تاثیر ارتفاع و موقعیت عمودی شکاف در کاهش آبشستگی پایه‌پل استوانه‌ای در بستر با رسوبات یکنواخت و نا‌یکنواخت

آبشستگی از مهم‌ترین عوامل تخریب پایه‌ی پل‌هاست. یکی از راه‌هایی که باعث کاهش قدرت جریان سیستم گردابی در مکانیزم آبشستگی می‌شود، استفاده از شکاف در پایه‌ی پل است. با توجه به این‌که رسوبات رودخانه‌ها از مصالح طبیعی (رسوبات غیریکنواخت) می‌باشد بررسی تاثیر و عملکرد شکاف در رسوبات غیریکنواخت حائز اهمیت است. در این پژوهش آزمایشگاهی، سه دبی (8، 10 و 12 لیتر بر ثانیه) و سه مدل شکاف، به همراه پایه بدون شکاف به عنوان پایه شاهد، در رسوبات یکنواخت و غیریکنواخت بررسی شد. نتایج نشان‌داد با تبدیل رسوبات یکنواخت به غیریکنواخت تا 77 درصد از عمق آبشستگی در مدل‌های مختلف پایه کاسته می‌شود. برای دو نوع شکاف با ابعاد یکسان، شکافِ هم‌تراز با بستر تاثیر بیشتری نسبت به شکاف نزدیک به سطح آب دارد. افزایش ارتفاع شکاف هم‌تراز با بستر به اندازه 2/6 سانتی‌متر (حداکثر عمق آبشستگی در دبی 12 لیتر بر ثانیه برای پایه شاهد در رسوبات یکنواخت) در زیر بستر در رسوبات یکنواخت و غیر یکنواخت، منجر به کاهش عمق آبشستگی به طور میانگین به ترتیب 47 و 43 درصد نسبت به پایه شاهد می‌شود. با افزایش دبی و عدد فرود جریان، عمق آبشستگی در مدل‌های مختلف پایه و بستر افزایش یافته است.

The Effect of Height and Vertical Position of Slot on the Reduction of Scour Depth around Cylindrical Bridge Pier in Uniform and Non-uniform Sediments

Introduction: Bridge scour is one of the most important challenges in river engineering. Every year, thousands of bridges around the world are destroyed due to scouring. Hence, the accurate prediction of maximum scour depth for saving financial resources and the safety of human lives is of paramount importance. One way to reduce scouring is to use slot at the pier of the bridge. Research on the effect of the slots, in reducing scour around the bridge pier, has been done bed sediments of uniform type and in most cases the depth of flow have been considered as a constant factor. The aim of the present study is to investigate the effect of sediment nonuniformity in the presence or absence of slots in the bridge pier and the relative effect of increasing the depth of flow (with increasing flow rate) on the scour depth around the cylindrical pier.Methodology: The present experiments have been conducted in hydraulic laboratory at BuAli Sina University, Hamadan (Iran). The pier model with a diameter of 4 cm was put inside a rectangular flume with dimensions of 0.5 m wide, 10.5 m long and 0.5 m height. Three different types of slots were used along with the pier without slot. In order to obtain the effect of sediments on scour depth, two types of bed were used. The bed of the first and second types had uniform and nonuniform sediments, respectively. In this experiment, flow rates of 8, 10 and 12 l/s were used with corresponding flow depths of 5.7, 6.6 and 7.4 cm, respectively, Melville and Chiew (1999) chose a time as equilibrium after which scour depth changes of less than 0.5% of the pier diameter over a 24hour period. According to this criterion, the time of all experiments was considered to be 6 hours.Results and Discussion: The percentage of scour depth reduction varies according to the height of the slot and its location. Comparison of S1 and S2 models for two slot models in the same dimensions showed that the closer the slot is to the bed, the greater its effect in reducing scour depth. Also, increasing the height of the slot under the bed (S3 compared to S2) is more effective in reducing the scour depth. Comparison of the first and second beds showed that by converting uniform to nonuniform sediments under constant particle diameter conditions, the maximum scour depth in models S0,S1,S2 and S3 at a flow rate of 8 l/s was 76, 78, 76 and 79%, is respectively reduced. The reason for this can be the formation and expansion of a layer of coarse particles on the bed surface (armor layer). As the flow rate increases due to the increase in flow depth, the scour dimensions increases. According to the obtained results, the scour hole in the nonuniform bed reaches equilibrium with less time than the uniform bed. The formation of an armor layer in a nonuniform bed is the reason for less equilibrium time in similar conditions than in a uniform bed. With increasing the Froude number of flow in all base models, the depth and dimensions of the scour hole have increased. Increasing the Froude number makes the downflow current stronger, and as a result, horseshoe vortices are formed and act with less time. In clear water conditions, the maximum scouring depth will occur in the state u/〖 u〗_c =1 (Melville and Chiew 1999.( The results showed that with increasing the ratio u/〖 u〗_c of clear water scouring conditions, scouring depth increases in a variety of models and beds. According to Guo (2012) if the ratio of flow depth to pier diameter is less than 6. By increasing this ratio, the scour depth in uniform and nonuniform sediments increases.Conclusion: The results of this research showed that the use of slots in Cylindrical bridge pier was more effective in reducing scouring than the use of bridge piers. With the conversion of uniform to nonuniform bed due to the formation of the armor layer, the scour depth has decreased by an average of 77% in different base models and also the time to reach the scour equilibrium has been reduced. Scour depth has increased with increasing flow rate, Froude number and ratio u/〖 u〗_c in different pier and bed models. In general, the closer the slot is to the bed, the more it reduces the scouring depth and the nonuniformity of sediments due to the formation of an armor layer around the pier causes a significant reduction in scouring depth.Keywords: Bridge Pier, Armor Layer, Nonuniform Bed, Slot