پيش بيني لايه بندي حرارتي سد در دست احداث بختياري با استفاده از مدل CE-QUAL-W2

Prediction of Thermal Stratification in Proposed Bakhtyari Reservoir With CE-QUAL-W2

In recent decades and by beginning of new century, one of earth planetʹs inhabitant populationʹs worries and particularly countries located in dry lands such as our country is water problem. Before dam construction, a few studies have been done by researchers on thermal stratification, because it needs calibration by future sampling in reservoir. Gunduz et al by use of two dimensional model(CE-QUAL-W2) simulated quality related behavior of proposed Isikli reservoir for provision of potable water. Encon consultant company by use of one dimensional model "DYRESM" studied effects of Yusufeli dam -which was under construction in Turkey- on thermal stratification of reservoir (ENCON, 2006). .In this article effect of various programming scenarios of inflow water resources to reservoir in formation of thermal stratification has been surveyed by use of CE-QUAL-W2. Bakhtyari dam is located in west south of Zagros chain mountainsʹ skirt over a river under the same name in border of Lorestan province (right wing) and Khuzestan province (Left wing).Surface of basin area is 6503 Sq Km and maximum altitude of the area is 4080 m from the free sea level. At normal level reservoirʹs altitude is 830 m in ratio to sea level, lake total volume is 4845 cubic million meters and live storage of that is 2647 cubic meters, area at full supply level is 55 Sq Km, length of that about 60 Km and maximum depth at dam wall is 300 meters. For simulation of reservoirs water quality CE QUAL-W2 model has used.CE-QUAL-W2 is a two dimensional (longitudinal-depth) hydrodynamic and quality related model. Due to lateral homogeneity assumption, it is appropriate for long water bodies. This two dimensional model based on finite difference, has ability of simulation of vertical and longitudinal distribution of thermal energy and chemical and biological parameters which has been selected in water bodies. Also in water bodies, it simulates appropriately volume, density, vertical and longitudinal concentration of quality parameters. Topographic and Geometrical information and of Bakhtyari reservoir has been excerpted from topological maps in scale of 1:20000. For define of reservoirʹs geometry in model, water body has been divided to 51 pieces with a length of 1.5 Km and 2 and 3 meters depth layers. (Fig. 1) In order to simulate Bakhtaran thermal behavior based on mean probable changes in inflow flow rate into reservoir, three scenarios has been defined. In first scenario reservoir thermal behavior due to inflow water decreasing based on one of a very dry year had been simulated. The given year has been 1970-1971.In the second scenario thermal gradients in mean inflow rate into reservoir has been surveyed. For this purpose normal aquatic year of 1988-1989 has been selected. In third scenario thermal regiment of reservoir for the year of 1993-1994 which was very wet year has simulated. Outlet structure level in all scenarios was selected 700 meters above sea level. 13239817 Dry year temperature profile(Winter) 860 800 740 680 620 560 500 Dry year temperature profile(Summer) 860 800 740 680 620 560 500 n t it± c o (0 > Q) LU C O CO > _a> LU 15 th Jan 15 th Feb 15 th Mar 15 th Jul 15 th Aug ♦ 15 th Sep 0 10 20 30 40 Water Temperature(C) 20 5 10 15 Water Temperature(C) Fig.2: Dry year vertical temperature profile It is observed that under 700 meters level which is selected as outlet structure level, temperature profile have stagnant behavior and seasonal changes could not be observed on that and water temperature would be uniform and about 5 degree centigrade. At preliminary test it has determined that depth of this stagnant area is a function of changes in outlet structure level. In dry year from the beginning days of April, layer forming has been begun from high altitudinal level and would maximize up to mid of September, in a way that 16 degree changes of temperature in depth of 100 meters is observable and surface water could reach to 30 degree centigrade. From the mid of September up to January depreciation of stratification would be commenced and again an isothermal layer would begun to form and during temperature decreasing, depth of the layer increase from 35 to 60 meters and January, February and March isothermal conditions are observed about 13 to 15 degree centigrade up to 100 meters depth. Resevoir is warm monomictic and the turnover at the beginning of fall would be happened and stratification is observed about 5 month. In comparison between two scenarios of normal and dry year, it could be concluded that, with regard to long detention time in dry year, thermal gradient is high, but the period of that is shorter. The reason of shortness of stratification could be referred to reaching of a low flow rate of water period and taking of water from middle layers which give absorption opportunity of sun light to upper layers. As well as it can be concluded that, more effects of dam on aquatic regiment (dry year), more is the expectation of circumstances closeness to lake like situation and yearly circulations more would be under affection of geometrical conditions, winding and as a whole effective processes on layer formation. But by decreasing dam effect in flow regiment (wet year), outflow water quality more would be under effect of inflow regiment and reservoirʹs effects whether on hydrodynamic parameters such as temperature or quality parameters such as dissolved oxygen , nitrogen and phosphorus changes in depth and etc. would decrease.