Mechanics and modeling of flow, sediment transport, and morphologic change in riverine lateral separation zones

Brandy Logan, Jonathan Nelson, Rich McDonald, and

Scott WrightUSGS Golden, COUSGS Sacramento CA

Courtesy of Scott Wright

A well calibrated flow model

does not necessarily accurately

predict morphologic change

Outline

• Project background• Flow model calibration• Morphodynamic modeling• Process not accounted for

in the model

Courtesy of Scott Wright

Background: 2008 High Flow Experiment

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Background: Study area

Flow direction

rapid

eddy

Eminence Break bar

DATA COLLECTED:• Detailed bathymetry• ADCP velocity profiles• Water-surface elevations• Sediment concentration

by size class• D50 of the bed surface

pool

Flow calibration: Delft3D Model

• 2D and 3D• Steady and unsteady flow• Several turbulence models• Various sediment transport

relationships

Flow calibration: water surface

Difference between measured and modeled ws

(m)

ks=0.03 and 1.5 ~zo=0.001 and 0.05

2D3D

zo=0.001 and 0.05

RMS=0.028 m for 3D and 0.043 for 2D

Flow calibration: velocity

Modeled eddy-eye is shifted upstream

2.1925 2.193 2.1935

x 105

5.9725

5.973

5.9735

5.974

x 105

Black = Measured

Blue = 3D

Red = 2D

RMS error is 0.34 for 3D and 0.36 m/s for 2D

xxx

Flow calibration: velocityMeasured Modeled

Flow calibration: Summary

• 2D and 3D models are well calibrated based on water surface elevations and velocity

• 3D flow model is marginally better than 2D

Morphodynamic modeling

• Input– HFE Hydrograph– Suspended sediment concentration– Sediment thickness on the bed– Initial bed grain size distribution

• Output– Evolution of bed through time– Suspended sediment concentration– Composition of bed

Morphodynamic modeling: Measured data

Elevation (m)

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March 2 17:00March 5 12:00March 5 17:00March 6 10:00March 6 15:00March 8 09:00March 10 16:00

Morphodynamic modeling: 3D results

Topography at end of flood event

3D Model Measured

Morphodynamic modeling: 2D results

Topography at end of flood event

2D Model Measured

Process not in model: measured data

Courtesy of Scott Wright and Matt Kaplinski

Erosion

No change

Deposition

Difference between surveys on the rising limb of the flood

March 2 17:00

Process not in model: measured data

March 5 12:00March 5 17:00Elevation (m) Slope (degree)

Process not in model: modeled data

Elevation (m) Slope (degree)

March 10 02:00

Conclusions:

• 2D and 3D models produce well calibrated flow solutions• 2D and 3D morphodynamic predictions are different• Both over predict deposition in the eddy• Erosional processes such as slumping are not accounted

for by the models

Questions?