01C070 - Hutchinson Creek near Acme Technical Notes: 2007 Water Year Chuck Springer The telemetered stream gaging station on Hutchinson Creek near Acme operated throughout water year 2007 with only one 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 2007 on Rating Table 2. Table 2 covers a range of discharge from 4.3 cubic feet per second (cfs) to 635 cfs. Only one of the 15 discharge measurements used to develop this rating was taken during water year 2007. The measured flows for this rating, ranging from 4.6 to 140 cfs, cover only 21% of the rating curve; however, flows exceeded the highest measured flows only 4% of the time while Table 2 was in effect during water year 2007. Flows greater than 140 cfs were modeled using a slope-conveyance model developed for this site. Since the stage-discharge relationship naturally formed a fairly straight line when viewed in logarithmic space, no offset was required to perform interpolations between measurements and extrapolations to half the lowest measured flow. The potential error for flows derived from this rating curve is ±12%. A moderate storm event in November 2006 caused a substantial amount of channel fill. This shift is represented by Rating Table 3. Table 3 covers a range of discharge from 14 to 635 cfs. All three of the discharge measurements used to develop this rating were taken during water year 2007. The measured flows for this rating, ranging from 28.1 to 194 cfs, cover only 27% of the rating curve; however, flows exceeded the measured range of flows only 10% of the time while Table 3 was in effect during water year 2007. Five percent of flows exceeded the lowest measured flow, and 5% exceeded the highest measured flow. Flows greater than 194 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=1.3) calculated from the stage-discharge relationship. The potential error for flows derived from this rating curve is ±11%. A moderate, but long-lasting storm event in March 2007 caused a substantial full-range channel fill, necessitating a complete rebuilding of the rating. This shift is represented by Rating Table 4. Table 4 covers a range of discharge from 0 to 943 cfs. Four of the eight discharge measurements used to develop this rating were taken during water year 2007. 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 2007. 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=1.4) calculated from the stage- discharge relationship. The potential error for flows derived from this rating curve is ±11%. Stage Record The station logged continuously throughout water year 2007 with one interruption from February 27 to March 10 due to a power supply failure. A continuous stage record for this time period was modeled by regressing stage at this site against stage at station 01F070 - S.F. Nooksack River at Potter Road, which is located 8.3 miles below the confluence with Hutchinson Creek (Y=0.297X+0.095, r2=0.62). 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 typically differed from manual wire weight gage readings by highly variable amounts, as much as 0.56 ft. Many of the large differences were attributed to the terminal end of the orifice slant pipe being buried in the sediment following the November 2006 event. 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 0.993, with a standard deviation of 0.03 ft. The regression identified one outlier, which was attributed to an improper staff gage reading (the “ramp-up” of water on the staff gage was likely included in the observed reading). The calculated potential error of the continuous stage data for this station is ±21%. This high potential error is largely due to the terminal end of the slant pipe becoming buried. Future Efforts Accumulation and subsequent blowout of a debris dam at this site has made for very unstable flow conditions during water years 2007-2009. Continued frequent discharge measurements will be necessary to ensure accuracy of the rating curve.