Transcript CROSS REGULATOR - Andhra Pradesh Water Sector …

The cross regulator is provided to effect equitable distribution of
supplies amongst the distributary and parent canal, to raise water
level when supply in the parent canal is low, to release surplus
water from canal, in conjunction with escapes, or to provide means
for cutting off supplies to the downstream side for repairs etc.
The criteria for the Hydraulic Design of cross regulators for canals
is as per I.S. code: 7114 – 1973 (reprint December, 1979).
The driving head is the difference between the water levels on U/S
and D/S side of the regulator. This is provided to allow the passage
of required discharge on D/S through the regulator at full supply
level.
Depending upon the driving head (fully utilizing the driving head)
the regular width may be flumed up to a maximum of 50% of the
canal width to economize the cost of the structure.
A hump is provided below the regulator gates creating a fall on the
D/S side for the following reasons.
(a) To trap silt carried by the water on the U/S side of the regulator.
(b) To reduce the depth of flow over the hump to increase velocity
through the vents and economize the gate structure.
(c) Hump is created in the glacis drops to increase the efficiency of
flow of water to D/S side.
(d) To negotiate the difference of levels if any in the canal bed levels
on the U/S and D/S side of the regulator.
Q = C Bt H3/2
WhereQ = D/S full supply discharge in m3 /sec
C = Co-efficient of discharge
Bt = Clear water way in metres.
H = Head over crest i.e. Full supply level on the U/S + head
due to velocity of approach – crest level.
The value of ‘C’ is determined using Malikpur graph (a graph drawn
between drowning ratio and co-efficient of discharge based on
experiments).
Crest level is calculated as per CWC, Manual i.e, Crest level = U/s
TEL – head over crest (H)
The height of crest above up stream bed level should not be more
than 0.4 H. Glacis profile is calculated as per CWC manual with 2:1
slopes to negotiate the levels and smooth curves at the junctions.
The radius of curvature to be adopted is H/2 on up steam and ‘H’
on downstream as specified therein.
D/s floor of the regulator is depressed to form a cistern to dissipate
energy. Since the U/s and D/s C.B.Ls and F.S.Ls are almost the
same in the NSP Canals and distributaries, the energy dissipation
arrangement is quite simple. To dissipate energy at low flows
through regulator the cistern with water cushion with a minimum
length and deflector wall at the end of the cistern are provided. On
main system the hydraulic jump calculations are to be done for
different opening conditions i.e., ¼, ½, ¾ and full supply. Further if
there are more than one vent, these calculations have to be made
for different conditions of vents opening. The height and length of
jump in each case is to be found. Based on these calculations the
depth and length of cistern will be fixed. Refer I.S:4997–1968 or
Small Dams by USBR.
When there is no water on D/s of the regulator and water at FSL
on U/s, the exit gradient is to be calculated and the thickness of
floor has to be designed for the uplift pressures at various
sections. The formula for exist gradient is:
GE =
1
x
(π √ λ)
H
d
Where: λ = 1 + 1 + α2
2
α=
b
d
H = difference between crest level and downstream bed level in m
b = length of impervious floor in m
d = depth of downstream curtain wall in m
Scour depths are to be calculated at the U/s and D/s transition ends
and the curtain walls to be taken up to 1.5 times the scour depth.
Piers to be designed considering hoist loads, load due to water
thrust on gates, wind pressure and water currents. Whenever a
road bridge is provided the live load moments, tractive force and
braking force etc., are to be considered while checking the
stability.
Abutments to be designed with super imposed loads, live load
moments, tractive force and braking force and the earth pressure
behind them.
Conventional Wings and returns to be designed for the earth
pressures with T.V.A. procedure considering Ф as 32 degrees and
δ as 16 degrees.
U/s and D/s canal bed and sides are to be protected with C.C.
lining in M 15 grade concrete with profile walls at the end. The
thickness of lining is normally the same as for the remaining
length of the canal in the reach.
It is in VRCC M 20 grade, designed for its self weight plus forces
transmitted through the screw rod or the hoist and crowd load of
400 kg/ sqm.
Either sliding type or fixed wheel type gates are provided depending
on the size of opening. Electrically or manually operated hoist
arrangement is to be made to operate the gates.
FIG.16
The canal fall or drop is required to be provided, whenever, the
natural slope of the country is steeper than the bed slope of the
canal and the difference in levels is adjusted by constructing a fall
or drop. Drops become necessary in the case of distributaries,
which are generally aligned along the ridge for commanding the
area on either side. There are two main types of falls.
In this type of fall, the nappe impinges clear into the water cushion
below. The dissipation of energy is effected by the turbulant
diffusion as the high velocity jet enters the deep pool of water
downstream.
This type utilizes the principal of standing wave for dissipation of
energy. This type of fall can be divided into following three
classes.
(a)
Straight glacis with baffle platform and baffle wall.
(b)
Straight glacis without baffle platform and baffle wall.
(c)
Modified glacis type.
The falls are further divided into:
(a)
Flumed or unflumed falls and
(b)
Meter or non – meter falls.
As per the Central Water Commission’s Manual on falls, the
following table indicates the type of falls to be selected for the
given discharge and height of drop.
Sl.
No.
Discharge (Q) in cumecs
Drop
(HL) in
metres
Type
Unflumed
Clear over-fall
1
5
High Discharge & High
Falls Q > 15
HL > 1
Baffle type (suitable up to
retrogression of 25% also)
Flumed
Drowned
Clear
over-fall
Drowned
Baffle type
Baffle
type
Straight Glacis
or Baffle type
2
High Discharge & Low
Falls Q > 15
HL > 1
Baffle type
Modified
Glacis type
Baffle
type
Straight Glacis
type
3
Low Discharge & High
Falls Q < 15
HL > 1
Baffle or Glacis type
Baffle or
Glacis type
depending
on merit of
each
Baffle
type
Straight Glacis
type
4
Low Discharge & Low
Falls Q < 15
HL < 1
Q<8
All
drops
Baffle or Glacis or Vertical type Modified
Baffle or Glacis type`
depending on economy and
Glacis type
Glacis
suitability at site
type
Vertical type is suitable, selection of other types depends on consideration
of cost
The Design Circular No. 35/1807 dated 2.2.1978 of CE., N.S.L.C.
stipulated the type of drop to be adopted for different discharges and
heights of drops.
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.
1)
2)
3)
Height of drop
Up to 1.0 m
Over 1.0 m to 3.0 m
Above 3.0 m
Percentage of fluming
66%
75%
85%
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
3)
Length of Crest: 2/3x D.
4) Height of Crest: Should not be greater than 0.4 D, above the
upstream canal bed level.
5) D/S Glacis: In the case of baffle type glacis drops, glacis slope is to
be 2/3: 1 joined tangentially to the crest on the U/S side and baffle
platform on the downstream side with radius equal to ‘D’. In the
case of straight glacis provide glacis slope of 2:1 with radius of
curvature as D at the junction with the crest at the upstream end
and pavement at the downstream end.
6) U/S Glacis: Glacis slope is to be ½: 1 joined tangentially to the crest
with a radius equal to D/2.
7) Protection:
(i)
Length of U/S protection: 3 times F.S.D. or as per the standard fixed
by the project authority. The protection is in CC M 15 grade with
profile walls at the end.
(ii)
8)
(i)
Length of D/S protection: 4 (d + h) where d = d/s F.S.D. and
h
= difference in F.S.Ls or as per the standard fixed by the project
authority. The protection is in CC M 15 grade with profile walls at
the end.
Glacis fall without baffle:
The hydraulic jump is calculated to be the most efficient means of
dissipating the energy. To ensure formation of the hydraulic jump,
it is necessary that the depth of tail water flowing at sub–critical
velocity in the canal downstream should bear the following relation
to hypercritical depth of flow at the toe of glacis:
dx = -d2
+
√ 2v2²
g
d2
+ d22
4
Where v2 = velocity of water at the formation of jump
d2 = hyper critical depth at formation of jump
dx = sub – critical depth in canal on downstream side
The values of d2 and dx are calculated from the following formulas
dx for unflumed falls = 0.985 q0.52 x Hx0.21
For flumed falls d1x = Hx - HL + dx (unflumed)
Where = Hx
HL
K 0.152
Hx = calculated drop in m
HL = actual drop in m
K = fluming ratio (D/S bed width / throat width).
d2 = 0.183 q0.89 x Hx - 0.35
ii)
Cistern: The cistern level is obtained by subtracting the value of
1.25 dx or 1.25 dx1, as the case may be, from the downstream full
supply level of the canal or 1.25 Ef2 from the downstream total
energy level, which ever gives the lower level. Ef2 is the energy of
flow in the canal after formation of the hydraulic jump.
The length of the cistern is equal to 5 Ef2. The cistern is joined to
the downstream bed at a slope of 1 in 5.
9) Glacis fall with baffle:
The dimensions of the baffle platform and baffle wall are determined
from the relationship given below:
(i) R.L of Baffle platform: D/S F.S.L. – d1x.
(ii) Height of Baffle wall (Hb) = dc – d2
Where, d2 = Hyper - critical depth at the point of formation of
standing wave.
d2 = 0.183 (q) 0.89
x
Hx -0.35
dc = Critical depth
dc =
q2

