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Discharge.-- <br />PLATTE RIVER BASIN <br />SOUTH PLATTE RIVER BELOW ANTERO RESERVOIR <br />Location.-- <br />Drainage and Period of Record.-- <br />Lat. 38°56’53”, Long. 105°41’02”, in the SW ¼ of the NE ¼ Sec.21, T125, R76W, Park County, Hydrologic Unit 10190001, on <br />left bank about 400 ft below the Reservoir. <br />Drainage area approximately 185 square miles; flows regulated by Antero Reservoir outlet works. 1976 to <br />current year. <br />Sutron 56-0540-400-DTR shaft encoder connected to a Sutron SatLink 2 satellite Data Collection Platform (DCP) with an <br />independent standalone Sutron Stage Discharge Recorder (SDR) in a concrete shelter over a 48 inch concrete well at a <br />sharp crested Cipolletti weir with broad-crested concrete overflow walls. The well is connected to channel by two 4-inch <br />intakes located at the same elevation. An electric tape gage (ETG) on a equipment shelf is the primary reference. There <br />are no supplemental gage provisions. The SDR was installed February 3, 2010 replacing the Stevens A-71 graphic water <br />-stage. Gage is operated in cooperation of the Denver Water Department (DWD) and the Colorado Division of Water <br />Resources (CDWR). <br />Equipment.-- <br />Hydrologic Conditions.-- <br />Gage-Height Record.-- <br />Datum Corrections.-- <br />Rating.-- <br />Discharge.-- <br />Special Computations.-- <br />Remarks.-- <br />Recommendations.-- <br />Drainage area approximately 185 square miles; flows regulated by Antero Reservoir outlet works. <br />The primary record is hourly averages of 15 minute satellite data with chart record and hourly averages of 5 minute SDR as <br />back up. Regular visits ensure that the blade of the Cipolletti weir remained clean. No corrections were necessary after <br />cleaning. Frequent visits by DWD and CDWR staff ensured good instrument calibration. The record is complete and <br />reliable. The large intakes (4 inch) create gage height (GH) fluctuation at higher flows. Base reference and instrument <br />calibration checks can be somewhat dubious at stages above 1.50 feet. <br />Levels were run to the ETG on October 2, 2009 using R.M. 2 as base. The gage was found to be reading accurately. <br />The control is a 10 ft. sharp-crested stainless steel Cipolletti weir centered in a rectangular concrete wall within a concrete <br />trapezoidal canal section. Flows can exceed the confines of the Cipolletti weir at which point the control becomes <br />compound with the concrete walls functioning as a broad crested weir. Initially the weir employed a theoretic rating based <br />on compound weirs. However, this proved inaccurate and a new rating, MOD10FTCIP was developed using the Cipolletti <br />rating to a stage of 3.16 ft and discharge of 192 cfs. Above 3.16 ft. the rating is defined to 316 cfs by measurements made <br />in 2002 and 2003. This rating was utilized for 2010. By agreement with DWD, CDWR will not use shifts on the Cipolletti <br />portion of the rating. The accuracy of the Cipolletti portion of the weir is thought (by Denver) to be somewhat better than <br />the accuracy of the Price meter made in a natural channel. No discharge measurements were made this year. However, <br />once above GH’s of about 3.16, the rating is based on a few measurements with only Fair accuracy due to surging GH <br />conditions. Currently there is no suitable location to measure higher flows. The measurement bridge over the stilling pool <br />in not usable due to eddies and extreme turbulence in the pool during high releases. The peak discharge of 234 cfs <br />occurred at 0910 on June 14, 2010 at a gage height of 3.40 ft. with a shift of 0.00 ft. <br />Shifts could be caused by moss growth and approach velocities. By agreement with the DWD, the rating was applied <br />directly to gage heights. Flow is controlled by low head slide gates which are prone to blockage by debris, causing spikes <br />in flow until the material breaks free or is flushed out by caretaker opening the gate. The peak flow was outside the <br />Cipolletti portion of the structure and flow conditions were not observed. For flows above the Cipolletti, some combinations <br />of gate operation and outlet size choice can yield flow conditions which are too violent to be defined by a rating. <br />Peak flow occurred as the larger conduit was placed into service while release was increased. The peak event duration <br />was less than fifteen minutes. Stage data for the peak was taken from SDR which records at five minute intervals providing <br />increased event resolution. <br />The record is good with the exception of the instantaneous peak discharge which is poor. The peak stage of 3.40 feet was <br />outside the upper limit of the Cipolletti weir where the rating is not well defined. Also, the peak flow conditions were not <br />observed, and the amount of stilling in the weir pool can be quite variable depending on outlet operations. Station <br />maintained and record developed by Mike Wild. <br />Due to limitations and or issues associated with performing conventional current meter measurements at this site, <br />investigation and evaluation of the weir’s rating by use of an Acoustic Doppler Current Profiler (ADCP) should be <br />considered. However, site condition suitability for ADCP use has not been evaluated as of yet, and excessive air <br />entrainment introduced by the baffle structure located in the weir pool may preclude ADCP use. <br />2010Water Year