Karluk
river Weir Kodiak, AK photo by Andrea CooneyThe physics of holding a weir in place are simple, yet amazing!
The primary support structures of a fish weir, the tripods, are lined up across the river, and weighted down. In areas of swiftly moving current, more mass is necessary on the tripod to press straight down on the structure. As shown in the photograph above, rocks are piled on the center surface area of the tripods, and act as force, MG where mass is the total masses of the rocks and G is the force of gravity. This force opposes the bouyant forces of the water that would otherwise lift the porous and less massive (in comparison to water) wooden tripods. This downward force exerted on the tripods is made to be enough (by adding more mass) to counter the bouyant forces of the water in the y direction, AND to make the frictional forces between the feet of the tripods and the river bed be greater than the forces exerted on the tripod in the x direction by the downstream-rushing water of the river.
The weir panels, the metal fence-like pieces on the upstream side of the weir, are placed flush to the bottom of the river, and parallel to the fronts of the tripods. It is not necessary to bolt them down because they are held against the tripods by the force of the water pushing against them as it flows down stream. These panels do provide more surface area for the water to exert force on as it flows downstream, which increases chance of moving the entire weir as more force downstream is being directly exerted on the weir, but more mass is added to the tripods to maintain placement of them.