12 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
2.5.2 PROCEDURE TO ESTABLISH THE DESIGN FLOW
1. Conduct a flow measurement at site during the dry season
(November-May). Preferably in February for snow-fed rivers
and March/April for other rivers. Not if the year is drier
than average, or wetter than average. This can be
established by talking to the locals. Consider if there are
significant abstractions by other water users, such as
irrigation and drinking water schemes upstream of the point
where the gauging was conducted.
2. Calculate:
Average monthly flows by using WECS/MIP methods.
Flow duration curve using WECS.
Instantaneous flood flows of different return periods using
WECS.
3. Compare the dry season mean monthly flows obtained by
WECS and MIP method. If the flow measured at site is
above average according to the local people, compare the
dry season mean monthly flow obtained by the WECS
method with that obtained by the MIP method, and use
the lower value. If the flow was measured at site during
either a wetter than average year or a drier than average
year, then use the value obtained by the WECS method.
4. Use the flow duration curve (FDC) to establish the probability
of exceedance of the value from step 3. The FDC is useful
because the power equivalent of the flow can be superimposed
onto it, so that it is possible to read off the amount of time
each year that certain power levels can be obtained. This is a
useful planning tool, allowing a choice of size of turbine to
be made, together with an indication of required variable
flow performance of turbine and an indication of the plant
factor constraints which will result from any particular choice
of turbine size. See Ref.1 for details.
5. Decide on what percentage of the flow established in step 3
can be diverted for power generation. If using a temporary
weir assume that 50 percentage of the flow can be diverted.
If the river presents formidable difficulties, assume less than
50 per centage. If using a permanent weir founded on bedrock
assume 95 per centage and for weir based on alluvium
foundation, allow for seepage losses and assume that 90
percentage of the flow can be diverted.
6. Calculate seepage losses for the water conveyance structures.
These losses must be deducted from the flow established
in step 5. Seepage calculation is covered in Section 2.5.3.
7. Consider if there are other water users such as irrigation
and drinking water downstream of the diversion works.
Establish the amount of flow that has to be released
downstream and deduct this amount from the flow from
step 6. This is the design flow.
A design example is included in Appendix A.
TABLE 2.3 Canal seepage losses
TYPE OF SOIL
Rock
Impervious clay loam
Medium clay loam
Clay loam or silty soil
Gravelly clay loam or sandy
clay or gravel cemented with clay
Sandy loam
Sandy soil
Sandy soil with gravel
Pervious gravelly soil
Gravel with some earth
SEEPAGE LOSS,
(l/S/1000 m2 OF
WETTED AREA)
<0.5
0.8 -1.2
1.2-1.7
1.7-2.7
2.7 - 3.5
3.5 - 5.2
5.2 - 6.4
6.4 - 8.6
8.6 -10.4
10.4 - 20.8
Example 2.1 Seepage Calculation
A 500 metre long unlined headrace canal is to be constructed
in sandy clay to convey a design flow of 0.1m3/s. A standard
trapezoidal section is proposed with a depth of 0.2 m, a bot-
tom width of 0.6 m and side slopes of 1:2 (V:H). Calculate the
seepage loss in the canal.
Solution:
The wetted perimeter (P) of the canal can be calculated using
the following equation:
P = B + 2 x H x (l+N2) (see chapter 4)
= 0.6 + 2 x 0.2 x (l + 22) = 1.49m
The wetted area
=PxL
= 1.49 x 500
= 747 m2
From Table 2.1, seepage loss in sandy clay is 3.5 l/s/1000 m2
of wetted area. The seepage loss is given by:
Qseep = 3.5 x wetted area/1000
= 3.5 x 747 /1000
= 2.6 l/s
Which is 2.6% of the designed flow of the canal.
2.5.3 SEEPAGE
It is imperative to examine the soil along the route of the
proposed canal and estimate the amount of seepage that a
canal may suffer, an important issue that is often overlooked
by micro-hydro designers. This is especially true for micro-
hydropower schemes with long unlined canals. Table 2.3 gives
canal seepage losses for different soil types. By calculating