A thunderstorm and rainfall event with an intensity several times greater than the 100-year storm was recorded in August 2010. Residents in the Skyline Heights area recorded precipitation measurements from 1.7″-2.5″ in a 20-minute period, whereas average annual precipitation is approximately 14 inches. The resulting stormwater runoff revealed several flooding-prone areas throughout north and west Great Falls.
The City contacted TD&H within weeks of the flooding to initiate a comprehensive stormwater study for an approximately 1200-acre basin. Current conditions modeling indicated several infrastructure deficiencies existed throughout the study area, including inlet, piping, and detention capacity limitations, inadequately designed street grades in insufficient or ineffective private runoff storage. Analysis required integrating local and global flooding impacts under an endless number of improvement scenarios. Following countless modeling iterations, several independent projects were recommended including regrading street intersections, upsizing and installing parallel mains, installing additional inlets, extending new outfall piping to the Missouri River and constructing a 1.4 acre detention pond to restrict flow and settle contaminants. Seven major design and construction phases were scheduled, five of which have been completed, while the outstanding projects are currently in land acquisition negotiations.