Visqueen1 is the linchpin of flood control.

When levees are threatened, the proper procedure is to line the side of the levee with Visqueen and weight it with sandbags to prevent erosion. When an isolated leak comes through the saturated ground, the area around the leak is covered with Visqueen, and the leak is surrounded by a horseshoe or cairn of sandbags, until the water runs clear (and is no longer eroding the earthworkwork). Attempting to stop the flow entirely is a recipe for destruction.

Without this handy, robust, and cheap material the economic and environmental, and human costs of flooding would be far greater.

1 I have always encountered it spelled as Visquine, which a quick web search proves to be wrong, even though it is the common spelling in government flood control literature

My information is from my flood control training as part of the California Conservation Corps in the early 90's. It is possible (but unlikely) that some of these procedures have changed.