presently being explored and developed, which has increased the scientific
focus in this area. Measurements from the slopes reveal large variability in temperature and velocity, and some of the observed
events are due to interactions between large-amplitude oscillations of the thermocline and the topography. The present study
focuses on the strong currents that are generated near the seabed during shoaling and breaking of internal waves along shelf
slopes. The parameter regime used is similar to the one for the Nordic Seas. The results show that, during shoaling of large
internal waves along (gentle) slopes, the energy is transferred towards smaller scales and strong velocities (over 1 m s − 1) can be generated. To resolve all scales involved is still not feasible, and therefore, the model results are sensitive to
the grid size and the subgrid scale closure. 相似文献
6.
The experimental studies done at high temperature and high pressure find that increased temperature can lead to dramatic velocity and strength reductions of most of rocks at high confining pressure[1,2]. What causes this phenomenon? Is it due to dehydrati… 相似文献
7.
Dev K. Niyogi Janet M. Bandeff Cem Selman Drew E. Menke 《Aquatic Sciences - Research Across Boundaries》2010,72(2):203-212
We examined nutrient flux, uptake, and transformation along a spring-fed stream in the Ozark region of Missouri, USA, over
the year 2006. Water in Mill Creek originates from several springs, with a single spring contributing over 90% of the stream
discharge during much of the year of study. Soluble reactive phosphate concentrations were usually low (<10 μg L−1) along Mill Creek, but peaked during high discharge. Concentrations of dissolved inorganic nitrogen (DIN) were relatively
high in the spring water, mainly as nitrate, but usually declined across a small pond and the 10-km length of Mill Creek.
During low flows in summer and early autumn, the stream removed over 300 μg L−1 of DIN over its 10-km length, or about 80% of the initial amount. DIN retention along the stream, as a percentage of the
DIN upstream, was related mainly to discharge, with higher flows having much higher DIN concentrations. The net uptake rate
of DIN uptake was 0.91 μg m−2 s−1 in the stream during summer baseflow. The uptake rate declined downstream for different reaches and was closely related to
DIN concentration. In experimental channels, uptake by epilithic algae was one significant sink for nitrate-N in Mill Creek.
In 2006, inorganic nutrient export during a single day after a spring storm was similar to export during 40–100 days of low
flow conditions in summer and early autumn. Our results suggest that significant nutrient retention can occur during baseflow
periods via biological uptake, whereas substantial export occurs during high flow conditions. 相似文献
8.
In order to investigate crustal structure beneath the eastern Marmara region, a seismic refraction survey was conducted across
the North Anatolian Fault (NAF) zone in north west Turkey. Two reversed profiles across two strands of the NAF zone were recorded
in the Armutlu Highland where a tectonically active region was formed by different continents. We used land explosions in
boreholes and quarry blasts as seismic sources. A reliable crustal velocity and depth model is obtained from the inversion
of first arrival travel times. The velocity-depth model will improve the positioning of the earthquake activities in this
active portion of the NAF. A high velocity anomaly (5.6–5.8 km s−1) in the central highland of Armutlu block and the low velocity (4.90 km s−1) pattern north of Iznik Lake are the two dominant features. The crustal thickness is about 26 ± 2 km in the north and increases
to about 32 ± 2 km beneath the central Armutlu block in the south. P-wave velocities are about 3.95 km s−1 to 4.70 km s−1 for the depth range between about 1 km and 5 km in the upper crust. The eastern Marmara region has different units of upper
crust with velocities varying with depth to almost 8 km. The high upper crust velocities are associated with Armutlu metamorphic
rocks, while the low velocity anomalies are due to unconsolidated sedimentary sequences. The western side of Armutlu block
has complex tectonics and is well known for geothermal sources. If these sources are continuous throughout the portions of
the crust, it may be associated with a granitic intrusion and deformation along the NAF zone. That is, the geothermal sources
associated with the low velocity may be due to the occurrence of widespread shear heating, even shear melting. The presence
of shear melting may indicate the presence of crustal fluid imposed by two blocks of the NAF system. 相似文献
9.
