Among the following river training works, which are aligned either perpendicular or at an angle to the banks, identify the one having a difference in alignment when compared to others.

Concept:

Groynes or Spurs:

• A groyne is a rigid hydraulic structure built either from the shore (in case of seas) or bank (in case of rivers) in order to dissipate the wave energy or to protect the banks from erosion by trapping the sediments.
• They are perpendicular or slightly oblique to the shoreline.

Guide banks:

• Guide banks are meant for guiding and confining the flow in a reasonable waterway at the site of the structure.
• The design of the guide banks is based on the theory developed by Mr. Bells. Hence, guide banks are also known as Bell’s bunds.
• The guide banks are constructed parallel or approximately parallel to the direction of flow. They extend both upstream and downstream of the abutments of the hydraulic structure.
• The guide banks may be provided on either side of the hydraulic structure or on one side as required.

Levees:

• It is a low ridge or earthen embankment built along the edges of a stream or river channel to prevent flooding of the adjacent land. 
• Levees are usually embankments of dirt built wide enough so that they will not collapse or be eroded when saturated with moisture from rivers running at unusually high levels.

Flood walls:

• When no space is available for the construction of a levee or when it is not suitable to construct the levee due to local site conditions, then a flood wall is constructed as local flood hazard prevention for some important area situated at a low level.
• There are masonry or concrete walls constructed just on the river bank.