River training methods and types of spurs

Methods of river training (3 marks)

River training works guide a river and protect its banks. The main methods are:

1. Guide bunds for the approach to a bridge or barrage.
2. Spurs (groynes) projecting from the bank.
3. Marginal (flood) embankments along the banks.
4. Revetment (bank pitching) with stone, concrete or gabion and a launching apron.
5. Cut-offs across meander necks.
6. Bed sills / check dams to arrest bed degradation in steep reaches.
7. Bio-engineering and pitched islands.

Types of spurs used (5 marks)

Spurs are classified by orientation, material and shape.

By orientation to the flow

 Repelling (up)    Deflecting (90)    Attracting (down)
  \  /              |   |               /   /
   \/ <-flow        |   |              /   /
 -----------       -----------        -----------
  bank              bank                bank

• Repelling spur points upstream (angle 100 to 120° to the bank). It pushes the current strongly away from the bank; the scour at its nose is large. Used on concave bank bends.
• Deflecting (neutral) spur at right angles. It gives moderate effect and is the most common type.
• Attracting spur points downstream (60 to 70°). It draws the current toward the bank and has less scour, but protects less.

By material and permeability

• Impermeable (earth core with stone pitching, stone, gabion): deflect flow completely.
• Permeable (pile and brushwood, wire-crate or bamboo): reduce velocity and induce silting.

By shape

• Straight spur with a round or tapered head.
• T-head spur and hockey (curved) spur: the head protects against scour and gives better protection downstream.

By height

• Submerged spurs (overtopped at high flood) and non-submerged spurs (above HFL).

Spurs are laid in a series with spacing of about 2 to 2.5 times the spur length on a concave bank.