Spillway Modifications, Miraflores Dam Panama Canal Zone
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s134 n0. 2-667 c.2 TECHNICAL REPORT NO. 2-667 SPILLWAY MODIFICATIONS, MIRAFLORES DAM PANAMA CANAL ZONE Hydraulic Model Investigation by T. E. Murphy P. S. Cummins, Jr. January 1965 Sponsored by The Panama Canal Company Conducted by U. S. Army Engineer Waterways Experiment Station CORPS OF ENGINEERS VichsIEI dvr RTURN TO ca 23 9 COAST AL ENINERNGRESEARCH GENTER I U58 TECHNICAL REPORT NO. 2-667 SPILLWAY MODIFICATIONS, MIRAFLORES DAM PANAMA CANAL ZONE Hydraulic Model Investigation by T. E. Murphy R. S. Cummins, Jr. January 1965 Sponsored by The Panama Canal Company Conducted by U. S. Army Engineer Waterways Experiment Station CORPS OF ENGINEERS Vicksburg, Mississippi ARMY-MRtC VICKSBURG, MISS. FOREWORD Model studies of the Miraflores Dam spillway were requested by the Panama Canal Company in a letter to the U. S. Army Engineer Waterways Experiment Station (WES) dated 10 October 1961. Tests were authorized by the Office, Chief of Engineers, in the first indorsement, dated 24 October 1961, to a letter from the WES, "Request for Authority to Conduct Model Studies of Miraflores Dam Spillway," dated 20 Octobe 1961. The tests were conducted at WES during the period February to September 1962 under the general supervision of Mr. E. Fortson, Jr., Chief of the Hydraulics Division, and Mr. F. R. Brown, Chief of the Hydrodynamics Branch, and under the direct supervision of Mr. T. E. Murphy, Chief of the Structures Section. In immediate charge of the model were Messrs. R. S. Cummins, Jr., and E. S. Melsheimer, assisted by Messrs. J. A. Conway and E. A. Graves. This report was prepared by Mr. Murphy with the assist- ance of Mr. Cummins. During the course of the investigation, Messrs. J. M. Cooke and A. R. Nard of the Panama Canal Company visited the WES to discuss test results and correlate these results with planning being done by the Panama Canal Company. Director of the WES during the testing program and the preparation of this report was Col. Alex G. Sutton, Jr., CE. Technical Director was Mr. J. B. Tiffany. iii CONTENTS Page 'FOREWORD " " " ! " " " " " " M " 4 " " ! " " " iii SUMMARY 000000. " " " " " i " " " " " " " " ! " " " " vii PART I: INTRODUCTION ....... """.. 1- The Prototype . .. ............... Purpose of Model Tests............... .. .. .. .. ... PART II: THE MODELS 0........... .......................... 3 Description...... " 3 Appurtenances ..... " " 5 Scale Relations . " 5 Test Conditions . .. " " 6 Presentation of Data " 6 PART III: TESTS AND RESULTS " Existing Conditions # 7 Stone Protection . " " 8 Stilling Basins .... " " 9 Recommended Design ... " M 10 PART IV: DISCUSSION.. .. .. 0 " A i " M A " " A M A " " " 12 TABLE 1 PHOTOGRAPHS 1-5 PLATES 1-16 V SUMMARY Miraflores Dam spillway is a gravity ogee section designed for a maximum head on the crest of 18 ft. The spillway is composed of eight 45-ft-wide bays with vertical-lift gates. The spillway crest is at elevation 38.67 and the downstream face of the spillway is terminated by a 38-ft radius which brings the toe tangent to a horizontal plane at elevation -15. No stilling basin is provided. Since the spillway was put in operation in 1917 a maximum flow of 25,290 cfs has been experienced. This flow is about one-fourth of the spillway design flood. Erosion of the rock downstream fromri the tae of the spillway has progressed until the dam is in danger of partial failure. Model investigations on a 1:36-scale general model and 1:20- and 1:50-scale section models were made to determine the suitability of various schemes for protection against further erosion and for develop- ment of design details of the selected scheme. Results indicated that adequate protection would be provided by addition of a stilling basin consisting of a 40-ft-long apron terminated by a 3-ft-high dentated end sill. vii ".Gm o ;Fe * PEDRO .0 * MIGUEL x# *vLOCKS ',. r MIRAFLORES LAKE MIAFORS A MIRARFLLRSREA SCALE IN! MILESLORS 5*0 ," 1 Fig.1.ocaton ac SPILLWAY MODIFICATIONS, MIRAFLORES DAM, PANAMA CANAL ZONE Hydraulic Model Investigation PART I: INTRODUCTION The Prototype 1. The Miraflores Dam spillway is used to discharge flow from Miraflores Lake which forms a section of the Panama Canal about 1.7 miles long near the Pacific Ocean (fig. 1). The lake level is maintained between elevations 53.67 and 56.67.* The spillway (plate 1) is a gravity ogee sec- tion with crest at elevation 38.67, and is composed of eight 45-ft-wide bays with vertical-lift gates. The downstream face of the spillway is terminated by a 38-ft radius which brings the toe tangent to a horizontal plane at elevation -15. No stilling basin is provided. When the spillway was constructed (in 1917), the top of what was considered relatively good rock immediately downstream also was at elevation -15. However, inspection of records indicates that this rock was closely jointed and that the strata dipped downstream. 