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 Technical Note DOER-N4
May 1999
The second nondimensional parameter Fgr represents particle mobility defined as the ratio of shear
forces to the immersed sediment weight:
1- n
υ*
n
Vwc
Fgr =
(10)
gD (s - 1) 32 log10 d
D
where d is the mean depth of flow (ft), and υ* is the shear velocity (ft/sec), which can be defined
from Chow (1959, 204) as:
gVwc
υ* =
(11)
Cz
where Cz is the Chezy coefficient defined by
(
)
1.48d 0.167
Cz =
(12)
mann
and mann is the Mannings N = 0.025. Initiation of motion of sediments can then be estimated from
the following value:
Fgr
-1
(13)
A
If this value is less than or equal to zero, there is no movement of a uniformly graded sediment with
the specified median grain size. If the value is above zero, then there will be sediment movement.
The rate of transport can also be determined by further analysis of the parameters (see Scheffner et
al. 1995). If the value of Equation 13 is only slightly above zero, transport will be minimal and
essentially zero. Therefore, under these conditions, transport and total erosion will be barely
noticeable.
Application of the above methods to determine initiation of motion is determined by the user first
specifying wave height, wave period, current velocity, and median grain size. The program then
estimates the water depth at which these wave conditions will begin breaking, i.e., the water depth
at which the above methods are valid. Each water depth between the breaking value and 250 ft is
analyzed at 1-ft increments until the value of Equation 13 is negative (no transport). If the sediment
does not move at the location of breaking, or the sediment is still mobile at 250 ft water depth, the
program will indicate to the user that the depth of residence cannot be calculated and will provide
the reason.
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