These structures are used in river training as contraction works to establish normal channel width; to direct the axis of flow; to promote scour and sediment deposition where required; and to trap bed load to build up new banks. Although less effective than training walls in rivers carrying small bed loads and in channels having steep gradients and swift currents, they are often more economical than longitudinal works since fewer gabions are required to protect the bank.

Design Considerations
The engineer must rely chiefly on experience and know-how in using these structures. Theory is of little help. There are no general rules for determining spacing, length or the angle they should form with the bank or current.
Experiments with models may be helpful if used wisely, but badly located groins may cause considerable damage at another point on the river.
Groins must not cause an abrupt change in the direction of the current, but rather, train it gently into the desired course. Control should begin at or before the point at which the current begins to deviate from the desired course (Fig. 24).

If the first groin is located downstream of this point, the current will likely thread its way around the wrong end of it.
Isolated structures should be avoided as they may cause dangerous eddies that will aggravate the condition. A group of three groins is generally the smallest effective system.