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Discharge.-- PLATTE RIVER BASIN 06695000 SOUTH PLATTE RIVER ABOVE ELEVENMILE RESERVOIR Location.-- Drainage and Period of Record.-- Lat. 38°58'03",Long. 105°34'51", in NE¼ sec. 32, T.12 S., R.73 W., Park County, Hydrologic Unit 10190001, on left bank 200 ft downstream from highway bridge, 2.5 mi upstream from water line of Elevenmile Canyon Reservoir, at elevation 8,561 ft. and 13 mi southeast of Hartsel. 880 mi²; 1933 to present. Graphic water-stage recorder and Sutron SatLink 2 satellite Data Collection Platform (DCP) with a Sutron 56-0540 shaft encoder (SE) in a wooden shelter over a concrete well at a 25-foot concrete Parshall flume. Primary reference gage is inside electric tape gage with supplemental staff gage on the right side at the Ha location. A Sutron Stage-discharge recorder (SDR) was installed August 11, 2010 and the graphic water-stage recorder was removed. Facilities are owned and maintained by the Denver Water Board. Adjacent land owned by City of Aurora. Satellite instrumentation is owned and maintained by the State Engineers Office. Equipment.-- Hydrologic Conditions.-- Gage-Height Record.-- Datum Corrections.-- Rating.-- Discharge.-- Special Computations.-- Remarks.-- The gage is approximately two miles below Spinney Mountain Reservoir, flow is controlled by reservoir releases. A small drainage empties in above the gage that is not controlled by the reservoir. This drainage can contribute significant flow after severe local rain events. The record is generally flat with step changes. The primary record is hourly averages of 15 minute satellite data with chart and SDR back up. The primary record agrees with the chart and SDR data to within 0.02 ft. Generally releases above 80 cfs from Spinney Mountain Reservoir keep this gage open year round. Lower flows can see ice affect at the flume in two ways: iIce jams upstream can cause a drop in flow followed by a surge. This can result in an accumulation of ice above the normal water line for that release period, as each brief surge hits the frozen flume walls. In this situation, the baseline GH’s will be lower than the ice layers and be good record, but the higher surge GH’s will be ice-affected. Ice jams downstream can cause backwater into the flume which can result in ice forming on the crest and walls of the flume. In either case, the GH record will not be the flat release expected and the computed flows will be higher than either Spinney release data or computed good record for that Spinney release period. Ice affects seen on days when a change is made in Spinney release are more complicated as there is a delay between gate changes at Spinney Reservoir and the gage, return flows are also seen following gate change reductions. The record is complete and reliable except for the following periods when the stage discharge relationship was affected by ice: November 24, 25, 27, 28, December 3, 4, 18, 26, 2009, January 9, February 10, 15, 2010. Moss was cleaned from the flume on July 13, 2010 causing a reduction in gage height following cleaning. Measurements were taken before and following the moss removal (867, 868) and were utilized to account for the cleaning correction. A tracked excavator was used to clean the weir pool approach to the flume on April 2, 2010. The work did not affect the GH record. Instrument calibration was verified by thirty visits made to the gage. Three SE adjustments of 0.01 ft each were made. Levels were last run on August 27, 2008. No instrument corrections were made. The control is a 25 ft. concrete Parshall flume. A standard rating has been used since the flume was installed in 1940. Rating No. 15, dated Oct. 1, 1970 is an expansion of the standard equation. Any flow by-passing the flume would be accounted for separately and not computed as part of this rating. The streambed is composed of compact cobble and gravel and silt, and a gravel bar often forms just upstream from the flume. The gravel bar will affect shifts and encourage more moss to form in the flume where the velocities have been slowed. Shifts are caused by gravel in the approach basin and moss. Sixteen measurements (Nos. 857 - 872) were made this year, ranging in discharge from 59.3 to 401 cfs. Using the USBR Water Measurement Manual, Third Edition, Figure 8-9, Page 8-44, the range of accurate (within +/-5%) discharge measurement for a 25 ft Parshall Flume is 15 to 1200 cfs. Anything above or below this range is outside the +/- 5% accuracy range. All flows during the 2010 water year were within this range. The peak discharge of 458 cfs occurred at 1900 on August 6, 2010 at a gage height of 2.64 ft. with a shift of 0.04 ft. It exceeded measurement No. 865 by 0.22 ft. in stage and 57 cfs respectively. Shifting control method was used all year. Stage dependent shifting was used with the exception of periods were moss accumulation could be identified; for those periods shifts were applied as defined by measurements and were distributed mainly by time with consideration to change in stage. Measurements for the water year show shifts varying from -0.04 ft. to 0.04 ft. Shifts were distributed as follows: October 1, 2009 to May 25, 2010: shifts are caused by moss growth at low stages and by approach conditions at higher stages. Shifts were run by time with consideration to stage, holding the shift from No.861 (0.01 ft) to the stage change on April 14. Shift from Msmt No. 862 was started at peak flow event April 17. Shifts on April 15, 16, 2010 were hand prorated. Measurement No. 858 was adjusted in relation to adjacent shifts after review of GH fluctuation. May 25, 2010 to June 10, 2010: Stage-shift table PLAHARCOVST1, developed from Msmts Nos. 863-865 made during the period and given full weight was applied. June 10, 2010 to June 25, 2010: Stage-shift table PLAHARCOVST2, developed from Msmts No. 865-866 made during the period and given full weight was applied. June 25, 2010 to September 30, 2010: shifts were run by time to account for moss growth with consideration to stage. Measurement No. 871 was adjusted to account for heavy aquatic plant material in meter during measurement. Extremely high flows can bypass the gage through a culvert to the south. This did not occur this year. Many ice days were estimated without significant loss of accuracy since these ice days fell in periods when the release by Spinney Reservoir remained constant. The remaining ice days were estimated from adjacent reliable record and from Aurora’s Spinney Mountain Reservoir accounting. Record is rated good, except for periods of ice effect, which are considered fair. Station maintained and record developed by Mike Wild. 2010Water Year