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Discharge.-- PLATTE RIVER BASIN 06720500 SOUTH PLATTE RIVER AT HENDERSON Location.-- Drainage and Period of Record.-- Lat. 39°55'19",Long. 104°52'00", in SE¼NE¼ sec. 34, T.l S., R,67 W., Adams County, Hydrologic Unit 10190003, on right bank 500 ft upstream from bridge on State Highway 22 and 0.2 mi northwest of Henderson. 4,768 mi². May 1926 to current year. Monthly data only prior to 1933. Periodic water quality data available starting in 1955. Graphic stage recorder and digital shaft encoder in 42-inch corrugated metal pipe shelter and well . A 10 ft x 20 ft building located 10 ft upstream from the well shelter houses a Sutron 8210. The shaft encoder connects to the DCP through conduit running between shelters. The primary reference is an electric tape gage in the shelter and well. A cantilevered wire weight gage just downstream of well is used as a supplemental reference. The stilling well is connected to stream by three 2-inch intakes, each equipped with flushing provisions. One moveable flushing funnel serves one of the intakes. The other two intakes are flushed using an electric pump in the gage with a reservoir tank attached to the intakes. Equipment.-- Hydrographic Conditions.-- Gage-Height Record.-- Datum Corrections.-- Rating.-- Discharge.-- Special Computations.-- Remarks.-- Recommendations.-- Reservoirs and diversions above 64th Avenue (and Metro Wastewater) often totally control the flow to that point, leaving the Henderson gage to be strongly influenced by the Metro effluent and Clear Creek. Low flows exhibit a strong diurnal due to the Metro effluent. High snowpack in Clear Creek basin will usually signal snowmelt runoff periods at Henderson. Peak flows are usually seen from storm events. Rainstorm runoff from the metro area in the spring and summer will contribute sharp high peaks, often in the early morning hours. Under conditions of high basin-wide runoff, releases from Chatfield Reservoir upstream are regulated to keep the flow at or below 5,000 cfs at Henderson. The primary record is hourly averages of fifteen minute data taken from satellite monitoring with chart backup. The record is complete and reliable. The shaft encoder remained in good calibration this year. Thirty-four visits were made to the gage this year ensuring instrument calibration. Only one calibration adjustment was required (August 13, 2009), which was caused by the beads becoming misaligned after maintenance work. Chart record was used during the period that the shaft encoder was out of calibration without loss of accuracy. The control has created a sand bar moving in the direction of the gage that caused the inlets to become buried. One inlet is currently free of sand with a working valve. Further maintenance is not being done as the gage is in the process of being moved to the other side of the river for water year 2010. Levels were run on September 24, 2009 using RM. No. 5 as base. A -0.05 foot correction was indicated but was not taken in lieu of further evaluation of the gage’s stability as well as potential relocation of the gage. The control is a grouted rock dam, established in 2002 as a grade control structure by the Urban Drainage and Flood Control District. The rock dam has a low flow notch in the right of center portion of the control, and will effectively regulate flow at all stages. Prior to the dam construction, the control was a shifting sand and gravel channel, with high flows influenced by the bridge opening approximate ly 500 feet downstream. The channel had been scouring during the entire history of the gage. The old station has had numerous ratings and has had the stilling well re-installed at a lower datum twice in the last 20 years. The rock control raised the channel bed and PZF by 2 ft. Rating Number 33, developed in WY 2004, was used for the entire water year. It is fairly well defined over a range of 55 to 7500 cfs. Twenty measurements (Nos. 549-568) were made this year, ranging in discharge from 79.5 to 7090 cfs. They cover the range of stage experienced well. The peak flow of 7200 cfs occurred at 1430 on June 2, 2009 at a gage height of 8.94 ft with a shift of 0.21 ft. It exceeded the high measurement (No. 560) made concurrent to the peak event by 0.05 ft. in stage.. The channel has a sand bottom and is continually scouring and filling. Measurements show shifts varying between -0.08 and 0.21 feet. Time dependant as well as stage dependant shifting was used. Shifts were applied as follows: October 1, 2008 through April 10, 2009; time dependent shifting as defined by measurement Nos. 548 – 557, all of which were given full weight except for No. 549 which was adjusted -5% to better fit the shift distribution. April 10, 2009 through April 18, 2009: time dependant shifting with consideration given to stage. April 18, 2009 through June 2, 2009: stage dependant shifting using variable shift table PLAHENCOVST0901 comprised of measurement Nos. 558-560 and No. 526 made in the 2007 water year, all of which were given full weight. June 2, 2009 through July 2, 2009: stage dependant shifting using variable shift table PLAHENCOVST0902 comprised of measurement Nos. 560-562 and No. 526 made in the 2007 water year, all of which were given full weight. July 2, 2009 through September 30, 2009: time dependent shifting as defined by measurement Nos. 564-569, all of which were given full weight except for Nos. 563 and 568, which were adjusted 1% and 5% to smooth the shift distribution. Due to rapid stage changes, high flow measurement gage heights are usually computed as a weighted average of discharge. This is accomplished by inputting the Aquacalc section discharges and times into a spreadsheet containing gage height data and times. As discussed below, wading measurements made some distance downstream can also require time-delayed gage heights. The lag time is estimated from the distance divided by the measurement average velocity. Record is rated as good. Many measurements were performed using half counts (20 second counts instead of the full 40 seconds). This method is employed to counteract the large and rapid changes in stage due to Denver-Metro Sewer releases. The measurement section is getting difficult to wade as it narrows down and moves. This is suspected to be due to the frequently changing, and increasing number of sand bars building both upstream as well as downstream of the control. As the measurement section changes to accommodate a favorable section, lag times may become an issue to consider since changes in stage vary so greatly during short periods of time. Further employment of weighted gage height determinations may become necessary. Station maintained and record developed by Jana Ash. Photos of the control at various high water stages would be helpful to determine the transition to channel control. These photos should be titled with the date and GH. 2009Water Year