CROSS REGULATOR

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).

1) DRIVING HEAD:

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.

2) FLUMING RATIO:

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.

3) HUMP AT THE REGULATOR SRUCTURE

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.

4) DISCHARGE FORMULA

Q = C Bt H3/2 Where Q = D/S full supply discharge in m3 /sec

C = Co-efficient of dischargeBt = 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).

5) HEIGHT OF HUMPCrest 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.

6) ENERGY DISSIPATION:

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.

7) EXIT GRADIENT AND FLOOR THICKNESS

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 H(π √ λ) d

Where: λ = 1 + 1 + α2

2

α = b d

H = difference between crest level and downstream bed level in mb = length of impervious floor in md = depth of downstream curtain wall in m

8) U/s & D/s CURTAIN WALLS

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.

9) PIERS

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.

10) ABUTMENTS

Abutments to be designed with super imposed loads, live load moments, tractive force and braking force and the earth pressure behind them.

11) WINGS AND RETURNS

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.

12) BED PROTECTION

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.

13) OPERATION PLATFORM

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.

14) GATES AND HOISTS

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

CANAL FALL (DROP)

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.

(i) Type I: Vertical drop:

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.

(ii) Type II: Glacis fall:

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: (d) Flumed or unflumed falls and(e) 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 Flumed

Clear over-fall Drowned Clear over-fall

Drowned

1 High Discharge & High Falls Q > 15

HL > 1 Baffle type (suitable up to retrogression of 25% also)

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 Baffle or Glacis or Vertical type depending on economy and suitability at site

Modified Glacis type

Baffle or Glacis type

Glacis type`

5 Q < 8 All drops

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.

Glacis type 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 Percentage of fluming1) Up to 1.0 m 66%2) Over 1.0 m to 3.0 m 75%3) Above 3.0 m 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 cumecC = 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) 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.

8) Glacis fall without baffle: (i) 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 d2 + d22

g 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 formulasdx for unflumed falls = 0.985 q0.52 x Hx

0.21

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 1/3 g

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) = hTop width (W) = 2h / 3Distance 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 311) 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

3ii) Depth of D/S curtain wall = D/S FSD subject to minimum of 0.50 m

2These 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 floord 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 wallα b length of impervious floorFind 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 wall1 = 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/bb = 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.

4D = 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 pressuresa) 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, λ = 1 + 1 + α2

2GE = exist gradient = 1 x H

∏ λ d

b) Uplift pressures: (a) U/S face of crest wall d= U/S CBL – Bottom of foundation concrete.

1 = dα bφ E is read from ‘Plate 17’ of CWC manual on falls

At the end of floor1 = dα 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 1:1Clear 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.

TABLE No. IDETAILS OF COMPONENTS OF VERTICAL TYPE DROPS WITH

Discharge

Q (cum)

Height

of drop

Depth of

cistern below D/S

B.L (x)

Length of

crest

(Lt.)

Throat width

(Bt.)

Depth of crest below U/S T.E.L (D)

Height of crest above U/S

B.L (D1)

Bottom width of drop wall (L

W)

Length of

apron (La

)

Width of

apron (Wa)

Thickness of

apron (ta)

1.5 to 1.0

0.8 1.0 1.2 1.5

0.15 0.17 0.19 0.22

0.8 0.3 0.8 0.3

Bed

width on U/S or

D/S whichever is

less

As per formulae

As per

formulae

0.80 0.80

0.90 1.10

3.4 3.8 4.2 4.7

Bed width on D/S

0.60 0.65 0.65 0.70

1.0 to 0.5

0.6 0.8 1.0 1.2 1.5

0.12 0.14 0.16 0.18 0.21

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

0.5 to 0.1

0.6 0.8 1.0 1.2 1.5

0.10 0.12 0.14 0.15 0.17

0.6 0.6 0.6 0.8

0.8

0.60 0.70 0.80 0.90 1.00

2.4 2.8 3.2 3.5 4.0

0.60 0.60 0.60 0.65 0.70

0.1 and below

0.6 0.8 1.0 1.2 1.5

0.07 0.08 0.09 0.10 0.12

0.6 0.6 0.6 0.6

0.8

0.60 0.60 0.70 0.80 1.00

2.0 2.3 2.6 2.8 3.2

0.60 0.60 0.60 0.60 0.70

THAN 1.5 CUMEC AND HEIGHT OF DROP LESS THAN 1.5 m

Discharge Q

(cum)