g
1/3
q = discharge per meter width.
R.L. of Baffle wall = R.L. of Baffle Platform + Hb.
(iii) Thickness of Baffle wall = 2/3 x Hb
(iv) Length of Baffle Platform Lb = 5.25 (Hb)
The baffle platform should join the toe of glacis with a radius equal to D
and the baffle wall with a radius R = 2/3 Hb
v) Cistern:
(a)
Depth of cistern: D/S FSD/10 subject to a min of 15 cm for
distributaries and minors and 30 cm for main canals and branches.
(b) R.L. of the cistern = D/S bed level – depth of cistern
(c)
Length of cistern = 5 times down stream F.S.D.
(d)
R.L. of the deflector wall = D/S CBL + D/S F.S.D/ 10
10)
Friction blocks and glacis blocks:
(i) Glacis fall with baffle
(a) If the height of drop is less than 2.0 meters, friction blocks and
glacis blocks are not required. If the height of drop is more than 2.0
m, two rows of friction blocks staggered in plan are to be provided.
Size of friction blocks:
Height (h) = 0.262 dx,
Length (L) = h
Top width (W) = 2h / 3
Distance between two rows = h.
The downstream edge of downstream row of friction blocks shall be
provided at a distance of one third length of cistern from the end of the
cistern floor.
b) Glacis blocks: Single row of glacis blocks of same size as friction
blocks is to be provided at the toe of the glacis.
(ii) Glacis fall without baffle
Four rows of friction blocks staggered in plan are to be provided in the
case of flumed falls. The upstream edge of first row of blocks may be at
a distance of 5 times the height of blocks from the toe of glacis.
Size of friction blocks:
Height (h) = D/S FSD
8
Height (L) = 3h
Height (W) = 2h
3
Distance between rows = 2h
3
11) Deflector wall:
In glacis falls, a deflector wall of height equal to one tenth of the
downstream FSD is provided at the downstream end of the cistern.
The minimum height should be 15 cm.
12) Curtain wall:
i) Depth of U/S curtain wall = U/S FSD subject to minimum of 0.50 m
3
ii) Depth of D/S curtain wall = D/S FSD subject to minimum of 0.50 m
2
These should be checked with scour depth formulae with suitable
factor of safety. Downstream cut off can be increased suitably to
reduce the thickness of floor.
13 (i) Exit gradient and uplift pressure:
H = difference between crest level and D/S CBL
d
depth of D/S curtain wall
b = length of impervious floor
d
depth of D/S curtain wall
After working out values of H/d and b/d, find the value of exit gradient
GE from the graph in plate 16 of CWC manual on falls. The GE depends
upon the soils, but it should be less than 0.30.
Uplift Pressure:
(a) U/S curtain wall:
1 = d = depth of D/S curtain wall
α b length of impervious floor
Find out corresponding value of φ E = from graph i.e., from plate 17 of
CWC manual on falls.
…% of residual head φ E1 = 100 - φ E
b) At the d/s cut off wall
1 = d = depth of D/S curtain wall
α
b length of impervious floor
Find out the corresponding value of φ E from graph i.e, from plate 17
CWC manual on falls.
ii) Thickness of floor: The uplift pressures at toe of glacis, at the end of
baffle and at the end of cistern are worked out by interpolation for
fixing the thickness of floor.
Thickness of floor at toe glacis:
% age of pressure @ toe of glacis
= φ E at D/s + (φ E1 − φ E D/s) X L/b
b = total length of impervious floor.
L = Length of floor up to toe of glacis from D/S end.
Thickness of floor at the toe of glacis
= %age of pressure @ toe of glacis x H
100 x (ρ − 1)
Where ρ is specific gravity of CC i.e., 2.4
Similar method is to be adopted for calculating thickness of floor at the
end of the baffle, at the end of cistern etc.
FIG.17
FIG.18
Vertical drop:
Design procedure:
1) a) Throat width Bt = B.W. of canal (If canal bed width on upstream and
downstream are different, lower of the two).
b) Crest Level:
Crest level is obtained by working out value of D (depth of crest below
upstream TEL) from the following formula.
Q = C x Bt D 1/6 x D 3/2
Lt
Where Bt = Throat width in m
C = Coefficient of discharge usually taken as 1.835
Lt = Length of crest in m
D = Depth of crest below upstream TEL in m
U/S T.E.L = U/S FSL + Velocity head
R.L. of crest = U/S TEL – D
2) Cistern:
A cistern is provided at the toe of the drop by suitably depressing the
floor below the downstream bed of the canal.
a) Depth of cistern = (HL x D) 2/3 in m.
4
D = depth of crest below U/s TEL.
R.L. of cistern = D/s CBL – depth of cistern.
b) Length of cistern = 5 (HL x D)½ in m.
(3) Length of throat or crest (Lt):
Lt = 0.55 √D in m subject to a min. of 0.50 m.
(4) Thickness of crest wall at base:
T = 0.5 x D1 in m, where D1 = RL of crest – RL of cistern
5) U/S and D/S Protections:
i) Length of U/s protection= 1 ½ times the U/S FSD or as per standard
fixed by the Project authority.
ii) Length of D/s protection = 3 times the D/S FSD or as per standard
fixed by the Project authority.
6) Exit Gradient & Uplift pressures
a) Exit gradient:
