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<br />Dolores River <br /> <br />As part of this study, Hydrology section of the Corps <br />Sacramento District reviewed the 1976 hydrology investigation for <br />adequacy and determined that the data presented are still <br />adequate. Streamflow records since that time have been such that <br />there would be little if any change in the results if the data <br />were reanalyzed at the present time. Therefore, the peak flow <br />values identified in the FEMA, Flood Hazard Information Report, <br />and Hydrology Office Reports, shown below, were used in this <br />study. <br /> <br />FLOW-FREQUENCY RELATIONSHIPS <br />DOLORES RIVER DOWNSTREAM OF LOST CANYON CREEK <br />(Draingage Area = 567 Sq Mi) <br /> <br />Return Period Peak Discharge <br />(years) (cfs) <br />10 6,800 <br />50 11,500 <br />100 14,500 <br />500 36,000 <br /> <br />HYDRAULXC CONDXTXONS <br /> <br />Present river hydraulic conditions at Dolores were assessed <br />to determine the backwater effect of the McPhee Reservoir and the <br />local levees on the flood plain. Since the data developed for <br />the Flood Hazard Information and Flood Insurance Studies predated <br />actual reservoir and levee construction, the town and State <br />wanted to know if the FEMA flood plain boundaries are still <br />fairly accurate. The two previous flood plain studies <br />incorporated USBR estimates on the then-future reservoir and <br />levees. However, until now, no hydraulic analysis has been made <br />using as-constructed data. <br /> <br />The first step was to obtain current topographic data. Due <br />to time and resource constraints, only a ~imited number of <br />critical cross sections could be resurveyed. On July 30, 1992, <br />cross sections 4, 7, and 9 (Attachment 3) were resurveyed by <br />Colorado Water Conservation Board and Corps staff. These cross <br />sections were at the downstream end of the study reach adjacent <br />to McPhee Reservoir where the USBR had constructed the main <br />backwater levee (between the Highway 145 and 4th Street bridges), <br />demolished several structures, and regraded the adjacent land <br />(which is now the town park). For the levees upstream of 4th <br />Street, survey data collected by the Colorado Water Conservation <br />Board in November 1985 were used. Both the 1985 and the 1992 <br />survey data are shown in Appendix E. <br /> <br />4 <br />