diversion head works ajitha miss

DIVERSION HEADWORKSDIVERSION HEADWORKS

CANAL HEADWORKSCANAL HEADWORKS

Canal head works– Structures/works constructed across river and at the

head of the off taking canal

Canal head works

Diversion head works

To raise water level in river and divert the required quantity

Storage head works

To store water on u/s of river and divert the required quantity

DIVERSION HEADWORKSDIVERSION HEADWORKS

Purposes

– Raises water level in the river

– Regulates supply of water into the canal

– Controls the entry of silt into the canal

– Provides some storage for a short period

– Reduces the fluctuations in the level of supply in river

TYPES OF DIVERSION HEAD WORKSTYPES OF DIVERSION HEAD WORKS

1. Temporary diversion head works

– Consists of a bund constructed across river to raise the water level in the river and will be damaged by floods

2. Permanent diversion head works

– Consists of a permanent structure such as a weir or barrage constructed across river to raise water level in the river

LOCATION OF CANAL HEAD WORKSLOCATION OF CANAL HEAD WORKS

Depends on the stages of flow (reaches) of river

(i) Rocky stage

(ii) Boulder stage

(iii) Trough stage or alluvial stage

(iv) Delta stage

Both rocky and delta stages are not suitable for location of diversion head works

SUITABLE SITE FOR DIVERSION SUITABLE SITE FOR DIVERSION HEAD WORKSHEAD WORKS

Having selected the reach of the river, selection suitable site in accordance with the following considerations

1. As far as possible, a narrow, straight, well defined channel confined between banks not submerged by the highest flood

2. Should be possible to align the off taking canal in such a way that command of its area is obtained without excessive digging

3. Materials of construction such as stone, sand etc. should be available in the vicinity of the site

4. Site should be accessible by rail or road

SUITABLE SITE FOR DIVERSION SUITABLE SITE FOR DIVERSION HEAD WORKSHEAD WORKS

COMPONENTS OF DIVERSION COMPONENTS OF DIVERSION HEADWORKSHEADWORKS

1. Weir or Barrage 2. Divide wall or divide groyne 3. Fish ladder 4. Pocket or approach channel 5. Under sluices or scouring sluices 6. Silt excluder 7. canal head regulator 8. River training works such as marginal bunds and guide

bunds

COMPONENTS OF DIVERSION COMPONENTS OF DIVERSION HEADWORKSHEADWORKS

WEIRWEIR

Weir is a structure constructed across river to raise the water level and divert the water into the canal

Weir aligned at right angle to the direction flow

Shutters are provided at the crest of the weir so that part of raising up of water is carried out by shutters

According to the material used for construction and certain design features

1. Masonry weirs with vertical drop walls

2. Rock fill weirs with sloping aprons

3. Concrete weirs with a downstream glacis

CLASSIFICATION OF WEIRSCLASSIFICATION OF WEIRS

MASONRY WEIR WITH VERTICAL MASONRY WEIR WITH VERTICAL DROPDROP

Weir consists of– Impervious horizontal floor or apron

– A masonry weir wall with either side vertical; or both faces inclined; or u/s face vertical and d/s face inclined

– Curtain walls or cutoffs or piles are provided at the u/s and d/s ends of the floor

– Block protection at the u/s end and graded inverted filter at the d/s end

– Longing aprons or pervious aprons after block protection graded filter

MASONRY WEIR WITH VERTICAL MASONRY WEIR WITH VERTICAL DROPDROP

ROCKFILL WEIRS WITH SLOPING ROCKFILL WEIRS WITH SLOPING APRONSAPRONS

Weir consists of

– A masonry weir wall

– Dry packed boulders laid in the form of glacis or sloping aprons

– Some intervening core walls

ROCKFILL WEIRS WITH SLOPING ROCKFILL WEIRS WITH SLOPING APRONSAPRONS

CONCRETE WEIRS WITH CONCRETE WEIRS WITH DOWNSTREAM GLACISDOWNSTREAM GLACIS

Floor made of concrete

Sheet piles of sufficient depth provided at the u/s and d/s ends

Sometimes intermediate piles are also provided

Hydraulic jump is developed at the d/s slope due to which considerable amount of energy is dissipated

