Monterey Bay Aquarium Research Institute
Marine Botany

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Net Force

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 zone.

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 * u2

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...

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copyright Elizabeth Nelson 1999  Non-profit educational uses permitted.    

Last updated: Feb. 05, 2009