H = R.L. of crest – D/S CBL.
d = depth of D/S curtain wall off = FSD/ 2 or as per the requirement
to bring the exit gradient within the limit.
b = Length of impervious Floor = Foundation offsets + width of drop
wall + length of cistern + width of curtain wall.
α = b/d,
GE =
∏ λ
λ = 1 + 1 + α2
2
exist gradient =
1
d
x
H
b) Uplift pressures:
(a) U/S face of crest wall
d =
U/S CBL – Bottom of foundation concrete.
1
=
d
α
b
φ E is read from ‘Plate 17’ of CWC manual on falls
At the end of floor
1 = d
α
b.
φ E is read from ‘Plate 11.1 (a) of CWC manual on
fall (enclosed)
Thickness of Floor at the d/s Face of drop wall is
interpolated considering the pressures at the face
of crest wall and at the end of floor.
Absolute pressure = (% Pressure) x H m of water
column. 100
 P = 75% of Absolute pressure for soils other
than pervious soils
Thickness of floor = P, where ρ = 2.40 ρ – 1
c) Friction Blocks:
For discharge exceeding 3 cumec, two rows of
friction blocks staggered in plan may be provided
in cistern. The downstream edge of downstream
row should be at a distance of one third the length
of the cistern from the downstream end of cistern
floor.
Size of friction blocks:
Length (L) = 1 x Downstream F.S.D.
8
Height (h) = 1 x Downstream F.S.D.
8
Top width (w) = 1 x height of subject to minimum of
8 cm, joined to floor on the
4 downstream side with a slope of 1:1
Clear space between rows = height of the blocks.
Vertical type core wall drop: (CE NSLC Circular No.
DW.150/ 3845 – S, 3-9-1980)
Various components of the vertical type drop with core
wall for different ranges of discharges i.e., 1.5 cumec to
1 cumec, 1 cumec to 0.5 cumec, 0.5 cumec to 0.1 cumec,
0.1 cumec and below and for various heights of drops
i.e., 0.6 m, 0.8 m, 1.0 m, 1.2 m and 1.5 m with clear over
fall are given in table I and II. The same may be
adopted for the drops on the distributories having
discharge 1.5 cumec and below.
For drops in silty or clayey soils the following
modifications may be adopted (Design Circular
No. 35/1807 dated 2.2.1978 of C.E., N.S.L.
Canals).
(a) For drops of 1.5 m and above, for all
discharges, wings and returns may be
provided.
(b) For drops less than 1.5 m height and discharge
above 1 cumec, wings and returns may be
provided.
Following are the recommendations of the Expert
Committees on design of drops on distributary
system.
(a) For drops with height of less than or equal to
0.60 m and discharge of less than 50 cusec,
unflumed core wall type drops may be
provided.
(b) For drops with height more than 0.60 m and
discharge between 50 and 100 cusec, unflumed
vertical drops with wings and returns may be
provided.
(c) For drops with discharges more than 100 cusec,
straight flumed drops may be provided. Where
fluming ratio as per codel provision could not
be adopted for drops of height less than 0.60 m,
unflumed vertical or unflumed core wall type
drop may be provided.
0.1 and
below
0.8 0.3
0.8 0.3
0.6 0.6
0.8 0.8
0.8
0.6 0.6
0.6 0.8
0.8
0.6 0.6
0.6 0.6
0.8
As per formulae
0.5 to
0.1
0.15
0.17
0.19
0.22
0.12
0.14
0.16
0.18
0.21
0.10
0.12
0.14
0.15
0.17
0.07
0.08
0.09
0.10
0.12
As per formulae
1.0 to
0.5
0.8
1.0
1.2
1.5
0.6
0.8
1.0
1.2
1.5
0.6
0.8
1.0
1.2
1.5
0.6
0.8
1.0
1.2
1.5
Bed width on U/S or D/S whichever is less
1.5 to
1.0
0.80
0.80
0.90
1.10
0.60
0.70
0.80
0.90
1.10
0.60
0.70
0.80
0.90
1.00
0.60
0.60
0.70
0.80
1.00
3.4
3.8
4.2
4.7
2.7
3.1
3.5
3.8
4 .3
2.4
2.8
3.2
3.5
4.0
2.0
2.3
2.6
2.8
3.2
Bed width on D/S
0.60
0.65
0.65
0.70
0.60
0.60
0.60
0.65
0.70
0.60
0.60
0.60
0.65
0.70
0.60
0.60
0.60
0.60
0.70
on
(ta)
Dis
cha
rge
De
Hei
Q
pth
ght
(cu
of
of
m)
cist
dro
Len
ern
p
gth
bel
of
ow
Thr
cre
D/S
oat
st
B.L
wid
(Lt.
(x)
th
De
)
(Bt.
pth
)
of
cre
st
bel
Hei
ow
ght
U/S
Bot
of
T.E.
to
cre
L
m
st
(D)
wid
abo
th
ve
of
Len
U/S
dro
gth
B.L
Wi
p
of
(D1
dth
wal
apr
)
of
Thi
l
on
apr
ckn
(L
(La)
on
ess
W)
(W
of
a)
apr
TABLE No. I
DETAILS OF COMPONENTS OF VERTICAL
TYPE DROPS WITH
0.1 and
below
0.6 0.8 1.0
1.2 1.5
3.4
3.8
4.2
4.7
0.60 0.65
0.65 0.70
0.6 0.6
0.8 0.8
0.8
0.60
0.70
0.80
0.90
1.10
2.7
3.1
3.5
3.8
4 .3
0.60 0.60
0.60 0.65
0.70
0.60
0.70
0.80
0.90
1.00
2.4
2.8
3.2
3.5
4.0
0.60
0.60
0.70
0.80
1.00
2.0
2.3
2.6
2.8
3.2
0.10 0.12 0.14
0.15 0.17
0.6 0.6
0.6 0.8
0.8
0.07 0.08 0.09
0.10 0.12
0.6 0.6
0.6 0.6
0.8
Bed width on D/S
0.6 0.8 1.0
1.2 1.5
0.12 0.14 0.16
0.18 0.21
0.80
0.80
0.90
1.10
As per formulae
0.5 to 0.1
0.6 0.8 1.0
1.2 1.5
0.8 0.3
0.8 0.3
As per formulae
1.0 to 0.5
0.15 0.17 0.19
0.22
Bed width on U/S or D/S whichever is less
1.5 to 1.0
0.8 1.0 1.2
1.5
belo
w
Heig
U/S
ht of
T.E.L
crest
Botto
(D)
abov
m
e U/S
widt
B.L
of
h
(D1)
Lengt
drop
h of
wall
Widt
apro
(LW)
of
nh(La)
apro
Thick
n
ness
(Wa)
of
apro
n (ta)
THAN 1.5 CUMEC AND HEIGHT OF DROP
LESS THAN 1.5 m
Disch
arge
Q
(cum
)
Heig
ht of
Dept
drop
h of
cister
n
belo
w
D/S
Lengt
B.L
h of
(x)
crest
Thro
(Lt.)
at
widt
Dept
hhof
(Bt.)
crest