2. The spillway is designed for a maximum discharge of about 100,000 cfs. Records of spillway operation show that between the years 1917 and 1957, inclusive, there have been 19 occasions on which the flow exceeded 10,000 cfs, and 5 on which flow exceeded 20,000 cfs. The maximum discharge, which occurred in 1932, amounted to 25,290 cfs. This discharge was passed through only five of the eight gates with three gates open 7.8 ft and two gates open 5.2 ft. 3. Erosion of the rock downstream from the toe of the spillway was noted following initial operation of the structure, and this erosion has progressed to such an extent that the dam is considered to be in danger of partial failure. Purpose of Model Tests 4. Several schemes were considered for protection of the Miraflores * All elevations are in feet referred to mean sea level. Dam spillway against further erosion at the downstream toe. Model tests were considered desirable for determination of the suitability of the various schemes considered and for development of design details of the selected scheme. 5. Specifically, model tests were used for study of details of pro- posed concrete aprons and precast concrete block protective works. In addition, study was made of flow conditions at the spillway training walls, the need for excavation in the exit area, and the desirability of a spe- cific gate operating schedule. PART II: THE MODELS Description 6. Three models were constructed for study of the problems at hand. A general model, at a scale of 1:36, was used for study of flow conditions at the training walls, determination of the need for excavation in the exit area, and development of a satisfactory gate operating schedule. A 1:20-scale section model was used for study of details of the proposed concrete aprons and precast blocks. Finally, a 1:50-scale section model was used for illustration of the flow conditions. 7. The general model of Miraflores Dam spillway reproduced about 500 ft of the approach, the spillway, and about 1700 ft of exit channel including the Miraflores Bridge (figs. 2 and 3). The overbank, approach, and exit areas were molded in cement mortar brought to grade through the use of sheet metal templets. The ogee crest, crest piers, training walls, and bridge were fabricated from sheet metal, and the stilling basin Fig. 2. General plan and model limits 3 ~1 - - Fig. 3. The 1:36-scale general model elements were constructed of wood treated to resist expansion. 8. The 1:20-scale section model was constructed in a flume which had glass panels for one side. The model reproduced a 45-ft-wide section of a short portion of the approach, the spillway, and 250 ft of exit channel. 9. The 1:50-scale section model was constructed in a 1-ft-wide glass-sided flume and was used primarily for illustrating flow conditions. A moving picture film of this model showing action at the toe of the spill- way under as-built conditions and with the recommended stilling basin apron installed was furnished engineers of the Panama Canal. Appurtenances 10. The water used in model testing was supplied by pumps from a sump system with flow rates measured by venturi and orifice meters. Steel rails graded to specific elevations were placed along the sides of the flumes and provided a datum plane for measurements. These rails were marked to indicate distances upstream and downstream from the spill- way crest. Velocities were measured using pitot tubes, and stopwatch timing of floatage. Current patterns were determined using confetti sprinkled on the surface and dye injected below the surface. Scale Relations 11. The accepted equations of hydraulic similitude, based on Froude's law, were used to express mathematical relations between dimensions and hydraulic quantities of the model and prototype. General relations for transference of model data to prototype equivalents are as follows: Dimension Ratio Scale Relations Length L 1:36 1:20 1:50 r 2 Area A = L 1:1296 1:400 1:2500 r r Velocity V = L1/2 1:6 1:4.47 1:7.07 Discharge Q = L5/2 1:7776 1:1789 1:17,678 12. Model measurements of discharge, water-surface elevations, and velocities can be transferred quantitatively to prototype equivalents by means of the preceding scale relations. Also, the limited experimental data which are available indicate that the prototype-to-model scale ratio is valid for scaling rock in the sizes used in this investigation. Test Conditions 13. Tests involving partial gate openings were conducted with the upper pool at elevation 56.67. All proposed remedial measures were tested at the design discharge of 100,000 cfs and minimum tailwater. Also, per- formance of the favorable designs was observed at a range of tailwater elevations and lesser discharges.