Environmental Engineering Reference
In-Depth Information
The energy principle holds true as long as proper allowance is made
for energy losses (dissipation of energy). In a steady flow with straight
and parallel streamlines, the centripetal acceleration is negligible and the
sum of the potential and pressure energy at any point gives the total energy
per unit of weight:
p
ρg
+
z
p
ρg
+
z
2
v
1
2
g
+
v
2
2
g
+
α
¯
α
¯
E
=
1
=
(Bernoulli equation)
(4.22)
In a real fluid the velocity head is multiplied with the Coriolis
coefficient
α
, and a loss of energy (due to friction and/or local losses)
has to be added:
v
1
2
g
+
v
2
2
g
+
α
¯
p
1
ρg
+
α
¯
p
2
ρg
+
z
1
=
z
2
+
h
f
(4.23)
The momentum principle is based upon the momentum passing a cross
section per unit of time:
mv
ρβQv
where
β
is the Boussinesq coefficient.
The change of momentum per unit of time is equal to the resultant of all
external forces acting on a body of flowing water during time dt. Between
two cross sections the change of momentum is
P
=
=
ρQ
(
v
2
−
v
1
). For any
ρgAy
1
.
cross section and assuming hydrostatic pressure the force
P
=
A
1
y
1
A
2
y
2
β
1
Q
2
A
2
g
β
2
Q
2
A
2
g
P
ρg
=
−
(4.24)
4.4.3
Hydraulics for some irrigation structures
For the conveyance of irrigation water from its source to the farmers field
an irrigation network is needed that consists of a large number of appur-
tenant structures that can be divided into a conveyance and an operational
part. The conveyance part includes the canals and fixed structures such as
aqueducts, siphons, bridges, culverts, drops and cascades, flow measuring
structures, etc. The operational part includes the structures that divide and
control the flow in terms of water level or discharge; they may be fixed
without any movable part for the control or movable such as gates.
The design of the conveyance part is normally based on hydraulic and
structural requirements, local conditions, available technology and cost.
The design of the operational part is based upon additional factors that
influence the operation and management e.g. water delivery mode, avail-
able manpower, acceptance by the users, ease in operation, transparency,
etc. In some designs and layouts different structural arrangements are used
within the same scheme.
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