Reservoir Southern Research Articles

Chemodynamics of Mercury (Hg) in a Southern Reservoir Lake (Cane Creek Lake, Cookeville, TN, USA): I—Estimation of the Kinetics of Photochemical Reduction of Aquatic Hg(II) Using Field-Measured Data of Hg Water/Air Exchange and Dissolved Gaseous Hg

An alternative, independent estimation of the kinetics of aquatic Hg(II) photochemical reduction featuring dissolved gaseous mercury (DGM) emission from water in consideration was obtained by using a mass balance box model. An interactive Excel spreadsheet was constructed to implement the model equations to yield the rate constants and the rates of the Hg(II) photoreduction. The model calculations used field-measured data of DGM paired with its emission flux coupled with the corresponding field sampling times. This data set came from a previous, separate, year-long field study conducted at a southern reservoir lake (Cane Creek Lake, Cookeville, Putnam County, TN). The mean value of the model-calculated rate constants (kDGM) of the Hg(II) photoreduction for the warm season (June–August) (4.5 fM h−1/pg L−1) is higher than that for the cold season (October–January) (2.2 fM h−1/pg L−1). The rate constants were found to be the highest (22.5 fM h−1/pg L−1) in August whereas the lowest (0.03 fM h−1/pg L−1) in January. The model-calculated rate constants are clearly higher in value than but comparable in order of magnitude to the published kinetic data. The model-calculated rates (rDGM) of the Hg(II) photoreduction are significantly higher, by one order of magnitude (102 vs. 101) than the apparent rates calculated using the same field DGM data without consideration of the Hg emission from the water. A sensitivity analysis of the model parameters points to a high sensitivity of Hg emission flux to the rate constant under modeled realistic environmental conditions. The initial Hg(II) concentration is also a sensitive model parameter under certain conditions. The results of our model study support the conclusion that DGM emission from water has a strong impact on the kinetics of aquatic Hg(II) photoreduction and the model calculation can provide an independent, valuable approach for estimating the kinetics of aquatic Hg(II) photoreduction.

Influência da não estacionariedade nas curvas de regularização de reservatórios de usinas hidrelétricas

ABSTRACT The reservoir storage-yield-reliability (S-Y-R) curve defines the required volume to meet a specific yield. It is typically obtained through the historical streamflow time series; however, as an effect of nonstationarity, the statistical properties of a streamflow series may vary, which might lead to a change in the reservoir’s operational risk. In this study we explore this issue by analyzing two sets of annual data: (i) natural energy flows to aggregated reservoirs, and (ii) streamflow time series of four hydropower plants currently in operation in Brazil. The study is supported by Monte Carlo simulations to account for the reliability of the S-Y-R curves. Results suggest that the time series from the Southern and Northeast regions exhibit upward and downward trends, respectively. Consequently, the regularization capacity of the Southern reservoir decreased, however only in relative terms. On the other hand, the Northeastern reservoir had an actual loss of its regularization capacity as an effect of lower average streamflow.

The clays involved in the 1963 Vajont landslide: Genesis and geomechanical implications

The catastrophic Vajont landslide that occurred on 9 October 1963 caused about 2000 deaths when 270–300 Mm3 of rock and debris slid from the northern slope of Mt. Toc into the newly created artificial reservoir, displacing some huge water waves that flooded the nearby villages. The 1963 slide was an en-block remobilisation of a prehistoric rockslide lying on the southern reservoir bank. Basal sliding took place within a complex lithostratigraphic sequence of cherty–marly limestones and clay interbeds belonging to the Fonzaso Formation of Upper Jurassic age. In this work, we investigate the geological origin and provenance of the Vajont clays according to the results of mineralogical and geochemical analyses performed on several clay samples which were collected from the landslide failure surface and the same limestone sequence in the surrounding area. The sampled clayey materials contained variable amounts of clay minerals (36–96%), calcite (4–64%) and quartz (0–6%). The dominant clay minerals were illite/smectite (I/S) mixed layers with a high illite content (85–50%), which was consistent with the high percentage of K2O (1.17–5.77%). The Vajont clays included in the Fonzaso Fm. can be referred to as K-bentonites and can be interpreted as distinct deposits of volcanoclastic materials (tephra), representing the sedimentary inputs of volcanic ashes that interrupted the “normal” calcareous sedimentation during the late Oxfordian–upper Tithonian (158–145 Ma). The soft clay interbeds played a different mechanical role in the occurrence of the prehistoric rockslide and the 1963 en-block remobilisation. When referring to the ancient slope failures, distinct clay-rich layers that were concentrated within some specific lithostratigraphic intervals of the Jurassic cherty limestone sequence caused a localised decrease in the available shear strength, allowing for relative shear displacements along the dip-slope bedding planes. This caused additional shear/tensile fracturing of limestone beds and the formation of a stepped basal failure surface. During the 1963 remobilisation, the clay layers contributed to the average shear strength decrease of the debris material forming the basal shear zone of the slide, but the main triggering factor was the increase in pore pressures caused by the reservoir-induced groundwater inflow.