A. S. N. Murty H. C. Tewari Prakash Kumar P. R. Reddy 《Pure and Applied Geophysics》2005,162(12):2409-2431
A 2-D subcrustal velocity model for the central Indian continental lithosphere has been derived by travel time and relative
amplitude modeling of a digitally normalized analog seismic record section of the Hirapur-Mandla DSS profile, using a ray-tracing
technique. Some prominent wave groups with apparent velocities slightly higher than the Moho reflection phase (PMP) are identified on the normalized record sections assembled with a reduction velocity of 6 km s−1. We interpret these phases as the wide-angle reflections from subcrustal lithospheric boundaries. Comparison of synthetic
seismograms with the observed record section shows that the observed phases cannot be explained either by multiples or by
the P-to-S converted phase (PMS) from the Moho. Subcrustal velocity models either with a velocity increase or with a single low velocity layer (LVL) also
do not provide a satisfactory fit. We infer that a subcrustal velocity model with two alternate LVLs (velocity 7.2 km s−1), separated by a 6-km thick high velocity layer (velocity 8.1 km s−1), can satisfy both the observed travel times and amplitudes. The prominent reflection phases are modeled at depths of 49,
51, 57 and 60 km. It is inferred that the subcrustal lithosphere in the central Indian region has a lamellar structure with
varying structural and mechanical properties. The alternating LVLs, occurring at relatively shallow depths below Moho, may
be associated with the zones of weakness and lower viscosity suggesting continued mobility, with a possible thermal source
in the upper mantle. This explains the source of observed high heat flow values in the central Indian region. 相似文献
10.
Vertical ash plumes were imaged at Santiaguito (Guatemala) using a thermal camera to capture plume ascent dynamics. The plumes
comprised a convecting plume front fed by a steady feeder plume. Of the 25 plumes imaged, 24 had a gas thrust region within
which ascent velocities were 15–50 m s−1. A transition to buoyant ascent occurred 20 to 50 m above the vent, where ascent velocities declined to 4–15 m s−1. Plumes that attained greater heights had higher heat contents, wider feeder plumes and higher buoyant ascent velocities. 相似文献
11.
A fluid dynamics approach to modelling the 18th March 2007 lahar at Mt. Ruapehu,New Zealand 总被引:2,自引:0,他引:2
Lahars are water-sediment mass flows from a volcanic source. They can be triggered by a variety of mechanisms and span a continuum
of flow rheology and hydraulic properties, even within the same event. Lahars are extremely powerful landscaping agents and
represent a considerable hazard potential. However, this highly dynamic character and a lack of direct measurements has made
modelling lahars difficult. This study therefore applies a fluid dynamics model; Delft3D, to analyse the 18th March 2007 dam
break lahar at Mount Ruapehu, New Zealand. The modelled lahar routed through the Whangaehu gorge in ~30 min, crossed the Whangaehu
fan in ~60 min, and then over a further 3 h travelled an additional ~22 km distance along the Whangaehu River to the Tangiwai
bridge. The modelled mean frontal velocity was 6.5 m s−1 along the gorge although peak velocity reached up to 19.6 m s−1. The modelled lahar flow front progressively slowed across the fan but along the River it accelerated from 2.1–3.3 m s−1. Calculated peak velocity along the River was <4.5 m s−1. These results generally compare well with gauged records, with historical records, and with other modelling approaches.
However, discrepancies in frontal velocity and time to peak stage arise due to (1) specifying roughness, which arises from
slope variations between adjacent computational nodes, and which is stage-dependant, and (2) due to rapid topographic changes
that produce frequent hydraulic jumps, which are inadequately accommodated in the numerical scheme. The overall pattern of
discharge attenuation, and of relationships between topographic and hydraulic variables, is similar to that calculated for
lahars on other volcanoes. This modelling method could be applied at other similar sites where a likely source hydrograph
and high-resolution topographic data are available. These results have important implications for hazard management at Ruapehu
and for examining geomorphic and sedimentary impacts of this lahar. 相似文献
12.
