Dames & Moore, 1999 - USDA Forest Service

Rating Calculations
Referring to Table 4.3-4, flow measurements were collected on the Site at RC-4 from April 16 to October 4,
1997. One flow measurement at RC-4 (June 16, 1997) and two flow measurements at RC-2 (July 10, 1997,
collected by Dames & Moore, and July 25, 1996, collected by the USFS) were excluded; the excluded flow
measurements appear to represent a physical shift in the stream channel conditions, probably due to scour
and bed movement during the peak flow of June 16. Rating curves were developed for stations RC-4 and
RC-2. A rating curve was not developed for RC-6 due to difficulties in collecting data at this station due to
the irregular cross-section.
Typically, the Railroad Creek streambed is armored and immobile during most flows; however, during peak
flows, the bed may become mobile (i.e., the stream power is capable of moving the cobble- to boulder-size
material which armors the streambed), which affects the stage discharge relation. Subsequent flow
measurements at these stations returned to the previous rating, indicating that the effect of the mobile bed
was short lived due to restabilization of the streambed. The rating curve data and rating graphs are provided
in Appendix H. The rating curves have been developed by fitting a single log-log curve to the stage-
discharge data using linear regression techniques. For both rating curves (RC-2 and RC-4), the regression
analysis showed very good predictability, with correlation coefficients of 0.994 or better. This indicates that
a single flow control over the range of flows measured adequately represents the channel flow conditions.
The best fit ratingequation for RC-4 is:
where:
Discharge = 26.2(G-GZF)"2.58 1
Discharge = flow estimate in cubic feet'per second (cfs)
G = the observed staff gage height (feet)
GZF = the gage height of zero flow (feet) = -0.4
The best fit rating equation for RC-2 is:
where:
GZF = -3.0
G:\WPDATA\OO5\REPORTSWOLDEN-ZUUWd.WC
17693-005419Uuly 19, 1999;4:J 1 P M;DW FINAL RI REPORT
Discharge = O.OS(G-GZF)"5.5 15