0.60 0.60
0.60 0.65
0.70
0.60 0.60
0.60 0.60
0.70
FIG.19
TABLE No. II
Table showing discharges and depth of crest
below U/S T.E.L. for vertical type drops with
rectangular opening and free fall.
Discharge Q = 1.835 Bt (D/ Lt)1/6 D3/2 in cumec
or D = {(Q/ Bt) x (Lt1/6/ 1.835)}3/5 in meters
Discharge per Meter run
of crest wall i.e., Q/Bt
Cumec
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
Depth of crest (D) below U/S T.E.L. in
meters for length of crest Lt
0.6 m
0.8 m
0.166
0.172
0.212
0.218
0.252
0.259
0.288
0.296
0.321
0.331
0.352
0.362
0.383
0.393
0.409
0.422
0.436
0.449
0.462
0.475
0.437
0.501
0.511
0.526
0.534
0.549
0.556
0.573
0.578
0.595
or D = {(Q/ Bt) x (Lt1/6/ 1.835)}3/5 in meter where Bt
= Width of crest = Canal Bed width in meters
Lt = Length of crest along axis of canal in meters
Notch type drop: (Trapezoidal/ Rectangular)
As per Irrigation manual by W.M Ellis.
Design procedure:
1) For half discharge, find out F.S.D. Usually it is 0.7
F.S.D.
2) Calculations of no. of notches:
No. of notches = Bed width
1.5 x FSD
Vide – Emperical rule No.4 page No. 229 of
‘Irrigation practice & Engineering’ by Etcheverry)
Find discharge per notch i.e., =
Q
No. of notches.
Silt level of drop = U/S CBL
3) For free fall notches:
Case I:
For free notch, the equation used for finding out
notch dimensions is
Q = 2.96 C d3/2 (L + 0.4 d n)
Where : Q = discharge in cumec
C = The coefficient of discharge of notch = 0.70
d = depth of water in metres over sill of the not
L = width of the horizontal sill of the notch in ‘m’.
n = 2 tan α, where α is the angle made by each of
the sides of the notch with the vertical.
If ‘n’ is Zero, then it becomes a rectangular notch.
Case II: For submerged notch:
Q= 2.96 C√ d-E
E +d L + 3 E2 + (d-E) E +
0.4 (d-E)2 n
2
4
Where E = the submersion depth of tail water
over the sill of the notch.
Q, C, d, L, n are the same as in the case – I.
Find L and n by using the above equations (free
fall or submerged) for full supply discharge and
half supply discharge conditions.
Substitute the values of L and n to get top width
of notch in the equation = L + nd.
4) Length of drop wall between abutments:
Length of drop wall between the abutments
should not be less than 7/8th of the canal bed
width on up stream. However in practice, the
length of drop wall is provided equal to
upstream bed width.
(5) Width of notch pier at FSL should not be less
than half of upstream F.S.D. ‘d’
Top width of notch is generally 0.75 d, where
notch is free and d where notch is submerged.
6) Water cushion:
The depth ‘x’ of the water cushion is worked out
from the following equation
X + d1 = 0.91 dc √ h
Where d1 = D/S F.S.D
dc = Depth of water over the crest.
h = height of drop (difference in FSLs).
7) Length of cistern:
Length of the horizontal floor of the cushion = 2
dc + 2 √ dc h subject to a minimum of
1.2 + 2√dc h.
It is to be designed on the basis of up lift
pressures and exit gradient if the soil is pervious.
8) Thickness of cistern floor = 0.55√ dc + h. This
should be designed on the basis of uplift
pressure and exit gradient, of the soil is
pervious.
Drop wall:
i) Top width of drop wall at sill level
(0.5d + 0.15) to (0.5d + 0.3)
ii) Bottom width of drop wall = H + dc+x
√ ρ
Where H = vertical height of the sill from the
apron, dc = depth of water over the crest
and x = depth of water cushion
10) Protection works:
i) Length of the U/S revetment = 3dc subject to
min of 3 meters
9)
ii) Length of the D/S revetment = 4 (d + h) subject
to min of 6 meters or as per standard fixed by
the Project Authority
(11) Scour depth calculations:
Scour depth = 1.34 q2 1/3 metres
f
q = discharge/ meter width
f = lacey’s silt factor.
(12) Check for uplift on floor: As per Khosla’s
Theory.
Trapezoidal notch core wall drops :( CE NSLC
Circular No. DW.150/ 3845 – S, 3-9-1980)
Various components of the notch type drop with
core wall for different ranges of discharges i.e., 1.5
cumec to 1 cumec, 1 cumec to 0.5 cumec, 0.5 cumec
to 0.1 cumec, 0.1 cumec and below and for various
heights of drops i.e., 0.6 m, 0.8 m, 1.0 m, 1.2 m and
1.5 m with clear over fall are given in table I and II.
The same may be adopted for drops on
distributaries' having discharge of 1.5 cumec and
less.
For drops in silty or clayey soils the following
modifications may be adopted.
i). For drops of 1.5 m and above, for all
discharges, wings and returns may be
provided.
(ii). For drops less than 1.5 m height and
discharge above 1 cumec, wings and returns
may be provided.
TABLE No. I (A)
DETAILS OF COMPONENTS OF NOTCH
TYPE DROPS WITH CORE WALL (FREE
FALL) FOR DISCHARGES LESS THAN 1.5
CUMEC AND HEIGHT OF DROP LESS
THAN 1.5 m
1
1.0 to 0.5
0.8 1.0 1.2
1.5
1
1
1
1
0.5 to 0.1
0.6 0.8 1.0
1.2 1.5
1
1
1
0.1 and below
0.6 0.8 1.0
1.2 1.5
1
1
1
1
1
1
1
Refer Table II (A)
Refer Table II (A)
7
8
9
10
0.09
0.6
0.6
0.6
0.6
0.6
0.6
0.08
0.60
0.60
0.60
0.60
0.06
0.04
Thickness of
apron (ta)
1
6
Length of apron
(La)
1
Length of drop
core wall
1.0 1.2 1.5
5
Bottom width of
drop wall
1.5 to 1.0
4
Top of drop wall
Lt
No. of notches
3
L + nd
Thickness of end
pier
Height of drop h
in m
2
L
X cushion
D/S Discharge in
Cum
1
Details of each notch
11
12
1.3
1.4
1.6
3.6
3.8
4.0
0.70
0.75
0.75
0.60
0.60
0.60
0.60
1.2
1.2
1.4
1.6
3.2
3.4
3.6
3.8
0.65
0.70
0.70
0.75
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.8
1.0
1.2
1.2
1.4
2.4
2.6
2.8
2.9
3.1
0.60
0.60
0.65
0.70
0.75
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.45
0.7
0.8
0.9
1.0
1.3
1.6
1.7
1.8
1.9
2.1
0.60
0.60
0.60
0.65
0.70
TABLE No. II (A)
Table showing width of Notches at sill and at
top of various discharges for Notch type drop
with free fall.
Discharge through each notch Q (cumec) is given
by
Q = 2.067 d3/2 (L+ 0.4 nd)
Where d = Depth of flow over sill (metres)
L = width of notch at sill in metres
n = 2 tanθ where θ is the angle made by each of
the sides of the notch with the vertical.
Top width of notch at F.S.L = L + nd.
Q/ d3/2
Width of Notch at
sill level ‘m’
Width of notch at
F.S.L ‘m’
2.2
0.546
1.844
2.1
0.521
1.760
2.0
0.496
1.676
1.9
0.471
1.592
1.8
0.446
1.508
1.7
0.422
1.425
1.6
0.397
1.341
1.5
0.372
1.257
1.4
0.347
1.173
1.3
0.322
1.090
1.2
0.298
1.006
1.1
0.273
0.922
1.0
0.248
0.838
0.9
0.223
0.754
0.8
0.198
0.671
0.7
0.174
0.587
0.6
0.149
0.503
0.5
0.124
0.419
0.4
0.099
0.333
Notch type drop with core wall:
In core wall type, the drop wall is combined with
a straight wall, which is extended into the banks
with proper keying. There are no wings & returns
on the U/S and D/S sides. But CC apron and side
protection with CC lining (better if double the
normal thickness provided) is provided.
i) Formulae adopted for working out the
rectangular notch
Q = 1.708 [ L – 0.1 nd] d 3/2
Where n = no. of notches
L = length of the drop wall in metres
d = depth of water in meters over the crest of drop
ii) Formulae adopted for trapezoidal notch is same
as discussed in the previous case.
iii) Length of apron, thickness of apron and water
cushion – same as discussed in the previous
case (trapezoidal notch).
 The drops can be combined with bridges
wherever possible. In such cases the clearance
between sill of drop to deck may be provided as
below:
N = h1 (hs + 0.3 m) from civil engineering hand
book volume II by ‘LELIAWSKY’.
OFF TAKE SLUICE:
Off takes are provided on the conveyance system to
irrigate the ayacut localized under branch or
distributary. As per World Bank norms, the water
distribution system is broadly classified as:
i) Supply system or conveyance system.
ii) Distributary System.
1)
Supply system or conveyance system:
Main canal, branch canals and majors carrying a
discharge above 5.66 cumec (200 cusecs) are
considered as supply system. They will run
continuously. The distributaries taking off from
these have gated structures if the carrying capacity
is 5.66 cumec (200 cusec) and above.
2) Distributary system:
 The distributaries have capacity less than 5.66
cumec (200 cusecs). These will run either full or
closed. The water will be distributed
proportionally through modules (APM or OFM).
No gated structures will be there on the
distributary system.
 In the first reach of distributary, a standing wave
flume which is used as a measuring device, is
provided.
Gated off – takes:
 These may be either:
 i) Rectangular/ square vents covered with R.C.C
slab or
 ii) Pipes


