arcgis desktop - Generating floodplain generation given river section polylines, flood level elevation points and DEM of land surface?

I am looking to generate flood plain polygons to represent 20 and 100 year flood zones, in the most accurate and efficient way possible, given the following datasets:

1. River cross section polylines


2. River flow polylines (center lines between opposing shorelines)


3. 20 year and 100 year elevation values for the vertices of each cross section


4. A digital elevation model (raster with an elevation value for each pixel) of the land surface

Here is my proposed methodology so far:

1. Form polygons delimiting each section, the river center line and inland areas well beyond the flood plain


2. For each polygon formed in the previous step, extract the intersecting raster surface (Extract by Mask)


3. Isolate the pixels of the DEM that are less than or equal to the flood elevation (20 and 100 year) value for the section using the Reclassify tool


4. Perform a raster (reclassified raster from previous step) to vector (polygons) to form floodplains between each section.


5. Merge the polygons together to form a continuous floodplain

I am looking for insight and suggestions to produce the most accurate results possible given the available data.

I have access to ArcGIS and the Spatial Analyst and 3D Analyst extensions.