تغييرات توپوگرافي بستر در قوس ملايم 90 درجه با نصب آبشكن مثلثي‌ شكل

Bed Topography Variations in a 90° Mild Bend Due to Triangle-Shaped Spur Dike

به منظور كنترل آبشستگي در قوس بيروني رودخانه ها از سازه هاي مختلفي مانند آبشكن، سرريز مستغرق، صفحات مستغرق و غيره براي اصلاح الگوي جريان رودخانه استفاده مي‌شود. از اندركنش اين سازه‌ها و جريان آب، در اطراف سازه آبشستگي بوجود مي آيد كه عامل اصلي تخريب آن‌ها است. فرض بر اين است كه تغيير هندسه و جانمايي سازه مي‌تواند توپوگرافي بستر را در جهت پايداري آنها تغيير دهد و اين از اهداف اين پژوهش است. پس در اين پژوهش آبشكن مثلثي شكل با زواياي مختلف (23، 30، 40، 60 و 70 درجه) درجهت بالادست و تحت شرايط هيدروليكي متفاوت (اعداد فرود 194/0، 214/0، 233/0 و 253/0) آزمايش شد. در همه آزمايش ها تك آبشكن در موقعيت بيشترين عمق آبشستگي قوس 90 درجه ملايم يعني زاويه 72 درجه نصب شد. عمق جريان در تمام آزمايش‌ها ثابت و برابر 13 سانتي‌متر بوده است. توپوگرافي بستر در انتهاي هر آزمايش برداشت و از روي آن محدوده آبشستگي، بيشينه عمق آبشستگي و نيز فاصله چاله آبشستگي و پشته رسوبگذاري پايين دست تا ساحل بيروني، تعيين شد. نتايج اين پژوهش نشان دادند كه به‌ طور كلي در محل پنجه آبشكن چاله‌ آبشستگي تشكيل مي‌شود كه به بالادست و پايين‌دست سازه كشيده ‌مي‌شود. زواياي 23 و 60 درجه به ترتيب داراي كمترين و بيشترين، عمق آبشستگي و فاصله خط القعر از ساحل بيروني است. بيشينه عمق آبشستگي زواياي 23 و 60 درجه به ترتيب 127/0 و 496/0 طول موثر سازه است درضمن فاصله بيشينه عمق آبشستگي (خط القعر) زواياي ياد شده از ساحل بيروني بترتيب 743/0 و 6/1 طول موثر سازه است. با افزايش زاويه، فاصله پشته رسوبگذاري شده از ساحل بيروني و توزيع آن در عرض مجرا، افزايش مي يابد. در زواياي 23 و 70 درجه فاصله پشته رسوبگذار از ساحل بيروني به ترتيب 228/0 و 365/0 طول موثر سازه است.

The erosion that occurs at the bends of rivers can damage the surrounding area during flood season and cause morphological changes in the river to some extent. Therefore, it engineers have become interested in developing measures that can control the scour process that occurs at bends in rivers. To control the outer bend of river from scour different structures such as spur dike, bendway weir and submerged vane can be used for modification of flow pattern. With interaction of these structures and water flow scour around structure occurs that is the main cause of their failures. It is assumed that the change of geometry and layout of structure can change bed topography for its stability and that is the purpose of the present study. Therefore, in this study a Triangle-shaped spur dike with different angles (23, 30, 40, 60 and 70 degrees) to the upstream direction under different hydraulic conditions (Froude numbers 0.194, 0.214, 0.233 and 0.253) was tested. For all experiments, The effective length (Le) of each Spur Dike, i.e., the horizontal distance from the outer bank to the interface of the Spur Dike and the bed, was 33% of the width of the flume (23.3 cm) and single Triangle-shaped spur dike was installed outside bend of flume in 72° position which maximum scour depth is usually occurring. For all experiments, flow depth was kept constant equal to 13 cm. The thickness of the sediment in the bed of the flume was 20 cm, and the sediment was composed of natural, uniform sand (geometric standard deviation of 1.22) with an average size of 1.5 mm. At the end of each experiment bed topography was measured and from that the scouring zone, maximum scour depth and distance of scour hole was computed. The results showed that the overall scour hole is created in toe of the spur dike that is expanded to the upstream and downstream structure. Triangle-shaped spur dike with 23 and 60 degree installation was found to create the minimum and maximum scour depth as well as its distance from the outer bank respectively. The maximum scour depth for spur dike with angles of 23 and 60° was measured to be 0.127 and 0.496 of the spur dike effective length respectively. In addition, the maximum scour depth distance of above mentioned angles from the outer bank was respectively, 0.743 and 1.6 the effective length of structure. Eroded sediments were deposited downstream the spur dike and point bars are developed. By increasing of spur dike angle, the point bars are formed away from the outer bank and extended in flume width. For spur dike angles of 23 and 70° the point bar distance from the outer bank was equal to 0.228 and 0.365 effective length of the structure respectively.