Acoustic investigation of fishery resources in Guanting Reservoir

This project uses acoustic methods to conduct a comprehensive survey of the fishery resources of the Guanting Reservoir. It uses the “Zigzag” survey route based on the actual situation of the reservoir, and uses the scientific fish finder (EK80, Simrad, Norway) as the main survey equipment. Post-processing software (Echoview, Myrix, Australia) analyzes the amount and distribution of biological resources. Statistical analysis of the existing reservoir resources and the spatial distribution of resource density showed that the fishery resources in the central part of the reservoir area are relatively high, the fishery resources in the waters near the dam in the southern reservoir area are second, and the northernmost fishery resources in the northern reservoir area. Resource Fengdu is relatively small. According to the analysis results of acoustic data, combined with the situation of biological sampling, various types of resources are also obtained. This survey provides basic data for the management of reservoir fishery resources and the formulation of enhancement and release plans, and provides a scientific basis for protecting the ecological resources and environment of the Guanting Reservoir.

Seasonal and Diurnal Variation of Air/Water Exchange of Gaseous Mercury in a Southern Reservoir Lake (Cane Creek Lake, Tennessee, USA)

A year-long field study of mercury (Hg) air/water exchange was conducted at a southern reservoir lake, Cane Creek Lake (Cookeville, TN, USA). The Hg air/water exchange fluxes and meteorological data including solar radiation (global solar radiation, Rg and ultraviolent radiation, UVA), water and air temperatures, relative humidity, and wind speed were collected to study the daily and seasonal trends of the Hg air/water exchange at the lake in relation to solar radiation and wind speed. The Hg exchange fluxes generally exhibited diurnal patterns with a rise in the morning, a peak around noontime, and a fall in the afternoon through the evening, closely following the change of solar radiation. There were cases that deviated from this general daily trend. The Hg emission fluxes were all below 3 ng m−2 h−1 with the daily mean fluxes < 2 ng m−2 h−1. The fluxes in the summer (mean: 1.2 ng m−2 h−1) were higher than in the fall (mean: 0.6 ng m−2 h−1) and winter (mean: 0.7 ng m−2 h−1). The daily and seasonal trends of the Hg air/water exchange fluxes are similar to the trends of the changes of the dissolved gaseous mercury (DGM) concentrations in the lake observed in our previous study. Solar radiation was found to exert a primary control over the Hg air/water exchange, while wind speed appeared to have a secondary effect on the Hg exchange. The two-thin-film model was used to calculate Hg emission fluxes from the Cane Creek Lake water.

3-D structural and seismic attribute analysis for field reservoir development and prospect identification in Fabianski Field, onshore Niger delta, Nigeria

