الفهرس 
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Abstract
When water falls over a spillway or passes under a gate, the potential energy is converted into very high kinetic energy due to the high velocity of flow. This excessive kinetic energy must be dissipated before it meets the unprotected downstream channel bed to prevent the possibility of sever scouring of downstream riverbed. The existence of significant local scour, which is defined as deepening of the channel downstream the control structure apron, may break the apron causing failure of the whole structure. Consequently, preventing or minimizing local scour downstream of the control structures is one of the most important considerations in the design of these structures. In the literature, many structures have been failed as a result of local scour downstream these structures such as; Islam Weir and Calder Wood Dam, Elevatorski (1959) [16]. Significant local scour was observed downstream Nile barrages with scour depths ranged from 3.0 m to 10.0 m, Gaweesh et al. (2004) [27]. These barrages are very precious hydraulic structures and very vital to the Egyptian national economy, where upstream these barrages, off-take channels withdraw water to irrigate most of the agricultural lands in Egypt. Local scour problems may threaten the safety of these important water control structures. Energy dissipators should be provided at the foot of hydraulic structures for dissipating the excessive kinetic energy. Energy dissipators convert this energy into turbulence and finally into heat for establishing safe flow conditions in the outlet channel. Several types of energy dissipators are used below hydraulic structures and can be classified as; 1) hydraulic jump type energy dissipators; 2) stilling basins; 3) energy dissipation by rough channels; 4) energy dissipation by counterflow; 5) energy dissipation by Air Entrainment; and 6) bucket-type energy dissipators.
The hydraulic jump type energy dissipators and stilling basins are most common uses types. Hydraulic jump type dissipates excess energy through formation of highly turbulent rollers within the jump. In the stilling basin, the exiting supercritical flow from the hydraulic structure is changed to subcritical flow by a hydraulic jump. To ensure that a stilling basin performs its function as an energy dissipator efficiently, basin should be designed in such a way that the elevation of tail water depth not be much less than the elevation of sequent depth of a hydraulic jump. Otherwise, sweep out of the hydraulic jump from the basin takes place and as a result scouring of downstream river bed will occur.
Design of stilling basin is governed by several parameters such as; nature of foundation; approach Froude number; impact angle of flow with respect to stilling basin floors; tailwater level; and economical considerations. Several appurtenances such as; baffle blocks; end sills; and chute blocks are installed along the basin floor to control and stabilize the jump, which helps to the dissipation of excess energy. The use of these devices acts as a safety factor against sweep out of the jump and reduce the cost of the project. Many types of stilling basins have been used throughout the world which are mainly based upon laboratory experiments, such as standard stilling basins. The best basin type suited for protection against scour at a particular location depends largely upon the conditions at this location. Therefore, no one solution can be applicable for all local scour problems.
OBJECTIVES OF THE STUDY
The main objective of this study is to investigate experimentally the effect of various types of stilling basins on the parameters of scour hole that developed downstream regulators. The main variables involved being:
1) Types of stilling basin with different dimensions.
2) Flow conditions such as discharges and tailwater depths.
SCOPE OF THE STUDY
This study was divided into two parts:
1) The first part is the theoretical approach using dimensional analysis (Buckingham’s п–theorem), which used to obtain dimensionless parameters expressing the scour parameters.
2) The second part is the experimental work to study the scour hole parameters under different flow conditions using several types of stilling basin.
The experimental study was carried out in the Hydraulic Research Institute (HRI), National Water Research Center, Ministry of Water Resources and Irrigation, Delta Barrages, Egypt. One hundred and twenty runs were conducted including four types of stilling basin with different dimensions. For each type of stilling basins, four values of discharge, Q = 15, 25, 30, and 35 Lit/s, were considered. For each discharge, three values of tailwater depths, yt = 10, 15, 20 cm were used. Scour hole profile was measured for each run at the centerline of the flume along the flow direction every 4 cm.
CONCLUSIONS
Based on the considered stilling basin types of different dimensions under the used flow conditions, the main conclusions drawn from this study could be summarized as follows:
1) As a general result, it was confirmed that, the dimensions of scour hole increased with the increase of discharge and decrease of tailwater depth.
2) For relatively higher values of tail Froude number, there were considerable differences in scour dimensions for the considered stilling basin types, while there were less differences for relatively smaller values of tail Froude number.
3) The length of steps in the dropped stepped basins had significant influence on scour hole parameters.
4) Most of the considered dropped upward stepped basins gave smaller values of maximum scour depth and shifted the position of the maximum scour depth closer to the apron than the dropped downward stepped basins. While, regarding to scour hole length, the comparison between the dropped stepped basins depended mainly on the length of the steps and the value of tail Froude number.