Suitable on pervious foundations

CONCRETE WEIRS WITH CONCRETE WEIRS WITH DOWNSTREAM GLACISDOWNSTREAM GLACIS

BARRAGEBARRAGE

Crest is kept at a low level

Raising up of water level is accomplished by means of gates

During floods, these gates are raised and clear off the high flood level

CAUSES OF FAILURES OF WEIRS ON CAUSES OF FAILURES OF WEIRS ON PERMIABLE FOUNDATIONSPERMIABLE FOUNDATIONS

Causes of failures

– Due to seepage or subsurface flow

– Due to surface flow

Due to subsurface flow– Piping or undermining

– By uplift pressure

Due to surface flow

– By suction due to hydraulic jump

– By scour on the u/s and d/s of the weir

CAUSES OF FAILURES OF WEIRS ON CAUSES OF FAILURES OF WEIRS ON PERMIABLE FOUNDATIONSPERMIABLE FOUNDATIONS

DESIGN OF IMPERVIOUS FLOOR FOR DESIGN OF IMPERVIOUS FLOOR FOR SUBSURFACE FLOWSUBSURFACE FLOW

Bligh’s creep theory

Khosla’s theory

BLIGH’S CREEP THEORYBLIGH’S CREEP THEORY Design of impervious floor or apron

– Directly depend on the possibilities of percolation in the porous soil on which the apron is built

Bligh assumed that– Hydraulic gradient is constant throughout the

impervious length of the apron– The percolating water creeps along the contact of base

profile of the apron with the sub-soil, losing head enroute, proportional to the length of its travel

– Stoppage of percolation by cut off (pile) possible only if it extends up to impermeable soil strata

Bligh designated the length of travel as ‘creep length’ and is equal to the sum of horizontal and vertical length of creep

BLIGH’S CREEP THEORYBLIGH’S CREEP THEORY

If ‘H’ is the total loss of head, loss of head per unit length of creep (c),

c-percolation coefficient

Reciprocal of ‘c’ is called ‘coefficient of creep’(C)


Design criteria

(i) Safety against piping

Length of creep should be sufficient to provide a safe hydraulic gradient according to the type of soil

Thus, safe creep length,

Where, C= creep coefficient=1/c


Design criteria

(ii) Safety against uplift pressure

Let ‘h’’ be the uplift pressure head at any point of the apron

The uplift pressure = wh’

This uplift pressure is balanced by the weight of the floor at this point


If, t =thickness of floor at this point G = specific gravity of floor material Weight of floor per unit area

=


LIMITATIONS OF BLIGH’S THEORYLIMITATIONS OF BLIGH’S THEORY

Bligh made no distinction between horizontal and vertical creep

Did not explain the idea of exit gradient - safety against undermining cannot simply be obtained by considering a flat average gradient but by keeping this gradient will be low critical

No distinction between outer and inner faces of sheet piles or the intermediate sheet piles, whereas from investigation it is clear, that the outer faces of the end sheet piles are much more effective than inner ones

Losses of head does not take place in the same proportions as the creep length. Also the uplift pressure distribution is not linear but follow a sine curve

Bligh did not specify the absolute necessity of providing a cutoff at the d/s end

LIMITATIONS OF BLIGH’S THEORYLIMITATIONS OF BLIGH’S THEORY

LANE’S WEIGHTED CREEP THEORYLANE’S WEIGHTED CREEP THEORY

An improvement over Bligh’s theory

Made distinction between horizontal and vertical creep

Horizontal creep is less effective in reducing uplift than vertical creep

Proposed a weightage factor of 1/3 for horizontal creep as against the 1 for vertical creep

KHOSLA’S THEORYKHOSLA’S THEORY

Dr. A. N. Khosla and his associates done investigations on structures designed based on Bligh’s theory and following conclusions were made

– The outer faces of sheet piles are much more effective than inner ones and the horizontal length of floor