An integrated 3-D seismic data, suite of eleven well logs and checkshot data from Fabianski field, Coastal Depobelt, Niger Delta basin Nigeria were analysed and interpretations carried out using relevant algorithm models in the Petrel workstation for detailed 3-D structural and seismic attribute analysis. The method adopted involves well log interpretation for identification of reservoirs tops and base, well correlation for determination of lateral continuity of the reservoirs, seismic structural mapping, seismic attribute analysis, lead(s)/prospect(s) mapping and risk assessment. Twenty-three (23) faults were interpreted to offset the stratigraphic packages within the field at different seismic horizons. These faults created structures that are favourable for hydrocarbon trapping and accumulation. Three reservoirs have been identified: FAB-1, FAB-2 and FAB-3. Correlation of the reservoirs across five wells along depositional dip shows that the reservoirs are laterally continuous with varying thicknesses at well points. FAB-1 reservoir was studied in detail because it corresponds to a seismic flat spot identified in seismic volume. Four prospects were identified within the FAB-1 reservoir based on structural contour closures: FAB-1 southern prospect, FAB-1 northern prospect, FAB-1 northeastern prospect and FAB-1 northwestern prospect. Only FAB-1 southern prospect shows evidence of hydrocarbon accumulation. FAB-1 southern reservoir is characterized by localized bright amplitudes of root mean square (RMS) and envelope attributes which indicates porous and fluid filled lithologies; constant drop-off high frequencies of instantaneous frequency (12 Hz–25 Hz), and very bright localized high values of sweetness attribute indicative of hydrocarbon saturated sand. Positive values of apparent polarity attribute validate the presence of hydrocarbons in the southern prospect. Spectral decomposition analysis shows that the FAB-1 southern reservoir is better imaged by a frequency range of 15 Hz–25 Hz. It is therefore concluded that spectral decomposition techniques integrated with seismic attributes can unravel subtle indications that are not independently conclusive, and delineate hydrocarbon charged reservoir geometry and structural discontinuities within complex tectonic settings.

Hydrochemistry as a tool for interpreting brine origin and chemical equilibrium in oilfields: Zubair reservoir southern Iraq case study

The oil reservoir brines are very common geological fluids coexisting with hydrocarbons. The chemistry of brines is a powerful tool for determining the brine evolution and its origin, constraining fluid flow at the basin and predicting reservoir scales. This study targeted the Zubair brines in seven oilfields and investigated the geochemical evolution of brine compositions. The composition of the Zubair brine is characterized by the high average of TDS (219,408 mg/l). The contribution of cations as epm% are Na (73.7), Ca (17.32), Mg (7.45), and K (1.27), while anions contribute as Cl (99.6), SO4 (0.23), HCO3 (0.08), and CO3 (0.005). The Zubair brines are of Na–Ca–chloride type, whereas sodium content is 6.3 times greater than seawater; Ca and Mg contents are thirty-four and three times greater, respectively, while Cl is seven times greater. The SO4 ion is depleted to 0.16, due to a biodegradation. The salinity of the Zubair brine is six times more than that of seawater. The Zubair brines are characterized by an acidic pH (5.24–5.68) with a specific gravity of 1.1436, hydrocarbon saturation in pore spaces of 39%, and water saturation of 61%. The mineral saturation model indicates that Zubair brines are unsaturated. The predicted hypothetical salts are NaCl (76%), CaCl2 (15%), and MgCl2 (7.5%). The salts Ca(HCO3)2, CaSO4, and KCl together form only 1.6%. The Zubair brines are characterized by Cl/Br (305) in average, greater than that of seawater (289) confirming the fluids (brines and hydrocarbons) migrated upwards from Sargelu Formation to Zubair reservoir through fractures and cracks in Gotnia Formation.

Changes in Extremes of Temperature, Precipitation, and Runoff in California’s Central Valley During 1949–2010

