What do groynes do? Difference between attracting and repelling groynes, permeable and impermeable groynes.

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      Groynes & Spurs      
 
These are a hydraulic structure constructed transeverse to the direction of river flow and extend from the bank of the river to the width of river upto a certain limit.
  • These are also called spurs.
  • Groynes are suitable in guiding the river flow,promoting scour, depositing of sediment where desired and traping the sediment load to build up new river banks.
  • spurs are suitable and hence more widely used in river training structures.
Functions of groynes:

You may be wondering that what do groynes do? How they work? And many more questions might be striking your mind. Here i will give you all your answers. 
  • These are used for training a river along a desired course by attracting,deflecting or repelling the flow in the river channel.
  • Protection of river bank is done with the help of  groynes by keeping the flow of water away from the bank.
  • These are used to silt up the area in the vicinty of the spur by creating a slack flow.
  • These are used to contract the width of river to increase the depth of flow for navigation.
 Classification of groynes :

 1.Classification based on methods and materials of construction 
  • Permeable groyne
  • Impermeable groyne 
 2. Classification based on height of groynes w.r.t. Flood level
     a. Submerged groyne 
     b. Non-submerged groyne 

 3. Classification based on function groyne serves 
     a. Atrracting groyne 
     b. Deflecting groyne 
     c. Repelling groyne 
     d. Sedimenting groyne

 4. Special type of groynes 
     a. Denehey's T-headed groyne
     b. Hockey type groyne 

# Difference between Atrracting and repelling groynes:

Repelling groyne :
  • Repelling groyne points towards upstream direction at an angle of 10 to 30 degree to the line perpendicular to the bank. 
  • The head of a groyne causes the current to be deflected in a direction nearly perpendicular to itself.
  • The current coming into contact with still water are adjacent to the groyne causes vertical eddies and deep scour. 
  • A still water pocket is formed on upstream and the suspended load that is brought by the river gets deposited in this pocket. 
Attracting groynes :
  • Attracting groyne points downstream in the direction of flow. 
  • This causes scour holes to form closer to the bank. 
  • Attracting groyne bears the frontal attack of the river on its upstream face. 
  • Hence, the upstream face has to be adequately strong. 
  • Comparatively less heavy protection is required on the downstream slope.
Deflecting groynes :
  • A deflecting groyne is generally taken in a river perpendicular to the bank to only deflect the flow without repelling the flow. 
  • Deflecting groynes have a shorter length than repelling groyne. 
  • Deflecting groyne gives only local protection. 
#Difference between Permeable groyne and Impermeable groyne :

Permeable groynes :
  • They allow the flow of water through them. 
  • Erosive action is reduced in the stream due to dampening of velocity.
  • Due to dampening of velocity, the groyne causes deposition of sediment near the groyne. 
  • Cost of construction is cheap and performance is better. 
  • Flow through Permeable spurs does not change abruptly (as it does around solid spur) and hence does not cause serious eddies and scour holes.
  • These are not strong enough to resist shocks and pressures from the debris (so, they are unsuitable for upper reaches of river).

Impermeable groynes :
  • Do not permit any flow through them. 
  • They are constructed by a cover of locally available material like sand, clay and gravel a d heavily protected on sides by stone pitching. 
  • An apron must be provided at the bottom of the slope to prevent slipping of stones. 

Sedimenting groynes :
  • A Sedimenting groyne dampens the velocity of flow and thus causes deposition of sediments carried by the river without repelling or deflecting the flow. 
  • These groynes are best suited for rivers carrying considerable amount of sediment in suspension. E.g. Permeable groynes. 

Hockey groynes :
  • Hockey groynes has a curved head. 
  • It behaves more like Attracting groynes and hence are not likely to be useful for bank protection. 

Pitched Island :
  • Pitched Island is an artificially created island in the river bed. 
  • It is protected by stone pitching on all sides. 
  • Pitched Island is constructed with sand core and boulder lining. A launching apron is also provided to protect from scouring. 
  • Pitched island gradually draw the main river current towards itself, thus, relieving attack on marginal bunds, guide banks, river bends etc. 


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