The effects of channelization on a high-energy river     

Ale F. Ritter 《Environmental Geology》1979,3(1):29-38

Extensive channelization of a high-energy river in Montana was accomplished during 1957–1958. The technique applied was the same throughout, even the fluvial characteristics are considerably different in the and downstream segments of the river. In the braided reach, rubble dikes composed of river bedload that was against the channel banks prevent the river from high-stage channels which traverse the lowest surface in the valley. The river has responded by altering its hydrologic characteristics at any given discharge and by accelerating the lateral erosion where banks have been left unprotected. Downstream, where the river is in a single-channel pattern, the dikes have been destroyed because the meandering thalweg, repeatedly shifting its position and impinging against the base of the dikes, provides the force needed to erode the boulders comprising the dikes. Some bank protection is provided by riprap composed of large sandstone blocks. This method works, however, only if long segments of the channel are riprapped. The various responses of the river in reaches having different channel patterns indicates that some knowledge of local fluvial mechanics must be obtained prior to the channelization.  相似文献   

Test of IGRF (1980) main field and secular variation in Europe     

W. Mundt  E. Ritter  A. Best 《Physics of the Earth and Planetary Interiors》1985,37(1):20-24

The IGRF-models for the main field and the secular variation (IGRF 1980 and IGRF-SV 1980–1985) have been compared with geomagnetic ground based data for epoch 1980.5 in the GDR, and also with the secular variation at 34 European observatories. The results for the main magnetic field were much better than for IGRF 1965. With a level-error of ～ 45 nT, the IGRF 1980 included ～ 75% of the magnetic crust field. The structure of the IGRF-field is similar to that of other regional reference fields.The IGRF-SV for 1965, 1975 and 1980–1985 have been compared with analytical SV-models, based on low-degree polynomial approximations. It is shown that the main features of SV in Europe are represented correctly. The mean differences between observatory data and IGRF-SV 1980–1985 are 1.5′/a for declination, and 7 nT/a for total intensity.  相似文献   

Spawning and egg development ofPerca fluviatilis in Lake Zürich     

Markus Zeh  Erich Ritter  Georg Ribi 《Aquatic Sciences - Research Across Boundaries》1989,51(2):100-107

Egg survival and development rate of perch eggs were investigated in Lake Zürich, Switzerland. A transplant experiment was carried out to examine whether individual females choose different depths for spawning because of an expected advantage for the development of their eggs. Egg masses which had been deposited on fir branches stuck into the ground of the lake were either left untouched or transplanted horizontally or vertically at depths of 3 and 6 m. Spawning occurred between 28 April and 9 June 1987. Egg survival to the eye pigment stage (EPS) was high in all treatments ( , minimum 92%). There was no significant difference in egg survival between treatments, suggesting that females do not choose the spawning site because of individual fitness differences related to the depth of spawning. Early egg masses which were exposed to lower temperatures developed more slowly (120–140 day degrees to the EPS) than late egg masses which were exposed to higher temperatures (80–100 day degrees). The observed differences are in agreement with the hypothesis of a constant sum of day degrees necessary for egg development using the corrected formula derived by Guma'a (1978) which deducts an estimated threshold temperature of 5°C below which development of perch eggs effectively stops, from the actual temperature.  相似文献   

Model selection in cosmology     

Andrew Liddle  Pia Mukherjee  David Parkinson 《Astronomy& Geophysics》2006,47(4):4.30-4.33

Andrew Liddle, Pia Mukherjee and David Parkinson discuss the new precision available to cosmological modellers.  相似文献   

The violent past of Cygnus X-2     

Ulrich Kolb  Melvyn B. Davies  rew King  Hans Ritter 《Monthly notices of the Royal Astronomical Society》2000,317(2):438-446

Cygnus X-2 appears to be the descendant of an intermediate-mass X-ray binary (IMXB). Using Mazzitelli's stellar code we compute detailed evolutionary sequences for the system and find that its prehistory is sensitive to stellar input parameters, in particular the amount of core overshooting during the main-sequence phase. With standard assumptions for convective overshooting a case B mass transfer starting with a 3.5-M_⊙ donor star is the most likely evolutionary solution for Cygnus X-2. This makes the currently observed state rather short-lived, of order 3 Myr, and requires a formation rate > 10⁻⁷–10⁻⁶ yr⁻¹ of such systems in the Galaxy. Our calculations show that neutron star IMXBs with initially more massive donors (≳4 M_⊙) encounter a delayed dynamical instability; they are unlikely to survive this rapid mass transfer phase. We determine limits for the age and initial parameters of Cygnus X-2 and calculate possible dynamical orbits of the system in a realistic Galactic potential, given its observed radial velocity. We find trajectories which are consistent with a progenitor binary on a circular orbit in the Galactic plane inside the solar circle that received a kick velocity ≤200 km s⁻¹ at the birth of the neutron star. The simulations suggest that about 7 per cent of IMXBs receiving an arbitrary kick velocity from a standard kick velocity spectrum would end up in an orbit similar to Cygnus X-2, while about 10 per cent of them reach yet larger Galactocentric distances.  相似文献   

Locating wind farms by seismic interferometry and migration     

Tobias Friedrich  Toni Zieger  Thomas Forbriger  Joachim R. R. Ritter 《Journal of Seismology》2018,22(6):1469-1483