This study presents a comprehensive trend analysis of precipitation, temperature, and runoff extremes in the Central Valley of California from an operational perspective. California is prone to those extremes of which any changes could have long-lasting adverse impacts on the society, economy, and environment of the State. Available long-term operational datasets of 176 forecasting basins in six forecasting groups and inflow to 12 major water supply reservoirs are employed. A suite of nine precipitation indices and nine temperature indices derived from historical (water year 1949–2010) six-hourly precipitation and temperature data for these basins are investigated, along with nine indices based on daily unimpaired inflow to those 12 reservoirs in a slightly shorter period. Those indices include daily maximum precipitation, temperature, runoff, snowmelt, and others that are critical in informing decision making in water resources management. The non-parametric Mann-Kendall trend test is applied with a trend-free pre-whitening procedure in identifying trends in these indices. Changes in empirical probability distributions of individual study indices in two equal sub-periods are also investigated. The results show decreasing number of cold nights, increasing number of warm nights, increasing maximum temperature, and increasing annual mean minimum temperature at about 60% of the study area. Changes in cold extremes are generally more pronounced than their counterparts in warm extremes, contributing to decreasing diurnal temperature ranges. In general, the driest and coldest Tulare forecasting group observes the most consistent changes among all six groups. Analysis of probability distributions of temperature indices in two sub-periods yields similar results. In contrast, changes in precipitation extremes are less consistent spatially and less significant in terms of change rate. Only four indices exhibit statistically significant changes in less than 10% of the study area. On the regional scale, only the American forecasting group shows significant decreasing trends in two indices including maximum six-hourly precipitation and simple daily intensity index. On the other hand, runoff exhibits strong resilience to the changes noticed in temperature and precipitation extremes. Only the most southern reservoir (Lake Isabella) shows significant earlier peak timing of snowmelt. Additional analysis on runoff indices using different trend analysis methods and different analysis periods also indicates limited changes in these runoff indices. Overall, these findings are meaningful in guiding reservoir operations and water resources planning and management practices.

Comparative precision of age estimates from two southern reservoir populations of paddlefish [Polyodon spathula (Walbaum, 1792)

The aim of the study was to determine whether location and sex affected the age precision estimates between two southern, reservoir populations of paddlefish [Polyodon spathula (Walbaum, 1792)]. From 589 paddlefish collected in Grand Lake and Keystone Lake, Oklahoma in 2011, ages from dentaries were estimated using three independent readers and precision was compared with coefficient of variation between locations and sexes. Ages were more precisely estimated from Grand Lake and from females.

Sediment Copper Concentrations, In Situ Benthic Invertebrate Abundance, and Sediment Toxicity: Comparison of Treated and Untreated Coves in a Southern Reservoir

Copper-based algaecides are used to control algae that compromise uses of lakes and reservoirs. However, there are concerns regarding potential adverse effects to benthic macroinvertebrates following long-term, repeated applications. Multiple lines-of-evidence are useful for evaluating potential ecological risks. These lines-of-evidence are encompassed in the sediment quality triad (SQT) and include sediment copper concentrations, in situ benthic invertebrate abundance, and sediment toxicity testing. The objective of this study was to measure potential ecological risks associated with long-term applications of copper algaecides in coves in Lay Lake, Alabama. Sediments from three coves treated for 7, 10, and 20 years were compared to sediments from three untreated coves in terms of copper concentrations, in situ benthic macroinvertebrate total abundance, and survival of Hyalella azteca and Chironomus dilutus in laboratory sediment toxicity tests. Sediment copper concentrations were not different between treated and untreated coves, with the exception of one treated cove (PC-1S) that contained elevated sediment copper concentrations compared to all other coves. However, the copper was not bioavailable to organisms based on in situ macroinvertebrate abundance and laboratory toxicity tests. In situ benthic invertebrate abundance was not different between treated and untreated coves. In all sediments tested, there were no measurable adverse effects to H. azteca and there were no significant differences in survival of C. dilutus between treated and untreated coves. Based on the weight-of-evidence approach utilized in this study, long-term copper use in three Lay Lake coves has not resulted in adverse effects to benthic invertebrates compared to untreated coves.

Gannet F field – Unexpected fluid flow between reservoirs identified from time-lapse seismic data

Time-lapse (4D) seismic data has been at the heart of the further development of the Gannet F field, from infill well planning, determination of fluid flow and identification of inter-reservoir connectivity. Repeated monitor surveys have allowed an insight into the spatial movement of fluids, even detecting fluid movement in reservoirs with no development well penetrations. Integration of 4D and high-definition 3D seismic inversion results with well data and geological conceptual models have been used to build and test a range of static and dynamic models to investigate connectivity concepts and reservoir distribution. From this improved understanding, the southern reservoir units in the field have been identified as having undeveloped crestal volumes.

