Agriculture Reference
In-Depth Information
(xiv) flow characteristics
(xv) gradient of channel
(xvi) wetness of the surrounding soil or season
(xvii) depth to groundwater table
(xviii) constraints on groundwater flow, e.g., presence of wells, drains, rivers,
and/or impermeable boundaries.
Permeability of soil is influenced by both pore size and percentage of pore space
(porosity). Soils consisting of a mixture of gravel and clay are almost completely
impervious, while coarse gravel may transmit water many times faster; thus a wide
range of seepage losses is possible. Seepage loss increases with the increase in
water depth in the canal. The distribution of seepage losses across the bed and
sides of the canal depends upon the position of the water table or impervious layer.
Seepage increases with the increase of the difference in water level in the canal
and water table. If the flowing water contains considerable amounts of suspended
material, the seepage rate may be reduced in a relatively short time. Even small
amounts of sediment may have sealing effects over a period of time. If the velocity is
reduced, the sediment-carrying capacity of the water decreases, resulting in the set-
tlement of part of the suspended materials. This forms a thin slowly permeable layer
along the wetted perimeter of the canal which decreases the seepage. In seasonally
used unlined canals, the seepage rate will be high at the beginning of the season
and gradually decrease toward its end. On most lined canals, seepage increases
with lapse of time (long period) for a variety of reasons and depending on the
material.
1.1.3 Expression of Seepage
The following terms are mostly used to express the amount of seepage:
(i) volume per unit area of wetted perimeter per 24 h or day (m
3
/m
2
/day)
(ii) volume per unit length of canal per day (m
3
/m/day)
(iii) percentage of total flow per km of canal (%/km)
Conveyance losses are sometimes expressed as a percentage of total flow for the
scheme or project basis.
Equivalents of the units (i) are
1m
3
/m
2
/d
3.2816 ft
3
/ft
2
/day
=
1ft
3
/ft
2
/day
0.3047 m
3
/m
2
/d
=
When comparing figures on seepage losses in lined canals with those in unlined,
attention should be paid to the following: For equal unit loss, the total volume lost
per unit length of canal is greater for an unlined than for a hard surface-lined canal,
since the wetted perimeter of a concrete-lined canal is about 30% less than that of
an unlined canal.
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