JUNE 19, 2007 FLASH FLOOD: EXTREME RAINFALL EVENT IN SMALL HEADWATER BASINS, CATSKILL MOUNTAINS, NEW YORK

A meteorological and hydrological analysis of an extreme flash flood event in the western Catskill Mountains of New York is presented. The meteorological analysis indicated that the environment associated with this event contained many characteristics previously found with convective flash floods. Important elements included unseasonably high atmospheric moisture content, a strong low-level jet associated with significant moisture flux convergence over the flash flood zone, and a thermal environment favorable for high precipitation efficiency.

The hydrological analysis indicated that extreme flash flooding occurred in five small, steep, heavily forested stream basins with 3-hour rainfall that doubled the 24-hour 100 year rainfall extreme for the area. Minor flooding initiated stream rises when estimated basin average rainfall totals reached 2.00 to 2.50 inches. Moderate flooding, affecting road infrastructure began in the 4.00 to 5.00 inch range. Major flooding began when estimated rainfall totals exceeded 5.00 inches. Peak rainfall during this event eventually reached 11 inches at several points. Slope-conveyance discharge estimation along Berry Brook was in excess of a 500-year return flow, consistent with extreme rainfall rates. Brief, extremely rapid rates of rise on Spring Brook, upwards to 2.00 feet per second, was likely caused by debris pileups giving way behind highway bridges. Mainstem river and reservoir systems witnessed the result of the rapid runoff from the flash flood zone many miles downstream.