Environmental Engineering Reference
In-Depth Information
The weed factor is defined as:
roughness including weed effect
roughness without weed effect
weed factor
=
(5.23)
The actual roughness can be calculated by using the weed factor: Actual
roughness
F w * Initial roughness.
Next the equivalent roughness for a cross section with obstruction can
be estimated by computing the initial roughness of the whole cross section,
depending on whether there is either a single roughness or an equivalent
roughness or based on the type of roughness of the bottom and sides. Next
the actual equivalent roughness is reduced by the weed factor F w .
=
5.2.5 Equivalent roughness for non-wide irrigation canals
The most common cross section of an irrigation canal has a trapezoidal
shape with a relatively small bottom width-water depth ratio. In this type
of cross section, the velocity distribution is heavily affected by the varying
water depth perpendicular to the sidewall and by the boundary condition
imposed on the velocity by the sidewall. An important interaction and
transfer of momentum between the side parts and the central part of the
canal will take place. The existing methods to estimate the composite
roughness were developed for rivers in which the cross section is divided
into a main canal and two flood plains. The assumptions made are not valid
for non-wide, trapezoidal or rectangular canals. The main shortcomings
are firstly, the lack of attention to the effect of the varying water depth on
the friction and secondly, the hypothesis that the mean velocity and/or the
hydraulic radius is the same in all the subsections.
Normally, the flow in irrigation canals is turbulent and it can be
assumed that the lateral velocity distribution in a trapezoidal canal is
more governed by the varying water depth above the sides than by
the boundary condition imposed by the wall. Based on this hypoth-
esis, the composite roughness can be derived by assuming that the
cross section consists of an infinite number of stream tubes (slices).
The water depth and the local friction of each stream tube govern the
resistance in the stream tube. To evaluate the resistance it is assumed
that there is no transfer of momentum between the stream tubes and
that the local resistance can be expressed by the de Chézy coefficient
(see Figure 5.10).
In irrigation canals the roughness of the bottom and the sides are
often different. Several cases of composite roughness in an irrigation
canal can be distinguished:
Rigid boundaries : When the boundary is rigid and unchanging then
the resistance only depends on the skin roughness. The composite
roughness depends on the material of the walls and bottom. Roughness
 
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