Flood Mapping

In July 2021, a small region in Otsego County New York experienced a very damaging flash flood. It caught me by surprise, for several reasons. The damage to roads was excessive. The damage was very localized to an area of about 50-60 square km (roughly 20 square miles). The damage was comparable to the flood of record in 2011 when the remnants of Hurricane Lee drenched the area. Because this happened literally in my backyard, I was confronted by the effects on a daily basis. I wanted to know--how big was this event? Why was it so big? Is this what we should expect with climate change in a warming world?

Since July, I have taken lots of photos, completed numerous structure from motion photogrammetry surveys, wrestled with suitable filming and RTK GPS techniques, scratched my head over how to make full use of photographs to measure high water marks, and have tried to find all of my photos and total station surveys from the past for the impacted areas. I have enlisted the help of Liv Bartik and Carly Eaton, students at SUNY Oneonta, to document channel cross section areas, and slowly build toward estimates of flood discharges at peak flow conditions, as well as gain some insight into how flood waters move through the landscape.

Floods are messy unruly monsters, especially in the hilly terrain of central New York. Beavers, inveterate dam builders, are found on most streams in the area. Their best laid plans often run awry during large runoff events, and dams breach. And these dam bursts raise the flood intensity to extremes. And once flows start exceeding thresholds for motion for woody debris, boulders and banks, all sorts of things happen. Culverts plug up, form their own ponds, then burst. Natural clogs from trees and rubbish divert flows, slow the flow, spread the flow, raise the flow and then break under the stress. Roadside ditches, recently "cleaned" of grass by local highway crews to facilitate drainage, become extremely erosive, scour their beds and dump their loads into culverts never designed to carry anything larger than a than fist sized particle, clog the culvert, which diverts the flow into all sorts of new and completely unintended and unanticipated mayhem. How can communities with limited resources manage these kinds of events? How do we minimize damage from future events?

Documenting this flood has become something of an obsession. OK. It's not "something of". It is obsession. I can't seem to stop gazing into the violent waters.

Morris Brook overtopped State Highway 51 south of Morris New York on July 17, 2021.

Morris Brook plugged then completely removed the culvert and road and guardrail at Pittsley Road, July 17, 2021.

Morris Brook in Dimmock Hollow unroofed gravel bars buried in floodplain silts and then mobilized the gravel. High water marks still litter the flood path, and provide a way of reconstructing flood extents.

The color shaded relief map above was created from a pixel point cloud. Width of channel and floodplain is about 100 m. Elevation range is about 11 m. Woody debris in the main channel on left hand side is readily visible. High water marks extended almost to the right hand side of the image. Flow direction from top to bottom of the map.

In the chart above, high water marks and high resolution elevation data permit one to measure channel cross section areas. Here, the flood was about 1.7 m deep. Measuring flood channel cross sections has proven quite instructive about flood extents and velocity changes due to "friction" encountered by the flow.

Dozens of cross sections along the streams document the flood heights, and serve as comparisons to older high resolution data sets.

So far, we have learned that the flood of 2021 reached similar heights to the regional floods in 2006 and 2011, even though the rainfall on the day was about 40% of what fell during those events. The ground was more saturated in 2021 on the lead up to the 3 to 5 inches of rain that fell over a couple of hours on July 17. Complications from floodwater interactions with infrastructure and other obstructions led to flood pulses that were very erosive.