Environmental Controls on the Surface Energy Budget over a Large Southern Inland Water in the United States: An Analysis of One-Year Eddy Covariance Flux Data

Abstract The authors analyzed the surface energy fluxes that were measured by an eddy covariance system over the Ross Barnett Reservoir in Mississippi for a 1-yr period in 2008. On a monthly basis over the course of the year, positive vertical temperature and vapor pressure differences were observed, though negative vertical temperature differences occurred occasionally during some short periods when overwater air masses were warmer than the water surface. Consequently, the unstable atmospheric surface layer (ASL) and sufficient mechanical mixing led to positive sensible H and latent λE heat fluxes. The quantities H and λE were distinctively out of phase with the net radiation Rn. The H and λE from the water to the ASL was still substantial on nights with a negative Rn and in winter when Rn was very small. From February to August, approximately 60%–91% of the Rn was used for H and λE, with the remainder being stored in the water. Fueled by the previously stored heat in the water, H and λE exceeded Rn by almost 3 times from September to January. Nighttime evaporation represented a large loss of water (i.e., λE = 82.8 W m−2 at night versus 91.4 W m−2 during the daytime). Intraseasonal and seasonal variations in H and λE were strongly affected by frequent passages of large-scale air masses that were brought in by different synoptic weather systems (e.g., cyclones or anticyclones). The authors’ analysis suggested that this southern reservoir responded to atmospheric forcings on both diurnal and seasonal scales in the same ways as northern lakes of comparable sizes and depths.

Made in Scotland (from girders?) BDS conference site visit to Loch Sloy dam and hydroelectric power station

This article provides a description of the site visit to Sloy dam at Loch Sloy, and the Sloy hydroelectric power (HEP) station at Loch Lomond carried out as part of the 16th biennial conference of the British Dams Society in Glasgow 2010. The visit took place on 28 June 2010 and included tours of the turbine hall, the control room, the high-pressure pipelines and the concrete buttress dam. The visit included a presentation on Scottish and Southern Energy's dams and reservoir assets as well as the construction of the dam and power station and future site improvement plans.

Hot spots and hot moments of planktonic nitrogen fixation in a eutrophic southern reservoir

Reservoirs have been identified as hot spots of biogeochemical activity, although they are known to exhibit pronounced temporal and spatial variability in planktonic community dynamics. Results from this 19-month study at 5 locations on a southern polymictic reservoir identify pronounced temporal (seasonal and interannual) and significant spatial variability in planktonic nitrogen fixation. Planktonic nitrogen fixation was found to be high during the warmest portion of the year and undetectable for most of the rest of the year. Interannual variability between 2 consecutive summer periods was high and likely due to differences in rainfall pattern and resulting differences in ambient nutrient concentrations and flushing rates. Rates during the dry summer exceeded those during the following summer when unexpected summer rains occurred. Spatial differences in areal and volumetric rates were also significant. Volumetric rates were highest in the inflow transition zone of a river draining a watershed impacted by dairy operations, with inflows exhibiting low N:P ratios. Areal rates were highest in the deeper and clearer portions of the reservoir. Areal rates of nitrogen fixation varied among the 5 stations by a factor of 4.4 in the dry summer and by a factor of 22 in the wetter summer.

Apparent rates of production and loss of dissolved gaseous mercury (DGM) in a southern reservoir lake (Tennessee, USA)

Apparent rates of dissolved gaseous mercury (DGM) concentration changes in a southern reservoir lake (Cane Creek Lake, Cookeville, Tennessee) were investigated using the DGM data collected in a 12-month study from June 2003 to May 2004. The monthly mean apparent DGM production rates rose from January (3.2 pg L − 1 /h), peaked in the summer months (June–August: 8.9, 8.0, 8.6 pg L − 1 /h), and fell to the lowest in December (1.6 pg L − 1 /h); this trend followed the monthly insolation march for both global solar radiation and UVA radiation. The monthly apparent DGM loss rates failed to show the similar trend with no consistent pattern recognizable. The spring and summer had higher seasonal mean apparent DGM production rates than the fall and winter (6.8, 9.0, 3.9, 5.0 pg L − 1 /h, respectively), and the seasonal trend also appeared to closely follow the solar radiation variation. The seasonal apparent DGM loss featured similar rate values for the four seasons (5.5, 4.3, 3.3, and 3.9 pg L − 1 /h for spring, summer, fall, and winter, respectively). Correlation was found of the seasonal mean apparent DGM production rate with the seasonal mean morning solar radiation ( r = 0.9084, p < 0.01) and with the seasonal mean morning UVA radiation ( r = 0.9582, p < 0.01). No significant correlation was found between the seasonal apparent DGM loss rate and the corresponding afternoon solar radiation ( r = 0.5686 for global radiation and 0.6098 for UVA radiation). These results suggest that DGM production in the lake engaged certain photochemical processes, either primary or secondary, but the DGM loss was probably driven by some dark processes.