Height of drop

Depth of cistern

below D/S B.L (x)

Length of crest (Lt.)

Throat width (Bt.)

Depth of crest

below U/S T.E.L

(D)

Height of crest above

U/S B.L (D1)

Bottom width of

drop

wall (LW

)

Length of

apron

(La)

Width of apro

n (Wa)

Thickness of apron

(ta)

1.5 to 1.0 0.8 1.0 1.2 1.5

0.15 0.17 0.19 0.22

0.8 0.3 0.8 0.3

Bed width on U/S or D/S whichever is less

As per formulae

As per formul

ae

0.80 0.80 0.90 1.10

3.4 3.8 4.2 4.7

Bed width on D/S

0.60 0.65 0.65 0.70

1.0 to 0.5 0.6 0.8 1.0 1.2 1.5

0.12 0.14 0.16 0.18 0.21

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

0.5 to 0.1 0.6 0.8 1.0 1.2 1.5

0.10 0.12 0.14 0.15 0.17

0.6 0.6 0.6 0.8

0.8

0.60 0.70 0.80 0.90 1.00

2.4 2.8 3.2 3.5 4.0

0.60 0.60 0.60 0.65

0.70

0.1 and below

0.6 0.8 1.0 1.2 1.5

0.07 0.08 0.09 0.10 0.12

0.6 0.6 0.6 0.6

0.8

0.60 0.60 0.70 0.80 1.00

2.0 2.3 2.6 2.8 3.2

0.60 0.60 0.60 0.60

0.70

FIG.19

TABLE No. IITable 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 (Lt

1/6/ 1.835)}3/5 in meters

Discharge per Meter run of crest wall i.e., Q/Bt

Depth of crest (D) below U/S T.E.L. in meters for length of crest Lt

Cumec 0.6 m 0.8 m0.10 0.166 0.1720.15 0.212 0.2180.20 0.252 0.2590.25 0.288 0.2960.30 0.321 0.3310.35 0.352 0.3620.40 0.383 0.3930.45 0.409 0.4220.50 0.436 0.4490.55 0.462 0.4750.60 0.437 0.5010.65 0.511 0.5260.70 0.534 0.5490.75 0.556 0.5730.80 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 metersLt = 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., = QNo. of notches.Silt level of drop = U/S CBL3) For free fall notches: Case I:For free notch, the equation used for finding out notch dimensions isQ = 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 notL = 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 equationX + d1 = 0.91 dc √ hWhere 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.

9) 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

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 widthf = 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

D/S Discharge

in Cum

Height of drop h in

m

No. of notche

s

Details of each notch

X cushionThickness

of end pier

Top of drop

wall Lt

Bottom width of

drop wall

Length of drop core

wall

Length of

apron (La)

Thickness of apron

(ta)

L L + nd

1 2 3 4 5 6 7 8 9 10 11 12

1.5 to 1.0 1.0 1.2 1.5 1 1 1

Refer Table II (A)

Refer Table II (A)

