In the size distribution analysis of coarse-bed materials, it is sometimes necessary to obtain
particle counts by number, rather than by sieving or visual accumulation tube analysis for a
part of the sample. Care must be taken in the interpretation of frequency distribution of part
of a sample obtained by sieving. Only if the size distribution in a sample follows log-normal
probability distribution can number counts be transferred to distributions by size, volume,
weight or surface areas directly. For other distributions, special numerical techniques have
to be used to transform the number distributions to weight or size distributions.
If the objectives of bed material sampling include bed roughness and channel response, then
the particles coarser than D84 or D90 need to be analyzed with more care. These sizes also
require large samples for their determination.
Resistance to Flow. In sandbed channels, the form roughness is the primary component of
channel roughness. Form roughness can be much greater than the grain roughness when
the bed forms are ripples and dunes. In coarse-material channels, the ripples never form
and dunes are rare. The main type of bed form roughness in such channels is the pool and
riffle configuration. With coarse material channels the grain roughness is the main
component of the channel roughness.
A coarse-material channel may have bed material that is only partly submerged during most
of the flows. It is difficult to determine the channel roughness for such beds. For other
cases, analysis of data from many rivers, canals and flumes (Anderson et al. 1968) shows
that the channel roughness can be predicted by various forms of Strickler's equation:
n = K u D1/ 6
(3.14)
x
V.T. Chow (1959)
(3.15)
Ku = 0.0417
(D50 in meters)
Ku = 0.0342
(D50 in feet)
(3.16)
Anderson et al. (1970)
Ku = 0.0482
(D50 in meters)
Ku = 0.0395
(D50 in feet)
(3.17)
Lane and Carlson (1955)
Ku = 0.0473
(D75 in meters)
Ku = 0.0388
(D75 in feet)
Ku = 0.0256
(D75 in inches)
(3.18)
U.S. Army Corps of Engineers (1991)
Ku = 0.046
(D90 in meters)
Ku = 0.038
(D90 in feet)
Equation 3.19 has been proposed by Limerinos (1970) and involves flow depth yo as a
parameter. Comparisons of several equations have been reported by Bray (1982) and
Simons and Senturk (1992).
3.30
Previous Page
