Laserfiche WebLink
Discharge.-- <br />PLATTE RIVER BASIN <br />NORTH FORK SOUTH PLATTE RIVER AT GRANT <br />Location.-- <br />Drainage and Period of Record.-- <br />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 <br />Grant, 1,550 ft downstream from Geneva Creek, and 1.3 mi downstream from east portal of Harold D. Roberts tunnel. <br />127 mi²; 1948 to present. <br />Graphic water stage recorder and satellite monitoring equipment in a wooden shelter over a concrete well at a concrete <br />trapezoidal channel section and spillway. The gage has residential power and is equipped with a heater and a heat lamp to <br />prevent freezing of the stilling well. The station, with the exception of satellite and other recording equipment, is owned and <br />maintained by the Denver Water Department. The satellite equipment is owned by the Office of the State Engineer. <br />Equipment.-- <br />Hydrographic Conditions.-- <br />Gage-Height Record.-- <br />Datum Corrections.-- <br />Rating.-- <br />Discharge.-- <br />Special Computations.-- <br />Remarks.-- <br />Recommendations.-- <br />Pine forest at 8500 ft to 9500 ft. Gage is affected by natural stream flows from Kenosha Creek, Geneva Creek and <br />discharges from the East Portal of the Roberts Tunnel. Rapid changes in stage are caused by the regulation of Roberts <br />Tunnel, ½ mile upstream. When Roberts Tunnel is operating, the gage is typically free from ice. <br />The primary record is hourly averages of 15 minute data taken from satellite monitoring with chart back up. The record is <br />complete and reliable, except for periods when Roberts’s Tunnel was off and the gage was affected by ice: November 7, <br />2008 , March 27, April 17-19, 2009 . <br />Levels were last run on August 27, 2008 and the gage was found to be reading within the ±.02 ft. tolerances. No instrument <br />corrections were needed. <br />The control through all stages up to 4.0 feet is a broad crested weir with slightly raised edges. Flow over control has free <br />getaway and should not become submerged. Both banks are clean p to 5 ft of stage. Channel is straight for 500+ feet <br />upstream and downstream. The streambed leading up to the broad crested weir consists of rock and cobble and is <br />affected by high flows during runoff and releases from Roberts Tunnel. Shifts are caused by scour and fill of the weir pool <br />and by gradual erosion of the control. Rating 12 has been in use since October 1, 2001 and was continued in use for the <br />entire year. It is defined by measurements from 15.6 to about 700 cubic feet per second (cfs.) Fourteen measurements <br />(Nos. 1063 – 1076) were made this year, ranging in discharge from 42 to 426 cfs. Flows outside the range defined by the <br />weir rating did not occur in 2009. The peak flow of 658 cfs occurred at 1930 hrs. May 24, 2009 at a gage height of 1.91 ft. <br />with a shift of -0.02 ft. It exceeded measurement number 1073 made July 20, 2009 by 0.29 ft. in stage. <br />Shifting control method was used all year. Shifts were distributed by time from October 1, 2008 to April 22, 2009. <br />Relatively stable gage heights during this period of time, allow for good representation by the shifting control method. <br />Measurements 1063-1069 made during this period varied in unadjusted shifts from -0.01 to +0.03 ft. All measurements <br />were given full weight except for No. 1063 which was adjusted +3% to smooth shift distribution. Stage-dependent shifting <br />was used from April 22, 2009 to September 30, 2009, mainly to properly capture a slight trend toward negative shifts at <br />higher flows. Variable shift tables gave greater consideration to the wide variety of stage within this time period, including <br />the peak flow for the year. Unadjusted shifts during this period varied from -0.02 to +0.01 ft. Two tables were used: Table 1 <br />was used from April 22 (measurement 1069) to July 20 (measurement 1073) and is based of measurement Nos. 1069- <br />1073, made during this period, plus Nos. 1075 & 1077. All were given full weight. Table 2 was used from July 20 (No. <br />1073) to September 30, 2009. It is based on all measurements made during this period—Nos. 1073-1076—plus <br />measurement 1077 made in 2010. All were given full weight. A hydrograph was used in the development of the shift table <br />and is included. <br />A spreadsheet is used to compute the daily difference between the Grant gage and Roberts Tunnel. This difference <br />represents the native flow in the North Fork without the Roberts Tunnel. Since this flow should follow trends and should <br />never be negative, the calculation is a reality check on the computation of both records. Discharges for all ice days were <br />computed by correcting hourly values using graphic estimates which cut off the ice peaks . These values are estimated <br />from adjacent good record and temperature trends. <br />The record is good with the exception of ice periods which are estimated and rated fair. Station maintained and record <br />developed by Patrick Tyler. <br />The Roberts Tunnel and North Fork of the South Platte at Grant record should be worked on a monthly basis. <br />2009Water Year