1. The Effects of Spatially Complex Shoreface Roughness on Boundary Layer Turbulence and Bed Friction
L. Donelson Wright1, Arthur C. Trembanis1, Malcolm O. Green2, Terry M.Hume2,
Carl T. Friedrichs1
1Virginia Institute of Marine Science, College of William and Mary,
Supported by the
National Science Foundation
INT
2National Institute of Water and Atmospheric Research,
New Zealand
Supported by the
NZ Foundation for Research Science and Technology
FRST-CO1X0015

2. Instrumented tripods were deployed on a complex shoreface over contrasting substrates in order to examine the effects of spatially varying bed roughness on boundary layer turbulence and bed friction.
Questions Addressed
How does spatially varying roughness type affect boundary-layer turbulence and bed friction?
How do temporal changes in bedforms affect drag and turbulence?
What is the appropriate drag coefficient at the local and shoreface scale?

3. Main Points
Estimates of bed stress and drag coefficients were made via the inertial dissipation method.
A sharp contrast in Cd exists between the rough and smooth sites such that the former is ~4-6 times as hydrodynamically rough as the latter.
The spatial gradient in the drag coefficient and bed roughness was at a maximum during the storm events
The morphodynamic behavior of bed roughness is spatially and temporally complex.

4. Location/ Observations
Coromandel Peninsula- East coast of North Island
Energetic wave dominated- Hs~1.0 m ; Ts~8-10 s
Microtidal- tidal range ~1.5 m
Sharply contrasting seabed substrates
36 day tripod deployment two storm events
Tropical
Cyclone
Paula
h = 22 m
z = 0.70 mab Ts
Speed (cm/s)
Period (s)
Uorb Uc

5. Field Methods Rough Smooth 1.70 1.6 0.20 16 Fine 3.8 8.7 0.75 22
900KHz 20m range
Rough
Smooth
1.70
1.6
0.20 16
Fine
3.8
8.7
0.75 22
Coarse
cr (dyn/cm)^2
s (cm/s)
D50 (mm)
H (m)
Facies

6. Methods Inertial Dissipation Method (IDM) to estimate shear stress
Compare turbulent characteristics temporally and spatially
Estimate Cd from shear stress
Estimate kb from ripple model

7. von Karman-Prandtl equation
Height above bed (cm)
Uc (cm/s)
Smooth Site
Rough Site

8. of vertical fluctuations is
Friction velocity via inertial dissipation method using the spectral density
of vertical fluctuations is
and the drag coefficient averaging instantaneous currents is
Computations followed the methodologies
of Stapleton and Huntley, 1995 and Feddersen and Guza, 2000

9. Turbulence ( ) in the Boundary Layer
Spectral density (m/s)^2
-5/3 slope
Frequency Hz
smooth
rough
Uc=7.7 cm/s
Uorb=42 cm/s
Uc=0.4 cm/s
Uorb=47 cm/s

10. Bed Friction
Estimate Drag coefficient from IDM estimate of shear stress
Estimate bottom roughness from Nielsen Model results
Estimate wave friction factor from Swart Model

11. Cd and kb Variation During Storm Event
Kb (m) Cd Cd
Kb (m) kb

12. Cd and fw Variation During Storm Event

13. Cd and kbestimates for rough site and smooth site
2.2 cm
0.0068
Smooth Uc>0.10m/s
11 cm
0.030
Rough Uc>0.10m/s Kb
(Nielsen) Cd
(IDM)
Site
N=10

14. Conclusions
Near-bed shear stress estimated via Inertial Dissipation Method
Velocity profile structure significantly altered by presence of large ripples
Drag coefficient (Cd) highly variable in space and time
Wave friction factor model (fw) does not capture the relationship between drag coefficient and bed roughness