canal falls or drops

8/12/2019 Canal Falls or Drops

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TRAINING PROGRAMME ON

ENGINEERING DESIGNS-CANAL STRUCTURES

GENERAL DESIGN PRINCIPLESCanal Falls or Drops

BY

ROUTHU SATYANARAYANA

CHIEF ENGINEER (Retired.)

FORMER ADVISOR, GOVERNMENT OF A.P


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THANK YOU


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Canal Falls or Drops

Introduction:i. Canal fall or drop:

A structure designed to secure lowering of the water surface in a canal and to dissipate safelythe surplus energy so liberated, which otherwise scour the bed and banks of the canal

Necessity:

Velocity in a canal is a function of the slope of the canal. There is a limit for the velocity, so thatthe canal bed can neither be scoured nor silted up. Hence there is a limiting surface slope in the

canal. The slope of the country, where the canal system has to run will naturally be steeper than the

surface slope the canal system that has to come up. To bring the velocity with in the permissiblevelocities, falls or drops are introduced at suitable locations.

Falls are combined with regulators, bridges, and escapes.

Classification or Type of Falls/Drops:1. Vertical drops.

2. Glacis falls

a. Straight Glacis with baffle (Inglis fall)

b. Straight Glacis without baffle


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3. Trapezoidal Notch falls

4. Well type, Cylinder Falls or Well Syphons.

5. Ogee falls (with and without raised crest)

6. Stepped falls

7. Meter and non-meter falls

Required Basic Data:

Site plan with contours showing the flow direction.

Hydraulic particulars of the canal both upstream and downstream

Cross sections of the canal with levels, such as BL, FSL, TBL, GL, marked on the CS.

TPs Particulars, taken up to hard strata or to a minimum depth of 2m below CBL orground level which ever deeper with soil classification.

Bearing capacity of the foundation strata.


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Canal Falls or Drops Hydraulic design:

Fixation of crest:

i. Throat width of drop wall,ii. Linear water way

Design of Stilling Basin:

i. Invert level of the cistern

ii. Length of the basin

iii. Level of end silliv. And other components.

Design of drop wall.

Transition lengths on upstream and downstream.

Check for Scour depth

Check for exit gradient

Check for apron thickness due to uplift.

Structural Design:

Design of body wall

Design of stilling basin

Design of wings, returns.


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Trapezoidal Notch Fall:

It consists of number of trapezoidal notches constructed in a high crest wall across thechannel with a smooth entrance and a flat circular lip projected down stream from eachnotch to spread out the falling jet.

The notches could be designed to maintain the normal water depths in u/s at any twodischarges, as the variation at intermediate value is small.

There would neither be drawdown nor heading up of water, as the channel approaches the

fall. These falls are Simpler, economical and popular. Design Trapezoidal Notch:

The crest level of the fall normally is u/s bed level,

The total linear water way should not be more than the bed width of the canal on thed/s side.

The discharge for one notch is computed and the number of notches designed by trial anderror method considering full supply discharge and half supply discharge...


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Discharge formula: Q = 2.25 [L + 0.4nH] H3/2Where

Q = discharge per notch in cumecs

n = a constant

L= Bottom width of notch

H = Depth of water.Top width of notch =L+ n.H and the thickness of pier notch >0.5H


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Glacis fall:

Straight Glacis fall:

It is a modern fall, a Straight glacis generally with a slope 2:1 is provided after a raisedcrest.

The hydraulic jump occurs on the glacis causing sufficient hydraulic jump.

Suitable up to 60 cumecs and 1.5m drop.

Baffle falls or Inglis falls: (used for flumed)

For all discharges and for the drops more than 1.5m. These drops are flumed.

These drops can be meters.

Design principles:

The following are main components:

1. Up stream approach

2. Clear width of the Throat (Bt), normally down stream bed width.

3. Down stream glacis

4. Down stream expansion

5. Down stream Energy Dissipation.


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Energy Dissipation : It is the portion of the fall on the d/s side of the crest, where the surplus energy is being

destroyed. It consists of: Sloping glacis (if any). The cistern Roughness devices and the deflectors to deflect the high velocities.

