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Ocean Engineering

Seabed Shear Stress Spectrum for Very Rough Beds

[+] Author and Article Information
Dag Myrhaug

Department of Marine Technology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norwaydag.myrhaug@ntnu.no

Lars Erik Holmedal

Department of Marine Technology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway

J. Offshore Mech. Arct. Eng 132(3), 031103 (Jun 17, 2010) (5 pages) doi:10.1115/1.4000501 History: Received June 23, 2008; Revised September 22, 2009; Published June 17, 2010; Online June 17, 2010

The spectrum of seabed shear stresses at beds with large roughness beneath ocean surface waves in finite water depth is derived by using the wave friction factor for large roughness. It is expressed in terms of the transfer function between the waves at the surface and the shear stress at the bed, and the wave spectrum in finite water depth, which is obtained by multiplying the deep water wave spectrum with a depth correction factor. The seabed shear stress spectrum for laminar flow, which is of practical interest for flow over muds, is also given in order to compare with the rough turbulent case. Examples are included to illustrate the applicability of the seabed shear stress spectrum for practical purposes using data typical for field conditions, both for very rough beds and mud beds.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

Example of results: deep water wave spectrum Sζζ(ω) (m2 s); shallow water wave spectrum Sζζ(ω,h=5 m) (m2 s); shallow water bed shear stress spectrum S(τ/ρ)(τ/ρ)(ω,h=5 m) (m4/s3) for very rough beds (c=9,18) and for laminar flow (mud beds); shallow water near-bed orbital velocity spectrum Suu(ω,h=5 m) (m2/s)

Grahic Jump Location
Figure 2

Sequences of time series of: near-bed orbital velocity u(t) (upper figure); bed shear stresses for very rough beds (c=9,18) (middle figure); bed shear stresses for laminar flow (mud beds, lower figure)

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