بررسي پديده فرا آب در بالادست پايه پل مستطيلي شكل در قوس رودخانه

Investigation of the flood phenomenon upstream 0n the base of the rectangular bridge in the river bend

ﺑﺎ اﺣﺪاث ﭘﻞ در ﻣﺴﯿﺮ رودﺧﺎﻧﻪ، ﻣﻌﻤﻮﻵ ﻋﺮض ﻃﺒﯿﻌﯽ ﺟﺮﯾﺎن ﮐﺎﻫﺶ ﯾﺎﻓﺘﻪ و ﻣﺎﻧﻌﯽ در ﺑﺮاﺑﺮ ﺟﺮﯾﺎن ﺑﻪ وﺟﻮد آﻣﺪه و ﻋﻤﻖ آب در ﺑﺎﻻدﺳﺖ ﭘﻞ اﻓﺰاﯾﺶ ﻣﯽﯾﺎﺑﺪ. اﯾﻦ اﻓﺰاﯾﺶ ﻋﻤﻖ را ﻓﺮاآب ﻣﯽﻧﺎﻣﻨﺪ. ﺑﻪ ﻣﻨﻈﻮر ﺑﺮرﺳﯽ ﻋﻤﻠﮑﺮد ﭘﺎﯾﻪ ﭘﻞ ﻣﺴﺘﻄﯿﻠﯽ ﺑﺎ اﺑﺘﺪا و اﻧﺘﻬﺎي ﺗﯿﺰ ﮔﻮﺷﻪ ﺑﺮ ﻓﺮاآب ﺑﺎﻻدﺳﺖ آن در ﻗﻮس 180درﺟﻪ، آزﻣﺎﯾﺶﻫﺎﯾﯽ اﻧﺠﺎم ﺷﺪ. ﺑﺮاي اﯾﻦ ﻣﻨﻈﻮر در ﯾﮏ ﮐﺎﻧﺎل ﻣﺴﺘﻄﯿﻠﯽ آزﻣﺎﯾﺸﮕﺎﻫﯽ از ﺟﻨﺲ ﭘﻠﮑﺴﯽ ﮔﻼس ﺑﺎ ﻗﻮس 180درﺟﻪ داراي ﺑﺴﺘﺮ ﺛﺎﺑﺖ در ﺷﺮاﯾﻂ آب زﻻل، آزﻣﺎﯾﺶﻫﺎﯾﯽ اﻧﺠﺎم ﺷﺪ. در اﯾﻦ ﺗﺤﻘﯿﻖ ﺑﺎ ﻗﺮار دادن ﯾﮏ ﭘﺎﯾﻪ ﻣﺴﺘﻄﯿﻠﯽ از ﺟﻨﺲ ﭘﻠﮑﺴﯽ ﮔﻼس در ﻣﺤﻮر ﻣﺮﮐﺰي ﮐﺎﻧﺎل، اﻗﺪام ﺑﻪ ﯾﮏ ﺳﺮي آزﻣﺎﯾﺶﻫﺎ ﺷﺪ. آزﻣﺎﯾﺶﻫﺎ ﺑﺎ ﭼﻬﺎر ﺿﺮﯾﺐ ﺗﻨﮓﺷﺪﮔﯽ، دو ﻧﺴﺒﺖ ﻃﻮل ﺑﻪ ﻋﺮض و در دﺑﯽﻫﺎي ﻣﺨﺘﻠﻒ اﻧﺠﺎم ﺷﺪ. در ﻫﺮ آزﻣﺎﯾﺶ دﺑﯽ ﮐﺎﻧﺎل اﺻﻠﯽ و ﻋﻤﻖ آب در ﺑﺎﻻدﺳﺖ ﭘﺎﯾﻪ در دو ﻧﻘﻄﻪ اﻧﺪازه ﮔﯿﺮي ﺷﺪ. ﻧﺘﺎﯾﺞ اﯾﻦ ﺗﺤﻘﯿﻖ ﻧﺸﺎن داد وﺟﻮد ﭘﺎﯾﻪ در ﻣﺴﯿﺮ ﺟﺮﯾﺎن آب ﺑﺎﻋﺚ ﺑﺎﻻ آﻣﺪﮔﯽ ﻋﻤﻖ ﺟﺮﯾﺎن در ﺑﺎﻻدﺳﺖ آن ﻣﯽﮔﺮدد. ﻫﻤﭽﻨﯿﻦ ﻋﺪد ﻓﺮود ﺑﺎ ﻓﺮاآب راﺑﻄﻪ ﻣﺴﺘﻘﯿﻢ و ﺑﺎ ﺿﺮﯾﺐ ﺗﻨﮓ ﺷﺪﮔﯽ راﺑﻄﻪ ﻣﻌﮑﻮس دارد. ﻧﺘﺎﯾﺞ ﻧﺸﺎن داد ﺑﺎ اﻓﺰاﯾﺶ ﻧﺴﺒﺖ ﻃﻮل ﺑﻪ ﻋﺮض، ﻓﺮاآب اﻓﺰاﯾﺶ ﻣﯽﯾﺎﺑﺪ

With the construction of a bridge along the river, the natural width of the stream is usually reduced and a barrier is created against the flow and the water depth upstream of the bridge increases. This increase in depth is called overflow. In this study, in order to evaluate the performance of the base of a rectangular bridge with a sharp beginning and end on the upstream overflow in a 180 degree bend, experiments with a physical model were performed. For this purpose, experiments were performed in a laboratory rectangular channel made of Plexiglas with a 180 degree bend with a fixed bed in clear water conditions. In this research, a series of experiments were performed by placing a rectangular base made of Plexiglas in the central axis of the canal. The experiments were performed with four coefficients of constriction, two length-to-width ratios and at different discharges. In each experiment, the main channel discharge and water depth upstream of the base were measured at two points. The results of this study showed that the presence of a base in the water flow path causes the flow depth to rise upstream. Also, the Froude number (Fr) is directly related to the flood and is inversely related to the coefficient of narrowing. Also results showed that with increasing the length-to-width ratio the overflow increases.