GLACIS DROP
By
CH. VENKATARAMAIAH
GLACIS DROP
• A drop is defined to be a work designed to secure
lowering the water surface in a channel and safe
destruction of surplus energy so liberated. It is
suggested to provide glacis type drop for canal
system carrying more than 1.5 cumecs irrespective
of height of drop .
Design Procedure:
(1) Clear width of throat (Bt): The fluming of Canal should not exceed
the limits given below subject to the condition that over all width
of throat is not more than Bed width of channel on the
downstream side.
Height of drop
(1) Up to 1.0 m
(2) Over 1.0 m to 3.0 m
(3) above 3.0 m
Percentage of fluming
66%
75%
85%
Bt = Percentage of fluming x bed width of canal
Design Procedure:
(2) Crest Level: The Crest level is fixed by working out ‘D’ using
formulae Q - = C. Bt. D 3/2
Where Q = discharge in cumec
C = co-efficient of discharge depending on the
drowning ratio. Up to 70% fluming
C = 1.84 can be adopted and above that, it is to be
read from Malikpur graph
Bt = Throat width in ‘m’.
D = Depth of crest below U/S TEL in ‘m’
After calculating value of D from the formula, crest level is
fixed with the equation:
Crest level = U/S TEL – D
Q = Canal discharge in cumec
V = Velocity in canals in m/s
Bt = Throat width
(i) Bt = Percentage of fluming x bed width of canal
(ii) Crest level:
Q = 1.84 x Bt x D3/2
D3/2 = Q/(1.84 x Bt)
D = Q/ (1.84 x Bt) 3/2
Crest level = U/S FSL + ( V2 / 2g) – D
Height of crest above U/S CBL = Crest level – U/S CBL < 0.4 h
(iii) Crest length Bt = 2/3 D
(iv) U/S glacis:
Crest joined to CBL with a slope of 0.5 : 1 and crest and slope
joined with a curve of radius equal to D/2.
(v) D/S glacis:
Straight glacis with slope of 2:1 and joined with crest and
the floor with a curve of radius equal to D.
(3) Cistern
q = Q/ Bt , k = fluming ratio = d/s bed width/ throat width
Subcritical depth dx = 0.985 x q0.52 x hx0.21
hx = hL / (k)0.152
hL = height of drop
sub critical depth of flow flumed fall d = hx + dx – hL
(i) Cistern level
Case (1)
Cistern level = D/S FSL + (V2/2g) – 1.25 d
Case (2)
V1 = q/d
Ef2 = d + (V12 / 2g)
Cistern level = D/S FSL (V2/2g) – 1.25 Ef2
Lower value of the two cases is adopted
Depth of cistern = D/S CBL – cistern level
(ii) Cistern length
Cistern length l = 5 x Ef2
Cistern joined to D/S CBL with a slope of 1 in 3
(iii) Deflector wall
Height of deflector wall above D/S CBL d1 = (1/10) x D/S FSD
4) Curtain walls
(i) U/S curtain wall = (U/S FSD/ 3) + 0.10 (minimum)
(ii) D/S curtain wall = (D/SFSD/ 2) (minimum)
These should be checked with score depth formula with suitable
factor of safety. D/s cutoff can be increased to reduce the
thickness
of the floor.
5) Inlet transition
U/S wing walls given a splay of 1 in 2 to join the crest with the
bed width.
D/S wing walls extended upto end of the cistern.
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