WEIR

What is a Weir?

WEIR

WEIR TYPES

Types of Weir

Gravity Weir

Vertical

Drop weir

Sloping

Weir

Masonry or Concrete Sloping Weir

Dry Stone Slope Weir

Parabolic Weir

Non Gravity Weir

Gravity weirs

The uplift pressure - seepage of water below the floor - resisted by the weight of the floor.

1.Verical Drop Weir• Vertical drop wall or crest wall - with or without

crest gates.• Cut off piles are provided at the U/S and D/S

ends.• To prevent scouring - aprons - at the U/S and D/S• Inverted filter - D/S end - relieve the uplift

pressure.• Are suitable for any type of foundation

Vertical Drop weir

Drop Weir

Masonry or concrete sloping weir• Recent origin.• Suitable for sandy foundations.• They are used when the difference in the crest

level and the D/S floor level is less than 3 meters.

• The main advantage - hydraulic jump is formed on the d/s glacis - dissipation of energy.

• Sloping weir - suitable for large drops.

Concrete sloping weir

Dry stone slope weirs• In addition to the main weir wall - number of

core walls. • The space between the core walls - fragments of

rock (called rockfill). • Requires a lot of rock fragments – economical -

rockfill is easily available• Suitable for fine sand foundation. • Obsolete these days.

Stone Weir

Parabolic weir

• Similar to spillway section of a weir.

• Body wall – similar to low dam

• Cistern is provided at the D/S to dissipate energy

• The U/S and D/S - similar to vertical dropweir

NON GRAVITY WEIR

• The floor thickness is kept less• Uplift pressure – resisted by bending action of

reinforced concrete floor.

CAUSES OF FAILURE OF WEIR

CAUSES OF FAILURE OF WEIR

PIPINGRUPTURE OF FLOOR DUE TO UPLIFT

RUPTURE OF FLOOR DUE TO

SUCTION OF STANDING WAVE

SCOUR ON U/S AND D/S

SIDE

PIPING• Occurs in permeable soils• Caused by groundwater seeping out of the bank face.• Grains – detached - transported away if there is sufficient

volume of flow. • The exit gradient of water - @ D/S base of the weir -

exceed a certain critical value of soil.• Result - the surface soil starts boiling & washed – by

percolating water. • The progressive backwash at the upstream results in the

formation of channel (pipe) underneath the floor of weir.

• Difference in head between U/S and D/S – constant flow of water

• Erosion continues – pipe formed – failure of D/S end

Remedies:• Decrease Hydraulic gradient - increase path of

percolation - sufficient length of impervious floor

• Providing curtains or piles at both upstream and downstream

PIPING OF SOIL

RUPTURE OF FLOOR DUE TO UPLIFT• Weight of floor < uplift pressure – floor bursts• Bursting – reduces – effective length of

impervious floor – failure of the floor

Remedies:• Impervious floor – sufficient length• Impervious floor – sufficient thickness• Providing pile @ U/S side

RUPTURE OF FLOOR DUE TO SUCTION OF STANDING WAVE

Standing wave – U/S of weir – suction

Floor thickness – insufficient – failure by rupture

Remedies:• Providing additional thickness of the floor• Constructing wall with concrete instead of

masonry

SCOUR ON U/S AND D/S OF THE WEIR• Natural waterway – contracted – water scours

– bed – U/S and D/S ends.• Scour holes – failure of the structure

Remedies:• Piles at greater depth than scour level • Launching aprons

BLIGH’S CREEP THEORY• The length of the seepage path transversed by

the water is known as the length of creep (percolation length). 

• Bligh - the dissipation of head per unit length of creep - constant throughout the seepage path.

• Percolation length is the path length from (a) to (b)

L` = 2 t + L

L` = L + 2 t + 2 S1 + 2 S2

LANE’S WEIGHTED CREEP THEORY

• Lane suggested that a weight of one third should be given to vertical creep and a weight of one to horizontal creep.

• Lw = Cw H

• Lane percolation length      L` = 1/3 L (horizontal) + L (vertical)

L` = 1/3 L + t + t + 2 S1 + 2 S2