Environmental Engineering Reference
In-Depth Information
Initiation of suspension : van Rijn (1984b) describes the initiation of
suspension by:
w s
16
D 2
θ cr =
for 1 < D
10
(5.80)
( s
1) gd 50
w s
θ cr =
16
for D > 10
(5.81)
( s
1) gd 50
These equations can also be written as:
u ,cr
w s =
4 D 1
for 1 < D
10
(5.82)
u ,cr
w s =
0 . 4
for D > 10
(5.83)
where:
θ
=
mobility Shields' parameter (dimensionless)
θ cr =
critical mobility Shields' parameter (dimensionless)
Re =
particle Reynolds number (dimensionless)
s
=
relative density ( ρ s / ρ )
u =
local shear velocity (m/s)
w s =
fall velocity (m/s)
D =
particle parameter (dimensionless)
d 50 =
median diameter (m)
acceleration due to gravity (m/s 2 ).
g
=
Irrigation canals are manmade canals and their design takes into
account aspects related to the irrigation criteria and sediment transport.
On the one hand, the canals should meet the irrigation requirements and
on the other hand, no deposition of the sediment entering into the system
or scouring of the parent material should occur. Suggested values for non-
scouring bottom shear stress are available in the literature. Kinori (1970)
and Chow (1983) give minimum values for the non-scouring shear stress
for water containing fine sediments in the range between 1.5 N/m 2 (fine
sand, sandy loam) and 15 N/m 2 (hard clay and gravel). Dahmen (1994)
suggests a maximum value for the design of irrigation canals of 3-4 N/m 2 .
Even though the design values for the shear stress can be reduced due to
changes in the operation strategies during the irrigation season, the value
of the remaining shear stress is still high enough to initiate motion and
further suspension of the previously deposited sediment. However, it is no
longer so high as to produce scouring of the parent material of the canal.
Figure 5.17 shows the Shields' curve for initiation of motion and the
criteria used by van Rijn (1993) to initiate suspension. This figure presents
 
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