Rectangular vents:
(1) Sill level: The sill of O.T is kept either at or
above the CBL of parent canal depending on
the ratio of discharges in distributary and
parent canal.
% of O.T. discharge to parent
canal discharge
Height of sill of sluice above the CBL of parent canal
when FSD in the parent canal is:
Above 2.14 m
2.14 to 1.22 m
Below 1.22 m
15% and above
0.075
-
-
10% to 15%
0.15
0.075
5% to 10%
0.30
0.15
0.075 m
2% to 5%
0.30
0.30
0.15 m
2% and less
0.30
0.30
0.30 m
(2)
(3)


Driving head:
The driving head at O.T. is arrived at normally
considering half supply discharge in the parent
canal when the full supply discharge flows into
the distributary channel.
Driving head = Supply level in parent canal for
half supply discharge FSL in distributary
The FSL of off take channel is generally fixed at 15
cm below the half supply level of parent canal for
the channels taking off from main canal and
branch canal and 7.5 cm for channels taking off
from the distributaries. However vent way is
designed with minimum driving head of 7.5 cm
(3”) for pipes. The level difference between the sill
level and C.B.L. of parent canal is negotiated by
proving suitable longitudinal slope.
(3)
(4)









Vent way:
The vent way for square or rectangular/
circular vents is calculated by the formulae.
Q = Cd. A. √ 2g H
= 2.746 A√ H
Where Q= Discharge of off take sluice in
cumec
Cd = 0.62 for square or rectangular openings
A= Area in sqm
H= Driving head in m.
The vent way for circular openings with
C = 0.75 is calculated by the formula:
Q = 3.322 . A√ H



The length of barrel is worked out with respect
to the position of D/S head wall. The flow
condition in the barrel is dependent on D/S
condition in the O.T. channel immediately
below the vent way. TELs at entrance and exit
of barrel are calculated and checked for
assumed level.
4) R.C.C. slab under head wall:
It is designed to withstand for the max stress at
the bottom of head wall (resting over the slab)
in addition to its self weight. The slab is
constructed in VRCC M 20 grade with HYSD
bars.
(5) R.C.C. slab under earth bank:
It is designed for weight of earth over it in
addition to its self weight. Live load is also to
be taken into consideration for the slab under
inspection path.
(6) Transitions:
The U/S and D/S transitions are provided
with 1 in 3 and 1 in 5 splay respectively as per
practice.
(7) Stilling Basin:
 The type of stilling basin to be provided depends
upon the velocity at the entry of barrel. If the entry
velocity is above 6.1 m/sec. (20 ft/sec) the barrel
floor is depressed both for rectangular and square
vents based on the hydraulic jump calculations. In
case of normal velocities which are of the order of 4
m/ sec the floor is at the same level and the floor is
checked for arch action for the uplift pressure.

