Order this information in Print

Order this information on CD-ROM

Download in PDF Format

     

Click here to make tpub.com your Home Page

Page Title: COMBINED CURRENT/WAVE SHEAR STRESS
Back | Up | Next

Click here for a printable version

Google


Web
www.tpub.com

Home


   
Information Categories
.... Administration
Advancement
Aerographer
Automotive
Aviation
Combat
Construction
Diving
Draftsman
Engineering
Electronics
Food and Cooking
Math
Medical
Music
Nuclear Fundamentals
Photography
Religion
USMC
   
Products
  Educational CD-ROM's
Printed Manuals
Downloadable Books
   

 

Technical Note DOER-N1
April 1998
COMBINED CURRENT/WAVE SHEAR STRESS
The method incorporated into LTFATE to estimate bottom stresses due to combined current and
wave actionbm in Equation 1) is described in this section. Large water bodies such as lakes, oceans,
and estuaries are subject to both current velocities and wave-generated orbital velocities. As a result,
the bottom shear stress will depend on the relative magnitude of both velocities and the angle
between them. In general, the bottom shear stress is assumed to follow a quadratic law:
ρ f  c u2
τcb =
2
(2)
where τcb is the near-bottom shear stress due to currents (lbf/ft2), ρ is the density of water, fc is the
current-related friction factor, and u is the current velocity outside the boundary layer. For pure
currents, fc ranges from 0.002-0.005, depending upon the bed roughness. If waves are present, a
similar equation for the wave-related shear stress τwb is assumed:
ρ fw U  2
=
τwb
2
(3)
where U is the near-bottom orbital velocity for the wave and fw is the wave-related friction factor.
The coefficient fw ranges from 0.002-0.05, depending on U and the wave period Ts.
Often, for simplicity, these two effects are simply added together. As will be described later, this
will produce a shear stress that is too low, often by a factor of two or more.
The physical problem is described by Grant and Madsen (1979). Summarizing the main points of
the wave-current interaction, the near-bottom flow is influenced by the relatively low-frequency
currents and high-frequency surface waves. The bottom boundary layer is assumed to consist of an
oscillatory wave boundary layer nested within a relatively steady current boundary layer. When the
maximum bottom orbital velocities of the waves are of the same order as the steady current
velocities, the small scale of the wave boundary layer causes the boundary shear stress that would
be associated with the wave to be much greater than that associated with the current alone. The
wave and current interact to generate a shear stress that is different from that generated by the sum
of the two components. This interaction is a nonlinear process and numerous assumptions are
necessary to derive it.
The process discussed here relies upon the same basic physical description as that provided by
Grant and Madsen (1979) but is somewhat simpler in its description of bottom shear stress. Bottom
shear stresses are predicted using a method developed by Christoffersen and Jonsson (1985),
referred to hereafter as CJ. Shear stresses predicted by the CJ model compare well with experimental
data.
4

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing

Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business