Strombolian explosive styles and source conditions: insights from thermal (FLIR) video 总被引:1,自引:3,他引:1
Matthew R. Patrick Andrew J. L. Harris Maurizio Ripepe Jonathan Dehn David A. Rothery Sonia Calvari 《Bulletin of Volcanology》2007,69(7):769-784
Forward Looking Infrared Radiometer (FLIR) cameras offer a unique view of explosive volcanism by providing an image of calibrated
temperatures. In this study, 344 eruptive events at Stromboli volcano, Italy, were imaged in 2001–2004 with a FLIR camera
operating at up to 30 Hz. The FLIR was effective at revealing both ash plumes and coarse ballistic scoria, and a wide range
of eruption styles was recorded. Eruptions at Stromboli can generally be classified into two groups: Type 1 eruptions, which
are dominated by coarse ballistic particles, and Type 2 eruptions, which consist of an optically-thick, ash-rich plume, with
(Type 2a) or without (Type 2b) large numbers of ballistic particles. Furthermore, Type 2a plumes exhibited gas thrust velocities
(>15 m s−1) while Type 2b plumes were limited to buoyant velocities (<15 m s−1) above the crater rim. A given vent would normally maintain a particular gross eruption style (Type 1 vs. 2) for days to
weeks, indicating stability of the uppermost conduit on these timescales. Velocities at the crater rim had a range of 3–101 m
s−1, with an overall mean value of 24 m s−1. Mean crater rim velocities by eruption style were: Type 1 = 34 m s−1, Type 2a = 31 m s−1, Type 2b = 7 m s−1. Eruption durations had a range of 6–41 s, with a mean of 15 s, similar among eruption styles. The ash in Type 2 eruptions
originates from either backfilled material (crater wall slumping or ejecta rollback) or rheological changes in the uppermost
magma column. Type 2a and 2b behaviors are shown to be a function of the overpressure of the bursting slug. In general, our
imaging data support a broadening of the current paradigm for strombolian behavior, incorporating an uppermost conduit that
can be more variable than is commonly considered. 相似文献
13.
Ice‐ and snow‐melted water flow over partially thawed frozen soil of cultivated slopes causes serious soil erosion, which results in soil degradation and affects productivity in Northeast China. Water flow velocity over frozen and nonfrozen soil shows importance in understanding meltwater erosion. In this work, a series of laboratory experiments were conducted to measure water flow velocity over frozen and nonfrozen soil slopes. Experiments were performed using the electrolyte trace method under the pulse boundary model, under conditions of 4 slope gradients (5°, 10°, 15°, and 20°), 3 flow rates (1, 2, and 4 L/min), and 7 sensors positioned at 0.1, 1.0, 2.0, 3.0, 4.0, 5.0, and 6.0 m away from the electrolyte injection point. Results showed that velocities over frozen soil slopes increased with flow rate and slope gradient. Flow velocities over nonfrozen soil slopes increased with flow rate and slope gradients from 5° to 15° and stabilized at 15°. Flow velocities over frozen soil slopes were 30%, 54%, 71%, and 91% higher than those over nonfrozen ones at slope gradients of 5°, 10°, 15°, and 20°. Flow velocities over frozen soil slopes under different flow rates of 1, 2, and 4 L/min were approximately 52%, 59%, and 79% higher than those over nonfrozen soil, respectively. This study can help in assessing the erosion of partially thawed frozen soil by meltwater flow. 相似文献
14.
A series of 188 rainfall plot simulations was conducted on grass, shrub, oak savanna, and juniper sites in Arizona and Nevada. A total of 897 flow velocity measurements were obtained on 3.6% to 39.6% slopes with values ranging from 0.007 m s‐1 to 0.115 m s‐1. The experimental data showed that shallow flow velocity on rangelands was related to discharge and ground litter cover and was largely independent of slope gradient or soil characteristics. A power model was proposed to express this relationship. These findings support the slope–velocity equilibrium hypothesis. Namely, eroding soil surfaces evolve such that steeper areas develop greater hydraulic roughness. As a result overland flow velocity becomes independent of the slope gradient over time. Our findings have implications for soil erosion modeling suggesting that hydraulic friction is a dynamic, slope and discharge dependent property. Copyright © 2018 John Wiley & Sons, Ltd. 相似文献
15.
