The Design of a Spill-Way - Lab Report Example

Hydraulics By Presented to September 9, Hydraulics A study was conducted to compare flow parameters over a standard ogee-crested and a ski jump spillway. The physical model was constructed using scale 1:50 and it was then put in a flume. Installation pressure taps was then done along the spillway. The research was conceived out of the desire to make comparison of the two models and come up with the design of a spill-way with superior characteristics. After the installation of pressure taps discharge and pressure data was noted. Results obtained from different spillways were used to plot non-dimensional curves. Pressures are examined at low, mid and also at high flow conditions. INTRODUCTION The spillway is one of the most important structures that should be given priority in a dam project. Spillways constructed along with the dams help in controlling excess water or flood hence ensuring the safety of the project. It is also vital that the design of the spillway facilities be done with enough capacity to help avoid overtopping of the dam, especially where an earth fill or rock fill dam type is used for the purpose of the project. This experiment studies the two types of spillways namely: i. Ogee spillway ii. Ski jump spillway The ogee-crested spillway has superior hydraulic characteristics. It has the ability to pass flow efficiently and safely when it is properly designed, with good flow measuring capabilities and this has enabled its application in a variety of situations by engineers. Though much is understood about the ogee shape and about its flow characteristics, it is also clear that a slight deviation from any conventional design parameter like change in the upstream flow conditions, a slightly modified crest shape or construction variations can significantly alter properties of the flow. These minor adjustments require that engineers carry out evaluation of the crest and also to determine whether the change or deviation will adversely affect the performance of the spillway or not. On the other hand a ski jump spillway usually directs the water in a horizontal direction and eventually down into a plunge pool. The ski jump spillways are basic in their design as they transfer the excess water from behind the dam. They usually have a “flip lip” and a dissipater basin which helps create a hydraulic jump and therefore protecting the toe of the dam from erosion. METHODOLOGY a) Ogee Spillway We attended the Hydraulics lab with each student carrying out the practical at their allocated slot . An Ogee spillway was properly installed and the highest discharge was chosen and then using the weir boards, we created a USBR Type 1 stilling basin, with a hydraulic jump being within the stilling basin. We also ensured that the gradient on the flume was set at 3%. • The discharge was then decreased from maximum of 10 m3/h to a minimum of 2m3/h in steps of 1 m3/h. • At each step the following measurements taken: The hydraulic jump Water depths Referenced longitudinal distances Flow over the spillways b) Ski jump spillway . Similarly a ski jump spillway was properly installed and the highest discharge was chosen and then using the weir boards, we created a USBR Type 1 stilling basin, with a hydraulic jump being within the stilling basin. We also ensured that the gradient on the flume was set at 3%. • The discharge was then decreased from maximum of 10 m3/h to a minimum of 2m3/h in steps of 1 m3/h. • At each step the following measurements taken: The hydraulic jump Water depths Referenced longitudinal distances Flow over the spillways RESULTS Table of Results – Ogee Spillway Discharge Q (m3/s) 10 – 2 Downstream depth Y3 (m) 10 – 2 Energy lost through hydraulic jump/10 Y2 Depth (m) 10 – 2 Y1 Depth (m) 10 – 2 Yc Critical depth (m) 10 – 2 Y0 Upstream depth (m) 10 – 2 Upstream Total Energy E0 E0 Downstream Total Energy E3 Downstream Total Energy E3 (Chezy) E = E0 - E3 E = E0 - E3 (Chezy) Froude no Downstream Fr3 Froude no Fr2 Froude no Fr1 Froude no Frc Froude no Upstream Fr0 0.28 9.6 0.76 1.7 1.65 5.1 21.1 0.21 0.10 0.096 0.11 0.11 0.342 4.59 6.40 1 0.07 0.25 9.4 0.71 1.7 1.4 4.8 20.8 0.21 0.10 0.094 0.11 0.12 0.315 4.10 6.97 1 0.07 0.22 9.1 0.72 1.6 1.1 4.5 20.5 0.21 0.09 0.091 0.11 0.12 0.291 3.95 7.90 1 0.06 0.19 8.8 0.66 1.6 0.9 4.2 20.2 0.20 0.09 0.088 0.11 0.11 0.265 3.41 8.74 1 0.05 0.17 8.4 0.58 1.6 0.8 3.8 19.8 