We present a case study on the detection and quantification of seismic signals induced by operating wind turbines (WTs). We spatially locate the sources of such signals in data which were recorded at 11 seismic stations in 2011 and 2012 during the TIMO project (Deep Structure of the Central Upper Rhine Graben). During this time period, four wind farms with altogether 12 WTs were in operation near the town of Landau, Southwest Germany. We locate WTs as sources of continuous seismic signals by application of seismic interferometry and migration of the energy found in cross-correlograms. A clear increase of emitted seismic energy with rotor speed confirms that the observed signal is induced by WTs. We can clearly distinguish wind farms consisting of different types of WTs (different hub height and rotor diameter) corresponding to different stable frequency bands (1.3–1.6 Hz, 1.75–1.95 Hz and 2.0–2.2 Hz) which do not depend on wind speed. The peak frequency apparently is controlled by the elastic eigenmodes of the structure rather than the passing of blades at the tower. From this we conclude that vibrations are coupled into the ground at the foundation and propagate as Rayleigh waves (and not as infrasound). The migration velocity of 320 m/s corresponds to their group velocity. The applied migration method can contribute to the assessment of local sources of seismic noise. This topic gets growing attention in the seismological community. In particular, the recent boost of newly installed wind farms is a threat to seismological observatories such as the Black Forest Observatory (BFO) and the Gräfenberg array (GRF) or gravitational wave observatories (e.g. LIGO, VIRGO) in terms of a sensitivity degradation of such observatories.  相似文献   

Sustainable livelihoods approaches to inform government‐local partnerships and decision‐making in vulnerable environments     

Peter Edwards  Lisa Sharma‐Wallace  Tim Barnard  Sandra J. Velarde  Tui Warmenhoven  Gerard Fitzgerald  Duncan Harrison  Loretta Garrett  Tina Porou  Pia Pohatu 《New Zealand geographer》2019,75(2):63-73

Sustainable livelihoods approaches used in international development are applied to a vulnerable New Zealand catchment. The Waiapu Catchment has a high proportion of indigenous residents, and is one of the most remote and deprived areas in the country. Linear and centralised approaches to indigenous development have failed to bring about desired changes. We identify “capitals” (social, human/political, physical, natural, financial and cultural) present in the catchment. From this assessment, we propose capital‐based holistic approaches to bring about community‐led change. The assessment and resulting approaches can be used in other vulnerable environments around the world.  相似文献   

The chemical pollution of the secondary of a cataclysmic variableby novae     

R. Stehle  H. Ritter 《Monthly notices of the Royal Astronomical Society》1999,309(1):245-252

We study the chemical evolution of population I and population II secondaries in cataclysmic variables (CVs) assuming that during nova explosions a part of the high metallicity nova ejecta is intercepted by the secondary and mixed into its convective envelope. We derive analytic expressions for the chemical composition of the envelope of the secondary as a function of the chemical composition of the nova ejecta X _i,ej and the cross-section of the secondary σ . For population I CVs we find that the increase of the metallicity of the secondary is comparable to its initial metallicity only if σ is larger by an order of magnitude than the geometrical cross-section. A significant accumulation is therefore possible only in those species that are highly overabundant in the nova ejecta. Because the changes in the abundances of even those species depend strongly on the poorly known cross-section σ , the predictive power of our model is weak for population I CVs as long as σ is not well determined. In the case of population II CVs the accumulation of heavy elements by this process dominates over the initial metallicity of the secondary even for values of σ that are smaller by an order of magnitude than the geometrical cross-section. Thus, within a short time after turn-on of mass transfer, the relative metal abundances in the envelope of the secondary reflect those in the nova ejecta. This is nearly independent of the cross-section σ .  相似文献   

Food web manipulation by extreme enhancement of piscivory: an invertebrate predator compensates for the effects of planktivorous fish on a plankton community     

Jürgen Benndorf  Bjrn Wissel  Anne F. Sell  Uwe Hornig  Pia Ritter  Wiebke Bing 《Limnologica》2000,30(3)

By enhancing the stock of piscivorous fish in a whole-lake experiment in Gräfenhain (Germany) since 1981 to such an extent that almost all planktivorous fish were exterminated, we examined the concept of “over-biomanipulation”. This hypothesis predicts that (a) extremely strong piscivory will allow uncontrolled development of large invertebrate predators and (b) these invertebrates can exert the same strong predation pressure on large herbivorous zooplankton as planktivorous fish. The hypothesis is tested the first time by a cross-comparison of the long-term response of the plankton community structure in the experimental lake (Piscivore L.) with (1) the intermediate response in the same lake and with (2) that of the long-term state in a nearby reference lake (Planktivore L.) densely inhabited by planktivorous fish (Leucaspius delineatus, a small cyprinid). The intermediate (1989–1992) response in Piscivore L. revealed a strong increase of the abundance of the invertebrate predator Chaoborus flavicans. Large daphnids were able to coexist with C. flavicans so that edible phytoplankton were suppressed and water transparency increased.As part (a) of the hypothesis predicts, the long-term response in Piscivore L. was characterized by the immigration of the larger predator Chaoborus obscuripes which displaced the smaller C. flavicans completely. The results support also part (b) of the hypothesis of “over-biomanipulation”: C. obscuripes-dominated Piscivore L. showed not much difference in biomasses of daphnids and total and edible phytoplankton as well as Secchi depth compared with fish-dominated Planktivore Lake. On the other hand, C. obscuripes-dominated Piscivore L. was characterized by distinctly lower biomass of daphnids, mean body volume of all crustaceans and Secchi depth as well as by higher biomass of edible phytoplankton compared with C. flavicans-dominated Piscivore Lake. We conclude that long-lasting success of biomanipulation cannot be achieved by extremely high piscivory leading to the almost complete extermination of planktivorous fish.  相似文献   

Deforestation, hunting and the ecology of microbial emergence     

Nathan D. Wolfe  Mpoudi Ngole Eitel  Jim Gockowski  Pia K. Muchaal  Christian Nolte  A. Tassy Prosser  Judith Ndongo Torimiro  Stephan F. Weise  Donald S. Burke 《Global Change & Human Health》2000,1(1):10-25

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