Below are several links to interesting articles about the historic event.

Severe Flooding on the Colorado Front Range: September 2013 Report
An initial assessment report compiled by Western Water Assessment

Detailed Daily Analysis of the September 2013 Flood By NWS Boulder Office

How much rain fell on the Front Range, and how historic was it? By Bob Hensen
Historic Rainfall and Floods in Colorado at Climate.gov
Colorado Flash Flooding: How It Happened, How Unusual? By John Erdman
Front Range Flooding - CoCoRaHS Deja Vu By Steve Hilberg