Question

Directions: Below question consists of two statements, one labelled as the 'Statement (I)' and the other as 'Statement (II)'. You are to examine these two statements carefully and select the answers to these items using the codes given below:

Statement (I): Spur length is kept longer than 1.5 to 2 times the depth of flow.

Statement (II): Shorter spur length in deeper rivers induces swirling motion on both the upstream and downstream sides of the spur.

1. Both Statement (I) and Statement (II) are individually true and Statement (II) is the correct explanation of Statement (I)
2. Both Statement (I) and Statement (II) are individually true, but Statement (II) is not the correct explanation of Statement (I)
3. Statement (I) is true, but Statement (II) is false
4. Statement (I) is false, but Statement (II) is true

Answer 1

Correct Answer - Option 4 : Statement (I) is false, but Statement (II) is true

Explanation:

Groynes or Spurs:

(i) Groynes or spurs are constructed transverse to the river flow extending from the bank into the river.

(ii) This form of river training works performs one or more functions such as training the river along the desired course to reduce the concentration of flow at the point of attack, creating a slack flow for silting up the area in the vicinity and protecting the bank by keeping the flow away from it. 

Length of Groynes:

(i) The length of groynes depends upon the position of the original bank line and the designed normal line of the trained river channel.

(ii) In easily erodible rivers, too long groynes are liable to damage and failure. Hence, it would be better to construct shorter ones in the beginning and extend them gradually as silting between them proceeds.

(iii) Shorter and temporary spurs constructed between long ones are helpful in inducing silt deposition. 

No general rule is formulated for the selection of spur length. It depends on the position of the existing bank line and the desired position of the bank line after the training works.

Hence Statement I is wrong, thus the correct answer is option (d).

Layout of groynes or spurs:

• Groynes are much more effective when constructed in series as they create a pool of nearly still water between them which resists the current and gradually accumulate silt forming a permanent bank line in course of time.
• The repelling spurs are constructed with an inclination upstream which varies from 10°  to 30°  to the line normal to the bank. In the T-shaped groynes, a greater length of the cross groyne projects upstream and a smaller portion downstream of the main groyne. 

Spacing of Groynes:

• Each groyne can protect only a certain length and so the primary factor governing the spacing between adjacent groynes is their lengths.
• Generally, a spacing of 2 to 2.5 times the length of groynes at convex banks and equal to the length at concave banks is adopted.
• Attempts to economise in cost by adopting wider spacings with a view to insert intermediate groynes at a later date may not give the desired results as the training of river would not be satisfactory and maintenance may pose problems and extra expenditure.
• T-shaped groynes are generally placed 800 m apart with the T-heads on a regular curved or straight line.