Extreme Drag Forces and the Survival of Wind- and Water-swept Organisms

A stationary organism exposed to steady turbulent flow is subjected to a drag force that fluctuates about a mean, and when the drag on the organism is characterized, it is traditionally this mean force that is cited. Important information is lost, however, when the fluctuations in drag are ignored. This is particularly true when extreme drag forces are relevant; for instance, when predicting the survival of benthic animals on wave-swept shores and in torrential streams, or of plants in windblown terrestrial habitats. This study reports on the probability distribution of drag fluctuations for five objects: a flat plate, large and small cylinders, a sphere and a limpet shell. Distributions vary substantially among different objects exposed to the same mainstream flow; the sphere and limpet exhibit larger fluctuations than the plate and cylinders. The distribution of extremes in drag is used to predict the likelihood that an organism will be dislodged. For organisms in which the applied fluid-dynamic stress is near the mean breaking stress (e.g. some corals, trees and mussels), calculations made using the extreme drag can yield a probability of dislodgment substantially higher than that calculated using the average.