Development Causing A Rise
Floodplains are natural low lying areas along streams which have some important natural functions. One of the major functions of the floodplain is to convey moving flood water during large storm events when the stream spills over its banks. When development in the floodplain disrupts this flowing water, it causes a rise in flood heights, similar to what happens when a dam is placed across a creek.
Development NOT Causing A Rise
Flood standards for New Growth Areas require No Net Rise, which means that development must be designed so that it doesn't increase flood heights. A similar standard has been historically required in a narrower band called a Floodway which is designated along certain stream channels. The flood standards for New Growth Areas extend this standard to the entire floodplain to make sure that development does not increase flood heights.
For reasons of flexibility, the standard outside of the Floodway allows development to cause flood heights to increase by up to five hundredths of a foot (0.05'). However, this is considered to be negligible in terms of its overall impact. Also, limited exceptions are allowed, particularly for bridges or culverts where meeting this standard may not be feasible, as long as certain conditions are met to minimize and mitigate adverse impacts.
For more information
Drainage Criteria Manual: 10.2 No Net Rise and Compensatory Storage (373 K)