ارزيابي مقاومت جريان در رودخانه هاي رسوبي با فرم بسترريپل درشرايط هيدروليكي مختلف

Evaluation of Flow Resistance in Alluvial Streams with Ripples in Various Hydraulic Conditions

در آبراهه‌هاي آبرفتي، حركت رسوبات و فرم بستر ناشي از آن، تأثير زيادي بر مقاومت در مقابل جريان آب دارد. در ﺗﺤﻘﯿﻖ ﺣﺎﺿﺮ ﺑﺮاي ﺑﺮرﺳﯽ ﺗﺄﺛﯿﺮ ﻓﺮم ﺑﺴﺘﺮ رﯾﭙﻞ ﺑﺮروي ﺗﻨﺶ ﺑﺮﺷﯽ ﺟﺮﯾﺎن آزﻣﺎﯾﺶﻫﺎﯾﯽ در ﯾﮏ ﻓﻠﻮم آزﻣﺎﯾﺸﮕﺎﻫﯽ ﺑﺎ ﺗﻐﯿﯿﺮ در دﺑﯽ، ﺷﯿﺐ و ﻋﻤﻖ ﺟﺮﯾﺎن اﻧﺠﺎم ﺷﺪ ﺗﺎ ﺗﺄﺛﯿﺮ اﯾﻦ ﭘﺎراﻣﺘﺮﻫﺎ ﺑﺮروي ﻣﻘﺎوﻣﺖ ﺟﺮﯾﺎن ﻣﻄﺎﻟﻌﻪ ﺷﻮد. در ﺑﺴﺘﺮﻫﺎي داراي ﻓﺮم، ﺗﻨﺶ ﺑﺮﺷﯽ از دو ﺑﺨﺶ ﺗﺸﮑﯿﻞ ﻣﯽﺷﻮد. 􀀂􀀁 ﮐﻪ ﺗﻨﺶ ﺑﺮﺷﯽ ﻣﺆﺛﺮي اﺳﺖ ﮐﻪ ﺑﺮ ﺗﮏﺗﮏ ذرات رﺳﻮب وارد ﻣﯽﺷﻮد و 􀀂􀀁 ﻧﯿﺰ ﺗﻨﺶ ﺑﺮﺷﯽ اﺳﺖ ﮐﻪ از ﻣﻘﺎوﻣﺖ ﺷﮑﻠﯽ ﯾﺎ ﻓﺮم اﯾﺠﺎد ﻣﯽﮔﺮدد. ﻫﻤﭽﻨﯿﻦ ﺑﺎ ﺗﺸﮑﯿﻞ ﺑﯿﺸﺘﺮ ﻓﺮمﻫﺎي ﺑﺴﺘﺮ ﺗﻨﺶ ﺑﺮﺷﯽ و ﺿﺮﯾﺐ زﺑﺮي اﻓﺰاﯾﺶ ﯾﺎﻓﺖ. ﻧﺘﺎﯾﺞ ﻧﺸﺎن داد ﮐﻪ ﺑﺎ ﺗﺸﺪﯾﺪ ﭘﺎراﻣﺘﺮﻫﺎي ﺟﺮﯾﺎن و در ﻧﺘﯿﺠﻪ ﺗﺸﺪﯾﺪ اﯾﺠﺎد ﺗﻼﻃﻢ در ﭘﺎﯾﯿﻦدﺳﺖ ﺗﺎج ﻓﺮم ﺑﺴﺘﺮ، ﻣﻘﺪار ﭘﺎراﻣﺘﺮﻫﺎي ﻣﻮرد ﻣﻄﺎﻟﻌﻪ اﻓﺰاﯾﺶ ﯾﺎﻓﺖ. ﻧﺘﺎﯾﺞ ﺑﯿﺎنﮔﺮ آن ﺑﻮد ﮐﻪ ﺑﺎ اﻓﺰاﯾﺶ ﺳﺮﻋﺖ ﺟﺮﯾﺎن، در ﻣﺤﺪوده ﻣﺸﺨﺼﯽ از دﺑﯽ )10 ﺗﺎ 14 ﻟﯿﺘﺮ ﺑﺮ ﺛﺎﻧﯿﻪ( و ﺷﯿﺐ ﮐﻒ )0 ﺗﺎ 0/015( ارﺗﻔﺎع رﯾﭙﻞﻫﺎ زﯾﺎد ﺷﺪه و ﻫﻤﭽﻨﯿﻦ ﺗﻌﺪاد رﯾﭙﻞﻫﺎي ﺗﺸﮑﯿﻞﺷﺪه ﺑﯿﺸﺘﺮ ﻣﯽﺷﻮﻧﺪ. ﺑﯿﺸﺘﺮﯾﻦ ﻣﺤﺪوده ﺗﻐﯿﯿﺮات ﻣﺸﺎﻫﺪه ﺷﺪه در 70 ﺳﺎﻧﺘﯽﻣﺘﺮي اﻧﺘﻬﺎﯾﯽ ﮐﺎﻧﺎل ﺑﻮد. اﻓﺰاﯾﺶ ارﺗﻔﺎع ريپل، موجب شد كه انتقال رسوبات نيز زياد شده و به‌ازاي افزايش 35 درصدي پارامتر بي‌بعد (ارتفاع فرم بستر به طول آن)، دبي رسوب بي‌بعد در حدود 40 درصد افزايش نشان داد. با بررسي روند تغييرات تنش برشي ناشي از فرم بستر نسبت به ارتفاع فرم بستر مشخص شد كه با افزايش 45 درصدي ارتفاع فرم بستر به‌ميزان 58 درصد به تنش برشي ناشي از ريپل‌ها اضافه شد.

Sediment transport and bedforms in alluvial streams, significantly affect the flow resistance. In the present research to discuss the effect of ripple bedform on the flow shear stress, a series of experiments were done and the influences of ripples on flow roughness coefficient were investigated. These experiments were carried out in a laboratory flume with changing discharge, slope and flow depth alternatively to create different hydraulic conditions. In flow surfaces having bedforms, shear stress includes two parts. The first part is the , which is imposed on each particle of the sediment and the second part is the , which is the stress created by bedforms. The results showed that the shear stress and roughness coefficient increased with formation of bedforms. Also, with intensifying the flow parameters the turbulences were grown in the downstream of the bedform crest and the studied factors increased accordingly. In a certain limit of discharge (10 to 14 lit/s) and slope (0 to 0.015), results showed that with increasing flow velocity, the height of ripple increased and the number of them increased too. The most variation area was at the last 70 cm length from the end of the channel. With ripple height increasing, sediment transportation was increased more and there was a 40 percent growth of dimensionless sediment discharge, when the (bedfrom height to the length) increased about 35 percent. Studying the shear stress caused by bedform in relation with bedform height, it was obvious when the ripple height increased about 45 percent, the bedform shear stress increased about 58 percent.