Point bar

Key facts and history

Water flowing around a bend creates vortex flow, where speed is fastest where the radius is smallest.^[3] In the boundary layer along the stream floor, secondary flow moves water toward the inside of the bend.^[1] This secondary flow sweeps and rolls loose particles such as sand, gravel, and small stones toward the point bar.^[1] Point bar materials are not deposited from suspension but are rolled into place by the secondary flow.^[2]

The secondary flow can be intensified by reflection from an irregular, scoured opposing bank.^[2] In low-gradient meandering streams, coarse sand and gravel cannot be held in suspension because the water moves slowly and has low turbulence.^[2]

What the cited sources add

Bowker further explains that these materials are swept and rolled into place by secondary flow, not carried in suspension.^[2] The 2002 paper by Chant in Journal of Geophysical Research and the 2003 encyclopedia by Hickin describe the secondary flow in the boundary layer as moving partly across the floor toward the inside of the bend.^[1] Bowker's 1988 Earth Sciences History article explains that in low-gradient, settled parts of a watercourse, the water's slow speed and low turbulence prevent it from suspending coarse sand and gravel.^[2] Hickin's 2003 encyclopedia entry notes that any fluid, including stream water, can only navigate a bend through vortex flow.^[3]