Usually what happens is the frazil ice accumulates on the upstream side of objects and sticks to them. The frazil ice accumulates as more gets deposited. The growth will extend upstream and increase in width until the point where the frazil ice accumulations bridge together and block the water. As more and more water flows against this block, the pressure on the upstream side increases and causes a differential pressure (difference in pressure from the upstream side and the downstream side). This will cause the growth of the bridge to extend downstream. Once this happens, flooding and damage is likely unless otherwise prevented.