Forest Reference Streams

Turbidity Equations and Coefficients

Samuel H. Austin March 3, 2006

Turbidity is a useful indicator of suspended sediment load. Natural turbidities change with stream flow. Reference stream turbidity measurements describe the normal range of stream water turbidity for each type of forest reference stream, serving as a useful guide.

Patterns of sediment discharge do not necessarily coincide with runoff patterns. At sediment source areas, erosion is usually highest at the beginning of a rainstorm when sediments are more readily available. These are washed into the stream with the first rivulets of overland flow, and as the sediment supply is exhausted the concentration drops quickly. Therefore, sediment concentrations will ordinarily be greater when the water is rising, with ValueMrse particles depositing quickly as the water begins to fall. Thus, the sediment concentration will be different at the same discharge on the rising and falling limbs of a hydrograph. This effect is called hysteresis and complicates the prediction of sediment concentration from streamflow (Gordon et.al. 2004).

Nevertheless, equations documenting change in stream water turbidity over the full range of water flows in forest reference streams can serve as useful guides to the normal variation in, and magnitude of naturally occurring turbidities. These measured, empirical values may be used to help determine the normal range of sediment concentrations in streams of similar Rosgen stream type.

The turbidity equations are power functions of the form y = bx^m, where y is average turbidity measured in nephelometric turbidity units (NTU) and x is the fraction of bankfull discharge. The coefficient b, and the exponent m are empirically derived. When plotted on log-log paper the slopes of the curves, described by m, indicate the average change in turbidity with change in water discharge. The dimensionless x value, fraction of bankfull discharge, serves as an index that allows comparisons to be made among streams of various sizes.

The following table lists values and coefficients of forest reference turbidity, organized by Rosgen stream type. Turbidity values shown are for bankfull conditions. Other values may be calculated using the equation and appropriate coefficient and exponent.

Note: x represents fraction of bankfull discharge.
y represents turbidity in nephelometric turbidity units.
b and m are coefficient and exponent, respectively.

Last modified: Monday, 10-Mar-2008 19:32:11 UTC