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.
The upstream groin of a system should be inclined at a small angle with the bank and extend to the edge of the proposed channel line. The remaining groins in the system are generally placed normal to the bank. The outer ends of the groins should all coincide with the edge of the desired channel. Spacing usually varies between three (3) and five (5) times the length of the groin, depending on the curvature of the bank. It is minimum on a concave curve and maximum for a convex one.
One method for locating groins is illustrated in Fig. 25.

Gabions are well suited for building groins because they can be placed in a flexible apron laid directly on the river bed.

The outer end of a groin must be at the level of low water and should be shaped as shown (Fig. 27). The bank end should be one (1) foot above high water level and well keyed into the bank.

The flexible apron is indispensable in a great majority of cases because of the heavy flow that generally scours the outer end of the groin. The extension of the apron is determined in relation to the estimated scour depth. See Gabion Aprons for details. As a general rule, the upstream part of the apron should be larger than the downstream part.