بررسی آزمایشگاهی تاثیر آب نگارهای دارای چولگی و زمان تداوم متفاوت بر تغییرات زمانی آبشستگی پیرامون تک پایه استوانه ای

آبشستگی بیش از حد در هنگام سیلاب به عنوان تهدیدی برای پایداری سازه‌های واقع در بستر فرسایش‌پذیر به شمار می‌رود. در این تحقیق به بررسی آبشستگی پیرامون پایه استوانه‌ای در شرایط جریان غیرماندگار با شبیه‌سازی هیدروگراف‌های مختلف سیلاب پرداخته شد. اثرات شاخه‌های خیزان و افتان هیدروگراف و همچنین زمان تداوم هیدروگراف بر روی توسعه زمانی عمق آبشستگی و حداکثر عمق آن مورد ارزیابی قرار گرفت. نتایج نشان داد برای هیدروگراف‌های با دبی پیک و زمان تداوم مشابه، زمان شاخه خیزان هیدروگراف بر روی توسعه زمانی تنش برشی و عمق آبشستگی به شدت تاثیرگذار است اما روی حداکثر عمق آبشستگی تاثیر ناچیزی دارد. علاوه بر این مشخص شد با کاهش زمان شاخه خیزان، اثرگذاری شاخه افتان هیدروگراف بر روی حداکثر عمق آبشستگی افزایش می‌یابد. بررسی نتایج اثر زمان تداوم هیدروگرف بر آبشستگی موضعی نشان داد با افزایش زمان تداوم هیدروگرف از 10 دقیقه به 160 دقیقه حداکثر عمق آبشستگی 69/44 درصد افزایش و شیب تغییرات زمانی عمق آبشستگی کاهش می‌یابد.

experimental investigation of the effect of hydrographs with different skewness and duration time on temporal variations of scour around a single cylindrical pier

for a bridge pier in the flow path, a three-dimensional and complex flow pattern is formed around the pier leading to the formation of a scour hole around it. the development of the scour hole will cause the instability of the bridge pier and ultimately the destruction of the pier and the bridge. this problem becomes very important during the floods, when the flow in the river increases rapidly and has the highest potential of destruction. most studies have investigated scouring in steady flow conditions. the maximum scour depth that occurs under a flood hydrograph can be much smaller than the equilibrium depth resulting from steady flow under peak discharge conditions (kothyari et al., 1992; lai et al., 2009). therefore, the use of flood peak discharge for design can greatly overestimate the maximum scour depth compared to the actual flood conditions (chang et al., 2004). considering the importance of scouring investigation in the conditions of unsteady flow and the limited available studies in this regard, more research in this field is necessary. the purpose of this research is to investigate the effect of the time of the rising and falling limbs of the hydrograph, as well as the duration of the hydrograph on the temporal variations of scour depth and its maximum value around the cylindrical pier.methodologythe experiments were carried out in a rectangular flume with glass walls and a straight length of 10 m, a width of 0.74 m and a depth of 0.6 m in the physical and hydraulic modeling laboratory of shahid chamran university, ahvaz. the test section in the flume was covered with uniform sand with an average size of d50=0.7 mm and geometric standard deviation σ=1.3. in order to achieve the goals of this study, a total of 13 experiments were examined. in order to investigate the effect of the time of the rising and falling limbs of the hydrograph on the temporal variation and the maximum scour depth, a number of 6 hydrographs with a constant duration of 100 minutes and the ratio of the time to reach the peak (tp) to the duration time (td) of the hydrograph (skewness) equal to 0.1, 0.2, 0.4, 0.6, 0.8 and 0.9 were designed. also, 7 hydrographs with gaussian distribution and duration times (td) of 10, 20, 45, 80, 100, 120 and 160 were simulated to investigate the effect of flood duration on scouring (fig. 3 and table 1). a hydrograph generation system was used to create unsteady flow in the flume. this system included a programmed inverter that was used to adjust the variable flow rate of the hydrograph. the inverter was connected to the pump on one side and to the electromagnetic flow meter on the other side and was run by a computer through a software.results and discussionthe results of the temporal variations of scouring showed that scouring starts from the sides of the pier and reaches the nose of the pier over time, and finally, the maximum depth of scouring occurs in the nose of the pier. the results showed that the maximum scour depth in the hydrograph with a gaussian distribution occurs after the peak time (about 10% of duration time) (fig. 5).