Environmental Engineering Reference
In-Depth Information
irrigation networks. Many flow conditions along the canal network and
during the irrigation season are the same in upstream and downstream-
controlled irrigation systems. Some specific adjustments in the module
for downstream-controlled irrigation systems will be discussed in this
section; as discussed in Section 6.1.4 the general framework of the
computer program consists of:
Hydraulic aspects
- The water flow can be modelled as a sub-critical, quasi-steady,
uniform or gradually varied flow (backwater as well as drawdown
curves);
- The irrigation canals have a rectangular or trapezoidal cross section;
only friction losses are considered: no local losses due to changes in
the bottom level, cross section or discharge will be taken into account;
also seepage losses are not included;
- The distance x is measured along the canal axis; at the most upstream
boundary
x
is 0 m; the bottom slope
S
o
(m/m) and the bottom elevation
above a reference level.
- The roughness is defined by the equivalent roughness coefficient (
k
s
);
the total friction factor follows from the composite roughness of the
cross section and the weed factor (see Section 6.1.4);
- The flow control structures in this module include AVIO and AVIS gates.
Downstream controlled canals are generally equipped with AVIO and
AVIS gates that control the water levels at the headworks and in the
canal reaches. These hydro-mechanical self-operating gates maintain
a constant water level at their downstream side. The AVIS gate have a
free surface flow and AVIO gates operate under orifice conditions. Two
types of AVIS/AVIO gates are available, namely the High Head and the
Low Head type. The high head gates are usually employed in irrigation
canals with narrow canal cross sections. The choice between the open
type and the orifice-type is determined by the maximum head loss that
will occur between the upstream and downstream water levels. Ankum
(2004) gives several details of the AVIO and AVIS gates, including
the available gate types, their index and dimensions. The manufacturer
of the AVIS/AVIO gates (GEC Alsthom, previously Neyrtec, Neyrpic
from France) provides head loss charts, which present the relationship
between the discharge and the upstream water level above the gate axis.
Both gates are water level and not discharge regulators; they are not
water-tight. If the flow has to be shut off completely, the gate needs
some stop-logs to close the structure.
The discharge computation for the AVIS/AVIO gates is a two-step
procedure. Firstly the gate opening follows from the downstream water
level and then this opening is used to compute the discharge. Munir
(2011) has given the head discharge relations that are incorporated in
SETRIC.
Search WWH ::
Custom Search