WEIR ON PERVIOUS SOIL

OBJEVTIVE OF WEIRS IN IRRIGATION CANALS

Proper distribution of water carried by a main canal among the branch canals depending upon it.

           

Reducing the hydraulic slope (gradient) in a canal (if canal water slope is greater than the allowable water slope)

Reducing head on existing structures

CLASSIFICATION OF WEIRS ACCORDING TO GEOMETRICAL SHAPE    

 FLOOR OF HEADING UP STRUCTURES

A weir on solid rock (impervious foundation) does not need long apron (Floor), but needs sufficient width “b” to resist soil stresses.

A weir on pervious soil needs length “L” to:

a)      Cover percolation length,

b)     Resist scour from falling water

Soil K (cm / min) Type of flow

Clean gravel 5000 – 50 Turbulent

Clean sand 50 – 0.05 Turbulent or laminar

Fine sand + silt 0.05 – 0.00005 Laminar

Clay < 0.00005 Always laminar

  

SEEPAGE OR PERCOLATION BELOW WEIRS ON PREVIOUS SOILS:

 

a weir may be subject to failure from under seepage

water head will force (push) the water to percolate through the soil voids

 if water velocity at D.S. end is not safe (> v critical) then undermining occurs.