Answer to Question 1
ANSWER: The floodplain materials on which levees are built are often composed of old permeable sand and gravel channels surrounded by less permeable muds. The floodplain muds beyond the current channel are sediments that were left behind by the river where it spilled over a natural levee to flow on the floodplain. A river migrates across all parts of that floodplain over a period of hundreds or thousands of years. Under a mud-capped floodplain, the broad layers of sand and gravel deposited in former river channels interweave one another. These permeable layers provide avenues for transfer of high water in a river channel to lower areas behind levees on a floodplain. If a flood is prolonged, seepage beneath the levee can transmit enough water to flood surrounding areas behind the levee.
Answer to Question 2
ANSWER: Removal of sediments from a stream often has consequences far from the site of removal, where a river deposits its sediment at the coast. Normally, much of the sediment deposited in the river delta is carried up or down the coast by longshore drift. When the sediment supply is reduced because of gravel mining or an upstream dam, the longshore movement of sediment along the coast continues but is not replenished. Beaches erode and may even disappear. Waves that normally break against the beach then break against the beach-face dunes or sea cliffs, causing severe erosion.
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