Change of Dissolved Gaseous Mercury Concentrations in a Southern Reservoir Lake (Tennessee) Following Seasonal Variation of Solar Radiation

A 12-month field study was conducted consecutively from June 2003 to May 2004 to quantify temporal variations of dissolved gaseous mercury (DGM) concentrations in Cane Creek Lake, a southern reservoir lake (Cookeville, TN). Diurnal changes of DGM concentrations in two periods (morning increase vs afternoon decrease with an around-noon peak) were observed, and the changes closely followed daily solar radiation variation trends. The diurnal patterns prevailed in the late spring and summer, but became vague in the late fall and winter. The monthly mean DGM concentrations peaked at 40.8 pg L(-1) in July and reached the lowest at 14.2 pg L(-1) in December and 21.9 pg L(-1) in January; this DGM concentration change closely followed the monthly mean solar radiation variation trend. The increase of the lake DGM concentration from January to July and its decrease from July to December mirror the typical daily rhythm of DGM concentration variations in the two periods. This finding supports the following hypothesis: The natural phenomenon of daily oscillation of freshwater DGM concentrations that follows diurnal solar radiation variation would manifest on a seasonal scale. High DGM concentrations were found in the spring and summer and low in the fall and winter (seasonal mean: 34.2, 37.5, 20.0, 24.4 pg L(-1), respectively). This seems to suggest an annual occurrence of two periods of the seasonal DGM level fluctuation (spring and summer high vs fall and winter low DGM levels). Linear relationships of the monthly mean DGM concentrations were found with the monthly mean global solar radiation (R2 = 0.82, P < 0.05) and UVA radiation (R2 = 0.84, P < 0.05). Linear relationships of the seasonal mean DGM concentrations were also found with the seasonal mean global solar radiation (R2 = 0.85, P = 0.08) and UVA radiation (R2 = 0.93, P < 0.05).

Diurnal variation of dissolved gaseous mercury (DGM) levels in a southern reservoir lake (Tennessee, USA) in relation to solar radiation

Variations of dissolved gaseous mercury (DGM) concentrations in a southern reservoir lake (Cane Creek Lake, Cookeville, TN, USA) in relation to solar radiation were investigated consecutively from June 2003 to May 2004. The daytime DGM levels in the lake exhibited a two-phase diurnal trend; the DGM concentrations rose in the morning, peaked around noontime and then fell in the afternoon through the evening; these trends followed the general pattern of diurnal solar radiation variations. The morning and afternoon phases appeared to be asymmetrical with the former relatively steep and the latter gradual. A variety of daytime DGM level variations other than the typical two-phase diurnal patterns were also observed. For the time spans studied, the daytime mean DGM concentrations of the lake ranged from 12 to 68 pg L − 1 (60–340 fM). The daytime mean DGM levels in the summertime (June, July, August) showed values above 30 pg L − 1 (150 fM) in most cases and a large number of peak DGM concentrations above 50 pg L − 1 (250 fM). The summer DGM levels in the lake appear to be comparable to those observed in the large northern lakes for the summertime. The daytime DGM levels in the lake were found to correlate with solar radiation to various degrees (cases of r values above 0.8: ∼ 12% and ∼ 18% of the total sampling days for correlation with global solar radiation and UVA radiation, respectively). Correlating trends are recognizable between the daytime mean DGM concentration and the corresponding mean global solar radiation ( r = 0.66, p < 0.0005) and between the daytime mean DGM concentration and the corresponding mean UVA radiation ( r = 0.62, p < 0.0005).