--

0.09

0.60.60.6

0.60.60.6

1.31.41.6

3.63.84.0

0.700.750.75

1.0 to 0.5 0.8 1.0 1.2 1.5

1 1 1 1

---

0.08

0.600.600.600.60

0.600.600.600.60

1.21.21.41.6

3.23.43.63.8

0.650.700.700.75

0.5 to 0.1 0.6 0.8 1.0 1.2 1.5

1 1 1 1 1

----

0.06

0.50.50.50.50.5

0.50.50.50.50.5

0.81.01.21.21.4

2.42.62.82.93.1

0.600.600.650.700.75

0.1 and below 0.6 0.8 1.0 1.2 1.5

1 1 1 1 1

----

0.04

0.450.450.450.450.45

0.450.450.450.450.45

0.70.80.91.01.3

1.61.71.81.92.1

0.600.600.600.650.70

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 metresn = 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 notchQ = 1.708 [ L – 0.1 nd] d 3/2 Where n = no. of notches

L = length of the drop wall in metresd = 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) Driving head:(3) 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) Vent way:(4) 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:

Min φ of pipe

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

(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 OUTLETSThe 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 belowArrive 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 proportionallyOpen 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 = CoefficientThe value of C is adopted as under :

Bt C

Above 6 cm & upto 9 cm 1.60 Above 9 cm & upto 12 cm 1.64Above 12 cm 1.66Length of Throat (Crest ) = 2 GSetting G =0.9 x D , where D =full supply depth in the parent canalMinimum modular working Head = 0.2 GCrest level = U/S F.S.L - 0.9 DU/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 Hs

1/2

Y =Height of opening in metres. Bt =Throat width (minimum 0.06 m )

G =Depth of water in parent canal over the crest in metresHs = Depth to under side of the roof block below FSL of parent canal.Hs = G – Y , Hs ≤ 0.80 D

y > (2/3 ) GSetting of crest, G = 0.750 x D , where D = Full supply depth in the parent canalSetting 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 DLength 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 moduleDesign criteriaThe 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(Vp

2/ 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 STRUCTURESGENERALProvision 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 flume2. Standing wave flume fall (associated with drop)3. Rectangular throat flume (to be adopted on canals having discharge less than 1 cumec) Design criteriaThe design is as per IS.6063 - 1971 "Method of measurement of flow of water in open channels using standing wave flume”

FIG.40

(1) DischargeDischarge 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 /15.2Where Z = Height of hump over U/S canal bed levelCf = Coefficient of friction

For Q Value of Cf

0.3 to 1.5 cumec 0.980.5 to 15 cumec 0.99above 15 cumec 1.00Modular Limit value 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 = 0.8 to 0.85Minimum modular head = 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.

1m 1/x

11m 2/3

-1(i) Z = d1 – D1 = d1 m 1/x 1- For channels running

with fluctuating discharge

where m = Qm = Any particularfraction of full supply discharge

Qx = approach channel indexd1 = upstream depth of water

in the canalD1 = Depth of water over the

crestWhere Q = dischargeC1 = 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 = 4x = 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 jumpThe 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 = L1

2 + Z2

2 Z

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) DischargeDischarge through standing wave flume ( Q ) in cumec is given by the equation given in sub-para (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 √ 3Where Q = discharge in cumec

Cf = Coefficient of friction having the following values:

0.97 for Q = 0.05 to 0.30 cumec0.98 for Q = 0.30 to 1.50 cumec0.99 for Q = 1.50 to 15.0 cumec1.00 for Q = 15.0 cumec and aboveBo = Overall throat width including piers

m = no. of piersb = thickness of each pierCc = 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 = 2 R - B1 – B 0 B1 – B0

√ 2 2

Where B1 = upstream bed width of channelB0 = overall throat width

The radius of curvature of hump (rh) in the bed is given by the following equation.(rh) = L1

2 + Z 2

2 ZWhen 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 cisternLength (l) = 1.5 to 3 hWidth (w) = 2/3 hClear distance between blocks = lClear distance between rows = wThe 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

cisternWidth (w) = hGap in the deflector = hInternal of gaps = 4hShort 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.eE = Y + V2 / 2g Take the value of 2 g equal to 19.2In Rectangular section , critical depth (Yc ) is equal to two thirds the Total Energyhead ,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 = 0.97b= throat width H = Yc = depth of flow at critical section Length of the crest is equal to 2H.