River channel morphology; meandering channel

River channel morphology is the study of the shape, pattern and cross-section of river channels, including width, depth, slope, plan form (straight, meandering, braided) and bed material, and how they change with time.

Engineering significance of the meandering channel

Meanders grow with time: erosion on the outer bank and deposition on the inner bank move the channel sideways and downstream.

        ___
   ___ /   \   outer bank: erosion (scour)
  /   \     \  inner bank: deposition (point bar)
 /     \___/

1. Bridges: the channel can shift away from the span or attack the abutments. Bridges should be built at straight reaches, and long approach embankments or river training works are needed.
2. Intake and headworks: place the intake on the outer (concave) bank, where the water is deep and carries less sediment. The convex bank silts up.
3. Bank protection: outer banks need revetments, spurs, gabions and boulder pitching.
4. Embankments and settlements: land and buildings on the outer bank are lost to erosion, so set-back distances are needed.
5. Flood level: meanders reduce the slope and the carrying capacity, so floods rise and spread.
6. Cut-off and straightening: natural cut-offs change the river's length, level and slope. Engineers sometimes make cut-offs artificially to lower flood levels, but this steepens the channel and causes upstream degradation.
7. Aggregate and groundwater: point bars supply sand and gravel; old channels and ox-bows leave saturated, soft soils.
8. Navigation and water supply are also affected.