Water Conveyance Loss and Designing Conveyance System - Practices of Irrigation and On-farm Water Management - page 9

Table 1.1 Seepage rates in some typical soils

Seepage rate (m 3 /m 2 /day)

Soil type Uncompacted Compacted Reduction by compaction (%)

Sandy 4.0 3.8 5

Loam 1.4 1.0 27

Clay 0.35 0.10 71

Water losses occur in earthen channels under both compacted and uncompacted
conditions due to seepage. Though in some soils the extent of seepage loss is very
low (Table 1.1 ) , canal lining is nevertheless proposed to overcome losses.
Lining a canal will not completely eliminate losses; therefore, it is necessary to
measure systematically present losses or estimate the losses that might reasonably
be saved by lining before a proper decision can be made. Roughly 60-80% of the
water lost in unlined canals can be saved by hard surface lining. Seepage data for
different soils and cost of lining materials can serve as a guide in cases where no
other data are available and where investigations are extremely difficult.
In canals lined with exposed hard surface materials, such as cement concrete,
brick masonry, and other types of lining, greater velocities are permissible than are
normally possible in earthen canals. The friction loss is less in such cases. For that
reason, to supply a given discharge, the surface area of the concrete lining can be
reduced. In addition, steep side-banks can be allowed. As a result, the canal requires
lesser cross-sectional area and thus lesser total land wastage.

1.1.7.2 The Decision on Canal Lining

The decision whether or not to line a canal essentially depends upon the permeabil-
ity of the soil in which the canal is to be excavated, seepage rate of water, cost of
lining, durability of the lining, cost of water, opportunity cost of water, and environ-
mental costs (e.g., damage due to waterlogging, salinity). In many practical cases,
this decision can be reached from the visual observations of the soil, provided that it
is of a type which obviously is very pervious or impervious. When permeability is in
doubt, the decision may be reached either by applying comparative seepage data or
by measuring seepage (may be in conjunction with the determination of hydraulic
conductivity “ K ”, by field tests). Economic analysis may be performed to judge the
lining need and to select from alternative options. Details of economic analysis have
been discussed in Chapter 12 ( Economics in Irrigation Management ), Volume 1.

1.1.7.3 Lining Materials

Lining may be done with a large variety of materials. Selection of a suitable one
depends mainly on cost, performance, durability, and availability of the material.
Normally, the brick lining and precast section (both semicircular and rectangu-
lar) are durable for about 15 and 10 years, respectively. The soil-cement, asphalt