In this receiver function study, we investigate the structure of the crust beneath six seismic broadband stations close to
the Sunda Arc formed by subduction of the Indo-Australian under the Sunda plate. We apply three different methods to analyse
receiver functions at single stations. A recently developed algorithm determines absolute shear-wave velocities from observed
frequency-dependent apparent incidence angles of P waves. Using waveform inversion of receiver functions and a modified Zhu
and Kanamori algorithm, properties of discontinuities such as depth, velocity contrast, and sharpness are determined. The
combination of the methods leads to robust results. The approach is validated by synthetic tests. Stations located on Malaysia
show high-shear-wave velocities (V
S) near the surface in the range of 3.4–3.6 km s − 1 attributed to crystalline rocks and 3.6–4.0 km s − 1 in the lower crust. Upper and lower crust are clearly separated, the Moho is found at normal depths of 30–34 km where it
forms a sharp discontinuity at station KUM or a gradient at stations IPM and KOM. For stations close to the subduction zone
(BSI, GSI and PSI) complexity within the crust is high. Near the surface low V
S of 2.6–2.9 km s − 1 indicate sediment layers. High V
S of 4.2 km s − 1 are found at depth greater than 6 and 2 km at BSI and PSI, respectively. There, the Moho is located at 37 and 40 km depth.
At station GSI, situated closest to the trench, the subducting slab is imaged as a north-east dipping structure separated
from the sediment layer by a 10 km wide gradient in V
S between 10 and 20 km depth. Within the subducting slab V
S ≈ 4.7 km s − 1. At station BSI, the subducting slab is found at depth between 90 and 110 km dipping 20° ± 8° in approximately N 60° E. A
velocity increase in similar depth is indicated at station PSI, however no evidence for a dipping layer is found. 相似文献
16.
Following an intersection of rising magma with drifts of the potential Yucca Mountain nuclear waste repository, a pathway
is likely to be established to the surface with magma flowing for days to weeks and affecting the performance of engineered
structures located along or near the flow path. In particular, convective circulation could occur within magma-filled drifts
due to the exsolution and segregation of magmatic gas. We investigate gas segregation in a magma-filled drift intersected
by a vertical dyke by means of analogue experiments, focusing on the conditions of sustained magma flow. Degassing is simulated
by electrolysis, producing micrometric bubbles in viscous mixtures of water and golden syrup, or by aerating golden syrup,
producing polydisperse bubbly mixtures with 40% of gas by volume. The presence of exsolved bubbles induces a buoyancy-driven
exchange flow between the dyke and the drift that leads to gas segregation. Bubbles segregate from the magma by rising and
accumulating as a foam at the top of the drift, coupled with the accumulation of denser degassed magma at the base of the
drift. Steady-state influx of bubbly magma from the dyke into the drift is balanced by outward flux of lighter foam and denser
degassed magma. The length and time scales of this gas segregation are controlled by the rise of bubbles in the horizontal
drift. Steady-state gas segregation would be accomplished within hours to hundreds of years depending on the viscosity of
the degassed magma and the average size of exsolved gas bubbles, and the resulting foam would only be a few cm thick. The
exchange flux of bubbly magma between the dyke and the drift that is induced by gas segregation ranges from 1 m3 s−1, for the less viscous magmas, to 10−8 m3 s−1, for the most viscous degassed magmas, with associated velocities ranging from 10−1 to 10−9 m s−1 for the same viscosity range. This model of gas segregation also predicts that the relative proportion of erupted degassed
magma, that could potentially carry and entrain nuclear waste material towards the surface, would depend on the value of the
dyke magma supply rate relative to the value of the gas segregation flux, with violent eruption of gassy as well as degassed
magmas at relatively high magma supply rates, and eruption of mainly degassed magma by milder episodic Strombolian explosions
at relatively lower supply rates. 相似文献
17.
