Bedforms

Question 1

What are bedforms?

Answer 1

Morphological features produced by the interaction of a fluid flow and sediment on the bed.

Question 2

What are the two types of bedform flows?

Answer 2

Unidirectional flows (air, water) and wave generated (oscillatory) flows (water).

Question 3

What are the three bedform scales?

Answer 3

Microforms (e.g. ripples), mesoforms (e.g. dunes), and macroforms (e.g. draa field).

Question 4

What is the Froude number (Fr) for?

Answer 4

A dimensionless number used to distinguish between different types of turbulent flow; ratio of flow velocity (U, inertial force) to speed of surface wave (gravitational force); Fr<1 subcritical, Fr=1 critical, Fr>1 supercritical.

Question 5

What is a hydraulic jump?

Answer 5

Standing wave created due to deceleration of supercritical flow; e.g. water flow over a weir dam - standing wave forms downstream marking critical flow boundary.

Question 6

What is the flow regime bedform hierarchy?

Answer 6

Lower flow regime: plane, ripples, sand waves, dunes.
Upper flow regime: plane (critical flow), anti-dunes, chutes/pools

Question 7

What are current ripples?

Answer 7

Small-scale, asymmetric bedforms formed in cohesionless bed by unidirectional traction currents; migrate downstream; form in coarse silt to coarse sand; straight to sinuous crests.

Question 8

What are the conditions for ripple formation?

Answer 8

Moderate flow velocities, hydraulically smooth bed (necessary for flow separation); form in shallow to deep water.