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).