Hurricane Irene: The Storm Event of August 28-29th, 2011

Rainfall
The storm event experienced in Ulster County during August 28-29 was greater than a 100 year rainfall event at some locations. Almost all of Ulster County received more than six inches of rain and some places exceeded eight inches of precipitation. The rainfall at Slide Mountain (11.53 inches) was considered at least a 200 year event while rainfall at Mohonk and in Rosendale was considered at least a 100 year event.

Stream Flooding- Accordingly, many streams also set flood records. Record flood levels were experienced across the County including along the Upper Esopus Creek, the Stony Clove Creek in Phoenicia, the Rondout Creek in Rosendale, and a near record crest at the Walkill River in Gardiner. This storm event is estimated to be an 80 year storm event on some of our larger streams and greater than a 100 year event in our mountain streams.

Lower Esopus
Although flooding was severe on the Lower Esopus Creek, flood elevations did not exceed the record flood of 2005. This is due in part to the void in the Ashokan reservoir which captured some flood water and delayed (attenuated) the flow from the Upper Esopus to the Lower Esopus. The storage and attenuation of the reservoir is evidenced by the gage data below.
 
The first peak is the rapid rise in water levels associated with the actual rainfall event. The second peak is associated with spill from the reservoir. The second flood peak was not as high as the first. This indicates that spill from the reservoir did not create the highest flood elevations. The highest flooding experienced along the Lower Esopus was from the actual rainfall event in the lower watershed and sub-watersheds (Sawkill, Plattekill), not from the spilling of the reservoir. Additional storage void in the reservoir would not have had any effect on lowering the first peak of the storm.

Irene crests at Mt. Marion:
08/28/2011 16:15 EST 25,200 cfs 25.39 ft
08/29/2011 14:45 EST 19,100 cfs 23.95 ft

Historical crests:
(1) 26.46 ft on 04/03/2005
(2) 25.10 ft on 04/26/1977

This storm demonstrated the protective ability of the reservoir to mitigate flooding on the lower Esopus. Coldbrook gauge on the upper Esopus Creek was 2.76 feet higher during Irene than in 2005 and 1.4 feet higher than its previous record (1980). Given the flood levels experienced on the Upper Esopus Creek, if the reservoir had been at capacity at the beginning of the rain fall event, we would have likely seen peak flood elevations of up to three feet higher on the Lower Esopus. Flood peaks at this level would have significantly exceeded the record flood of 2005 and caused even more damage. 

The Ashokan Waste Channel was releasing at 600 MGD during the first peak in creek level. The Waste Channel was shut off at 8/28/2011 18:35:00. As the stage of the Mt. Marion gauge rises, the contribution of 600 MGD release to stage height decreases as follows (600 MGD = 930 cfs):

@ 17 ft, 930 cfs = 0.75 ft of additional height
@ 18 ft (Action Stage), 930 cfs = 0.6 ft of additional height
@ 20 ft (Flood Stage), 930 cfs = 0.5 of additional height
@ 22 ft (Moderate Flood Stage), 930 cfs = 0.27 ft of additional height
@ 24 ft (Major Flood Stage), 930 cfs = 0.22 ft of additional height

At the crest of 25.39 ft, Ashokan Release was contributing less than 3 inches of additional height at Mt. Marion. Of the 25,200 cfs at peak flow, the release was contributing 3.69%.

The East Basin started spilling at 8/28/2011 17:55:00 at 3100.00 MGD and peaked at 8/28/2011 22:10:00 at 9540.00 MGD, 10 hours after Coldbrook gauge crest. The second crest at Mt. Marion occurred 16.5 hours after spill crest and 26.5 hours after Coldbrook gauge crest.

Coldbrook crest
08/28/2011 12:00 EST 75,800P 23.34P

Historical crests:
(1) 21.94 ft on 03/21/1980
(2) 20.70 ft on 03/30/1951
(3) 20.58 ft on 04/03/2005

Tidal Surge
Flooding along Lighthouse Drive in Saugerties in the tidal portion of the Esopus Creek was primarily due to storm surge that traveled up the Hudson River from the Atlantic Ocean. Based on water level data from observation stations along the Hudson River, storm surge from Irene coincided with high tide at New York Harbor/Battery Park shortly after 8 AM on Sunday, 8/26, and rolled up the Hudson River, reaching Poughkeepsie at about 12:45 PM, Saugerties around 2:00 PM and Albany at roughly 5:15 PM. Storm surge added nearly 4 feet to the high tide height at the Battery. At Poughkeepsie, river levels were 3 feet above normal high tide. At the Saugerties Lighthouse, the crest in water level was measured at 8.15 feet, which was about 3.5 feet above normal high tide. As a general rule, flood runoff is rarely enough to raise the mainstem Hudson River above sea level further south than Catskill. Irene was an exception to this rule but did not affect river levels near as much as surge. Runoff peaked after the surge event and contributed roughly 1 foot of height at the subsequent high tides on 8/29 at the Saugerties Lighthouse.