26 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
value, the orifice surface will be scoured. For micro-hydro, the
recommended velocity (V) through the orifice during normal
flow is 1.0-1.5 m/s. Starting with a small orifice opening for
normal flow (i.e., high velocity) will limit excess flow during
floods, since the discharge through the orifice is proportional
to the square root of the difference between the water level in
the river and the headrace canal (hr - hh). However, if the
orifice is directly at the river (without a trashrack) the velocity
should be less than 1.0 m/s to avoid drawing bedload into the
intake. The size of the orifice is calculated as follows:
Assuming a maximum velocity of 1.5 m/s through the orifice,
calculate the required area of the orifice opening using
Q = V x A.
For a rectangular opening, A = W x H
where W is the width and H is the height of the orifice. Set
H according to the river and ground conditions and calculate
W.
To ensure submerged condition, arrange the orifice opening
such that the water surface level at the headrace canal is at
or slightly higher (say up to 50 mm) than upper edge of the
orifice. Note that the design of headrace canal is covered in
Chapter 4. Hence the design of different micro-hydro
components are interdependent. Now calculate hr for the
design flow conditions. The hr is the water level that needs
to be maintained in the river during normal conditions. If
the actual level in the river is less, repeat the calculations
with larger width and smaller height of the orifice. If the
actual river level is still less provide a weir with weir crest
level at hr
Calculate the flow through the orifice for flood condition
(hr = design flood level). The excess flow (i.e., flow during
flood less the design flow) will have to be spilled back into
the river or nearby gullies in the initial reach of the
headrace. This is discussed in the next Chapter.
An example of an orifice sizing is shown in Example 3.1
3.6 Diversion weirs
3.6.1 GENERAL
A weir is required if the flow cannot be diverted towards the
side intake without raising the river water level, especially
during the low flow season. The weir may be of temporary,
semi-permanent or permanent construction. A temporary weir
is the preferred option for micro-hydro schemes. In planning
Example 3.1 Sizing of an orifice
Choose a suitable size of an orifice for a design flow of 2501/s. The normal water level in the river is 0.8 m above the bed level.
The design flood level is about 0.7 m above the normal water level. What is the discharge through the orifice during such a
flood?
Q = 0.250 m3/s
Set V = 1.2 m/s
Orifice area (A) = Q = 0.25 = 0.21 m2
V 1.2
Set
orifice
height
(H)
=
0.20
m
and
Width
of
orifice
(W)=
A
H
=
0.21
0.20
=1.05
m
Set bottom of orifice 0.2 m above the river bed level. This will minimise the bed load. Also, set the datum at the river bed level.
Set water level at headrace canal, hh = 0.5 m with respect to the datum as shown in Figure 3.6 (i.e. 100 mm above the upper
edge of orifice to ensure submerged condition). Note that later the headrace canal will have to be designed accordingly.
Q = AC 2g (hr-hh)
Assume C = 0.6 for roughly finished masonry orifice.
Q = 0.21 x 0.6 2 x 9.8 x (0.8-0.5)
= 0.31 m3/s or 3101/s
Qrequired = 2501/s: Therefore orifice size is OK.
Discharge through the orifice during flood flow:
hr - hh = 0.8 + 0.7 - 0.5 = 1.0 m
Qflood = 0.21 x 0.6 2 x 9.8 x 1.0 = 0.56 m3/s
Qflood = 560 1/s
The dimension of the orifice and the levels are shown in Figure 3.6. Note that the excess flood flow
can be discharged via a spillway at the gravel trap or another suitable location. A second option is to
install another orifice (double orifice system) downstream.