account. The effect of fine sediment on resistance to flow is a result of its effect on the
apparent viscosity and the density of the water-sediment mixture. Generally, the fine
sediment is uniformly distributed in the stream cross-section. The method of defining and
treating the fine-material load computations is discussed in Chapter 4.
3.4.11 Bedform Predictor and Manning's n Values for Sand-Bed Streams
In Figure 3.13, the relation between stream power, median fall diameter of bed material, and
form roughness is shown. This relation gives an indication of the form of bed roughness one
can anticipate if the stream power and fall diameter of bed material are known. Flume data
were utilized to establish the boundaries separating plane bed without sediment movement
and ripples, and ripples and dunes for bed material with D50 finer than 0.64 mm, and plane
bed without sediment movement and dunes for D50 coarser than 0.64 mm. The lines dividing
dunes and transition and dividing transition and upper regime are based on flume data and
the following field data: (1) Elkhorn River, near Waterloo, Nebraska (Beckman and Furness
1962); (2) Rio Grande, 32 km (20 mi) above El Paso, Texas; (3) Middle Loup River at
Dunning, Nebraska (Hubbell and Matejka 1959); (4) Rio Grande at Cochiti, near Bernalillo
and at Angostura heading, N. Mexico (Culbertson and Dawdy 1964); and (5) Punjab canal
data upper regime flows that have been observed in large irrigation canals that have fine
Figure 3.13. Relation between stream power, median fall diameter, and bed configuration
and Manning's n values.
Observations by the authors on natural sandbed streams with bed material having a median
diameter ranging from 0.1 mm to 0.4 mm indicate that the bed planes out and resistance to
flow decreases whenever high flow occurs. Manning's n changes from values as large as
0.040 at low flow to as small as 0.012 at high flow. An example is given in Figure 3.14.
These observations are substantiated by Dawdy (1961), Colby (1960), U.S. Army Corps of
Engineers (1968) and Beckman and Furness (1962).