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Integration of Stream Stability, Reference Condition & Sediment Rating Curves

Stream stability shifts are reflected in sediment rating curves, where measured sediment values are regressed against measured discharge. The upward shift in the slope and/or intercept values of the sediment rating curve are due to increased sediment supply as a result of a variety of sources. The upward shift in the sediment rating curve exponentially increases the sediment yield for selected increments of streamflow. Land uses that increase streamflow magnitude and duration can thus be very instrumental in accelerating flow-related increases in sediment due to characteristically unstable channels with elevated sediment rating curves. Examples of sediment rating curves influenced by land use are shown in Missouri gully erosion (Piest et al. 1975), silvicultural impacts (EPA 1980), Colorado silvicultural and reservoir impacts (Rosgen 1996), silvicultural impacts (Rosgen 2001c), west Tennessee channelization for land drainage and flood control (Simon and Hupp 1989), and Arizona silvicultural impacts (Lopes et al. 2001).

Many of the ongoing adjustments to the stream system involve events and/or past perturbations associated with the various stages of morphological types. It is very important to identify the potentially stable stream type of the existing river in contrast to its present state. A reference stream type can not only provide the morphologically stable form of dimension, pattern and profile, but also the corresponding rates of sediment supply, bank erosion rates and other characteristics representing a stable (quasi-equilibrium) geomorphic condition.

An example of the use of a reference stream to assess changes in sediment yields is that of the Hatchie and South Fork Forked Deer Rivers in West Tennessee. The sediment consequence associated with a shift in stability and evolution stages was presented by Simon (1989). The Hatchie River, a stable E5 stream type, meandering, sand bed stream with a well developed floodplain and a low width/depth ratio, had an annual sediment yield of 57 metric tons/year/km2 (163 tons/mi2). Channelization for land drainage was applied to the South Fork Forked Deer River in the same basin as the Hatchie River. The results of the channelization entrenched the river and initiated a channel evolution sequence from stage I to III/IV (Simon and Hupp 1986). The changes in channel evolution through successional stages corresponded to a shift from stream types E5 to G5 to F5, or scenario #5 in Figure 42 (Rosgen 1994, 1999, 2001b). The F5 stream type is a relatively straight, incised, high width/depth ratio, sand bed stream. The resultant unstable, entrenched (F5 stream type) was producing 872 metric tons/year/km2 (2490 tons/yr/mi2) (Simon 1989). The majority of the sediment supply is from streambank erosion. The U.S. Geological Survey's suspended sediment rating curves for the Hatchie and South Fork Forked Deer Rivers appear in Figure 53 (Simon, 1989).

The F5 stream type is a relatively straight, incised, high width/depth ratio, sand bed stream. The resulting unstable, entrenched, F5 stream type was producing 872 metric tons/year/km2 (2490 tons/yr/mi2) (Simon, 1989). The majority of the sediment supply was from stream bank erosion. The U.S. Geological Survey (Simon, 1989) measured suspended sediment rating curves on the Hatchie and South Fork Forked Deer Rivers (Figure 53).

Figure 53

Figure 53. Suspended sediment rating curves for South Fork Forked Deer and Hatchie River (from Simon 1989).

The sediment rating curves were converted to dimensionless ratios (Figure 54). In this relation the data from Simon (1989), mg/l were converted to tons/day to match the published rating curve. The E5 stream type Hatchie is a stable reach whereas the F5 stream type South Fork Forked Deer is an unstable reach. Subsequently, a statistical analysis performed on the two dimensionless sediment rating curves resulted in a p value of 0.00000029, at the 1.95 level, indicating that the South Fork Forked Deer River curve is statistically significantly different than the reference Hatchie River curve.

Figure 54

Figure 54. Conversion of suspended sediment rating curves into dimensionless relation for the South Fork Forked Deer and Hatchie Rivers (from Simon 1989).

The direct disturbance that created the accelerated sediment supply has existed since the 1950s and the river is yet to reach an equilibrium state. These channel adjustments have resulted in long-term adverse sediment effects. The channel stability shifts and corresponding changes in stream types are associated with measured quantitative morphological relations involving channel dimension, pattern, profile, and channel materials. The corresponding sediment consequence due to streambank erosion is obvious.

The prediction of a shift or departure from a reference condition in measured sediment rating curves, ranking good to poor stability ratings (Pfankuch 1975) by stream type (Rosgen 1994, 1996) was conducted by Troendle et al. (2001). In this relationship, dimensionless sediment rating curves for suspended sediment and bedload sediment were established from measured data, using measured bankfull discharge and sediment values for normalization. Study results demonstrated that there was a statistically significant difference (departure) of the curve compiled from streams with poor stability ratings when compared to the dimensionless "reference" curve for stream types with good/fair stability. Stream channel stability analysis can thus be used to show a departure from the reference sediment rating curve.

Similar results have been obtained in other sediment studies. Measured suspended sediment rating curves in the Redwood Creek drainage were plotted by channel stability rating categories, indicating that sediment concentration increases exponentially with discharge as a function of channel stability ratings (Figure 55) (Leven 1977, EPA 1980, and Rosgen 2001b). Measured bedload sediment rating curves by stream type from Colorado rivers show the variation in sediment rating curves associated with various stream types (Rosgen 1996, 2001b). Stream type change often is associated with instability and corresponding changes in sediment supply. It is thus important to realize that sediment relations must address both stream type and stability assessment.

Figure 55

Figure 55. Suspended sediment rating curves by channel stability ratings of various reaches of Redwood, CA (from Leven 1977, EPA 1980, and Rosgen 2001b).

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