Environmental Engineering Reference
In-Depth Information
Table 5.4. Weed factor for different types of vegetation at full growth (derived from
Chow's (1983) suggested n for vegetation).
Category
Description
Weed factor (Chow)
- dense growth of flexible turf grass ( h/h g = 2-3)
Low
1.25-1.5
- supple seedling tree switches ( h/h g =
3-4)
Medium
- turf grasses ( h/h g =
1-2)
- stemmy grasses, weeds or tree seedlings ( h/h g =
2-3)
1.5-2.5
- brushy growth, moderately dense
High
- turf grasses ( h/h g =
1)
2.5-3.5
- willow or cottonwood trees 8-10 years old
- bushy willows
Very high
- turf grass ( h/h g =
0.5)
3.5-6.0
h
=
water depth and h g =
height of vegetation.
mei is a parameter that is a function of stem density, elasticity modulus
and second moment of area and is:
for green grass: mei
=
319 h 3 . 3
g
25 . 4 h 2 . 26
g
for dead grass: mei
=
where:
h g =
height of the vegetation (m)
k s =
equivalent roughness height (m)
local shear stress (N/m 2 )
Another simple way to incorporate the effect of weed is given by
Chow (1983), who has suggested a classification of vegetation and a
correction to be added to the Manning's n .
Concerning the weed effect it can be mentioned that the analysis of the
sediment transport process assumes that the canal bed is free of weeds;
however, the side slopes may have weed effects. In the case that weed
grows on the side slope then the roughness due to surface irregularities
is neglected. It is assumed that the vegetation fully covers the surface
irregularities. The equivalent roughness of a canal with bed forms on
the bottom and vegetation on the side slopes depends on the degree of
obstruction by the weed growth. Querner (1993) presents typical variation
of the relative obstruction and elative roughness coefficient during the
weed growth period (see Figures 5.7 and 5.8). This variation gives only
tendencies for the changes in relative obstruction and roughness.
To obtain the actual roughness the friction factor has to be increased
by a weed factor F w . The weed factor depends on local flow conditions,
vegetation characteristics (growing period, type etc.). Méndez (1998)
mentions values of F w between 0.1 (a very densely overgrown canal) and
1 (an ideally cleaned canal). Table 5.5 shows some values of the weed
factor F w , which is based on the data from Nitschke (1983). These values
are only applicable for the specific flow conditions and vegetation
described in that research.
τ
=
 
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