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Why A Low Level Outlet Protocol Is Necessary
Monday, Jan. 23, 2010 held promise for being a day to remember like Jan. 19, 1996. There existed a snow pack in the Catskills with a water content of 2.2” and a storm was headed our way that threatened to bring copious rainfall accompanied by warm temperatures. For those who remember the flood of Jan. 19, 1996 ranking #1 on the list of recorded floods on the Schoharie, the similarities between a past nightmare and what threatened to unfold on Monday the 25th, 2010, were both obvious and frightening.
What happened this time to lessen the impact of heavy rainfall and extensive snowmelt on water levels in the Schoharie? The answer can be found in several factors that affect stream flow. 1) Time or duration of snowmelt. Prior to the 1996 flood there was a 5 day period of unusually warm weather. 2) Water content of snowpack and amount of accompanying rain. Total rainfall in the 1996 event was 3.3”. This fell in the 314 sq. mi. catchment of the Schoharie Reservoir when it was already full and spilling over the 1130' high Gilboa Dam spillway. By contrast, the “warm up” prior to the Jan. 25, 2010 event was of a much shorter duration, less than 24 hours as opposed to 5 days in 1996 and (it is a little word of great importance) the Schoharie Reservoir was drawn down 10' below the crest of the spillway at the onset of the storm at el. 1119.46' as opposed to full pool el. 1130' at the beginning of the Jan. 19, 1996 storm. This Jan. 25, 2010 starting elevation of 1119.46' provided a “void” or storage space for the warm weather induced snow melt run off prior to its spillage through the 220' x 5.5' deep “notch” in the Gilboa Dam spillway. Once the reservoir water reached notch level el. 1124.5' it took about 6 hours to reach the top of the 1326' long spillway.
Preview a copy of the Hydrograph showing spillage at the Gilboa Dam 1/25/10. It is very instructive to see how quickly a 10' void on the 1.8 sq. mil Schoharie Reservoir fills in time of heavy rain accompanied by snow melt. During the 6 hour period the water level rose from 1124.5' to 1130', and up to 8,140 cfs of water passed through the notch before it was topped around 7 pm. While the notch spillage was “buying time” or attenuating spillage, the storm was passing out of the Schoharie Reservoir Watershed. While this attenuation of “filling and spilling” was happening at the Gilboa Dam courtesy of the siphons and notch, some very interesting things were occurring in the Schoharie Creek upstream of the Schoharie Reservoir. 5.2” of rain fell at Tannersville, NY in the Catskill Mountains, near the headwaters of the Schoharie Creek causing the creek to run at 18,100 cfs at the Lexington gauge station #01349705 at 4:00 pm on 1/25/10. (Interestingly, this is a USGS gauge station that NYCDEP wanted to stop funding last spring. Parenthetically, did anyone notice the absence of the gauges that NYCDEP did cease to fund at Toad Hollow and the Batavia Kill? These gauges are vital prediction tools that certainly show their value in time of high water.) By 5:15 pm stream flow at the Prattsville gauge #01350000 was at 28,700 cfs. This is important because the total drainage above that gauge is 237 sq. mi. while the catchment or drainage of the Gilboa Dam is 314 sq. mi. Included in the 314 sq. mi. catchment are Toad Hollow, no longer measured, and the Bear Kill gauge, #1350035, on the west side of the reservoir and the Mannor Kill, gauge #01350080, on the eastern side. These were adding an additional amount of water but, as these gauges were ice bound, no actual flood figures were available. However, it is safe to say their stream flow was significant. The input into the Schoharie Reservoir on 1/25/10 was greater than its spillage 9/18/2004, when 27,659 cfs was recorded at the Gilboa Dam. The flood of 9/18/04 is ranked #8 by NYCDEP on its list of the top 10 floods. The amount of water measured at Prattsville on 1/25/10 exceeded the spillage at the Gilboa Dam on 9/18/04 by over 1000 cfs; not counting the input of the Manor Kill or the Bear Kill!
In spite of this great quantity of water going into the Schoharie Reservoir, the maximum spillage elevation over the dam on Tuesday 1/26/10 was el. 1131.55' with a maximum spillage of less than 16,000 cfs. Such is the function of a reservoir void and the Notch. With this in mind it is important that a protocol for the operation of the low level outlet works, which will replace the siphons when the Gilboa Dam is rebuilt, be in place so that similar voids can be created in the Schoharie Reservoir ahead of future flood events that are certain to occur.