The design of hydraulic jump basin for energy
dissipating arrangements can be followed from
‘Small Dams’ by U.S.B.R. or as per I.S. 4997 – 1968.
For shooting flows, an impact type basin - VI with
R.C.C. baffle wall is to be provided. The baffle wall is
to be designed for the maximum water thrust with
50% impact factor when full discharge is let out in
the canal.


Off – takes with hume pipes:
(1)The minimum diameter for off takes from
main/ branch canal and distributaries is as
follows:
Main/ Branch
canal
Distributary
0.90m
i) 14.15 to 2.83
cumec
discharges –
0.23m φ
ii)2.83 cumec and
less – 0.15m φ
Min φ of pipe
(2) Laying of O.T. Pipes:
 The condition of laying of off – take pipes such
as “Negative projecting condition”, and
“Trench condition” etc., relevant to the
individual cases are followed as per IS. 783 –
1985 for laying and jointing. For the selection of
proper size of pipe for the vents, IS. 458 is to be
followed.
 Controlling arrangements
 The following controlling arrangements are
followed.
 Type of control to be adopted:
i) For pipe sluices of 6” (150 mm) dia and below
and vents of
 equivalent area with F.S.D of parent canal not
exceeding
No control
 4ft (1.22 m). and O.T discharge 1.5 c/s and less
(ii) For pipe sluices of diameters above 6” and
upto and including 12”(300 mm) with F.S.D of
parent canal not Stem shutter exceeding 4ft
(1.22 m).
iii)For all sluices where the FSD in the parent
canal Screw is more than 4ft (1.22 m) and for
sluices of larger ventways. gearing shutter
SEMI MODULAR OUTLETS
The Expert Committee (Core Committee) suggested to
provide Semi modular outlets (ungated ) for the
outlets with discharge of 0.5 cumec and less, taking off
from channels having discharge less than 25 cusec
(about 0.7 cumec)
Definition of semi modular outlets (flexible
modules)
The outlets whose discharge is independent of the
water level of the outlet channel but depends on the
water level of the distributary so long as minimum
working head required for their working is available.
The discharge through such an outlet will therefore,
increase with the rise in the distributary water surface
level and vice versa. The common examples of this
type of modules are
1. Open Flume Module (O.F.M)
2. Adjustable Orifice Semi module (A.O.S.M)/
Adjustable Proportional Module (A.P.M)
3. Pipe Semi - module -free fall pipe outlet (P.S.M)
1) Open flume module:
 It is weir type outlet with a constricted throat and
an expanded flume on D/S side. Due to
constriction, super critical velocity is ensured in
the throat and thereby allowing formation of jump
in the expanding flume. The formation of
Hydraulic jump makes the outlet discharge
independent of water level in the outlet channel,
thus making it a semi - module.
(2) Adjustable Orifice Semi - Module (A.O.S.M):
 An adjustable orifice semi - module consists of an
Orifice provided with gradually expanding flume
on the d/s side of the orifice. The flow through the
orifice is super critical, resulting in the formation of
hydraulic jump in the expanded flume portion. The
formation of jump makes the discharge independent
of water level in the out let channel.
3) Adjustable Proportional Module (A.P.M):
This type is the most commonly used outlet in this
class. In this, the CI roof block is fixed to the check
plates by blots, which can be removed and depth of
outlet adjusted after masonry around is dismantled



(4) Pipe Semi - Module (P.S.M):
Pipe outlet discharging freely into atmosphere is
the simplest and the oldest type of flexible outlet.
The discharge through such an outlet will depend
only upon the water level of the distributary and
will be independent of water level in the outlet
channel so long as the pipe is discharging freely.
This can be provided where sufficient level
difference between distributary and outlet channel
is available.]
The suitability of the type of the semi module
outlet is determined based on the ratio of parent
canal discharge (Q) to the discharge of the out let
(q) and the throat width (Bt) as detailed below.
i) for (Q/q ) < or = 20 and B t ≥ 6 cm Open
Flume Module( OFM)
ii)
for (Q/q ) < or = 20 and B t < 6 cm
Adjustable Proportional module ( APM )
iii) for (Q/q ) > 20
If the above requirements do not suit the site
condition, provide pipe semi module (where
possible) with diaphragm of required diameter
inserted at the first joint. The minimum diametre
of pipe used will be 150 mm.
The above conditions are further explained as
below
Arrive at the ratio of parent channel / out let
channel.
If it is < or = 20, select OFM. Calculate the Bt
( throat width ), using weir formula.
If Bt is > 6 cm it is ok.
Otherwise select A.P.M.
Work out the Bt using the sluice formula
setting the crest of outlet at less than 0.80 D
from FSL of Parent Channel and adjusting the
height of outlet opening.
If Bt = or > 6 it is ok

Otherwise go for pipe semi module (PSM), if it
is possible to do so. Check for proportionally
Open flume module
Discharge through the out let (q) in cumec is
given by the formula:
Discharge through the out let (q) in cumec is
given by the formula:
q = C x Bt x G1.5
Where,
Bt =Throat width in 'm'
G =Depth of water in the Parent Canal over the
crest in 'm '
D = U/S FS Depth in 'm '
C = Coefficient
The value of C is adopted as under :
Bt
C
Above 6 cm & upto 9 cm 1.60
Above 9 cm & upto 12 cm
1.64
Above 12 cm
1.66
Length of Throat (Crest ) = 2 G
Setting G =0.9 x D , where D =full supply
depth in the parent canal
Minimum modular working Head = 0.2 G
Crest level = U/S F.S.L - 0.9 D
U/S approach wings to the throat
one
Curved and diverging and another straight
D/S expansion Splayed to 1 in 10 to meet the
bed width of out let channel
Adjustable Orifice Semi Module (A.O.S.M) or
Adjustable Proportional Module (APM)
Discharge through outlet in cumec.
Q = 4.03 Bt Y Hs1/2
Y =Height of opening in metres.
Bt =Throat width (minimum 0.06 m )
G =Depth of water in parent canal over the
crest in metres
Hs = Depth to under side of the roof block
below FSL of parent canal.
Hs = G – Y , Hs ≤ 0.80 D
y
> (2/3 ) G
Setting of crest, G = 0.750 x D , where D = Full
supply depth in the parent canal
Setting of crest shall not be below D/S B.L.
Minimum modular head Hm = 0.75 Hs for
modularity between full supply and any
fraction of full supply.
Crest level
≈
U/S FSL- 0.75 D
Length of throat
=
width of roof
block + G
U/S slope of glacis
=
curve with
radius 2G.
U/s approach wings
=
one curved and
the other straight, top at FSL + 0.15 m
D/S expansion
=
1 in 10 to meet
bed width of outlet channel

