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The Effects of Spatially Complex Shoreface Roughness on Boundary Layer Turbulence and Bed Friction

The Effects of Spatially Complex Shoreface Roughness on Boundary Layer Turbulence and Bed Friction. L. Donelson Wright 1 , Arthur C. Trembanis 1 , Malcolm O. Green 2 , Terry M.Hume 2 , Carl T. Friedrichs 1. 1 Virginia Institute of Marine Science, College of William and Mary, Supported by the

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The Effects of Spatially Complex Shoreface Roughness on Boundary Layer Turbulence and Bed Friction

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  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-9987936 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 Period (s) Speed (cm/s) Uorb Uc

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

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

  8. Friction velocity via inertial dissipation method using the spectraldensity 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. rough Uc=0.4 cm/s Uorb=47 cm/s smooth Uc=7.7 cm/s Uorb=42 cm/s Spectral density (m/s)^2 -5/3 slope Frequency Hz Turbulence ( ) in the Boundary Layer

  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 Kb (m) Cd Cd Kb (m) kb Cd Cd

  12. Cd and fw Variation During Storm Event Cd fw fw Cd Cd fw fw Cd

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

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