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<br />due to changes in the pressure profile along the crest and velocity <br /> <br />profiles in the vertical over the crest of the structure with changing <br /> <br />flow depths and upstream bed levels. This seemed to be borne out by the <br /> <br />fact that there was essentially no variation of C when the bed was level <br /> <br />with the crest of the structure. Actual measurements of velocity and pres- <br /> <br />sure profiles were not made, however. <br /> <br />The change of C with p represents a disturbing phenomenon in <br /> <br />attempting to obtain a stable stage-discharge relationship of the flow over <br /> <br />the structure. A Shifting rating curve for the artificial control below <br /> <br />a discharge of about 10,000 cfs would be unsuitable in the Rio Grande. <br /> <br />vi eir C <br /> <br />Weir C was conceived as a variation of Weir A to provide a more <br /> <br />stable discharge coefficient. The stability of C is evident in Fig. 13 <br /> <br />when compared to the coefficients of vleirs A and B. While a more stable <br /> <br />coefficient was obtained for this structure, the undesirable feature of <br /> <br />this weir was that high velocities were created on the crest of the struc- <br /> <br />ture because of the convergence of the streamlines downstream of the sill. <br /> <br />The high velocities would render difficulty in obtaining current meter <br /> <br />measurements on the structure. Furthermore, at certain tailwater condi- <br /> <br />tions considerable wave action was generated, and a ilydraulic jum;p could <br /> <br />also develop. Under these conditions meaningful stage and discharge <br /> <br />measurements would be impossible. <br /> <br />Weirs D and E <br /> <br />Weirs D and E were tested subsequently to overcome the undesirab~e <br /> <br />features of Weir C while still maintaining the stability of the discharge <br /> <br />-20- <br /> <br />.- , ,. :--'" <br />,; '..r t:::., . J <br />