– The intermediate sheet piles, if smaller in length than the outer ones were ineffective

– Undermining of floors started from the tail end. If hydraulic gradient at exit is more than the critical gradient, soil particles will move with water and leads to failure

– It is absolutely essential to have reasonably deep vertical cutoff at the d/s end to prevent undermining


Khosla and his associates carried out further research to find out a solution to the problem of subsurface flow and provided a solution

– Khosla’s theory

– Considered the flow pattern below the impervious base of hydraulic structures on pervious foundations to find the distribution of uplift pressure on the base of the structure and the exit gradient


KHOSLA’S METHOD OF KHOSLA’S METHOD OF INDEPENDENT VARIABLESINDEPENDENT VARIABLES

A composite weir section is split up into a number of simple standard forms

The standard forms(a) A straight horizontal floor of negligible thickness with a sheet pile either at the u/s end or at the d/s end of the floor

(b) A straight horizontal floor of negligible thickness with a sheet pile at some intermediate point

(c) A straight horizontal floor depressed below the bed but with no vertical cutoff


These standard cases were analyzed by Khosla and his associates and expressions were derived for determining – The residual seepage head (uplift pressure) at key points

(key points are the junction points of pile and floor, bottom point of pile and bottom corners of depressed floor)

– Exit gradient

– These results are presented in the form of curves


The curves gives the values of Φ (the ratio of residual seepage head and total seepage head) at key points

The directions for reading the curves are given on the curves itself

The curves are for specific cases only In actual practice

– consider the assembled profile with piles at u/s end, d/s end, intermediate point, floor has some thickness and slope

– combination of simple profiles needs to be considered– Corrections need to be applied

1. Correction for thickness of floor

2. Correction for mutual interference of piles

3. Correction for slope of the floor

(i) Straight floor of negligible thickness with pile at u/s end (ii) Straight floor of negligible thickness with pile at some

intermediate point (iii) Straight floor of negligible thickness with pile at d/s end The pressure obtained at the key points from curves are then

corrected for (i) Thickness of floor (ii) Interference of piles (iii) Sloping floor

CORRECTION FOR THICKNESS OF FLOORCORRECTION FOR THICKNESS OF FLOOR

Pressure at actual points C1 and E1 can be computed by considering linear variation of pressure between point D and points E and C

When pile is at u/s end, Correction for

Pressure at

For the intermediate pile,

Correction for

Correction for

When pile at d/s end,

Correction for

CORRECTION FOR THICKNESS OF FLOORCORRECTION FOR THICKNESS OF FLOOR

Percentage correction for mutual interference of piles (C)

d- depth of pile on which the effect of another pile of depth D is required to be determined

D- depth of pile whose effect is to be determined on the neighbouring pile of depth d

CORRECTION FOR MUTUAL CORRECTION FOR MUTUAL INTERFERENCE OF PILESINTERFERENCE OF PILES

This correction is positive for points in the rear and subtractive for points in the forward direction of flow

For example, if we want to find the interference of pile no. 2 on pile no.1, the correction will be positive as point C is on rear side of pile 2

CORRECTION FOR MUTUAL CORRECTION FOR MUTUAL INTERFERENCE OF PILESINTERFERENCE OF PILES

CORRECTION FOR SLOPECORRECTION FOR SLOPE

The % pressure under a floor sloping down is greater than that under a horizontal floor

The % pressure under a floor sloping up is less than that under a horizontal floor

Correction is plus for down slopes and minus for up slopesSlope (vertical/horizontal) Correction (%)

1 in 1 11.2

1 in 2 6.5

1 in 3 4.5

1 in 4 3.3

1 in 5 2.8

1 in 6 2.5

1 in 7 2.3

1 in 8 2.0

The corrections given table are to be further multiplied by the proportion of horizontal length of slope to the distance between the two pile lines in between which the sloping floor is located

The slope correction is applicable only to that key points of pile line which is fixed at the beginning or end of the slope

CORRECTION FOR SLOPECORRECTION FOR SLOPE

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