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<br />c. <br /> <br />~ <br /> <br />in order to permit the determination of a tailwater <br />rating curve for power'calculations. <br /> <br />Task 7 - Analysis of Flood Hydrology and Sedimentation <br /> <br />a. Flood Hydrology <br /> <br />The flood hydrology of the Una Reservoir drainage basin <br />shall be studied to develop an estimated Probable <br />Maximum Flood (PMF), which PMF shall be used as the <br />basis for designing the flood-passing capacity of the <br />project. A frequency curve of lesser floods will also <br />be developed and may be used for design of certain <br />aspects of the project, including diversion during <br />construction. The Contractor shall take thE following <br />steps in order to complete the flood hydrology study: <br /> <br />(1) Determine flood frequency curve of annual peak <br />flows at the site using the U.S. Water Resource <br />Council guidelines. Data from the long-term USGS <br />gaging station "Near Cameo" will be adjusted to <br />develop an annual flood series for the station <br />"Near DeBeque". <br /> <br />(2) Review USBR PMF studies for projects in the <br />Colorado River basin with similar hydrologic <br />characteristics. Particular attention will be <br />given to the USBR studies for the Una site (done in <br />the 1960s) and for the more recently proposed <br />Dominguez Project, as well as to the studies for <br />the existing Curecanti Project dams. <br /> <br />(3) Based upon the review in (2) above, and upon a <br />recently completed detailed PMF analysis performed <br />by the Contractor for the proposed Tri-County <br />Reservoir Hydroelectric Project on the Gunnison <br />River, an estimate of the project PMF will be made <br />considering differences in the respective project <br />drainage areas and basic hydrologic characteris- <br />tics. This estimated project PMF shall be subject <br />to the State's review and approval before it is <br />used in subsequent work tasks. <br /> <br />It is understood and agreed by the State that although <br />this level of PMF analysis will be sufficient for the <br />purposes of this appraisal-level study, a more detailed, <br />site specific analysis based upon watershed hydrograph <br />calibrations and specific estimates of the project <br />probable maximum precipitation and probable maximum <br />snowpack will be necessary prior to final project design <br />and Federal Energy Regulatory Commission (FERC) <br />licensing. <br /> <br />-6- <br />