River Meanders

River Meanders

Outline• Primary flow characteristics within a

meander bend• Flow and sediment transport within

meander bend• Controls on meander wavelength• Meander migration and shift

Red River, near Aubigny, southern Manitoba

Turkey River, Clayton County, IA

Pecatonica River near Mineral Point, WI

(Bridge, 2003)

Plan

Cross section

Primary Flow Characteristics• Flow-transverse component of water surface

slope towards the inner bank• Secondary flow pattern (spiral or helical flow)

due to– (a) curvature-induced centrifugal forces and– (b) pressure gradient of the transverse sloping

water surface– Effects proportional to velocity and inversely

proportional to radius of curvature • Flow field is stage-dependent

(Hey and Rainbard, 1996)

Point bar (inner bank) Outer bank

Secondary flow pattern in a meander bed

Balance between centrifugal force and pressure gradient

rmuFc

2

force lCentrifuga nhg

ru

2

(Bridge, 2003)

Observed for River South Esk

Simulated

Mean Flow Velocity

Predicted

(Bridge, 2003)

Outside Outside InsideInside

Simulated

(Bridge, 2003)

Observed for River South Esk

Simulated

Water Surface Topography

Predicted

(Bridge, 2003)

Observed for River South Esk

Simulated

Flow Depth

Predicted

(Bridge, 2003)

Observed for River South Esk

Simulated

Relative BedloadTransport Rate

Predicted

(Bridge, 2003)

Observed for River South Esk

Simulated

Bedload Grain Size

Predicted

(Knighton, 1998)

Maximum shear stress Flow field

Secondary flowFlow structure

(Julien, 2002)

Controls on Meander Wavelength

L WL ~ 12.34W

L rc

L ~ 4.59rc0.98

Combining: rc~2 to 3W

(Julien, 2002)

Controls on Meander Wavelength

L Qbb; b~0.5

(Knighton, 1998)

Meander Movement

(Knighton, 1998)

Types of Meander Growth and Shift

(Julien, 2002)

Shear Stress and Meanders

tt0 hU2/rc

tt0t0

tt max qmW-1.61

tt high when rc~2.5W

Meanders and River Restoration

• Links channel process with form• Designing or creating meandering rivers

appears to be optimal channel form used in river restoration

• Provides the boundary conditions for the hydraulic stability of a meandering planform

River Meanders

Conclusions• Spiral or helical flow in meander bend is

a balance of centrifugal and pressure gradient forces

• All flow, bed, and sediment parameters then are modified by this flow pattern

• Meander wavelength can be determined from width and discharge relations