5-Year Flood Event in Oliverea

Posted on: November 1st, 2021 by Tim Koch

The Esopus Creek headwaters and Birch Creek both experienced a 5-year flood event as a result of heavy rains on October 25th and 26th. The McKinley Hollow bridge was briefly overtopped due to downed trees blocking the inlet. Oliverea Road (County Route 47) was closed for a number of hours due to flooding in a low lying area where the road and stream are at the same elevation. Ulster County DPW promptly responded to reopen the bridge and road.

McKinley Hollow bridge after large trees were removed from the inlet and placed on the downstream side of the bridge.
Ulster County DPW crews at work reopening a low-lying section of Olivera Road.

A 5-year flood event is the discharge that has a 20% chance of occurring in any given year, based on statistical analysis of at least 10 years of continuous flow data from a USGS stream gage. The “100-year flood” has a 1% chance of occurring in any given year.

The longer the period of record at the gage, the more accurate the probabilities will be for a flood of a given magnitude. The Ashokan Reservoir watershed is one of the most heavily monitored watersheds in New York State, both in terms of the number of gages and the length of time.

The Esopus Creek gage at Allaben has continuous flow data since 1968 (53 years) and the Birch Creek at Big Indian gage has been in operation since 1998 (23 years). The Esopus Creek at Coldbrook gage has been continuously monitoring flow since 1931 (90 years), with monthly flow data dating back to 1914, when the Olivebridge dam and the Ashokan Reservoir were still being constructed.

USGS stream gage on the Esopus Creek at Coldbook, in continuous operation for 90 years. Photo courtesy of USGS.

The Ashokan Watershed Stream Management Program is continuing to assess this flood event and assisting local highway departments with emergency stream repairs as requested. If you need advise on how to manage flooding or erosion on your property in the Ashokan Reservoir watershed, call the stream program office at (845) 688-3047.

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