Dealing with the rising water

Des Moines had time to prepare for the rising water and, when it arrived, many precautions were in place. I ran across this sandbagged culvert near the downtown area; beneath all of that was a manhole cover that evidently was expected to flood. The water would be allowed to come up through the manhole into the culvert until it reached the level of the nearby river. Once the water in the culvert was at the same level as the water in the river, the water in the culvert wouldn't rise any more and therefore, wouldn't flood this area... as long as it didn't go over the top of the culvert.