Environmental Engineering Reference
In-Depth Information
u ,cr
w s =
0 . 4
for D > 10
(5.64)
Discharge of the sediment per unit width can be written as:
h
q s,c =
uc d z
(5.65)
a
h
u
u
c
c a d z
q s,c =
c a u
(5.66)
a
where:
c
=
sediment concentration at height z from the bed
c a =
reference concentration at reference height a above the bed level
h
=
water depth (m)
volumetric suspended transport (m 2 /s)
q s,c =
u
=
velocity at height z above the bed (m/s)
depth-averaged flow velocity (m/s)
For the solution of the equation, the velocity profile, concentration
profile and concentration at reference level should be known. For steady
and uniform conditions of water and sediment, the concentration profile
of sediment in the vertical is in equilibrium. A number of relations are
available for the prediction of suspended sediment transport rates. Some of
them are based on analytical approaches, but they still need experimental
results to derive certain parameters.
u
=
Initiation of motion
Due to the fact that the sediments entering irrigation canals are from
external sources (for instance, rivers), the particle size of the sediment is
usually different from the parent bed material. The incoming particle size
depends on the operation of the sediment trap or intake structure at the head
of the canal network. Normally the sediments entering into the irrigation
canals are in the range of fine sand, silt and clay (Worapansopak, 1992).
Larger particles are preferably excluded from entering the canal system
by a careful skimming of the water at the intake or have been allowed to
settle in a sediment trap in the first reach of the canal system (Dahmen,
1994). In view of these provisions at the head of an irrigation network, the
sediment sizes are assumed to be in the range of 0.05 mm < d 50 < 0.5 mm.
It is also assumed that only non-cohesive material will be present in the
irrigation system, despite some degree of cohesion that is present for the
smaller particle sizes.
 
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