Pipe semi module
Design criteria
The discharge through pipe semi module is
given by
Q =Cd . A (2g hc )1/2
Where Cd = 0.62 for free pipe out let
hc =head on U/S above the centre of pipe
hc should be more than 1.5 times the dia of the
pipe proposed.
The above formulae can be reduced to
Q =0.62 x √ (2x 9.81 ) A √ (hcnt)
=2.746 A hc 1/2
For free fall condition set the F.S.L of OT
Channel below the pipe sill level keeping in
view the command under the pipe sluice .It is a
simplest type and the users will appreciate.
Throttling the vent way of existing pipe out
lets: (From design guidelines for structured
irrigation network to suit to RWSS).
When the existing diametre of pipe is more
than required then, to reduce the size of the
pipe a sleeve pipe is introduced whose
diametre is worked out by equating operating
head to the headloss.
h = Ki (Vs2/ 2g) + (Vs – Vp )2 / 2g + f x (Lp/ Dp) x
(Vp2/ 2g) + Ko(Vp2/ 2g)
Where, Ki = 1, Loss coefficient at inlet
Lp = Length of pipe
Ko = 1, Loss coefficient at exit
Dp = Diametre of pipe
f = Friction loss coefficient = 0.02
Vs = Velocity in sleeve pipe
Vp = Velocity in the pipe
Substituting the values in the equation find out the
Vs, then the area of sleeve pipe As
Find
out the dia of sleeve pipe Ds = (4 As/ 3.14)0.5. The
length of sleeve pipe shall be 5 Ds
FIG.24
FLOW MEASUREMENT STRUCTURES
GENERAL
Provision of "measuring structures/devices"
shall conform to the following guide lines
given by Sri R.K. Malhotra, World Bank
Consultant.

A measuring structure is to be provided downstream
of every off-take of major from the main canal/ branch
canal, distributory from a major, minor from the
distributory and sub-minor from the minor etc.
Measuring structure is also to be provided at off-take
of branch canal from the main canal and also in the
main canals.


Types of measuring structures shall be broadly:
"Standing Wave Flumes” in concrete (SWF) and
Parshall Flumes & Cut Throat Flumes (CTF) in fiber
glass reinforced plastic material with their hold-fasts
to be embedded in concrete structures. Standing Wave
Flumes may be provided in the main & branch
canals; Cut Throat Flumes /Parshall Flumes in the
majors/distributaries, while Cut Throat Flumes may
be provided in the minors/sub-minors. The Parshall
and Cut Throat Flumes in fiber glass reinforced plastic
(FRP) material shall have engraved gauge markings in
centimeters as well as in liter/second.
Division Boxes shall be constructed in concrete.
Likewise, turn-outs shall be constructed in concrete.



STANDING WAVE FLUME
Standing wave flume is a critical depth flume.
The discharge through this is independent of
water level on downstream and varies with
water levels on upstream. The hydraulic
behavior is same as that of a broad crested
weir. Since only one gauge reading is required
to be taken for measuring the discharge and
due to ease of construction, standing wave
flumes are recommended as a flow measuring
device.
The following are the three types of flumes
proposed for adoption
1. Standing wave flume
2. Standing wave flume fall (associated with
drop)
3. Rectangular throat flume (to be adopted on
canals having discharge less than 1 cumec)
Design criteria
The design is as per IS.6063 - 1971 "Method of
measurement of flow of water in open channels
using standing wave flume”
FIG.40
(1) Discharge
Discharge through standing wave flume ( Q )
in cumec is given by
Q = 2 2 g . Cf Bt . H3/2
3√ 3
= 1.705 . C f . Bt . H 3/2
Where B t = Throat width in ‘m’
H = Height of specific energy over the crest in
‘m’.
= Depth of flow over the crest on upstream (d1)
+ head due to velocity of approach (v)
= d1 - Z + v 2 /15.2
Where Z = Height of hump over U/S canal
bed level
Cf = Coefficient of friction
For Q Value of Cf
0.3 to 1.5 cumec
0.98
0.5 to 15 cumec
0.99
above 15 cumec 1.00
Modular Limitvalue of submergence ratio of
H2/H1 at which the real discharge deviates by 1 %
of Q calculated by discharge equation. It should be
between 0.7 to 0.95
With straight transition from throat width to
downstream bed width in a length of 4 H
Modular Limit H2 /H1
Minimum modular head
=
=
0.8 to 0.85
0.15 H to 0.2 H


2) Height of hump :
The height of hump is the difference between the
u/s canal bed level and the sill level of the flume.
Height of hump, for proportionality between full
supply and any fraction of full supply between the
channel and weir is given by the equations.
1
m 1/x
1
1
m 2/3
-1
(i) Z = d1 – D1 = d1 m 1/x 1- For channels running
with fluctuating discharge

where m = Qm =
Any particular
fraction of full supply discharge
Q
x = approach channel index
d1 = upstream depth of water in the canal
D1 =
Depth of water over the crest
Where Q = discharge
C1 = a coefficient
d1 = depth of water in the channel
x = index, which varies from 1.5 to 2


From the discharges Q,Q',Q'',Q''', etc , for the flow of
depths of d1,d'1, d''1,d'''1, etc respectively, the value of x
in the equation is estimated by least square method by
considering 4 sets of d and corresponding Q.
∑ log Q . Log d - (∑ log Q ) ( ∑ log d) Where M =
No. of sets = 4
x =M
∑ ( log d )2 - ( ∑ log d )2 M
Figure 2 gives the height of hump required for
various values of x and fluctuations. In case of
channels which run either full or closed, a flume
which gives proportionality at full supply discharge
is desirable. In the case of channels, in which
discharge varies considerably, bulk proportionality is
preferable. Figure 3 gives the heights of hump for bulk
proportionally.
(3) Head loss:
The head loss consists of the following losses:
(i) Approach transition,
(ii) Exit transition,
(iii) Friction in structure, and
(iv) Hydraulic jump
The loss in approach and exit transitions depends
on the amount of fluming and its gradualness. The
friction loss is usually very small. The loss in
hydraulic jump is given by the equation:
HL = (d2 – d1)2
4 d1 d2
Where d1 = depth of flow before jump
d2 = depth of flow after jump
(4) Approach transition
The radius of side walls of the bell mouth
entrance should be 3.6 H 1.5 metres. If ‘H’ is less
than 0.30m, the radius may be 2H from the
throat. The curvature (formed from the throat)
should continue till it subtends an angle of 600,
from where, it should be continued tangentially
to meet the side of the channel upstream.
The bed convergence should begin on the same
cross section as the side convergence. The radius
of curvature of hump in the bed should be:
rh = L12 + Z2
2Z
Where rh = radius of curvature of hump
L1 = length between the junction of side wall
with the bed of upstream channel and
upstream end of the throat measured along the
axis.
Z = height of hump above u/s bed level.
FIG.42
FIG.43
5) Throat
Sides of throat should be vertical and length
should be 2.5 H. Width of the throat way be
calculated from the formula given in sub-para
(1) of 6.1.1.
(6) Downstream glacis
The length of downstream glacis should be
equal to 4H, which is also the length of the side
walls along the glacis. The slope of the glacis is
usually 1 in 20 or flatter. The divergence of side
walls should be 1 in 10 or flatter so as to make
the width at the toe of the glacis equal to or less
than the downstream canal bed width.
(7) Gauge (Stilling) well
The stilling well should be so located as to
measure the water upstream of the sill, where
there is no curvature of flow. This could be
ensured by locating the stilling well intake pipe
at a distance of
4 Hmax upstream of the bell
mouth entrance. Hmax is the maximum value of
upstream head over the sill (including velocity
head).
STANDING WAVE FLUME FALL
(ASSOCIATED WITH A DROP)
Standing wave flume fall is a flow measuring
device .It acts as a control point to maintain design
supply level in the canal on u/s of the structure. A
measuring device to be provided at the head of the
distributory shall be a standing wave flume
combined with fall if it exists at reasonable distance
from head before first drawal. In case any existing
drop is damaged, requires reconstruction and
satisfies the above condition, it can be reconstructed
with standing wave flume. The drops downstream
of the outlets may be designed as the standing wave
flume fall, wherever necessary.

