Fluvial Geomorphology – Secondary Currents in Channels In river engineering and physical geography, what is the technical term for the spiral or corkscrew-like secondary current in a stream that is induced by channel shape, curvature, and banks?

Introduction / Context:
Rivers rarely move as simple straight, uniform currents. When channels curve or banks and bedforms impose constraints, the flow develops secondary circulations that wrap like a corkscrew. Understanding this phenomenon is essential for explaining meander migration, bank erosion, point-bar deposition, and river training works.

Given Data / Assumptions:

• The question refers to a spiral, corkscrew-like motion in streams.
• The mechanism is driven by channel shape and curvature, not by a purely straight reach.
• We need the accepted geomorphological/ hydraulics term for this secondary current.

Concept / Approach:
As water negotiates a bend, centrifugal acceleration pushes high-velocity surface water outward, while bed friction slows near-bed water. The imbalance creates a cross-stream pressure gradient, setting up a secondary circulation: water moves outward near the surface and returns inward near the bed. This three-dimensional circulation is known as helical flow (also called secondary flow). It is distinct from laminar or fully turbulent descriptions, which classify flow regimes rather than this specific spiral pattern.

Step-by-Step Solution:

Verification / Alternative check:
Laboratory flumes and field dye-tracing show inward near-bed motion and outward surface motion in bends, confirming the corkscrew circulation characteristic of helical flow. This pattern explains point-bar build-up on inner banks and erosion on outer banks.

Why Other Options Are Wrong:

• Stream flow: A generic phrase; does not specify spiral secondary currents.
• Laminar flow: Implies orderly layers at low Reynolds number; rivers are usually not laminar.
• Turbulent flow: Describes eddy-rich regimes but not the organized bend-induced spiral.

Common Pitfalls:
Confusing turbulence with secondary circulation. Turbulence is random; helical flow is a systematic, curvature-driven pattern superimposed on the main current.

Final Answer:
helical flow