Stratification, reservoirs

Fig. 5. Typical thermal stratification of a lake, reservoir, or poorly mixed estuary in summer which, because of density differences, estabUshes discrete 2ones...

The reason for this decoupling between climate and water temperature in the reservoir is explained in Fig. 3. Moreno-Ostos et al. [37] demonstrated that the degree of stratification and the location of the thermocline both depended on... 

Fig. 10 Temperature, dissolved oxygen saturation, and conductivity profiles recorded in reservoirs from the Llobregat basin during the summer of 2005. Despite 6 months of stratification, oxygen saturation still showed high values except in La Baells. Compare these profiles with those from the Ter River reservoirs in Fig. 8...

Also, the alterations can be classified depending on their intensity. The more intense or frequent the intensity, the easier to predict the consequences. When the alterations are slighter or less frequent, it may occur that the natural factors are more important than the alteration itself [3]. The effect produced by the reservoirs depends on different factors, like the size of the reservoir, the residence time, the stability of the thermal stratification and the withdrawal depth. Moreover, there is a certain interannual variability in the magnitude as well as in the timing of the alterations [4]. Among these factors, the most important one is the depth at which water is released. 

In reservoirs with discharge of hypolimnetic water, the effects on the water temperature have been widely studied. If there is stratification, water temperature downstream from the dam is higher in winter, colder in summer, the daily and annual thermal amplitude is reduced and the maximum annual temperature is delayed [5-7]. Recently, it has been observed that hypolimnetic discharges can reduce the variability of the temperature of the water in reduced time scales, in the range of days to weeks [8]. The low temperatures in summer can modify the composition of the fluvial community, but they can also bring the river to a previous stage of the river continuum [9, 10]. 

Like many other reservoirs in the temperate regions, Mequinensa is monomictic. The thermal stratification begins in spring, intensifies and attains its maximum in summer. In the autumn the water column mixes and water temperature is uniform in the vertical dimension in winter. The summer stratification is more intense close to the dam [39]. In this area, during the stratification period the surface temperatures can attain 24-27°C, while at the bottom remain around 14—16°C [36-39]. 

The thermocline is located at about 14—25 m deep [38, 39]. In the hypolimnion, the stratification can produce oxygen depletion and anoxic water with H2S [36, 38, 39]. In winter the minimum water temperature can be around 5-9°C [38, 43]. The surface water temperature of the reservoir can be estimated as [28, 43]... 

Roura [46] modelled the thermal and hydrodynamic behaviour of the reservoir using the computer model DYRESM. According to the simulation results, the stratification initiates at the end of June and reaches its maximum at the end of September. Since then, the intensity of the stratification weakens, so that in January the water column is completely mixed. 

The reservoir of Riba-roja presents a weak stratification in summer, so that surface water temperatures are 24—27°C and 18-21°C at the bottom [36, 39], The thermocline is situated between 14 and 21 m deep [38, 39], During this time of the year, the water entering the reservoir of Riba-roja from the reservoir of Mequinensa is colder than the water coming from the Segre River. In consequence, the denser Ebro River water sinks and flows along the bottom of the reservoir while the Segre River water floats and flows along the surface [40, 41], The deep water can be anoxic [36, 38, 39], In winter, the water temperature is uniform in the water column and the minimum values are around 11°C [38],... 

Navarro E, Bacardit M, Caputo L, Palau T, Armengol J (2006) Limnological characterization and flow patterns of a three-coupled reservoir system and their influence on Dreissena polymorpha populations and settlement during the stratification period. Lake Reservoir Manage 22 293-302... 

Thermal stratification in reservoirs ( lake-type versus river-type )... 

Generate a statistical description (means, trends, variances and correlations) of the reservoir flow field. Doing this requires a rather massive amount of data primary sources are well data, outcrop analogues, seismic profiling and "type functions based bn stratification types and depositional environment. In an ideal case, there should be such a statistical description for every input variable for the reservoir simulator. 

The vertical structure of the ocean features a decrease of temperature with increasing depth and a correspondingly stable stratification. A shallow surface layer of 50-100 m thickness is vertically well mixed due to agitation by wind force. This portion represents only a small subvolume, but it is of crucial importance to the exchange of C02 with the atmosphere. Compared with the bulk of the ocean, the mixed layer responds quickly to changes in the atmosphere and it must be treated as a separate reservoir. The depth of the mixed layer is variable. We adopt the recommendation of Bolin et al. (1981) and use a value of 75 m. This is a seasonal average obtained by Bathen (1972) from measurements in the Pacific Ocean. 

A comparison of observed and calculated helium and heat fluxes at ocean ridges suggests that a deep mantle reservoir supplies most of the He and heat to the upper mantle (O Nions and Oxburgh 1983 Kellogg and Wasserburg 1990). The involvement of such a deep reservoir in the upper mantle heat and mass balance appears to make it inescapable that some form of stratification is present within the mantle, and that the mantle source of some ocean islands, such as Hawaii and Iceland, lies below the mantle source for ocean ridges. 

Excluding the highly cemented tight-zone, reservoir quality can generally be related to aeolian facies and stratification type (Marzano et al. 1988 Dixon et al. 1989 Thomson Stancliffe 1990). All examples of the interdune facies have extremely low porosity and permeability and do not contribute to hydrocarbon production (Fig. 8). Sands exhibiting dune avalanche cross-stratification have, as a population, higher porosity and permeability than... 

Fig. 8. Porosity versus air permeability for Norphlet sandstones from Mobile Bay. The data plotted are from the reservoir interval and are divided according to stratification type (determined by examination of core). Tight-zone samples have porosity <8% and air permeability < 1 mD regardless of stratifieation type and are excluded from this plot.

Sampling in valley dams over 10 m high is carried out at several predetermined points (profiles) from a boat. Marking of the sampling points by anchored buoys simplifies position fixing. Zonal sampling should be performed to follow the stratification and motion of in-flow water in reservoirs. 

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