Comparison of stilling basins with rigid and gabion equipment

Document Type : Research Article


1 Assistant Prof., Dept. of Civil Eng., Faculty of Eng., University of Bu-Ali Sina Hamedan, Iran. 2- M.Sc. Graduated, Dept. of Civil Eng., Faculty of Eng., University of Bu-Ali Sina Hamedan, Iran

2 Civil Engineering Faculty Engineering Bu-Ali Sina University Hamedan Iran


Introduction: Hydraulic jump occurred widely in most of hydraulic structures, such as spillways in high dams, downstream ‎weirs and sluice gates, where is high velocity, the condition of occurrence of a hydraulic jump is to change flow suddenly from ‎supper critical flow (low depth with high velocity) to subcritical flow (high depth with a low velocity).
Stilling basin must be designed perfectly to ensure efficient operating over a wide range of flow. Additional devices may be used to ‎stabilize the jump, reduce the length and height of the jump and increase the energy dissipation. ‎
‎ Baffle blocks one of these devices which used to stabilize the jump and dissipate energy as a result of impact action. Baffle block ‎used with different shapes such as cubic and trapezoidal (trapezoidal shape in section), the cubic shape is effective when the best ‎dimensions of height, width, spacing, and the best location in the basin were used. United States Department of the Interior Bureau ‎of Reclamation (USBR) recommended that the corners of baffle block must be not be rounded because the corners are effective in ‎producing of eddies which help in energy dissipation. Previous studies showed that, the model of baffle blocks which have an ‎ability to circulate the jet of water in the vertical transverse direction behave best than others for dissipation of energy, also the ‎rotation in jet of water prevents the jump action to the extent, in this case there is a complete energy dissipation and reduction in the ‎stilling basin length.‎
However, an insufficient number of studies on gabion baffle blocks and gabion end sill have been developed more recently ‎compared with ‎studies on fixed baffle blocks and fixed end sill.‎ Also, another insecurity that exists for the baffle blocks of stilling ‎basin is cavitation around them, which can be reduced if gabion baffles are used due to changes in the flow pattern conditions.‎
In this research a hydraulic jump stilling basin equipped with baffle blocks and end sill in two condition was to be tested in ‎laboratory to determine the dissipation and other characteristics of hydraulic jump for various type of baffle blocks and end sill. In ‎one condition the bed and baffle blocks and end sill of stilling basin were rigid and in the other condition the bed was rigid but the ‎baffle blocks and end sill were permeable and were constructed with gabion by different porosity.‎

Methodology: Experiments were carried out at the Hydraulic Laboratory of the Faculty of Engineering, Bu-Ali Sina ‎University of Hamedan in a flume 15 m long and 0.6 m wide and 0.6 m deep. In this research a hydraulic jump stilling basin ‎equipped with baffle blocks and end sill in different conditions. The bed of stilling basin was rigid but various type of baffle blocks ‎and end sills with different permeability were used. The concrete was used for constructing rigid baffle blocks and rigid end sill and ‎gabion was used for constructing baffle blocks and end sill by 30, 45 and 60 percent permeability. Sand and gravel were used to ‎make permeable baffle blocks and end sills. A sluice gate was used for creating of flow by high velocity and low depth to create ‎high Froude Number. The sluice gate was placed in 8 meters from beginning of flume. The tailwater depth was adjusted by a ‎butterfly gate at the end of flume.‎
The Baffle blocks and end sill dimension and positions were according to USBR III recommendations. All experiments were done ‎in three Froude numbers of 3.8, 4.3 and 5.1. The flow depth was measured on a grid 5 cm5 cm using a depth gauge with an ‎accuracy of 0.1 mm. ‎

Results and discussion:
After measuring the flow depth, the parameters of conjugate depth ratio, relative depth, relative ‎length, relative energy loss and water surface profile for hydraulic jumps formed in the stilling basin with rigid floor and baffle ‎blocks and end sill with different porosities 0, 30, 45% and 60% were calculated. Then the changes of these parameters were ‎investigated according to the Froude number of the inlet flow to the stilling basin. Advantages shows that by increasing the porosity ‎of baffle blocks and end sill. The results show that by increasing the porosity of the baffle blocks and end sill, the ratio of conjugate ‎depths decreases and the amount of this decrease increases with increasing the Froude number of the inlet flow to the stilling basin. ‎So that for Froude number equal to 5 and porosity 60% of baffle blocks and end sill, this reduction reaches about 40%. The change ‎in the relative depth and relative length of the hydraulic jump has a similar trend to the conjugate depth due to the increased porosity ‎of the baffle blocks and the end sill. For the Froude number equal to 5 and the porosity to 60% of the baffle blocks and the end sill, ‎the relative length and relative depth of the hydraulic jump are reduced by about 77% and 22%, respectively. While the relative ‎energy loss increases with increasing the porosity of the baffle blocks and the end sill and reaches about 45% for Froude number ‎equal to 5 and porosity 60%. ‎

Conclusion: In this study, in addition to rigid baffle blocks and rigid end sill, the permeable baffle blocks and permeable end ‎sill with porosities of 30 45 and 60 percent were used in the USBR type III stilling basin. Then the important parameters of ‎hydraulic jumps in stilling basin were computed and compared for different Froude number. The results show that the permeable ‎baffle blocks and permeable end sill by changing the flow pattern in the stilling basin reduces the relative depth and relative length of ‎the hydraulic jump, while increasing the relative energy losses. The results also show which increasing the permeability of ‎baffle ‎blocks and end sill increases the relative energy losses and decreases relative depth and relative length of hydraulic jumps.‎


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