Furthermore they tend to channel the water into storm sewer systems and concrete lined drainages, all of which increase runoff and decrease infiltration. Levee Failures Natural levees are constructed as a result of flooding, as we saw in the discussion last lecture. But, natural levees tend to be relatively low and do not offer much protection from large discharge because they can easily be overtopped. Human made levees, such as we see on the Mississippi River along much of its length, are much higher and are constructed to prevent flooding from high discharges on the River. Such levees often give a false sense of security for those living on the floodplain the levee was built to protect, because failure of such levees can lead to flooding, either because discharge can become great enough to overtop the levees or the levees can become weakened and fail.
Levees can fail for three main reasons.
If high discharge in the river leads to a river stage that is higher than any point on a levee, the water will overtop the levee and start to flow onto the floodplain. Because the initial gradient from the river to flood plain is relatively high, the velocity of the stream as it overtops the levee will be high. High velocities can result in high rates of erosion, and thus the levee that is initially overtopped will soon become eroded and a channel through the levee will soon be created. Undercutting and slumping of levee Higher discharge in the river will lead to higher velocities with the stream trying to increase its width and depth. Higher velocities can lead to higher rates of erosion along the inner parts of levees and thus lead to undercutting and slumping of the levee into the river.