Point Bar - Formation

Formation

Any fluid, including water in a stream, can only flow around a bend in vortex flow. In vortex flow the speed of the fluid is fastest where the radius is smallest, and slowest where the radius is greatest. (Tropical cyclones, tornadoes, and the spinning motion of water as it escapes down a drain are all visible examples of vortex flow.) In the case of water flowing around a bend in a stream the secondary flow in the boundary layer along the floor of the stream does not flow parallel to the banks of the stream but flows partly across the floor of the stream toward the inside of the stream (where the radius of curvature is smallest). This movement of the boundary layer is capable of sweeping and rolling loose particles including sand, gravel, small stones and other submerged objects along the floor of the stream toward the point bar.

This can be demonstrated at home. Partly fill a circular bowl or cup with water and sprinkle a little sand, rice or sugar into the water. Set the water in circular motion with a hand or spoon. The secondary flow will quickly sweep the solid particles into a neat pile in the center of the bowl or cup. The primary flow (the vortex) might be expected to sweep the solid particles to the perimeter of the bowl or cup, but instead the secondary flow along the floor of the bowl or cup sweeps the particles toward the center.

Where a stream is following a straight course the slower boundary layer along the floor of the stream is also following the same straight course. It sweeps and rolls sand, gravel and polished stones downstream, along the floor of the stream. However, as the stream enters a bend and vortex flow commences as the primary flow, a secondary flow also commences and flows partly across the floor of the stream toward the convex bank (the bank with the smaller radius). Sand, gravel and polished stones that have travelled with the stream for a great distance where the stream was following a straight course may finally come to rest in the point bar of the first stream bend.

Due to the circular path of a stream around a bend the surface of the water is slightly higher near the concave bank (the bank with the larger radius) than near the convex bank. This slight slope on the water surface of the stream causes a slightly greater water pressure on the floor of the stream near the concave bank than near the convex bank. This pressure gradient drives the slower boundary layer across the floor of the stream toward the convex bank. The pressure gradient is capable of driving the boundary layer up the shallow sloping floor of the point bar, causing sand, gravel and polished stones to be swept and rolled up-hill.

The concave bank is often a cut bank and an area of erosion. The eroded material is swept and rolled across the floor of the stream by the secondary flow and may be deposited on the point bar only a small distance downstream from its original location in the concave bank.

The point bar typically has a gently sloping floor with shallow water. The shallow water is mostly the accumulated boundary layer and does not have a fast speed. However, in the deepest parts of the stream where the stream is flowing freely, vortex flow prevails and the stream is flowing fastest where the radius of the bend is smallest, and slowest where the radius is greatest. The shallows around the point bar can become treacherous when the stream is rising. As the water depth increases over the shallows of the point bar, the vortex flow can extend closer toward the convex bank and the water speed at any point can increase dramatically in response to only a small increase in water depth.