Third for the brick split in half. Wrap your cord around it when the water rises it will unwind off the brick. When it drops go out and wind them all back up.
The ones on my friends are 2 railroad spikes welded together. 15 foot cords slip knotted to one side and then wrapped. Can half hitch to one leg of the spikes for transport and easy dropping then. They unwrap as water rises, and then you pick up and rollup when it drops and put em back. The ones he and I rigged have 15 foot anyhow, a couple of the other guys rigged 10 so those were strewn over 800 yards with the rising water. Took us 2 hours to chase em all down.