عنوان مقاله [English]
نویسندگان [English]چکیده [English]
One of the common methods to dissipate energy of the flow passing over a dam is using flip bucket spillways to create free falling jet to the downstream. However, the collision of the falling jet to the downstream bed will cause topography changes due to scour threatening the stability and safety of the hydraulic structures. In case of precise analysis of the downstream topographic conditions due to the falling jet, such structures perform better than other dissipaters. Experimental results show that the dimensions of scour hole are dependent upon upstream water depth, discharge and tail water depth. Due to water and sediment flow complexities and the effect of different geometric-hydraulic parameters on bed profile variations, more studies are needed.
In the present study, the effect of flip bucket tooth and upstream gate opening on downstream bed topographic changes due to falling jet was investigated using a flat flume having dimensions of 2m*0.5m*0.5m. The free falling jet was controlled using a sluice gate located upstream of the flume. Falling jet hit a downstream sedimentation basin having dimensions of 2 m long, 1.5 m wide and 0.7 m deep. Tail water depth was controlled using a gate located at the end of the sedimentation basin. Spillway and flip bucket of the Karoon III dam were modeled considering a 1.165 scale. Experiments were carried out on a normal (simple) and slotted flip buckets. Slot dimensions were selected as 2 cm according to USBR (1987) recommendations. To control the water surface level at upstream of the spillway for different discharges, a sluice gate was used at the spillway inlet. To investigate bed topographic changes, sedimentation basin was filled with non-uniform silica sediments with a mean particle diameter of d50=2.76 mm, the geometric standard deviation of ?=2.11 mm, uniformity coefficient of Cu=4.07 and a specific gravity of ?s=1.52 gr/cm3 according to grading curve up to a depth of 40 centimeters. Researches show that several geometric-hydraulic parameters influence bed topography changes at downstream of the flip buckets. Most important parameters include input flow velocity to the flume (V), tail water depth (Yt), flow density ( ?_w), kinematic viscosity (?), acceleration of gravity (g), specific or mean diameter of bed particles (d50), bed particles density (?_s), falling jet height from the bucket edge to sediment level (Hw), gate opening height (L), bucket tooth height (h), Manning coefficient of shoot surface (n), flip bucket radius (R), time (t), sedimentation basin width (B) and geometric standard deviation of non-uniform materials (?). Using dimensional analysis and combining dimensionless parameters, the dimensionless densimetric Froude parameter (Frd) is achieved.
In this study, after depiction of the longitudinal profile changes of the hole and mound scour due to falling jet, variations of the obtained dimensionless parameters were first investigated. Then, using statistical analysis, experimental relations to estimate the scour hole depth and downstream sediment mound height were presented. For this purpose, the effect of a tooth located at the end of a flip bucket on downstream bed topography changes was experimentally investigated considering different discharges, spillway gate opening percentages and tail water depths. After leveling bed sediment surface at the beginning of each experiment, discharge was adjusted using the electromagnetic flow meter and water depth was controlled using a spillway sluice gate. Besides, tail water depth was stabilized using a gate available at the end of the sedimentation basin and the falling jet was finally released to the basin. In order to determine experiment’s equilibrium time, several primary experiments were carried out considering different discharges in different time periods. By drawing equilibrium time curves, 360 min time duration equal to formation time of 88% of the total scour hole depth was considered as experimental time duration. Therefore, a total of 54 experiments were carried out considering the studied variables in this study. Water jet was released from 57 centimeters height of the bucket end level to the non-uniform bed sediments during 360 minutes experiments. Bed topography changes were measured at the end of each experiment using a dot sounder with 0.01 mm precision and depicted using Surfer V. 9.0. By calculating the changes of bed sediment volume, the influence of the slot on reduction of the scoured sediment volume can be evaluated in comparison to the normal condition. It was found that scour hole dimensions enlarge with discharge, but increase in tail water depth shows a descending procedure. In addition, in the case of slotted flip bucket in comparison to the normal one, by increasing gate opening by 33%, the hole volume increased by 125.64 %, and increasing gate opening by 66% and 99%, the hole volume decreased by 45.92 % and 22.85 %, respectively.