Design criteria
The standing wave flume fall is essentially a broad
crested weir and IS: 6062 - 1971 "Method of
measurement of flow of open channels using
standing wave flume fall" and "Manual on canal
falls" by Central Water Commission are followed
for the design of standing wave flume fall . The
design calculations are similar to that of standing
wave flume. The main difference between the two
is in the energy dissipation arrangements. In the
case of normal standing wave flume, head loss is
considerably low and does not require any special
energy dissipation arrangements. In the case of
standing wave flume combined with fall or drop,
energy dissipation arrangements are provided as
per the requirements for the falls.
(1) Discharge
Discharge through standing wave flume ( Q )
in cumec is given by the equation given in subpara (1) of para 6.1.1, similar to that for standing
wave flume without fall.
In case, piers are provided in the flume, the
discharge is given by the formaula:
Q= 2
2 g . Cf (Bo – mb – 2Cc m H) H1.5
3 √ 3
Where Q = discharge in cumec
 Cf = Coefficient of friction having the following
values:
0.97 for Q = 0.05 to 0.30 cumec
0.98 for Q = 0.30 to 1.50 cumec
0.99 for Q = 1.50 to 15.0 cumec
1.00 for Q = 15.0 cumec and above
Bo = Overall throat width including piers
m = no. of piers
b = thickness of each pier
Cc= coefficient of contraction having values of
0.045 for piers with round nose and 0.04 for
piers with pointed nose.
H = head over sill including velocity head given
by equation
H = D1 + Va2
15.2
Where D1 = upstream depth of water over sill,
and Va = velocity of approach
(2) Height of hump:
(3) Throat :
The length of throat is equal to 2.5 H. The throat
width is calculated from the discharge formula
in sub-para (1) of para 6.2.1.
The width of throat shall not be less than 1.5 H
(4) Inlet transition:
The radius (R) of the side walls of bell mouth
entrance should be 3.6 H 1.5. The curvature should
continue till it subtends an angle of 600, from where
it shall be continued tangentially to meet the side of
the channel. However when the curved walls meet
the sides of channel when it subtends an angle of
600, it is not necessary to continue the walls further.
The length of inlet transition (L1) may be found out
knowing B1 , B2 and the radius of bell mouth
entrance R using the relation:
L1 =

2R√
B1 – B 0
2
B1 – B0
2
Where B1 = upstream bed width of channel
B0 = overall throat width
The radius of curvature of hump (rh) in the bed
is given by the following equation.
(rh) = L12 + Z 2
2Z
When the total head above the standing wave
fall (SWF) sill becomes considerable, say 1.2 m,
the height of hump ‘Z’ becomes insignificant as
compared to ‘L1’ so that the radius becomes
large and the U/S end of the throat may be
joined by a straight line to the channel bed
U/S.
5) Design of Glacis :
The glacis should have a slope of 2:1 connected
with the throat upstream by a curve of radius
2H and with the cistern downstream by a curve
of radius H. The side walls should be straight
over glacis portion. With steeper glacis slope of
2:1 and greater loss of head, proper expansion
should be provided. For controlling the issuing
flow, parallel sides should be extended down
to the toe of glacis followed by hyperbolic
expansion in the cistern using equation:
By B0 B3 L
L B3 - ( B 3 - B o ) y
Where, By = width at any distance ‘y’
Y = distance from beging of expansion of
hyperbola,
B0 = over all throat width of flume at the contracted
section (excluding piers)
B3 = bed width of downstream channel
L = length of cistern
(6) Cistern
The cistern is provided at the toe of the glacis.
The length of cistern (L) varies from 4 times to 6
times the downstream FSD (d3) of the channel
depending on the nature of soil in the channel
bed.
4 d3 for shingle bed
L = 5 d3 for good earthen bed
6 d3 for sandy bed
If the channel is lined with CC, length of cistern
may be taken as 4 d3.
In order to stabilize the flow, bed of cistern
should be made steeper in the center by 25%
compared to the sides.
(7) Control blocks
Two rows of control blocks, staggered in plan
should be provided downstream of the toe of
the glacis in the cistern. The size of the blocks
should be as follows:
Height (h) = 1/6 depth of water in mid cistern
Length (l) = 1.5 to 3 h
Width (w) = 2/3 h
Clear distance between blocks = l
Clear distance between rows = w
The first row of blocks should be at 3 to 5 times
the height of the blocks from the toe of glacis.
8) Deflector
A deflector should be provided at the
downstream end of the cistern.
Size of deflector:
Height (h) = 1/12 depth of water in mid cistern
Width (w) = h
Gap in the deflector = h
Internal of gaps = 4h
Short walls of same height should be placed close
to the upstream of gaps.
(9) Gauge well
RECTANGULAR THROAT FLUME:
The discharge in a open channel may be
measured by means of a flume. Consisting
essentially of contractions in the sides and / or
bottom of the channel forming throat. When the
dimensions are such that critical flow occurs in
the downstream, (in other words it is free
flowing) discharge can be determined from the
single upstream depth measurement. This device
is called “Critical Depth Measuring Flume ".This
structure may be adopted for measuring smaller
discharges less than 1 cumec.
Design criteria:
(a) Rectangular throat with hump
(b) Let 'Y' be the depth of flow and velocity be
"V" m/sec in the normal section. Then total
energy
head is equal to depth of flow and due to
velocity of approach i.e
E = Y + V2 / 2g
Take the value of 2 g equal to 19.2
In Rectangular section , critical depth (Yc ) is
equal to two thirds the Total Energy
head ,i.e Yc = 2/3 E
The throat width is worked out by discharge
equation , which is given as follows :Q = 2/3 √ 2/3 x g . C f . b . H1.5 = 1.705 C f b
H1.5
where C f = co-efficient of friction = 0.97
b= throat width
H = Yc = depth of flow at critical section
Length of the crest is equal to 2H.