Une méthode de modélisation hydrogéologique inverse appliquée à un problème d’ingénierie : l’abaissement du niveau d’un réservoir minier ennoyé dans le bassin ferrifère lorrain

Le bassin ferrifère lorrain est un système hydrodynamique et chimique complexe, formé par la superposition de deux grands aquifères en excellente relation hydraulique : un calcaire fracturé et localement karstique (nappe du Dogger), et un aquifère artificiel constitué de grands vides anastomosés (réservoir minier). Une nouvelle galerie de débordement en cours de création à l’exutoire du réservoir Sud sera achevée en 2003, dans le but d’abaisser de façon définitive le niveau du réservoir de 3,6 m. Cette opération permettra de supprimer les nuisances (inondations chroniques) dues à l’existence de fuites du réservoir vers la plaine alluviale constituant le soubassement de la ville de Moyeuvre-Grande. On présente dans cet article la méthode de modélisation mise en œuvre en 1999, dont l’objectif était de fournir les éléments chiffrés au dimensionnement du nouvel ouvrage. Les résultats de cette première modélisation sont validés par les résultats obtenus avec les nouvelles données acquises depuis lors : nouvel ajustement du modèle en 2001 avec deux années supplémentaires de données, et comparaison aux résultats de mesures obtenus lors de l’opération de pompage à gros débit en 2002. La méthode de modélisation globale inverse mise en œuvre pour résoudre ce problème pratique d’ingénierie civile s’est avérée efficace, économe en temps de calcul comme en temps de mise en œuvre, et bien adaptée au problème posé.

Effects of gizzard shad on benthic communities in reservoirs

Effects of gizzard shad Dorosoma cepedianum on benthic communities in a large southern reservoir (Lake Texoma, U.S.A.) were examined during two field enclosure and exclosure experiments in which enclosures were stocked at high and low densities in 1998 and 1999, respectively. In both years, chironomid abundance significantly increased in treatments that excluded large fishes from foraging on sediments. Mean abundance of chironomids and ostracods were significantly higher (P &lt; 0·05) in exclosures than enclosures stocked with gizzard shad at 1140–1210 kg ha−1. In 1999, benthic invertebrate abundances did not differ (P &gt; 0·08) between exclosure and enclosures stocked at 175–213 kg ha−1. Per cent organic matter, algal abundance and abundance of other macroinvertebrates in sediments did not differ significantly among treatments in either year. Although chironomid abundance was reduced in gizzard shad enclosures in 1998, food habits from this and other studies showed that adult gizzard shad in Lake Texoma only consumed detritus and algae. It is likely that high sedimentation rates in Lake Texoma limit the ability of gizzard shad to regulate algae and detritus in benthic sediments. Thus, it is concluded that disturbance of benthic sediments by gizzard shad caused the observed reduction in chironomid abundance, rather than through consumption or competition for resources.

Habitat Use by Striped Bass in Relation to Seasonal Changes in Water Quality in a Southern Reservoir

Adult striped bass Morone saxatilis (n = 61; 597-914 mm total length) were captured by hook and line throughout Lake Murray, South Carolina, and by electrofishing in the Greenwood Dam tailrace and upper Saluda River above Lake Murray, implanted with temperature-sensitive radio transmitters, and tracked biweekly. During late winter-early spring, striped bass were concentrated in the upstream portions of the reservoir. By midsummer, they were primarily located in the lower embayment of the reservoir, but several fish remained in the tailrace of the upstream dam as well as in a thermal refuge in the Saluda River. After the reservoir began to cool in fall, fish dispersed from the lower embayment and moved upstream toward the headwaters of the reservoir, where they had been captured the previous spring. Several fish returned to locations within 10 m of their original capture locations. Mean movement rates were lowest in winter and summer and highest in spring and fall. Low movement rates in summer were associated with a severe reduction of suitable habitat. In addition to the standard biweekly sampling, a 7.5-km2 section of the lower embayment of Lake Murray was searched every 2 h over a continuous 48-h period from 10 to 12 August 2000. During this period, striped bass were observed to use the same areas on a seasonal basis as they did on a diel basis. However, mean hourly rates of movement were greater than the movement rates calculated for the normal 2-week interval between samples. Changes in location between biweekly samples may not indicate displacement but rather only randomly chosen locations in normal use areas.