Environmental Engineering Reference
In-Depth Information
APPENDIX C
C.1 INTRODUCTION
In many cases, profitable and sustainable agricultural production needs
an irrigation network that brings water from the source to the fields at the
right place, at the right time, in the right quantity and at the right level
(FAO, 1992). An irrigation network can be divided into main, secondary,
sub lateral and tertiary canals. The design capacity of all the canals and
appurtenant structures should be sufficient to handle the maximum envis-
aged flow in a convenient and reliable way. The design of the physical part
of the irrigation network should enable the operation of the canal network
according to the following criteria (Dahmen, 1999):
•
The required flows are passed at design water levels.
•
No erosion of canal bottoms and banks occurs.
•
Any sediment that enters the system will be carried along the network
and will discharge through outlets either to the fields, or to the natural
drainage, or will settle in special designed silt traps.
The demand for irrigation water is not constant during the irrigation
season, but is affected by the water requirements of the crops growing on
the fields. Despite the fact that the water requirements vary during the
irrigation season, the design discharge is defined as the maximum flow
that can be handled in a proper way. The design discharge of a canal reach
is the sum of the simultaneous, maximum flows through the outlets in
this reach and the outflow into the next downstream reach plus seepage
and other losses, including those due to the operation of the network. The
simultaneous, maximum flow through the outlets will cover the maximum
crop water requirement at field level, taking into account the number of
farms irrigated at the same time and the application and distribution losses
at farm and tertiary unit level. The water losses in a canal reach due to
the operation are commonly included in the conveyance efficiency. When
seepage losses in the canal reach are important they are separately taken
into account. Water losses as a result of breakage will normally not be
considered.
As stated before, the canal design should result in the right canal dimen-
sions to convey the flows at the required water levels and with the allowable
Search WWH ::
Custom Search