بررسي هيدروليك فرآيند رسوب‌شويي تحت‌فشار با توسعه مجراي تخليه‌كننده تحتاني در مخزن سد

Hydraulic investigation of pressure flushing with expanding bottom outlet channel within the dam reservoir

در هنگام رسوب‌شويي تحت‌فشار با تخليه رسوبات نهشته شده از مخزن سد به كمك جريان خروجي از دريچه‌هاي تخليه‌كننده تحتاني، آب‌شستگي به شكل مخروط مقابل تخليه‌كننده به وجود آمده و توسعه مي‌يابد. در اين پژوهش، تاثير توسعه مجراي تخليه‌كننده تحتاني در مخزن، بر ابعاد مخروط رسوب‌شويي و همچنين هيدروليك اين فرآيند بررسي شده است. آزمايش‌ها با عمق و دبي‌هاي مختلف جريان در طول‌هاي مختلف توسعه مجرا براي تعيين ژيومتري مخروط رسوب‌شويي طراحي و انجام شد. براي بررسي هيدروليك جريان نيز، اندازه‌گيري سرعت جريان با دستگاه سرعت‌سنج صوتي داپلر انجام شد. نتايج نشان دهنده تاثير مثبت توسعه مجرا در مخزن بر ابعاد مخروط رسوب‌شويي است، به طوري ‌كه توسعه مجرا به ميزان نسبي 5/0، 1و 5/1 برابر ارتفاع رسوبات در مخزن سبب افزايش طول مخروط رسوب‌شويي به ميزان 48، 83 و 113 درصد و افزايش حجم مخروط به ميزان 50، 74 و 96 درصد نسبت به حالت بدون توسعه مجرا مي‌شود. نتايج بررسي هيدروليك جريان نشان مي‌دهد، بردارهاي سرعت جريان با نزديك‌تر شدن به دهانه ورودي مجرا، حالت گسترش يافته خود را از دست داده و گراديان سرعت در نواحي مجاور آن به شدت افزايش مي‌يابد. ‌همچنين با استفاده از داده‌هاي آزمايشگاهي، روابطي با قابليت برآورد بالا براي تخمين ابعاد مخروط رسوب‌شويي ارايه شده است.

Sedimentation in dam reservoirs is an important issue which requires to be considered within the operation life of the dam. In order to maintain long time storage in dam reservoirs, sediment removal from reservoirs is an essential issue. There are numerous methods to achieve this goal. One of the well known sediment removal methods is the hydraulic flushing. This method is classified into free and pressure classes. Pressure flushing is considered as an effective method in removing local accumulated sediments behind the dam and around the location of valves and turbines. During the flushing operation, water level in the reservoir creates pressure on the sediment, causing the removal of the sediment and after a while of flushing a funnel shaped crater is created in the vicinity of the bottom outlet opening. In this study, the effect of expansion of bottom outlet channel within the reservoir on the dimensions of the flushing cone were investigated from the hydraulics view point. The experiments were conducted with a physical model of a rectangular box with 7m length, 1.4m width, and 1.5m height, consisting of three parts, namely the inlet of the model, the main reservoir, and settling basin. In the inlet of the model, the turbulence of the inflow is disappeared and a uniform flow enters the main reservoir. Since in pressure flushing, the water surface level inside the reservoir is constant during the operation, the water level is adjusted by using a spillway positioned in the inlet of the model during the whole procedure. The main reservoir of the model was 5m long and the sediments were placed within this part of the model. The sediment particles used were non-cohesive silica sediments with uniform size and with median diameter (d50) 1.15mm and geometrical standard deviation (?g) 1.37. In order to perform flushing, an outlet made of Plexiglas with the diameter of 5cm aligned with the central line of flow was used. To investigate the effect of the bottom outlet channel expansion within the reservoir on the dimensions of flushing cone, the experiments carried out with different discharges and water depths above the bottom outlet in different expansion size of outlet channel in constant sediment level of 20cm above the center of the channel. Also experiments in the same hydraulic conditions without channel expansion within the reservoir were conducted as control tests. To perform the experiments, first the Plexiglas tube was positioned in its place as the outlet channel. The model was slowly filled with water until the water surface elevation reached to a desired level. The bottom outlet was manually opened until the outflow discharge, become equal to the inflow discharge. After a while, the sediments were discharged with the water flow in very high concentrations through the outlet channel (sudden discharge) and a funnel shaped crater was formed in front of it. At the end of each experiment, the flushing outlet was closed in which the incoming discharge was set to zero, then water was carefully and slowly drained from the main reservoir. After the run of