Research in the lab of Professor
Mark Denny at Hopkins Marine Station has addressed the relative
magnitudes of these forces. His lab has also developed a variety of methods
for measuring water speed and the forces exerted by waves in the surf
The thesis work of Brian Gaylord (Stanford, 1997) addressed the relative
importance of the acceleration reaction. He found
that the surf zone's chaotic flow patterns prohibit uni-directional acceleration
for long enough to develop significant accelerational force.
Visualizing your weed in flow helps to estimate the relative magnitude
of the remaining two forces (drag and lift).
By conforming to flow, most macroalgae extend parallel to the substratum.
The net force on such a deforming object is directed parallel to the substrate.
Since lift has no component of force in this direction, the net force is
predominantly due to pressure drag.
In summary, we find that the following formula estimates
drag on intertidal weeds:
Drag Force = 1/2 * r * CD *Aprojected *
Using values of the drag coefficient from the literature, estimated projected
areas and measured wave speeds we can estimate the forces macroalgae experience
in the intertidal. A good first estimate of water velocity is wave speed,
which we just measured with our unspeakably cool radar
gun. However, both the Denny lab and civil engineers measuring forces
on breakwaters and piers have observed forces which drastically exceed predicted
forces. This work is still in progress...
copyright Elizabeth Nelson
educational uses permitted.