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Water: WARSSS

Channel Stability & Sediment Rating Curves

Applying the same test as in Troendle et al, (2001), dimensionless suspended and bedload rating curves were tested against the measured sediment data associated with three stream reaches. These reaches were the Lower Wolf Creek, Lower West Fork San Juan River, and the Weminuche River. All rated as poor stability, all were C stream types. Upper Wolf Creek, West Fork San Juan River and Fall Creek, also all C stream types that rated good to fair, were compared with the reference dimensionless suspended and bedload sediment rating curves. The results showed statistically significant differences between the reference model curves and both the bedload and suspended sediment curves for the poor stability streams. The model represented the reference dimensionless sediment rating curves for suspended sediment and bedload for the good stability streams. The three good to fair streams were not statistically different from the reference relations, as one would expect.

The statistical tests of significance were made for all the streams at the 95 percent confidence level. This information is important in that it shows that poor stability rivers have a statistically significant increase over the good and fair stability reaches in measured sediment supply as indicated by the sediment rating curves for each river type and stability. This information is important for prediction as it helps to associate changes in sediment rating curves with instability. The linkage between the two is intuitively obvious; however, there have been limited data to show statistical significance from past studies.

Stream channel stability indicators can be consistently applied by various observers. It is advantageous to identify a potential problem early in the channel adjustment phase in order to apply mitigation and/or restoration, rather than waiting until the instability is such to shift the sediment rating curve of the river. This particular test demonstrated that instability can significantly increase sediment supply and shift the suspended and/or bedload rating curves of the river. The poor-stability suspended and bedload sediment rating curves are shown in Figure 62 and Figure 63, with corresponding equations of:

(eq.II-16)   Bedload (poor):                   y = -0.2445 + 1.2519x

(eq.II-17)   Suspended sediment (poor): y = 0.0989 + 0.9213x3.659

Where: y = dimensionless sediment
x = dimensionless discharge

Figure 62

Figure 62. Dimensionless suspended sediment rating curves for three unstable "poor" streams, Pagosa Springs, Colorado

Figure 63

Figure 63. Dimensionless bedload rating curves for three unstable "poor" streams, Pagosa Springs, Colorado.

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