FLOOD SURGE RECONSTRUCTION IN BEDROCK REACHES FOR THE JULY 17, 2021 FLASH FLOOD IN MORRIS NEW YORK

Located in central New York, the Morris Brook watershed produced a severe flood event which occurred on July 17, 2021. This study documents flood wave characteristics in bedrock reaches of Morris Brook. Bedrock reaches of the stream were the focus of this study because highwater marks were clearly visible, the channel sinuosity and slope varied between reaches, and superelevation of floodwaters on opposing banks could be easily documented. The bedrock reaches are all within 400 m of each other without significant tributary inputs between them, so peak discharges must have been comparable between reaches. RTK GPS and structure from motion photogrammetry were used to collect highwater marks and generate high resolution x-y-z-color point clouds. Agisoft Metashape software provided a robust environment in which to generate point clouds, scale and align the point clouds, generate digital elevation maps, and locate highwater marks. We gathered 5-10 channel cross sections for each reach. Reaches were typically straight channels with 1.2-1.7 % channel gradient. However one reach wandered through a bedrock gorge where channel gradients ranged from 1.7% to 11%. Flood channel cross section comparisons reveal large differences in cross section area related to variable flow resistance that can partly be attributed to channel sinuosity and steepness, but also to dynamic complications from transport of large woody debris, bedload and other material caught up in the flow. Our work demonstrates the utility of using pole mounted cameras as a survey tool and documents survey methods and workflow that allows for high resolution highwater mark documentation and channel characterization.