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11.
Olivier Planchon Norbert Silvera Raphael Gimenez David Favis‐Mortlock John Wainwright Yves Le Bissonnais Gerard Govers 《地球表面变化过程与地形》2005,30(7):833-844
This article introduces the SVG (salt‐velocity gauge), a novel automated technique for measuring flow velocity by means of salt tracing. SVG allows a high measuring rate (up to one every 2 seconds), short control section length (down to 10 cm), high accuracy (+[sol ]?1·5 cm s?1), and unbiased calculation of the mean velocity in experimental conditions with turbulent, supercritical flow. A few cubic centimetres of saturated salt solution (NaCl) are injected into the flow at regular time intervals using a programmable solenoid valve. The tracer successively passes two conductivity probes placed a short distance downstream. The transformation of the signal between the two probes is modelled as a one‐dimensional diffusion wave equation. Model calibration gives an estimation of the mean velocity and the diffusion for each salt plume. Two implementations of the SVG technique are described. The first was an outdoors simulated rainfall experiment in Senegal (conductivity probes at 40 cm apart, 8 Hz measurement rate, salt injections at 10 second intervals). Mean velocity was estimated to range between 0·1 and 0·3 m s?1. The second was a laboratory‐based flume experiment (conductivity probes at 10 cm apart, 32 Hz, salt injections at 2 second intervals). Another SVG with probes at 34 cm apart was used for comparison. An acoustic Doppler velocimeter (ADV) was also used to give an independent assessment of velocity. Using the 10 cm salt gauge, estimated mean velocity ranged from 0·6 to 0·9 m s?1 with a standard deviation of 1·5 cm s?1. Comparisons between ADV, 10 cm SVG and 34 cm SVG were consistent and demonstrated that the salt‐tracing results were unbiased and independent of distance between probes. Most peaks were modelled with r2 > 90 per cent. The SVG technology offers an alternative to the dye‐tracing technique, which has been severely criticized in the literature because of the wide interval of recommended values for the correction factor α to be applied to the timings. This article demonstrates that a fixed value of α is inappropriate, since the correction factor varies with velocity, diffusion and the length of the control section. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
12.
Mulamba Teddy Bacopoulos Peter Kubatko Ethan J. Pinto Gerard F. 《Climatic change》2019,152(3-4):533-550
Climatic Change - Salinity response to sea-level rise is evaluated for a low-gradient, tidally active estuary, the lower St. Johns River, Florida. A high-resolution numerical model is forced by... 相似文献
13.
Andrew J. Davies Gerard C.A. Duineveld Tjeerd C.E. van Weering Furu Mienis Andrea M. Quattrini Harvey E. Seim John M. Bane Steve W. Ross 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(2):199-212
The Lophelia pertusa community at Viosca Knoll (VK826) is the most extensive found to date in the Gulf of Mexico. As part of a multi-disciplinary study, the physical setting of this area was described using benthic landers, CTD transects and remotely operated vehicle observations. The site was broadly characterised into three main habitats: (1) dense coral cover that resembles biogenic reef complexes, (2) areas of sediment, and (3) authigenic carbonate blocks with sparse coral and chemosynthetic communities. The coral communities were dominated by L. pertusa but also contained numerous solitary coral species. Over areas that contained L. pertusa, the environmental conditions recorded were similar to those associated with communities in the north-eastern Atlantic, with temperature (8.5–10.6 °C) and salinity (~35) falling within the known species niche for L. pertusa. However, dissolved oxygen concentrations (2.7–2.8 ml l?1) and density (σΘ, 27.1–27.2 kg m?3) were lower and mass fluxes from sediment trap data appeared much higher (4002–4192 mg m?2 d?1). Yet, this species still appears to thrive in this region, suggesting that L. pertusa may not be as limited by lower dissolved oxygen concentrations as previously thought. The VK826 site experienced sustained eastward water flow of 10–30 cm s?1 over the 5-day measurement period but was also subjected to significant short-term variability in current velocity and direction. In addition, two processes were observed that caused variability in salinity and temperature; the first was consistent with internal waves that caused temperature variations of 0.8 °C over 5–11 h periods. The second was high-frequency variability (20–30 min periods) in temperature recorded only at the ALBEX site. A further pattern observed over the coral habitat was the presence of a 24 h diel vertical migration of zooplankton that may form part of a food chain that eventually reaches the corals. The majority of detailed studies concerning local environmental conditions in L. pertusa habitats have been conducted within the north-eastern Atlantic, limiting most knowledge of the niche of this species to a single part of an ocean basin. Data presented here show that the corals at VK826 are subjected to similar conditions in temperature, salinity, and flow velocity as their counterparts in the north-east Atlantic, although values for dissolved oxygen and density (sigma-theta: σΘ) are different. Our data also highlight novel observations of short-term environmental variability in cold-water coral habitat. 相似文献
14.
