Groynes and their types

A groyne (or spur) is a structure built out from the river bank, roughly at right angles or inclined to the current, to deflect the flow away from the bank, reduce velocity near the bank and induce silting. The root is anchored in the bank and the head projects into the stream.

Types of groynes

1. According to the effect on the current (inclination)

• Repelling groyne: inclined upstream (100 to 120° with the bank). It pushes the current toward the middle, but the scour at the head is heavy.
• Attracting groyne: inclined downstream (60 to 70°). It attracts the current, and has little scour.
• Deflecting or neutral groyne: at right angles to the bank. The commonest type.

 Repelling         Deflecting        Attracting
  \                 |                  /
   \ flow->          |                 /
 ---------        ---------        ---------

2. According to permeability

• Impermeable groynes are of earth, stone, concrete or gabion. They block the flow.
• Permeable groynes are of piles, bamboo or brushwood, or wire crates with stone. They let a slow flow pass, causing silt deposits, and have little scour; they suit sandy, low-velocity rivers.

3. According to shape of the head

• Straight groyne, T-head groyne and hockey (curved) head groyne. A T-head gives better protection and reduces scour at the nose.

4. According to the height

• Submerged groynes are overtopped in floods.
• Non-submerged groynes stand above HFL.

5. According to the material

• Earth with stone pitching, boulder or stone masonry, concrete block, gabion and bio-engineering types.

Design features

• Length limited to about 20 % of river width; spacing about 2 to 2.5 times the length on concave banks.
• Top width 3 to 6 m and top level at HFL plus a margin.
• Heads protected with pitching and a launching apron.