H(1800/1.7169.55)2/33.39 m.Hence the RL of weir crest300.08 3.39296.69 m AOD. Since the
allowable afflux is 1 m,
downstream HFL300.00 1.00299.00 m AOD.Check whether the flow is free (modular) or submerged (non-modular).
For the flow to be modular, i.e. not affected by submergence, the ratio
H 2 /H 1 , where H 1 andH 2 are the upstream and downstream heads above
the weir crest, is less than 0.75 (BSI, 1969; Bos, 1976):
upstream head, H 1 300.00 296.693.31 mand
downstream head, H 2 299.00 296.692.31 m.Therefore, the submergence ratio,
H 2 /H 1 2.31/3.390.7 (0.75).Hence the flow is modular, and the weir discharges the design flow with
the desired upstream and downstream water levels.
Note that if a structure is submerged its discharge capacity is
reduced. The submerged flow, Qs, may be estimated by using the modular
flow (Qm) equation with a correction factor, f(i.e.QsfQm). The correc-
tion factor depends on the type of structure, the submergence ratio H 2 /H 1 ,
and the ratio P 2 /H 1 , where P 2 is the crest height above the downstream
channel bed (Table 9.1).
The régime scour depth (equation (9.10)),
Rs0.475(1800/1)1/35.78 m.Provide cut-offs for (1) upstream scour depth1.755.7810.11 m and
(2) downstream scour depth2.005.7811.96 m. The RL of the bottom
of the upstream cut-off300.00 10.11289.89 m AOD and the RL
of the bottom of the downstream cut-off299.00 11.96287.04 m
AOD. Therefore the depth of upstream cut-off pile, d 1
293.00 289.893.0 m, and the depth of the downstream cut-off pile,
d 2 293.00 287.006.0 m. The pool level, i.e. the upstream storage level
or canal FSL upstream of the head regulator299.0 m AOD. Therefore,
the maximum seepage head (assuming tailwater depth0),
Hs299.00 293.006.00 m.