Sugarcane is grown on the floodplains of northern Queensland adjacent to the Great Barrier Reef lagoon. Sediment and nutrient loss from these sugarcane areas is considered a potential threat to coastal and marine ecosystems. To enable sugarcane cultivation, farmers have structured the landscape into different elements, comprising fields, water furrows, ‘headlands’ and drains. In order to apply appropriate management of the landscape and reduce export of sediment, it is important to identify which of these elements act as sediment sources or sinks. In this study erosion and deposition rates were measured for the different landscape elements in a subcatchment of the Herbert River and used to create a sediment budget. Despite large uncertainties, the budget shows that the floodplain area is a net source of sediment. Estimated sediment export varies between 2 and 5 t ha?1 y?1. The relative importance of the landscape elements as sediment sources could also be determined. Plant cane is identified as the most important sediment source. Water furrows generate most sediment, but are a less important source of exported sediment due to their low connectivity. Headlands and minor drains act as sediment traps. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
15.
16.
In this paper we describe the transport of pollution in groundwater in the neighbourhood of a well in a uniform background flow. We compute the rate at which contaminated particles reach the well as a function of the place of the source of pollution. The motion of a particle in a dispersive flow is seen as a random walk process. The Fokker-Planck equation for the random motion of a particle is transformed using the complex potential for the advective flow field. The resulting equation is solved asymptotically after a stretching transformation. Finally, the analytical solution is compared with results from Monte Carlo simulations with the random walk model. The method can be extended to arbitrary flow fields. Then by a numerical coordinate transformation the analytical results can still be employed. 相似文献
17.
J. -C. Gerard 《Planetary and Space Science》1975,23(12):1681-1684
Measurements of the O2(A3Σ − X3Σ) Herzberg system in the night airglow have been made with the ESRO TD-1 satellite in the wavelength range 2400–3100 A. The slant emission rate varies from 3.5 to 15 kR, indicating an irregular structure of the atomic oxygen near the turbopause. A statistical maximum intensity is found near the tropic in the winter hemisphere. The intensity profile is consistent with excitation by three-body recombination of oxygen atoms. The observed total emission rate can be accounted for by reasonable atomic oxygen densities and an O2(A3Σ) production efficiency of about 20% if quenching by N2 occurs at the rate deduced from laboratory and other airglow measurements. 相似文献
18.
19.
Assessment of gully erosion rates through interviews and measurements: a case study from northern Ethiopia 总被引:3,自引:0,他引:3
Jan Nyssen Jean Poesen Maude Veyret‐Picot Jan Moeyersons Mitiku Haile Jozef Deckers Joke Dewit Jozef Naudts Kassa Teka Gerard Govers 《地球表面变化过程与地形》2006,31(2):167-185
Gullying has been widespread in the Ethiopian Highlands during the 20th century. It threatens the soil resource, lowers crop yields in intergully areas through enhanced drainage and desiccation, and aggravates flooding and reservoir siltation. Knowing the age and rates of gully development during the last few decades will help explain the reasons for current land degradation. In the absence of historical written or photographic documentation, the AGERTIM method (Assessment of Gully Erosion Rates Through Interviews and Measurements) has been developed. It comprises measurements of contemporary gully volumes, monitoring of gully evolution over several years and semi‐structured interview techniques. Gully erosion rates in the Dogu'a Tembien District, Tigray, Ethiopia, were estimated in three representative case‐study areas. In Dingilet, gullying started around 1965 after gradual environmental changes (removal of vegetation from cropland in the catchment and eucalyptus plantation in the valley bottom); rill‐like incisions grew into a gully, which increased rapidly in the drier period between 1977 and 1990. The estimated evolution of the total gully volume in the other areas show patterns similar to those of the Dingilet gully. Average gully erosion rate over the last 50 years is 6·2 t ha?1 a?1. Since 1995, no new gullies have developed in the study area. Area‐specific short‐term gully erosion rates are now on average 1·1 t ha?1 a?1. The successful application of the AGERTIM method requires an understanding of the geomorphology of the study area and an integration of the researchers with the rural society. It reveals that rapid gully development in the study area is some 50 years old and is mainly caused by human‐induced environmental degradation. Under the present‐day conditions of ‘normal’ rain and catchment‐wide soil and water conservation, gully erosion rates are decreasing. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
20.
The use of riparian vegetated filter strips to reduce river sediment loads: an overestimated control measure? 总被引:1,自引:0,他引:1
The spatially distributed soil erosion and sediment delivery model WATEM/SEDEM was used to simulate the impact of riparian vegetated filter strips (RVFSs) on river sediment delivery at different spatial scales. For a field plot with a straight slope, sediment reduction by the RVFSs is comparable to results obtained through experimental set‐ups elsewhere (i.e. >70%). However, at the scale of an entire catchment, sediment reduction is much less (i.e. ±20%) due to (1) overland flow convergence, which reduces the sediment trapping efficiency of an RVFS, and (2) because part of the sediment bypasses the RVFSs through ditches, sewers and road surfaces. These results suggest that, at the catchment scale, RVFSs should be accompanied with other conservation techniques that are more appropriate for reducing river sediment loads, and that also reduce on‐site soil erosion. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献