01C070 - Hutchinson Cr. nr Acme Technical Notes: 2009 Water Year Chuck Springer The telemetered stream gaging station on Hutchinson Creek near Acme operated throughout water year 2009. During the water year, nine discharge measurements were made and ten discrete manual stage readings were taken at this station. Rating Curve This station began water year 2009 in transition from Rating Table 6 (discussed in water year 2008 technical notes) and Rating Table 7, due to beaver activity adding woody debris to the control for the gage. Table 7 covers a range of discharge from 6 cubic feet per second (cfs) to 943 cfs. One of the three discharge measurements used to develop this rating was taken during water year 2009. The measured flows for this rating, ranging from 12.1 to 106 cfs, cover only 10% of the rating curve. Flows exceeded the measured range of flows 60% of the time during the one-and-a-half months that Table 7 was in effect during water year 2009. Fifty-percent of flows exceeded the lowest measured flow, and 10% exceeded the highest measured flow. Flows greater than 106 cfs were modeled using a slope-conveyance model developed for this site. The rating curve was interpolated between discharge measurements and extrapolated to half the lowest measured flow using Johnson’s method to temporarily straighten the rating curve using a log offset (e=2.2) calculated from the stage-discharge relationship. The potential error for flows derived from this rating curve is ±12%. A moderately large storm event in November 2008 caused substantial channel scour, shifting back to the stage-discharge relationship reflected by Table 5. Table 5 covers a range of discharge from 0 to 943 cfs. One of the six discharge measurements used to develop this rating was taken during water year 2009. The measured flows for this rating, ranging from 55.1 to 119 cfs, cover only 7% of the rating curve. Flows exceeded the measured range of flows 50% of the time during the two months Table 5 was in effect during water year 2009. Flows greater than 119 cfs were modeled using a slope-conveyance model developed for this site. The rating curve was interpolated between discharge measurements and extrapolated to point of zero flow using Johnson’s method to temporarily straighten the rating curve using a log offset (e=1.2=PZF) calculated from the stage-discharge relationship. The potential error for flows derived from this rating curve is ±9%. During a large storm event in January 2009, a large woody debris jam 100 ft. downstream of the station blew out, and the channel scoured, shifting the stage-discharge relationship throughout the flow range. This shift is represented by Rating Table 8. Table 8 covers a range of discharge from 0 to 1,490 cfs. Seven of the eight discharge measurements used to develop this rating were taken during water year 2009. The measured flows for this rating, ranging from 4.8 to 168 cfs, cover only 11% of the rating curve; however, only 5% of flows exceeded the highest measured flow during the nine months Table 2 was in effect during water year 2009. Flows greater than 168 cfs were modeled using a slope-conveyance discharge model developed in July 2009. The initial model results were 30-40% higher compared with measured flows. The extrapolated results were then scaled using linear regression to calibrate the model to the measured flows. The calibrated model results were, on average, within 4% of the measured flows, indicating a very good fit of the model to measured flows after model calibration. The rating curve was interpolated between discharge measurements using Johnson’s method to temporarily straighten the rating curve using a log offset (e=0.92) calculated from the stage-discharge relationship. The potential error for flows derived from this rating curve is ±15%. Stage Record The station logged continuously throughout water year 2009 without interruption. The staff gage at this site is generally readable within 0.02 ft during low-flow conditions, and the readability deteriorates as much as ±0.10 ft during high flows. During the January 2009 storm event, the staff gage was severely damaged, and was replaced with a laser level. The laser level readings are taken on the opposite bank, and typically considered accurate within 0.02 ft. Conditions surrounding the terminal end of the bubbler orifice are similar to those around the staff gage. The stage height readings typically differed from the manual staff gage and laser level readings by variable amounts, as much as 0.13 ft. Drift at this station was variable and unpredictable, leaving very few periods of time that could be considered stable. Time-weighted corrective adjustments were made to the continuous stage record whenever the staff gage readings and datalogger readings differed. All adjustments are documented in the Hydstra Data Workbench. Where drift conditions were considered unstable, and adjustments resulted in a 20% or greater change in discharge for any given day, the data for that day were qualified as estimates. Days with a 50% or greater change in discharge were qualified as questionable estimates. For water year 2009, 62 days were qualified as estimates, and 5 days were qualified as questionable estimates. Quality control measures were taken to identify potentially erroneous staff gage and laser level observations. A linear regression of staff gage/laser level observations versus tape down observations had an r2 of 0.99, with a standard deviation of 0.06 ft. The regression did not identify any obvious outliers. However, with the moderately high standard deviation, minor outliers would be difficult to identify. The calculated potential error of the continuous stage data for this station is ±16%. Future Efforts In January 2009, this station experienced a dramatic change in channel geometry. While the high-flow model has since been updated, and measurements taken since January 2009 have shown that the channel has for the most part stabilized, continued frequent discharge, and point of zero-flow measurements will be required to maintain an accurate discharge record.