420 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



From the lay-out on the sea-level model it is apparent that the 

 designers are thinking of a retractable-type dam across the navigation 

 pass. And, since the dam must be capable of withstanding a 10-foot 

 hydrostatic head on either side, a triangular cross section steel box 

 has been chosen. Mounted on rollers, the prototype dam would be 

 80-foot high, extending 60 foot below and 20 foot above the sea bot- 

 tom. One contemplated way of operation is by hydraulic pressure, 

 introducing water into the retracting chambers to close the dam and 

 evacuating the water to open it. 



The water-control structure involves a normal tainter gate set-up, 

 its purpose being to bring the pool elevation inside the navigation 

 pass to the tide elevation outside as early in the tide cycle as possible 

 so that the navigation dam can be open the maximum length of time. 

 Under the tentative criterion of a maximum permissible current of 

 1.8 knots (3 miles per hour) the tests indicate that the navigation 





Constru 

 bypass cf>annel 



Sector gale recess 



Reiroctoble gale recess 



Figure 5. — One of the tide-control schemes studied locates the necessary structures 

 in the vicinity of the present Miraflores Locks. A tide lock is placed in the exist- 

 ing Third Locks Cut, a navigable pass, with retractable-type gates, in the present 

 canal, and a tainter-gate water-control structure in an adjacent bypass channel. 



pass can be used on an average of one-third of the time, or from 1^ 

 to 33^ hours out of each 6-hour tide change depending upon the time 

 of year. 



How the tidal variations in the model are produced is of interest. 

 The model itself, built to a geometric scale of 1 :100, is a half-mile- 

 long slab of 4-inch concrete laid to the alignment of the present lock 

 canal and supporting concrete curbs to simulate the canal slopes and 

 define the channel. At each end are basins representing Balboa and 

 Cristobal Harbors, and to these a constant amount of water is sup- 

 plied. Then, by wasting varying quantities of this water through an 

 electrically operated gate, the desired tide elevations in the harbors 

 are obtained. 



The waste gate is controlled by a special tide apparatus, chief ele- 

 ment of which is a cam whose shape was scribed by a polar plot of 

 the tide elevations at each hour of the cycle. Connected to a time 

 mechanism this cam keeps the waste gate as far open or closed as 



