The Origin of Spits, Bars, and Related Structures

Building and lengthening of the superaqueous portion of a spit or bar is caused by the transportation of material from the shore toward the distal end of the structure by the swash and backwash when the waves break obliquely on the shore. If the water surface has a constant elevation, the submerged part of a spit or bar is not brought above the surface by waves moving in a direction perpendicular to its axis. Such wave action results only in widening the submerged ridge. Hooks are caused by the work of refracted waves which cause transportation of material across the end of the spit. A change of wind direction or the presence of tidal or hydraulic currents is not necessary to their building, although if such currents are present they may aid the process. Looped bars are built by refracted waves in the same way as are hooks. However, in the looped bar the process operates more uniformly and continues until the bar rejoins the shore. The windward end of a looped bar originates in shallow water on a gently sloping bottom. The location of the point at which the return to shore begins depends on the depth of water, the amount of sediment being carried by beach drifting, and the energy of the waves. Extensive observations indicate that the lengthening of spits and bars takes place only when the waves move in a direction such that they cause the swash and backwash to move material along the shore toward the end of the spit. Waves from any other direction cause it to be widened and shortened.