50 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
4.3.4 DESIGN PROCEDURE
The headrace canal design procedure is as follows:
1. Decide on canal type as per site conditions (e.g. earth canal,
stone masonry in mud mortar or stone masonry in cement
mortar).
2. Choose a suitable velocity (V) such that it is less than the
maximum velocity given in Table 4.1. Note that
unacceptable headloss may result if chosen velocities are
close to maximum velocity. Also choose the
corresponding roughness coefficient (n) from Table 4.1.
Then calculate cross-sectional area (A) from the following
equation:
A = Q/V
3. Using Table 4.2 decide on the side slope (N). Note that N is the
ratio of the horizontal length divided by the vertical height of
the side wall ( i.e., N = h/v as shown in Figure 4.6 ).
X = 2 (1+N2) - 2N
H=
A
X+N
B = HX
T = B + (2HN)
4. Calculate the optimum canal height (H), canal bed width
(B), and the canal top width (T) using the following
equations:
Note that in case of a rectangular canal, N = 0 and X = 2,
so:
H = A/2 and T = B = 2xH
Hence, for a rectangular canal the hydraulically optimum shape
is when the width is twice the height. These symbols are
schematically shown in Figure 4.6.
If an optimum canal shape is not possible due to site specific
conditions (such as narrow width along a cliff) then either
the width or the height should be selected to suit the
conditions. Then the other dimension can be calculated.
5. To ensure stable and uniform flow in a the velocity must be
less than 80% of the " critical velocity limit" Vc = Ag/2
where Vc is the critical velocity. Note that for a rectangular
canal Vc = Hg
6. Calculate the wetted perimeter (P) using the following
equation
P = B + 2H (1 + N2) note that for a rectangular canal,
P = B + 2H
7. Calculate the hydraulic radius (R) as follows: R = A/P
8. The slope (S) can now be found from Manning’s equation:
s = (nV/R0.667)2
Now all dimensions required for the construction of the
canal are known.
9. Headless = L S (L is the length of the canal section).
Sometimes S is fixed by the canal route, which has already
been decided and surveyed. Another example of fixed slope
(S) situation is when an existing irrigation canal is proposed
to be used for a micro-hydro scheme (and higher flows as
well as less leakage are required). In such situations different
cross sectional area should be assumed (i.e., trial and error)
such that the velocity is less than the allowable maximum
velocity for the design flow and the type of canal proposed.
This can be done by rewriting Manning’s equation as
follows:
Q
=n([BBH
+
+
NH2)5/3 S
2H (1+n2)
]2/3
With a known design flow (Q), select the appropriate side
slope (N) according to the type of canal chosen. Then fix
either B or H and calculate the other using the above
equation. Finally from Table4.1, check that the velocity for
the canal type.
10.Calculate the size of the largest particle that will be
transported in the canal:
d = 11 R S
• Water level being above the design level due to
obstruction that for a rectangular canal V = Hg
If this is less than the possible size in the canal, repeat the
design using a higher velocity.
11.Allow a freeboard as follows:
300mm for Q<500 1/s
400mm for 500 1/s < Q < 1000 1/s (such flows are unusual
for micro-hydro shcemes).
Such freeboard allows for:
Uncertainties in the design (e.g. the value of *n* may differ
by 5% to 10% from estimate).
Water level being above the design level due to obstruction
in the canal or during emergencies.
Deterioration of the canal embankment.
12. Check that possible flood flow in canal can be accommo-
dated without using more than 50% of the freeboard.
13. Find the total head loss. If this is too high or too small,
repeat the calculations with a different velocity. Consider
using different types of canal keeping the overall cost in
mind.
Avoid a canal width of less than 300 mm as narrow canals can be
easily blocked. Also for stone masonary canals, smaller sizes
are difficult to construct.