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Water: WARSSS

Dimensionless Bedload & Suspended Sediment Rating Curves

Establishing bedload and suspended sediment rating curves requires measuring flow and sediment over a wide range of temporal and spatial variability. It is expensive and time consuming. It is also difficult to expect a bedload or total-load equation to predict actual sediment rating curves. By normalizing actual measured bedload and suspended sediment relations with measured bankfull values of bedload, suspended sediment and streamflow, reference sediment rating curves can be established. The "reference" is associated with streams of similar type and stability ratings. When departure from reference occurs in stream channel stability and is reflected in increased sediment supply, there is often a shift in the sediment rating curves (Rosgen 2001c). This shift may be detected by establishing a river-specific sediment rating curve, making it dimensionless, then testing against the reference curve as in Troendle et al. (2001).

Flow-related increases in sediment are generated from a flow model and a sediment rating curve, as described in WRENSS (EPA 1980). Since sediment rating curves are difficult to establish, sediment transport equations have been developed to provide rating curves. Unfortunately, without actual measured sediment data to calibrate these models, the values can be many orders of magnitude from actual data. The application of the reference dimensionless ratio sediment rating curve provides a new method of sediment rating curve prediction. This is made possible by collecting sediment and discharge data at the bankfull stage, and converting the dimensionless ratio to actual values as described previously.

Examples of the application of this relationship are shown where locally derived bedload and suspended sediment dimensionless relations were tested elsewhere on similar stable, snowmelt-dominated streams. The majority of the suspended sediment and bedload sediment data tested is from U.S. Geological Survey measurements in Idaho (Emmett 1975, and personal communication, 2001). For model validation, eighteen rivers were tested whose data were independent from those rivers used in the development of the relation. The reference dimensionless ratio suspended and bedload rating curve data was developed from stable, snowmelt runoff streams in southwestern Colorado (Figures 56 and 57).





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Figure 56

Figure 56. Dimensionless suspended sediment rating curves for "Good/Fair" streams/stability - Pagosa Springs, Colorado.

Figure 57

Figure 57. Dimensionless bedload sediment rating curves for "Good/Fair" streams/stability - Pagosa Springs, Colorado.

The computations for the test involved obtaining the bankfull discharge and bankfull suspended and bedload sediment values for each river, then applying the appropriate reference equation from dimensionless bedload sediment (Equation II-14) and dimensionless suspended sediment (Equation II-15).

(eq.II-14)   Bedload (good/fair):                   y = -0.0113 + 1.0139x2.1929

(eq.II-15)   Suspended sediment (good/fair): y = 0.0636 + 0.9326x2.4085

Where: y = dimensionless sediment
           x = dimensionless discharge

The multiplication of the bankfull values for the test rivers times the ratios generated a sediment rating curve. The comparison of predicted to measured values for suspended sediment and bedload rating curves for the eighteen rivers are shown in Figures 58 to 61.

Figure 58.(PDF, 34 kb, 1 p.) Examples of predicted versus measured suspended sediment data using reference dimensionless rating curve

Figure 59. (PDF, 49 kb, 1 p.) Examples of predicted versus measured bedload and suspended sediment data using reference dimensionless rating curve.

Figure 60. (PDF, 32 kb, 1 p.) Example of predicted versus measured bedload and suspended sediment data using dimensionless reference curve (Data from Emmett, 1975)

Figure 61. (PDF, 32 kb, 1 p.) Example of predicted versus measured suspended sediment data using dimensionless reference curve (Data from Emmett, 1975)

The relations developed and tested appear to be exceptionally good, especially considering the difficulty of predicting sediment rating curves from bedload and suspended sediment transport formulas. The applications of this approach make it feasible for developing both suspended sediment and bedload sediment rating curves from the bankfull measurements. This assumes that the bankfull measurements are properly obtained. If great variability occurs during the same discharge at the bankfull stage, then several measurements should be taken to establish average values. Nonetheless, these results are very encouraging. Coupled with a water yield analysis converted to flow-duration curves, flow-related sediment increases can be reasonably obtained. The contributions of sediment integrated in the sediment rating curves can be proportionally determined by more process-specific predictions. These sediment prediction procedures will be presented in the prediction level assessment section (PLA) of the WARSSS methodology.

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