Make sure you pick a day when temperatures have already reached freezing. Don't put it out there in sub freezing or it will freeze to slowly. Also you need the dented area to be submerged at least a few inches on both sides and not half or 3/4's filled with water like a canal. This is so that the volume of water as it expands 9% can generate enough pressure. I would be aggressive, pull the header off like its normally done and fill that baby, then stick it out there in the middle of freezing temperatures.