A COMPARISON OF THE JULY 17, 2021 FLASH FLOOD IN MORRIS NEW YORK TO PAST FLOODS OF RECORD IN ALLUVIAL REACHES OF MORRIS BROOK

Morris Brook in Otsego County New York suffered a damaging flash flood on July 17, 2021. Excessive damage and reports from local residents suggested that this event was similar to two other large floods, in 2006 and 2011. This study documents 2021 flood heights in alluvial sections of Morris Brook using structure from motion photogrammetry to create x-y-z-color point clouds and RTK GPS surveys to locate highwater marks and channel bed and banks. High resolution elevation data was read into GIS software, and channel cross sections were extracted. Highwater marks allowed for an approximation of flood height over the channel and an estimation of flood cross section area. We take advantage of airborne lidar elevation data collected by FEMA and NY State in 2007, total station surveys in 2008, and NY State aerial imagery in 2005, 2010, and 2014 to show flood effects from the 2006 and 2011 events.

We find that 2021 is comparable in its flood height and effects to earlier but larger regional flood events. We think peak flood height was strongly influenced by obstructions at roadways and within the channel from mobilized woody debris and boulder size bedload, as well as by breaching of a beaver dam upstream. As obstructions failed or were bypassed, pulses of floodwater waves traveled down Morris Brook, giving the appearance of a much larger flood.