0.20 0.09 0.084 0.11 0.12 0.254 3.05 9.25 1 0.05 0.14 8.0 0.93 1.1 0.7 3.4 19.4 0.19 0.08 0.080 0.11 0.11 0.225 4.41 9.86 1 0.04 0.11 7.0 0.90 0.9 0.6 3.0 19.0 0.19 0.07 0.070 0.12 0.12 0.216 4.68 10.61 1 0.04 0.08 7.0 1.28 0.7 0.5 2.5 18.5 0.19 0.07 0.070 0.11 0.12 0.157 4.97 11.58 1 0.03 0.06 6.4 2.11 0.4 0.2 1.4 17.4 0.17 0.06 0.064 0.11 0.11 0.135 8.62 18.17 1 0.02 0.03 5.7 7.70 0.1 0.1 1.3 17.3 0.17 0.06 0.057 0.12 0.11 0.080 34.48 25.73 1 0.01 Table of Results – Ski-Jump Spillway Discharge Q (m3/s) 10 – 2 Down stream depth Y2 (m) 10 – 2 Energy lost through hydraulic jump/10 Y1 Depth (m) 10 – 2 Yc Critical depth (m) 10 – 2 Y0 Upstream depth (m) 10 – 2 Upstream Total Energy E0 Downstream Total Energy E2 Downstream Total Energy, E2 (Chezy) E = E0 – E2 E = E0 – E2 (Chezy) Froude no Downstream Fr2 Froude no Fr1 Froude no Frc Froude no Upstream Fr0 0.28 10.5 0.4 2.7 5.1 21.1 0.212 0.110 0.105 0.10 0.11 0.30 4.85 1 0.11 0.25 9.7 0.5 2.1 4.8 20.8 0.209 0.102 0.097 0.11 0.11 0.30 5.57 1 0.10 0.22 9.3 0.6 1.9 4.5 20.5 0.206 0.097 0.093 0.11 0.11 0.28 5.87 1 0.09 0.19 8.9 0.7 1.6 4.1 20.1 0.202 0.092 0.089 0.11 0.11 0.26 6.41 1 0.08 0.17 8.5 1.1 1.1 3.8 19.8 0.199 0.088 0.085 0.11 0.11 0.25 7.82 1 0.07 0.14 8.0 1.1 1.0 3.4 19.4 0.194 0.082 0.080 0.11 0.11 0.22 8.17 1 0.06 0.11 7.5 1.1 0.9 3.0 19.0 0.190 0.076 0.075 0.11 0.12 0.19 8.59 1 0.05 0.08 7.0 11.7 0.1 2.5 18.5 0.185 0.071 0.070 0.11 0.12 0.16 26.19 1 0.04 0.06 6.3 9.5 0.1 2.0 18.0 0.180 0.064 0.063 0.12 0.12 0.14 26.00 1 0.03 0.03 5.8 8.0 0.1 1.3 17.3 0.173 0.058 0.058 0.12 0.12 0.08 25.73 1 0.02 ANALYSIS OF RESULTS Critical Depth Yc is the depth at which the velocity of flow of water is highest. Plotting a graph of Critical depth against discharge gives a rating curve Ogee spillway rating curve Ski jump rating curve Froude number is given by Coefficient of discharge Cd is given by , where; Qa is the actual discharge obtained from the appendices Qt is the theoretical discharge Discharge and elevation are also related by the following equation Where: C is the coefficient of discharge For the ogee spillway Taking values at critical depth of m and m Solving the two equations simultaneously gives: Cd =1.87 For the ski jump spillway Taking values at critical depth ofm andm Again solving the two simultaneously gives: Cd=1.47 DISCUSSION Energy dissipation As water passes over a spillway and down the chute, the potential is converted into an increasing kinetic energy. Failure to dissipate the waters energy may lead to scouring and erosion that may occur at the dams toe (base).This can cause spillway damage and undermine the dams stability Ski jump Dissipation of energy is enabled by construction of a ski jump since it directs water both horizontally and downstream to a pool plunge. Stilling basin A stilling basin helps to control erosion and improves dissipation of energy. They are often filled with relatively shallow depths and are constructed using concrete lining. Several parts which help in reducing velocity are also be incorporated into the design of the stilling basin to include, baffle blocks, chute blocks, surface boils, wing-walls among others. Recommendations The following recommendations should be carefully observed by the client who plans on making installation of a spillway in general. They are applicable to both the Ogee and Ski jump spillways: Spillway gates often operate suddenly and without warning, under remote control. Trespassers within the spillway run high risk of drowning in such cases. It is therefore recommended that spillways are fenced and also should be equipped with locked gates to help prevent casual trespassing within the structure. Warning signs and other safety measures should also be set in place to warn the users of the downstream area in cases of a abrupt release of water. References i. Savage BM, Johnson MC (2001). Flow over ogee spillway: Physical and Numerical model case study. J. Hydraulic Eng. ASCE., 127(8): 640- 649. ii. Christodoulou, G. C., “Energy Dissipation on Stepped Spillways,” Journal of Hydraulic Engineering, ASCE, 119, (5): May 1993. iii. Chow, V. T. (1959). Open-channel hydraulics, McGraw-Hill, New York, p365–380. iv. Kim DG, Park JH (2005). Analysis of flow structure over ogee-spillwayin consideration of scale and roughness effects by using CFD model. J. Civil Eng. KSCE., pp. 161-169. Appendix (supporting results) Ogee spillway Ski-jump spillway Read More