آبشستگي در اطراف پايه مربعي با دماغه سهمي شكل در حضور اجسام شناور چوبي در جلوي پايه

Scour around a square pier with parabolic nose in presence of woody debris in front of pier

هدف از اين پژوهش، تحليل و بررسي آزمايشگاهي تاثير تجمع اجسام شناور در جلو ي پايه مربعي با دماغه سهمي شكل بر عمق گودال آبشستگي است. آزمايش ها در شرايط آب زلال و در دو حالت با و بدون تجمع اجسام شناور در جلوي پايه با دبي هاي 20، 30 و 40 ليتر بر ثانيه و براي ابعاد مختلف اجسام شناور انجام گرفت. نتايج نشان داد كه وقتي اجسام شناور بالاي سطح آب، هم سطح آب و زير سطح آب باشند عمق آبشستگي، به ترتيب، بيشينه تا 2/2، 36/2 و 44/1 برابر نسبت به نمونه شاهد افزايش مي يابد. علاوه بر اين، زماني كه درصد انسداد (نسبت مساحت اشغال شده جريان به وسيله‌ي اجسام شناور به سطح مقطع جريان) برابر با %29/35 و اجسام شناور در هم سطح آب قرار گرفته باشند، بيشينه عمق آبشستگي (36/2 برابر نمونه شاهد) رخ مي‌دهد. همچنين در حالتي كه اجسام شناور وجود ندارند، ميزان آبشستگي پايه مربعي با دماغه سهمي شكل نسبت به حالت پايه مربعي با دماغه تيز، 5/1 برابر كمتر است. بعلاوه وجود اجسام شناور در جلوي پايه مربعي با دماغه سهمي تاثير قابل ملاحظه اي بر كاهش عمق، عرض و طول گودال آبشستگي نسبت به پايه مربعي با دماغه تيز دارد (3/2 برابر). همچنين با استفاده از آناليز ابعادي معادله ي جديدي براي پيش‌بيني بيشينه عمق آبشستگي با وجود تجمع اجسام شناور چوبي در جلو پايه مربعي با دماغه سهمي شكل ارايه شد.

Woody debris accumulates in front of bridge piers, reduces the flow area, deviates the flow and increases the velocity around the bridge piers. Debris accumulation in front of bridge piers increases the downward flow, and shear stress around the bridge pier, therefore, the scour hole depth increases and bed degradation accelerates. Most of previous researchers focused on the scour depth around the bridge piers, and less researchers have investigated the effect of debris accumulation on the scour depth. To the best of our knowledge, the effect of debris accumulation on the scour depth has not been reported in previously published literature, as explored in this work. The purpose of this study is to experimentally investigate the effect of accumulation of woody debris in front of a square pier with a parabolic nose on the scour depth. The experiments were performed in clear water condition, with and without debris accumulation and with 20, 30 and 40 l/s discharges and for different debris dimensions. The experiments were performed in a sixty centimeter width channel at the hydraulic laboratory of Shahrekord University. Previous field studies showed that most debris accumulates in front of bridge piers in rectangular shapes, therefore three different rectangular shapes debris are designed and are placed in front of bridge piers during the experiments. Since, performed experiments at twelve hours showed the maximum equilibrium scour depth occurs at the first seven hours, therefore, all experiments are done in 420 minutes. The results showed that when woody debris is placed over, at, and below the water surface respectively, the ratio of scour depth to the scour depth of control sample is 2.2, 2.36 and 1.44, respectively. Moreover, when the percentage of blockage (ratio of the occupied flow area by debris to the flow cross section) is 30% and when debris is located below the water surface, the maximum scour depth will occur (2.36 with respect to the control sample). Also, the comparison between a square pier with parabolic nose and a sharp nose piers at the same hydraulic condition, show when there are no debris, the scour depth around the square pier with a parabolic nose is less than the scour around a sharp nose square pier (1.5 times). For the case in which debris are placed in front of bridge pier, the scour depth, width, and length around a square pier with a parabolic nose are significantly decreased with respect to a sharp nose square pier (2.3 times). Also, using dimensional analysis an equation is presented for predicting the maximum scour depth around a square pier with a parabolic nose in presence of woody debris accumulation. The results show that there is good agreement between predicted and observed scour depth.