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Gilboa Dam Research Proposals
To: Schoharie County Board of Supervisors
From: Dam Concerned Citizens, October 2007
Re: Gilboa Dam Research/Proposals
On Tuesday, August 14, 2007, representatives from the New York City Department of Environmental Protection conducted a state mandated, semi-annual meeting regarding water discharge from the Schoharie Reservoir via the Shandaken Tunnel, into the Esopus Creek. This meeting was held as part of New York Cities efforts to comply with the NYS Pollution Discharge Elimination System or SPDES permit, as it is usually referred to. This particular meeting was held in the town of Shandaken, Ulster, Co. The previous meeting was held in March, 2007, in Kingston, NY. At the August meeting, the following information, of great importance to Schoharie and Mohawk Valley residents, was disclosed.
Dredging and removal of sediment from the area immediately in front of the Shandaken tunnel inlet, at an area on the Schoharie Reservoir marked by the presence of the Gate House, will begin on October 15th 2007 and will continue through December 31, 2007. The accumulated sediment, actually good top soil, has partially occluded or blocked water from entering the ÒTunnelÓ and has rendered 3 of the 9, 3Õx7Õ intake gates inoperable. An area of about 150Õx300Õ will be dredged and 100,000 cubic feet or 3,700 cubic yards of soil will be removed. In order to comply with the SPDES permit and not discharge turbid or muddy water into the Esopus Creek via the Shandaken Tunnel, the tunnel will be effectively closed during the time of dredging. Also, in order to comply with minimum combined flow levels as mandated by the SPDES permit, roughly 90 cfs will be drawn by hose from a less turbid area upstream of the hate house dredge site and sent through the Shandaken Tunnel. So, from Oct. 15th, 2007 until the end of the year 2007 there will be no means of lowering or drawing down the water levels of the Schoharie Reservoir other than the four siphons, which operate best above elevation 1110. Water level, as of sept. 21, 2007 is at 1,111.
As we all know, we are now approaching a time of flooding caused by tropical storms and hurricanes that come up the eastern seaboard and drop copious rainfall in the Catskill Mts. Four of the tope 10 historic crests at the Gilboa Dam have occurred in the months during which the Shandaken Tunnel is scheduled to be closed. Two in Sept, the second largest flood of record on October 16, 1955, and one in Dec.
It was also announced at the Aug. 14, 2007 meeting that the trigger point for engaging the siphons would be reached when the reservoir level attained 1,130 elevation, or spill crest level. By the time that level is reached, over 8000 cfs will be spilling through the ÒNotchÓ. That point of siphon engagement is too LATE! It would be more prudent to Òturn on the siphonsÓ when water levels approach 1,124.5Õ or notch level. Thus, there is a better chance of reducing high water levels down stream by attenuating or drawing out the water discharge time through the notch before level 1,130 is reached and water spills over the entire 1,324Õ long spill way. Secondly, lower water levels behind the Gilboa Dam create more of a void to take up water from spring snow melt, or God Forbid, rapid snow thawing, accompanied by warm rain as occurred on Jan. 18, 1996.
This brings us to another important point. We have been told by NYCDEP that, there is not sufficient room for 4 more siphons to be located in an area where they could operate efficiently on the Gilboa Dam. This is due to the configuration of the reservoir floor and accumulation of sediment. We accept this explanation believing it was given in good faith. However, the people downstream of the Gilboa Dam should not have to wait, as they did last year, for the siphons to be Òthawed outÓ. there was 4 feet of ice on the Schoharie Reservoir in Feb. 2007 and there was like amount inside the siphon tubes. The protracted thawing out process wasted valuable time and interfered with vital siphon operation. Therefore, DCC would strenuously recommend that compressed air be used to agitate water in the upstream side of the siphons to prevent ice formation. Compressed air bubbling units are routinely used in boat yards and marinas to keep vessels from becoming ice bound. It is the most efficient and economical method available, that is compatible with reservoir water. Obviously, anti freeze or other methods of chemical de-icing are out of the question. These compressors should be installed now! Anytime in the next 3 and 1/2 months a storm can occur that will fill the reservoir to capacity and make bubbling compressor installation an impossibility. The siphons are there for a reason, not ornamental purposes.
The matters of siphon engagement when reservoir levels reaches the notch elevation of 1124.5Õ and the installation of de-icing mechanisms are of short duration. That is, the siphons will not be present when dam renovation is complete. The next issue is of long term significance to people residing in the Schoharie and Mohawk Valleys below the Gilboa Dam. In conversations between representatives of DCC, the engineering firm of Gannet Fleming, and NYCDEP, DCC has recommended that an additional flood gate system be installed on the top of the existing spillway at 1130 to accommodate a combined flow with 8000 cfs capacity of the notch totaling 20,000 cfs. Such a flood gate system would greatly reduce the flood impact downstream of the Gilboa Dam by increasing reservoir capacity as well as attenuating dam spillage, i.e. longer periods of discharge with lower amounts of volume. The Obermeyer gate system, that NYCDEP intends to use in the 220Õx5.5Õ notch, lends itself well to the installation of a similar set of gates on the top of the spillway at Gilboa. If such a system were installed at Gilboa, with a combined discharge capacity of 20,000 cfs, true flood mitigation would be a reality in the Schoharie Valley. Historically, floods have occurred in the Schoharie Valley since the ice age ended 11,000 years ago. Floods have occurred at various rates of frequency since written records have been kept. At the present time, we seems to be entering a period of more frequent flooding. Since the flood of April 3, 2005, 3 more flood events ranking in the top 10 historic floods have occurred. A meaningful flood gate system, properly utilized, will, in times of flood, greatly reduce water level down stream of the dam and reduce the severity of subsequent damage, loss of property and possible loss of life. This recommendation needs to be given a great deal of consideration.
DEP data indicates how beneficial the notch is in attenuating floods. Basically peak inflow into the reservoir is reduced 12 percent without any notch. With the notch, this reduction is increased to 25% (Tina Johnstone stated the 25% value). What we want to do is set up a flood gate system that optimizes the discharge and storage (within the feasible constraints) such that the inflow peak is reduced greater than 25%. A multi-level system will significantly enhance this benefit.