CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
53
discussed in Chapter 3. The spillway should be sized such
that the entire flood flow can be diverted away from the
canal. This is because the micro-hydro system could be closed
during flood or there could be an obstruction in the canal.
Choose a spillway profile and determine Cw. In the Nepalese
context, a broad, round edged profile (Cw =1.6) is suitable
since it is easy to construct.
Spillway crest level should be 0.05 m above normal canal
water level. No more than 50% of the freeboard should be
used. Therefore, with a generally used freeboard of 300
mm, the available hovertop is 0.5 x 030 - 0.05 = 0.10 m. The
required length can then be calculated for the chosen hovertop
and flood flow.
Where there is no ponding immediately downstream, such
as in the headrace canal, the spillway length calculated using
the weir equation should be multiplied by 2: this accounts
for the gradual decrease in head over the spillway, until the
required level is reached at the downstream end of the
spillway. In this case only the excess flow (Qflood - Qdesign)
should be used for (Qspillway) Note that in such cases, locating
the spillway immediately upstream of an orifice will increase
the flow through the weir. The design of a spillway is
presented in Example 4.2.
Example 4.2 Design of a headrace canal and a spillway
Design a headrace canal to convey a flow of 285 l/s. Site conditions indicate that the canal would be stable if stone masonry in
mud mortar is used. The expected flow through the intake during a 20-year return flood is about 480 l/s. Design an adequate
spillway.
Design procedure:
Canal type: stone masonry in mud mortar
Q = 0.285 m3/s
From Table 4.1:
Roughness coefficient n = 0.035
choose V = 1.0 m/s
From Table 4.2, for gravelly earth, select side slope, N = 0.5, (1h/2v).
Cross sectional area, A = 0.285/1.0 = 0.285 m2
X = 2 (1+N2) -2N
X = 2 (l+0.52) - 2 x 0.5
X = 1.236
Calculate the water depth in the canal, H:
H= A
(X+N)
H = 0.285
1.236+0.5
H = 0.405 m
Calculate the bed width, B = HX