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Discharge.-- PLATTE RIVER BASIN NORTH FORK SOUTH PLATTE RIVER AT GRANT Location.-- Drainage and Period of Record.-- Lat. 39°27'26",Long. 105°39'29" in NW¼ sec. 10, T.7 S., R.74 W., Park County, Hydrologic Unit 10190002, on left bank at Grant, 1,550 ft downstream from Geneva Creek, and 1.3 mi downstream from east portal of Harold D. Roberts tunnel. 127 mi²; 1948 to present. Graphic water stage recorder and satellite monitoring equipment in a wooden shelter over a concrete well at a concrete trapezoidal channel section and spillway. The gage has residential power and is equipped with a heater and a heat lamp to prevent freezing of the stilling well. The station, with the exception of satellite and other recording equipment, is owned and maintained by the Denver Water Department. The satellite equipment is owned by the Office of the State Engineer. Equipment.-- Hydrologic Conditions.-- Gage-Height Record.-- Datum Corrections.-- Rating.-- Discharge.-- Special Computations.-- Remarks.-- Recommendations.-- Pine forest at 8500 ft to 9500 ft. Gage is affected by natural stream flows from Kenosha Creek, Geneva Creek and discharges from the East Portal of the Roberts tunnel. Rapid changes in stage are caused by the regulation of Roberts Tunnel, ½ mile upstream. When Roberts Tunnel is operating, the gage is typically free from ice. The primary record is hourly averages of 15 minute data taken from satellite monitoring with chart back up. The record is complete and reliable, except for periods when the gage was affected by ice due to Roberts’s Tunnel being turned off: October 29-31; November 2, 3, 5; November 28, 2009 thru March 20, 2010. November 1, 4, 27 had minor affect but were estimated without loss of accuracy. Levels were last run on August 27, 2008 and the gage was found to be reading within the ±.02 ft. tolerances. No instrument corrections were needed. Levels were not run this water year. The control through all stages up to 4.0 feet is a broad crested weir with slightly raised edges. Flow over control has free getaway and should not become submerged. Both banks are clean up to 5 feet of stage. Channel is straight for 500+ feet upstream and downstream. The streambed leading up to the broad crested weir consists of rock and cobble and is affected by high flows during runoff and releases from Roberts Tunnel. Shifts are caused by scour and fill of the weir pool and by gradual erosion of the control. Rating 12 has been in use since October 1, 2001 and was continued in use for the entire year. It is defined by measurements from 15.6 to about 700 cfs. Sixteen measurements (Nos. 1077 – 1092) were made this year (including ice measurements), ranging in discharge from 13.2 to 540 cfs. Daily flows for 2010 did not exceed the defined range for the control. The peak flow of 605 cfs occurred at 2315 on June 7, 2010 at a gage height of 1.84 ft. with a shift of -0.01 ft. It exceeded measurement number 1087 made June 10, 2010 by 0.08 ft. in stage. Shifting control method was used all year. Shifts were distributed using two variable shift tables; PLAGRACOVST01 before the winter shut down of Roberts Tunnel and PLAGRACOVST02 after the tunnel resumed operation and for the remainder of the water year. Stable gage heights are typical when Roberts Tunnel is on, however this water year saw many adjustments in flow as well as the tunnel being turned off eight times. Variable shift tables gave greater consideration to the wide variety of stage within both time periods, including the peak flow for the year. Unadjusted shifts through the year varied from -0.03 to 0.00 ft. The tables were applied as follows: Table 1 (PLAGRACOVST01) was used from October 1 to November 27, 2009. This table is similar to that used at end of water year 2009. It was adjusted slightly to cover the flows seen through this time period and is based on measurement Nos. 1074-1077, 1084 and 1085. All were given full weight. Table 2 (PLAGRACOVST02) was used from March 21 (No. 1073) to September 30, 2010. It is based on all measurements made during this period: Nos. 1084-1092 plus measurement 1093 made in WY2011. All were given full weight except measurements 1086, 1091 and 1093, which were adjusted from -1 to -3 percent to smooth shift distribution. A spreadsheet is used to compute the daily difference between the Grant gage and Roberts Tunnel. This difference represents the native flow in the North Fork without the Roberts Tunnel. Since this flow should follow trends and should never be negative, the calculation is a reality check on the computation of both records. A hydrograph was used in the development of the shift tables. Discharges for all partial ice days were computed by correcting hourly values using graphic estimates which cut off the ice peaks. Ice affected days when inlets were frozen (Nov 28—March 20) were estimated using winter measurements and temperature trends. The record is good with the exception of ice affected periods which are estimated and poor. Station maintained and record developed by Patrick Tyler. The Roberts Tunnel and North Fork of the South Platte at Grant record should be worked on a monthly basis. 2010Water Year