Cofferdams are useful when you need move water out of a certain area in order to work on a pier or perform some of type of other hydraulic work. Cofferdams are typically made by driving steel sheetpiling into the bed to form a watertight fence; however, this application is not always suitable. A simpler method that is suitable in many instances is to use bulk bags full of sand to quickly install a watertight border. These can be used to reroute flowing water or section off an area of standing water to pump out. They are quick and easy to install and remove.
Of course, any cofferdam must be sturdy and well-braced to withstand the horizontal forces from the surrounding body of water when the water inside is pumped from the cofferdam or from the flowing water pushing against it. Bulk sandbags are strong and heavy enough to handle this force.