Sediment Transport Concepts - Sediment Transport in Irrigation Canals: A New Approach - page 102

be uniform over the whole cross section. The value of n can be related to
de Chézy's roughness coefficient for the side part only by:

R 1 / 6

s
n

C s = (5.20)

where:
C s =

Chézy's roughness coefficient on the side slope only (m 1 / 2 /s)

R s = hydraulic mean radius of the sides (m)

Next the de Chézy's roughness coefficient can be related to the
roughness height of the sides as:

18 log 12 R s
k ss

C s = (5.21)

Roughness caused by surface irregularity may be due to the type of mate-
rial, construction methods and workmanship, ageing of the side slope,
rain cuts, slides of the banks, etc. Chow (1983) gives a classification of
the surface irregularities and correction factors for Manning's coefficient
for four categories:
- Ideal : refers to the best attainable surface for the construction
material. Newly constructed or well-maintained canals with perfect
workmanship belong to the ideal type of canal. No correction is needed
for this type of surface. For earthen canals the value of Manning's n
is 0.018;
- Good : refers to newly constructed or weathered but well maintained
canals with good to moderate finishing. A value of 0.005 is added to
the value of Manning: n 0.023;

- Fair : the surface of the canals that are moderately to poorly excavated
and also includes the canals that have been excavated by machines and
have eroded side slopes. A value of 0.01 is added ( n

=

0.028);
- Poor : badly eroded or sloughed side slopes, large rain cuts and
excavations not in the proper shape. A value of 0.02 is added ( n

=

= 0.038).

Vegetation reduces the effective flow area and increases the roughness
(see Table 5.4). Vegetation growth is more pronounced in clear water;
however, the nutrients in water with sediment may help the growth of
weeds. The degree of obstruction by vegetation is highly variable and
depends upon the type, height, density and flexibility of the vegetation,
submerged or un-submerged conditions, water level, and flow velocity
(Paudel, 2010). Kouwen (1988) gives an empirical relation to calculate
the equivalent roughness height for the given vegetation:

0 . 14 h g ( mei/τ ) 0 . 25
h g

1 . 59

k s =

(5.22)