01C070 - Hutchinson Creek near Acme Technical Notes: 2008 Water Year Chuck Springer The telemetered stream gaging station on Hutchinson Creek near Acme operated throughout water year 2008 without interruption. During the water year, ten discharge measurements were made and 20 discrete manual stage readings were taken at this station. Rating Curve This station began water year 2008 on Rating Table 4. Table 4 covers a range of discharge from 0 cubic feet per second (cfs) to 943 cfs. Four of the eight discharge measurements used to develop this rating were taken during water year 2008. The measured flows for this rating, ranging from point of zero flow (PZF) to 119 cfs, cover only 13% of the rating curve; however, flows exceeded the highest measured flows only 2% of the time while Table 4 was in effect during water year 2008. 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 half the lowest measured flow using Johnson’s method to temporarily straighten the rating curve using a log offset (e=1.2) calculated from the stage- discharge relationship. The potential error for flows derived from this rating curve is ±11%. A moderate storm event in February 2008 caused a minor amount of scour on the section control for the gage. This shift is represented by Rating Table 5. Table 5 covers a range of discharge from 0 to 943 cfs. Five of the six discharge measurements used to develop this rating were taken during water year 2008. The measured flows for this rating, ranging from 55.1 to 119 cfs, cover less than 7% of the rating curve. Flows exceeded the lowest measured flow 62% of the time while Table 5 was in effect during water year 2008. 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 an estimated PZF using Johnson’s method to temporarily straighten the rating curve using a log offset (e=1.2) calculated from the stage- discharge relationship. The estimation of PZF was consistent with surrounding rating tables, in that it fell between “scoured” and “filled” tables; however, since there was no direct measurement of PZF taken during this time period, the rating is considered estimated from the PZF to half the lowest measured flow. The potential error for flows derived from this rating curve is ±8%. Between early June and late July 2008, a gradual filling occurred on the section control for the gage. This shift is represented by Rating Table 6, which is “phased in” between discharge measurements taken on June 11 and July 21. Table 6 covers a range of discharge from 0 to 943 cfs. All five discharge measurements used to develop this rating were taken during water year 2008. The measured flows for this rating, ranging from PZF to 119 cfs, cover only 13% of the rating curve;however, flows exceeded the highest measured flows only 1% of the time while Table 6 was in effect during water year 2008. 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 half the lowest measured flow using Johnson’s method to temporarily straighten the rating curve using a log offset (e=1.6) calculated from the stage-discharge relationship. The potential error for flows derived from this rating curve is ±8%. Stage Record The station logged continuously throughout water year 2008 without interruption. The staff gage at this site is generally readable to within 0.02 ft during low flow conditions, and the readability deteriorates to as much as ±0.10 ft during high flows. Conditions surrounding the terminal end of the bubbler orifice are similar to those around the staff gage. The stage height readings differed from manual wire weight gage readings by varying amounts, as much as 0.19 ft. Time- weighted corrective adjustments were made to the continuous stage record whenever the staff gage observations and datalogger readings differed by amounts exceeding 0.02 ft. All adjustments are documented in the Hydstra Data Workbench. Quality control measures were also taken to identify potentially erroneous staff gage observations. A linear regression of staff gage observations versus tape down observations had an r2 of 0.985, with a standard deviation of 0.03 ft. The regression did not identify any obvious outliers, indicating that the staff gage and tape down observations made during water year 2008 are at least reasonably accurate. The calculated potential error of the continuous stage data for this station is ±10%. Future Efforts Accumulation and subsequent blowout of a debris dam at this site created unstable flow conditions during water years 2007-2009. Continued frequent discharge measurements will be necessary to ensure accuracy of the rating curve.