The easiest way to estimate peak flow rates is using the Regression Equations developed by the USGS for each state. Here is a link to the latest version of the National Flood Frequency software that contains all of the regression equations for the U.S.
http://water.usgs.gov/software/nff.html

Another source of the regression equation data is the West Virginia DOT drainage manual/criteria.

A quick program for estimating flood elevations after determining the flow using the regression equation is the Quick2 program from FEMA. Here is a link to where you can get the quick2 program.
http://www.fema.gov/mit/tsd/frm_soft.htm

John G. Morgan
john.morgan@cte-eng.com