Cistern: The object is three fold: To reduce the intensity of impact of the dropping jet against the d/s floor.

Length and depth of cistern are more crucial in this case To provide cushion to destroy the energy of the drop by formation of hydraulic

jump. In a sloping glacis a reverse curve at the lower end to turn the hypercritical jet to

horizontal to a horizontal before it impinges against the sub critical flow of the

lower channel, thus creating an hydraulic jump. Thus the invert level of the cistern be kept at 1.25 times below the d/s energy

line(Ef2) or a minimum 300mm below d/s bed level. The length of cistern be 5Ef2 to 6Ef2

To produce reverse flow by providing suitable end wall to ensure an impact in thecistern


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In case the hydraulic jump occurs on the glacis the energy dissipation will beincomplete and additional roughening devices such as friction blocks, endsill, deflectors etc.,

Cistern without impact: In this case hydraulic jump does not form, in case high drowning ratio and

low falls.



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Canal Falls or Drops Glacis fall:

Minimum fluming ratio:Drop height Fluming ratio1. Up to 1m 66%2. 1m to 3m 75%3. Above 3m 85%4. Maximum Bed width (d/s bed)

Coefficient of discharge:Discharge over the crest Q = C Bt D3/2Where

C= Coefficient of dischargeDischarge in Cumecs Value of C

0.06 to 0.30 1.660.31 to 1.50 1.671.51 to 15.00 1.68Over 15 1.70D = Head over the crest.Crest level = u/s TEL- D

Crest level should not > 0.4 d ( where d=u/s FSD)Length of crest = 2/3 x D


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Canal Falls or Drops When there are piers Bt will be reduced as (Bt - 0.2nH), where n= No. of piers.

Crest length = 2/3 D and for vertical falls it should be 0.55 D1/2, a minimum 0.5m.

Down stream Glacis:

1. D/S slope 2:1 can be increased to 1:1

2. If baffle platform is introduced the slope can further be increased to 2/3:1

Baffle Platform: It is a low weir constructed at the end of cistern, working on the principle of horizontal

impact for energy dissipation.

The jump is held stable on a horizontal platform by means of baffle wall.

To head up water to upstream to form hydraulic jump.

To with stand the impact of the high velocity jet to dissipate the energy.

Fixing Sill Level:

Sill level of the platform = Down stream FSL-dx

Where dx= Hx+ dx- HL

dx = 0.985d0.52.Hx

HL= K0.52. Hx

HL = Actual height of dropK = Bed width of canal on d/s/ Over all width of wall


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Baffle wall:

Height of Baffle wall (hb) = dc-d2Where

dc = (q2/g)1/3 and d2 = 0.183.(q) 0.89 (Hx) -0.35

q = discharge per meter width.

Thickness of baffle wall = 2/3 hb

Length of Baffle plat form = 5.25 hb

Cistern:

The platform below the baffle wall up to the deflector wall is known as the cistern.

Depth of cistern = 0.1 times down stream FSD and should not


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Well Type or Syphon Well drops:

It consists of well with pipe or an RCC box at its bottom, carrying water from inlet wellto a downstream well or a Cistern.

Down stream well is necessary if the discharge is greater than 0.29 cumecs and the fall is >1.8m

The water falls in to the inlet well through a trapezoidal notch constructed in the steining ofthe well.

Simple vertical Drop:

It is a simple fall broad crested weir with high raised sill, The nappe impinges into the watercushion below

There is no hydraulic jump and the energy dissipation is brought about by the turbulentdiffusion, as the high velocity jet enters the deep pool of water down stream.

The discharge is computed using the formula:

Q in cumecs = 1.66L.d3/2

Where

L is the bed width on d/s side and dis depth of water above crest in m.


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canal falls or drops

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