Another major function of the floodplain is to store flood water that floods outside of the stream banks and is temporarily slowed down in the floodplain outside of the creek. One way to think about storage vs. conveyance of flood water is to relate it to traffic at rush hour. If water in a floodplain from a big storm were a bunch of cars heading in the same direction on a freeway at rush hour, the water being conveyed would be the cars still moving on the freeway in the faster inner lanes, and the water being stored would be the cars in the slower outer lanes.
Another analogy is helpful when you think about what happens when development impacts the floodplain's ability to store flood water. Imagine that the floodplain during a large storm is like a bathtub filled to the brim with water. If you put a big brick in the tub, it will displace the water, which will spill onto the floor. The same thing can happen with development in the floodplain. Fill or other structures in the floodplain displace flood water just like a brick displaces water in a tub, and the displaced water moves onto another property.
Compensatory Storage is a standard for New Growth Areas which preserves the ability of the floodplain to store water. Compensatory storage means that loss of flood storage due to buildings or fill dirt in the floodplain is compensated for by providing an equal volume of storage to replace what is lost. In other words, if you bring one bucket in you need to take one bucket out. Limited exceptions are allowed as described in the section on 'No Net Rise.'
Drainage Criteria Manual: 10.2 No Net Rise and Compensatory Storage (373 K)