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Symbiosis - The crest wall
The strict biological definition of symbiosis is “the living together, in more or less intimate association, of two or more dissimilar organisms”. It often involves a host organism and a parasite. An example is the pilot fish, parasite, which cleans the teeth of certain species of sharks, host organism, and actually swims in and out of the shark’s mouth with impunity. A more general definition offered in the dictionary is “a co-operative relationship between two or more persons as groups”. This is what the relationship should be between the NYCDEP, owner of the Gilboa Dam and Schoharie Reservoir and the local residents living both up and downstream of the dam. In the 82 years of the Gilboa Dam’s diversion of Schoharie Creek waters to the Ashokan Reservoir and on to New York City, the relationship between NYCDEP and its host watershed counties has been, in a cost vs. benefit scenario, pretty top sided.
Water diverted through the Shandaken Tunnel, stream piracy if you will, so diminishes northerly stream flow of the Schoharie Creek during the summer months, there is often more water at Prattsville, NY, 29 miles from the head waters than there is at Burtonsville, Montgomery County, 65 miles from the source of the Schoharie Creek in Greene Co. NY. So complete is the interception and diversion of Schoharie Creek waters by the Gilboa Dam, that for several months each year the Schoharie Creek literally starts itself anew north of the Gilboa Dam.
Because of the small size of the Schoharie Reservoir, 1.8 sq. miles compared to its 314 sq. mile catchment basin, over three times the reservoir’s storage capacity of 19.5 billion gallons spills, annually, over the Gilboa Dam. The NYCDEP refers to this as “waste water”. This unregulated, and occasionally extreme spillage over the 1,324’ long spillway has contributed to the floods that periodically vex residents of the Schoharie and Mohawk valleys. From 1926, the year the Schoharie Reservoir went into operation until 2006, “waste water” spillage at Gilboa was unregulated. Whatever water that did not flow through the Shandaken Tunnel simply filled the reservoir and spilled over the dam, often after a major storm, hurricane or rapid thaw of the snow-pack.
With the disclosure of serious structural deficiencies at the Gilboa Dam in October 2005, releases from the Schoharie Reservoir, both over and through the Gilboa Dam, were subjected, to a limited extent, to actual control. This was accomplished by the installation of 4, soon to be removed, siphons capable of handling up to 900 cfs in total, conducting water up and over the dam. These siphons helped in creating a “void” or storage capacity behind the dam and in lowering water levels in the reservoir. Of far grater significance was the cutting of a 220’ long by 5.5’ deep notch in the Gilboa Dam spillway. This ungated opening can accommodate a volume of water up to 8,600 cfs or 64,000 gallons of water per second, before the dam is “topped” and water begins to flow over the entire 1,324’ long spillway. Thus, 5.5 feet of storage on a lake of 1.8 sq. mi. is created before full spillage occurs at reservoir el. of 1,130’. Often times the storm has passed or the snowmelt has diminished before the run-off of the 314 sq. mi. upstream drainage can fill the reservoir and reach the present crest level of 1,130 above sea level.
What the existing, soon to be gated, notch does, in effect, is to passively control spillage up to its full capacity. This regulated passage of water through the notch functions like a traffic light, so to speak, slowing or attenuating the drainage the way a flashing traffic light controls the flow of motor vehicles. Ultimately in both cases the volume of flow is the same. The way traffic lights help to prevent uncontrolled congestion, the notch has helped to prevent uncontrolled spillage for it takes time to fill up the 2 billion gallon void behind the notch, from el. 1124.5 - 1130’. This passive, for it takes no one to operate it, notch has shown its worth in its 2 years of existence. While the neighboring counties of Delaware and Otsego suffered terrible flooding in late June of 2006, the “notch” at Gilboa combined with preemptive draw down of the Schoharie Reservoir via the Shandaken Tunnel and the 4 siphons spared the residents of the Schoharie Creek corridor the effects of a major flood.
To improve upon the benefits observed as a result of the notch, Dam Concerned Citizens, Inc, 501C3, is recommending the construction of a crest wall or addition to the top of an eastern portion of the existing Gilboa Dam spillway, el. 1130’. This proposed crest wall would be designed to add additional storage behind the dam while passively discharging a manageable flow over the spillway where the crest wall is not constructed. The new spillway crest design would be capable reducing the impacts of the more frequent flooding events. For example, a 10-year storm event which delivers a peak inflow of approximately 40,000 cfs to the Schoharie reservoir can be reduced to a peak of approximately 28,000 to 25,000 cfs spilling over the dam with the addition of a 4 feet high crest wall. This is an estimated storm peak attenuation exceeding 30 percent, which transforms the 10-year event to a more manageable 5-year event. Coupled with the potential to add storage behind the dam by release of excess water through the proposed low level outlet (with the capacity of 1,800 to 2,500 cfs) there exists a prospect for real relief from the impact of future floods.
Two major advantages of the “crest wall notch system” are:
a). simplicity of design; it acts passively to control down stream discharge from the Gilboa Dam.
b). economy of construction, relatively small volume of concrete.
The crest wall will not eliminate flooding; floods have occurred throughout history at varying rates of frequency. We seem, unfortunately, to be entering a time of increased flooding. While we cannot stop flooding, we can reduce the impacts through simple practical efforts such as the design and construction of a new spillway that utilizes the benefits of crest walls (adding storage) and a notch (providing controlled release). The flood attenuation effort constructively advances our symbiotic relationship with the City of New York.
As a final note the DCC is pleased to see that the Delaware Energy Cooperative is exploring ways to use the hydro-electric energy potential of NYCDEP’s west of Hudson Reservoirs. In these challenging times, the multiple use of existing infrastructure only makes sense. If a reservoir can provide flood mitigation as well as hydro-electric power and pure wholesome drinking water, then it should! This would be true symbiosis.
Many of us remember the most recent severe floods of April, 1987, January, 1996 and April, 2005. Had a “NOTCH” and “CRESTWALL” passive release works been in place during these three flood events, flood damage levels downstream of the Gilboa Dam would have been reduced. It is in your best interest to contact your local, state and federal representatives about the benefits offered by this practical, common sense approach to Flood Mitigation.