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<br />crest width. In establishing a stage-discharge curve, measurements of <br /> <br />discharge and flow depth should be made within this middle third. Measure- <br /> <br />ments closer to the sill or the upstream end of the crest will encounter <br /> <br />extreme curvatures in the streamlines and should be avoided. <br /> <br />Stable Rating Curve - Studies were made to determine the effect of <br /> <br />changing upstream bed level. The stage-discharge curves for various <br /> <br />values of p are shown in Fig. 17. The tailwater curve used during these <br /> <br />studies is also shown on the same figure. The data shows that there is no <br /> <br />appreciable difference in stage with changing upstream bed level, where <br /> <br />stage is measured 5 feet upstream from the upstream face of the sill and <br /> <br />if the tailwater curve remains unchanged. If however, the tailwater rat- <br /> <br />ing curve changes, there will be an effect on the stage-discharge curve, <br /> <br />unless the structure is sufficiently high to be out of its influence. An <br /> <br />extreme case of change in tailwater and the subsequent change in stage is <br /> <br />shown in Fig. 18. Since the proposed Ria Grande control structures are to <br /> <br />have stable stage-discharge curves for discharges from 100 to 10,000 cfs, <br /> <br />it would be necessary to set the sill elevations of the weirs in favor- <br /> <br />able positions relative to the tailwater expected at the highest design <br /> <br />discharges so that changes in the tailwater rating curves could not affect <br /> <br />the stage-discharge curves. <br /> <br />Figure 19 shows the, relationship between upstream depth of water <br /> <br />above the sill and tailwater elevation relative to the top of sill for <br /> <br />Weir G, for various unit discharges. If the submergence is less than <br /> <br />60 percent, stage will be independent of tailwater level. Tllis chart applies <br /> <br />only to the configuration of the structure shown. <br /> <br />By use of Fig. 19 and. <br /> <br />-26 - <br /> <br />"'\.'/ r.. . . of' <br />V~4'~..i..L <br />