HOW BIG WAS THE JULY 17, 2021 FLASH FLOOD IN MORRIS NEW YORK?

In July 2021 a flash flood damaged infrastructure in the Towns of Morris and Butternuts in Otsego County New York (~60 square km). We compare flood reconstructions for this flood to two larger floods in 2006 and 2011, each a flood of record at the time. We document and quantify the flood size and effects with RTK GPS and structure from motion (SfM) photogrammetry. Pole mounted point and shoot cameras (Canon G9 and Yi 4K Action) yielded imagery that resulted in high resolution point clouds at sub decimeter spacing. We used Agisoft Metashape software to construct scaled and aligned-to-gravity point clouds and to identify and record highwater marks in 3D oriented photographs. CloudCompare, an open source software for analyzing point cloud data, provided useful tools to extract several channel cross sections per surveyed reach and to measure channel slope, relief and surface roughness. We find substantial similarity between flood channel cross section areas for 2006, 2011, and 2021 even though the 2021 storm total was about 40% of 2006 and 2011.

Given comparable scales of flood heights based on our cross section and highwater measurements, we argue that local complicating factors made 2021 more devastating than precipitation amounts alone would predict. Cumulative rainfall leading into the peak flood event was higher in 2021, so ground was saturated. At peak flow gravel bars buried by floodplain silt were unroofed and mobilized. Channels widened, and small upland first order tributaries show clear evidence for incision. Mobilized woody debris and boulder size bedload plugged culverts and diverted flows which bypassed or overwhelmed obstructions and resulted in multiple flood waves pulsing down channels. In addition to infrastructural obstructions, some beaver dams failed. These complications made the flooding in 2021 appear comparable to floods of record.