Speaking as a geologist I would say that you are almost completely correct.
It's not the streams necessarily are rushing "faster" the velocity of the streams increase, but they are also deeper too meaning the over all discharge of the stream/river (meters squared of water per second or m2/s) increases. This is mostly due to what we call runoff. Runoff is basically water flowing overland and into the river or stream system. There is another factor with is called through flow. This is water that percolates into the soil and then underground downgradient to the stream/river.
Now, normally there is a lag time between when there is a massive discharge of water and when a flood reaches it's highest flood stage. This is because it takes a while for water to percolate through the soil and travel downgradient. When this occurs in natural areas that have had a minimum of development floods tend to be less high and occur over a longer period (several days to weeks). If a flood occurs in a heavily urbanized area with lots of concrete and developed land you get higher flood stages in a shorter period of time (hours to days). This is because of the increase in Runoff due to most of the soil and dirt being covered by concrete. Runoff gets water to stream systems faster than through flow, because it takes time for water to soak into soil. Thus more water faster to the rivers making for higher and more destructive floods.
Another thing to think about: New Orleans has been a sinking city for sometime. Part of this is due to natural subsidance of the delta, but another factor is from over pumping of aquifers. If you pump too much water out of the aquifer, there is no water to support the overlying rocks and soil and the land sinks to fill the void.
Thus ends my brief lecture on hydrology and the nature of floods.
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timeasmymeasure
no subject
Date: 2005-08-31 06:13 pm (UTC)It's not the streams necessarily are rushing "faster" the velocity of the streams increase, but they are also deeper too meaning the over all discharge of the stream/river (meters squared of water per second or m2/s) increases. This is mostly due to what we call runoff. Runoff is basically water flowing overland and into the river or stream system. There is another factor with is called through flow. This is water that percolates into the soil and then underground downgradient to the stream/river.
Now, normally there is a lag time between when there is a massive discharge of water and when a flood reaches it's highest flood stage. This is because it takes a while for water to percolate through the soil and travel downgradient. When this occurs in natural areas that have had a minimum of development floods tend to be less high and occur over a longer period (several days to weeks). If a flood occurs in a heavily urbanized area with lots of concrete and developed land you get higher flood stages in a shorter period of time (hours to days). This is because of the increase in Runoff due to most of the soil and dirt being covered by concrete. Runoff gets water to stream systems faster than through flow, because it takes time for water to soak into soil. Thus more water faster to the rivers making for higher and more destructive floods.
Another thing to think about: New Orleans has been a sinking city for sometime. Part of this is due to natural subsidance of the delta, but another factor is from over pumping of aquifers. If you pump too much water out of the aquifer, there is no water to support the overlying rocks and soil and the land sinks to fill the void.
Thus ends my brief lecture on hydrology and the nature of floods.