Peter T. Harris Michael G. Hughes Elaine K. Baker Robert W. Dalrymple Jock B. Keene 《Continental Shelf Research》2004,24(19):2431-2454
Current metre deployments, suspended sediment measurements and surface sediment samples were collected from three locations within distributary channels of the tidally dominated Fly River delta in southern Papua New Guinea. Net bedload transport vectors and the occurrence of elongate tidal bars indicate that mutually evasive ebb- and flood-dominant transport zones occur in each of the distributary channels. Suspended sediment experiments at two locations show a phase relationship between tidal velocity and sediment concentration such that the net suspended sediment flux is directed seaward. Processes that control the export of fluid muds with concentrations up to 10 g l−1 from the distributary channels across the delta front and onto the pro-delta are assessed in relation to the available data. Peak spring tidal current speeds (measured at 100 cm above the bed) drop off from around 100 cm s−1 within the distributary channels to <50 cm s−1 on the delta front. Gravity-driven, 2-m thick, fluid mud layers generated in the distributary channels are estimated to require at least 35 h to traverse the 20-km-wide, low-gradient (2×10−3 degrees) delta front. The velocities of such currents are well below those required for autosuspension. A 1-month time series of suspended sediment concentration and current velocity from the delta front indicates that tidal currents alone are unable to cause significant cross-delta mud transport. Wave-induced resuspension together with tides, storm surge and barotropic return-flow may play a role in maintaining the transport of fine sediment across the delta front, but insufficient data are available at present to make any reliable estimates. 相似文献
18.
The fate of inflows into lakes has been extensively studied during summer stratification but has seen relatively little focus
during the weak winter stratification, with or without ice-cover. Field observations are presented of groundwater inflow into
a shallow bay of a subarctic lake. Atmospheric forcing of the bay during the study period was extremely variable and coincided
with spring ice-cover break-up. Two dominant wind regimes were identified; (1) weak wind-forcing (wind speed <5 m s−1 or land-fast ice-cover), and (2) strong wind-forcing (wind speed >5 m s−1 and open water). At a relatively constant temperature of ~3.3°C, the groundwater inflow was closer to the temperature of
maximum density than the water in the main body of the lake, which during the observed winter stratification is ~1.2°C. During
weak wind-forcing, the stratification within Silfra Bay approximated two-layers as this denser groundwater formed a negatively
buoyant underflow. A calculated underflow entrainment rate of 2.8 × 10−3 agrees well with other underflow studies. During strong wind-forcing, the water column out to the mouth of the bay became
weakly stratified as the underflow was entrained vertically by wind-stirring. Observed periods of mixing can be predicted
to occur when turbulent kinetic energy (TKE) production by wind stirring integrated over the underflow hydraulic residence
time in the bay exceeds the potential energy associated with the stratification. A decrease of ice cover, as observed in the
studied subarctic lake over the last decade, will result in the underflow being more frequently exposed to the strong wind-forcing
regime during winter, thereby altering the winter distribution of groundwater inflow within the lake. 相似文献
19.
Yunyun Ban 《国际泥沙研究》2023,38(1):97-104
Rills are primary sediment sources and hillslope water/sediment runoff transport channels. Water flow velocities in rills are easily affected by bed condition over eroding and non-eroding slopes, which is an important hydrodynamic process in soil erosion research. This research is done to demonstrate the poorly understood “feedback mechanism” related to slope independence of flow velocity to slope gradient. A series of experiments were done on silt loam soil slopes to measure water flow velocity... 相似文献
20.
We report on an experimental study conducted to investigate the influence of small-scale wind waves on the airflow structure
in the immediate vicinity of the air–water interface. PIV technique was used to measure the two-dimensional velocity fields
at wind speeds of 3.7 and 4.4 m s−1 and at a fetch of 2.1 m. The flow structure was analyzed as a function of wave phase. In the near-surface region, significant
variations were observed in the flow structure over the waveform. The phase-averaged profiles of velocity, vorticity, and
Reynolds stress showed different behavior on the windward and leeward sides of the wave in the near-surface region. The influence
of wave-induced velocity was restricted within a distance of three significant wave heights from the surface, which also showed
opposite trends on the windward and leeward sides of the crest. The results also show that the turbulent Reynolds stress mainly
supports downward momentum transfer whereas the wave-induced Reynolds stress is responsible for the upward momentum transfer
from wave to wind. In the immediate vicinity of the air–water interface, the momentum is transferred from waves to wind along
the windward side, whereas, the momentum transfer is from wind to waves along the leeward side. 相似文献