how can building up natural levees with stone and concrete often make flooding worse downstream?

Answer 1

Natural levees permit a river to overflow into adjacent flat lands that are at a lower elevation than the levee. Flooding may be seasonal and may vary from year to year. For example a 100 year storm may produce the maximum flood expected in a 100 year period, based on previous experience. Building up the natural levee with stone and concrete prevents flooding of adjacent terrain that would remove large quantities of water from the river. More water flowing downstream may cause more flooding there. Overflowing a natural levee may also remove some sand and silt from the river system that may otherwise settle downstream, reducing the river's capacity to flow without a flood.

Answer 2

Natural Levee

Answer 3

By channelling the water downstream. If it is allowed to flood up stream the water can distribute over the whole length of the river, but if build levees and dikes the water will rise till it breaks a bank.

Answer 4

The concrete prevents water from soaking into the ground and can speed up the flow of water since there is less turbulance. The excess speed lessens the time the water has to soak into the ground, and the concrete prevents the water from soaking into the ground. Also, the levees don't allow a high water level river from releasing water energy higher up on the river, leaving the lower reaches to accept the water energy.

Answer 5

Well, if the levee breaks, then there will be a massive rush of water that will sweep through downstram and wipe everything out. Also, dams and levees often get silt build up at the base of them that could eventually get to the point where it overflows (or the levee breaks) and it oozes down over everything.