رسوب‌شويي تحت فشار با توسعه مجراي تخليه كننده تحتاني در مخزن سد

Pressure Flushing with Expanding Bottom Outlet Channel within Dam Reservoir

درحال حاضر سدهاي مخزني متعددي در دنيا، به دليل حجم زياد رسوبات در مخزن آنها و به ويژه در اطراف آبگيرها، با مشكلات بهره‌برداري روبه‌رو است. در اين ميان رسوبشويي تحت فشار به عنوان يك روش موثر براي خارج كردن رسوبات انباشته شده در پشت سد كه محل جانمايي دريچه‌ها و توربين‌ها مي‌باشد براي حل اين مشكل مطرح شده است. اما در اين نوع رسوبشويي رسوب‌هاي مجاور دريچه هاي تخليه كننده به شكل موضعي حذف مي‌شوند و ميزان گسترش حد تاثير آن محدود است، از طرفي از آنجايي كه اتلاف آب در اين روش نسبتا پايين است، بنابراين ارايه يك راهكار مناسب براي افزايش حد تاثير فرآيند رسوبشويي تحت فشار مي تواند گامي براي افزايش عمر سد و همچنين نجات آبگيرها با كمترين اتلاف آب باشد. در اين پژوهش، به بررسي آزمايشگاهي تاثير توسعه مجراي تخليه كننده تحتاني در مخزن بر روي حجم و ابعاد مخروط رسوبشويي به عنوان يك راهكار پرداخته شده است. آزمايش‌‌ها با عمق و دبي‌هاي مختلف جريان در طول‌هاي مختلف توسعه مجرا، طراحي و انجام شد. نتايج پژوهش‌هاي اوليه نشان مي‌دهد، كه توسعه مجراي تخليه كننده تحتاني در مخزن تاثير مثبت و محسوس بر ابعاد مخروط رسوبشويي دارد، به گونه‌اي كه توسعه مجرا به ميزان نسبي 5/0 ، 1 و 5/1 برابر ارتفاع رسوب‌ها در مخزن باعث افزايش حجم مخروط رسوبشويي به طور متوسط، به ميزان 50 ،74 و 96 درصد نسبت به حالت بدون توسعه مجرا مي‌شود. همچنين با آناليز ابعادي پارامتر هاي موثر روابط بدون بعدي با قدرت تخمين بالابراي تخمين ابعاد مخروط رسوبشويي ارايه شده است.

Currently, large dams in the world, due to high amount of sediments in the reservoir, especially around the intake, have operational problems. One of the solutions for this problem is pressure flushing which is an efficient method for extracting the accumulated sediments behind the dams, where, the valves and turbines are placed over there. In this type of flushing, previously deposited sediments are removed by opening the bottom outlets. Sediment is scoured and a funnel shaped crater is created in the vicinity of the bottom outlet opening. Amount of the flushed sediments depend on many parameters such as water depth on the bottom outlets, discharge released through bottom outlets, size of the outlets, geometry of the reservoir, size and kind of the deposited sediments in the reservoir. But the extent of flushing impact range is limited. On the other hand, since in this method, the waste of water in the reservoir is relatively low; therefore, an appropriate solution for increasing the efficiency of the pressure flushing process, would be able to increase the dam’s lifetime with minimal amount of water waste. In laboratory experiments carried out in this study, the effect of expansion of bottom outlet channel within the reservoir is investigated on the volume and dimensions of the flushing cone. In order to achieve the objective of this study, experiments done by means of a physical model with length 7.1 m, wide 1.4 m and height of 1.5 m. Experiments performed with three bottom outlet channel lengths 10, 20 and 30 cm, three water heights 47.5, 55 and 64.5 cm over center of outlet and three discharge flows equal to 1, 2 and 3 (l/s) for each height. Preliminary results showed that, expansion of bottom outlet channel within the reservoir has positive and tangible effects on the size of the flushing cone. As, with the expansion of bottom outlet channel within the reservoir, new hydraulic conditions are introduced, which affects the mechanism of flushing and by increase in the length of outlet channel, dimension of flushing cone increases. But the rate of increase in dimensions decreases with increment in expansion amount. So the relative amount of bottom outlet channel expansion for 0.5, 1 and 1.5 times height of the sediment in the reservoir, leads to increase in flushing cone volume for average amount of 50, 74 and 96% compared to the case with the no developed bottom outlet channel. according to the experimental data, non-dimensional equations are derived for estimation of the flushing cone size. These equations show high regression coefficients and provide good estimations. Also the results indicate that, In the higher discharges of flow, effect of the expansion size of bottom outlet channel on the amount of the sediment discharged is high.