مقایسه آزمایشگاهی روشهای مختلف از نظر کنترل رسوب و آبشستگی در آبگیرهای 90 درجه

در این تحقیق بصورت آزمایشگاهی، تاثیر ترکیب سازه های آستانه، آبشکن و صفحات مستغرق بر کنترل رسوب بستر ورودی به آبگیر جانبی 90 درجه از یک کانال مستقیم و نیز ابعاد آبشستگی در پایین دست آبگیر در کانال اصلی مورد بررسی قرار گرفته است. تحقیقات در چهار حالت بدون سازه، حالت فقط آستانه، حالت آستانه و آبشکن و حالت آستانه، آبشکن و صفحات مستغرق و در سه نسبت آبگیری 0.12، 0.15 و 0.18 انجام شده است. در هر حالت نسبت رسوب انحرافی، نسبت حجم رسوب انباشته در آبگیر ، پروفیل عرضی بستر و طول، عرض و عمق ناحیه آبشستگی مورد تحقیق قرار گرفته است. نتایج نشان میدهد در نسبتهای آبگیر مذکور، در حالت آستانه و آبشکن بطور متوسط 90 درصد رسوب ورودی به آبگیر کاهش می یابد ولی آبشستگی در پایین دست آبگیر در مقایسه با حالت بدون سازه بیش از 55 درصد افزایش می یابد در حالیکه حالت آستانه، آبشکن و صفحات مستغرق علاوه بر اینکه توانسته است تقریباً از ورود تمام رسوبات بستر به آبگیر جلوگیری کند و راندمان 100 درصدی داشته باشد، در نسبتهای آبگیری بالاتر از 0.15 میزان آبشستگی را نسبت به حالت بدون سازه، کاهش داده و در نسبت آبگیری 0.18 به کمتر از نصف رسانده است.

Experimental comparison of different methods with regard to sediment control and scour at the 90 degree water intakes

An important goal of deviation of water flowing in a river is to control sediment and supply required water with minor sediment. This goal can be achieved using some sediment control structures, such as sill, spur dike and submerged vanes. These structures, however, can influence the flow pattern in a water intake, which induces scouring at downstream of the intake in main channel. For instance, generated helical motion due to secondary circulation induced by submerged vanes is the main cause of scouring at downstream of these structure. As for spur dikes, the generated helical motion is also the main cause of scouring in the main channel downstream, which occurs through increased flow velocity and curvilinear flow near the spur dikes. The induced scouring may impact the stability of coastal region near the channel/river that must be taken into account in designing process. In this study some experimental tests were carried out to understand the effect of sill, spur dike and submerged vanes on sediment control and scouring in the downstream of intake in the main channel. Four different cases were considered to be discussed; in the first case, there was no sediment control structure installed. In the second one, however, the effects of a sill with a height of one third of the flow depth at the entrance of the intake was evaluated. In the third case the effect of installing both sill and spur was studied. In this case, in addition to a sill installed at the intake entrance, an impermeable direct nonsubmerged spur, with a length of 1/4 of width of the main channel at upstream, was mounted on the opposite side of the intake at upstream of the main channel. In the fourth case, submerged vanes were added up to the two other mentioned structures earlier. The submerged vanes were put in two parallel rows in front of the intake in the main channel. All experimental tests were conducted in a flume equipped with a recirculating sediment system and a 90 ° lateral diversion channel. Three important parameters including the ratio of bed sediment transport into intake (Gr), the volume fraction of sediment deposited within intake (Vr), and the dimensions/volume/area of scouring at upstream in the main channel were evaluated in this study under three different discharge ratios of 0.12, 0.15 and 0.18. The experimental results indicate that the mentioned parameters are mainly determined by the discharge ratio and the mechanism of sedimentation control. It can be noted that Gr increases with Qr, whereas Vr decreases as Qr raises up. It was also observed that all sediment control structures play an important role in sediment control at the intake entrance, although the influence of spur dike and submerged vanes is greater as compared with that of sill, which causes a significant reduction in Gr and Vr. It was also found that the dimensions/volume/area of scouring area at upstream in the main channel is mainly controlled by existence of those structures. Generally speaking, the dimensions/volume/area of scouring area is mainly controlled by the velocity of water in contact with the downstream bank of intake, Qr, the power of induced secondary flow by the submerged vanes and the spur dike, and the cumulative sediment in front of the intake due to existence of sill. It was also noticed that in some cases both the submerged vanes and the spur dike may result in scour increase. The dimensions/volume/area of scouring area demonstrated different behavior over different Qr ranges that could be described through this fact that these parameters are influenced by many causes simultaneously. To approximate Gr, Vr and dimensions/volume/area of scouring area in different situations, some relationships have been presented in this study. A comparison has also been made between the results obtained from the current study and those presented by other authors, based on which the most proper structure was chosen with respect to sediment control and scouring. Eventually, the third and fourth cases were found as the most desirable system able to control sediment more efficiently comparing to other cases.