These are notes that I am writing to help me learn our industrial history. They are my best understanding, but that does not mean they are a correct understanding.
This dam is another example of a spillway using a tumble bay or side channel. But in this case the side channel is shallow but wide.
When new, the cut stone facing looked nice. The vanes across the top were added when it was discovered during the spillway's first operation that the water shot over the first step.
NYC Water Schoharie Reservoir. Gilboa Dam. Contract 203. Gilboa Division. Looking southeasterly across the pool area of the spillway channel. Practically the entire length of the spillway section of the dam and of the spillway channel are shown in center of picture. August 17, 1927. (Digital ID: p001747)
Unless noted otherwise, the following information comes from the Project Description prepared for the NYC Department of Environmental Protection (NYCDEP) available from nyc.gov
The gravity dam was constructed with cyclopean concrete covered by 3-5 feet thick blocks of cut-stone masonry. Cyclopean means that large boulders, called plums, are added during the pour to reduce the amount of cement used. But the cement savings is offset by the labor requirements. The pouring of the concrete has to be alternated with the placement of boulders to ensure that at least 4" of concrete is between each boulder. So this technique was popular for structures requiring a lot of cement such as gravity dams between 1870's-1930's.
The 11-foot Shandaken Tunnel is 18.6 miles long with the intake located 3 miles upstream from the dam. The outlet empties into the Upper Esopus Creek, which is in the watershed of the Ashokan Reservoir. Because part of the path of the outlet flow includes a river, the "flow, turbidity, temperature and phosphorous levels" are regulated. The Gilboa Dam captures water that is flowing north towards the Mohawk River.
Project Description
Construction began on the dam in 1919 and the reservoir was full in 1927. It has a capacity of 17.6 BG (16% of the NYC water supply). It is 160' high with a 1,326' spillway and a 700' earth embankment with a concrete corewall. The stair-step spillway empties into a tumble bay or side channel that varies in width from 80 to 270 feet and drops 125'. The side channel empties into a plunge pool.
The stone facade of the stair-step spillway proved to be a maintenance problem because of the harsh winters (freeze-thaw cycles) and heavy seasonal flows over the spillway. "The City implemented a costly and time consuming maintenance program which entailed repointing the damaged mortar joints and replacing damaged, broken or missing stones. This practice continued through the 1970’s but the maintenance program could no longer keep pace with the continuing cycle of deterioration to the Dam’s façade." The stone covering was allowed to disappear. This altered the geometry of the spillway and reduced the energy dissipating effectiveness of the stair-steps. This, in turn, started destroying the side channel.
Project Description
Since significant advances have been made in flood estimating and dam design, a study was done as to why the dam deteriorated so badly. "It was discovered that over the life of the dam, several floods have slightly exceeded the Dam’s original spillway design flow (SDF) of 52,700 cfs." So in 2005, NYCDEO decided the dam's design and condition was bad enough that they should contract a study as to how to fix it. In November 2005, a "red alert" was declared. (Actually, "the NYCDEP issued a declaration of emergency") The study determined that this high-hazard dam (based on its height, size of reservoir and downstream towns) could slide during a 100-year flood event. Sliding is the failure mode for a gravity dam where the force of water against the dam overcomes the friction within planes of the bedrock and the whole dam is shoved downstream. The Jan 19, 1996 flood that sent 70,800 cfs over the crest with a depth of 6.68' was a 60-year flood. The study indicated that a 100-year flood of 72,580 cfs and an 8' depth over the crest could push the dam. So NYCDEP immediately lowered the Schoharie Reservoir until emergency repairs could be completed. Just a month later they started those repairs in anticipation of the final reconstruction of the dam scheduled for 2008. (This 2005 study gives an interesting meaning to the phrase "ignorance is bliss.") The emergency repair "was the installation of post-tensioned anchors in the Spillway and its foundation." But before they started that work, they cut a notch above the plunge pool and installed siphons to keep the work zone dry. They also installed a debris boom.
HazenAndSawyer
Gilboa Dam, Gilboa, NY – Shown undergoing emergency repairs with construction of the notch and emergency siphons for flow diversion complete.
Even so, sometimes Mother Nature forced the repair work to stop. The installation of the anchors was completed in Dec 2006. Hurricane Irene in 2011 caused a flow depth of 8' over the crest.
HazenAndSawyer
Gilboa Dam, Gilboa, NY – Shown with water spilling over the entire dam spillway, during emergency repairs
This photo shows why a debris boom was installed to keep the work zone free of debris.
The emergency repair work met the dam safety requirements. The remaining work was to improve the operation, maintenance and aesthetics of the dam. And to meet safety requirements for another 100 years.The major remaining work consisted of refacing and reconstructing the concrete surfaces, installation of a new Low Level Outlet (LLO), and installation of crest gates in the existing spillway notch to improve "flood attenuation under a snowpack-based reservoir management program." The new spillway is designed to handle 311,400 cfs, which is 6 times the original design capacity. A 1:20 scale model predicts that larger flow would create a 17.4' spillway head. A 1:40 scale model was then built to fine-tune the design of the new dam configuration.
Project Description A reproduction of the current dam design to validate the modelling. When tested with the updated max flow rate, the water crested with a 15' head and shot over all of the steps. Thus there was no energy dissipation and all of the water landed on the side channel with full force. 30 modifications were then tested with the updated flow.
Project Description The model also tested 100-year, 500-year, and 1/2 Probable Maximum Flow rates. It also tested the impact of the notch for the emergency repairs. The conclusion was that the notch "would have no effect on the Spillway’s ability to pass the flow parameters."
The west side of the river just downstream of the dam has shown symptoms of landslide activity. The side of the river is glacial till rather than rock. So another modification is to extend the West Training Wall to help protect the hillside.
Figure 1-11
"Based on the 1:20 physical model, the optimal Spillway configuration was determined to be a mitered crest Spillway with a stair-step configuration consisting of seven 3-foot steps (step heights varied from 2.24 feet to 3.17 feet and step widths varied from 5.34 to 5.4 feet), six 6-foot steps (step heights were all 6 feet and step widths varied from 5.34 to 5.4 feet) and eight 12-foot steps (step heights were all 12 feet and step widths were all 10.8 feet). In addition, straight vanes were installed at the crest to provide additional energy dissipation. Figure 1-11 shows the profile of the Spillway stairs and crest for the recommended design configuration. Such a configuration provides more controlled, less turbulent flow over the face of the Spillway as flowing water strikes all steps uniformly and with equal pressure, thus limiting the damage over time to the Spillway façade and extending the serviceable life of the Dam."
NYC Water, Oct 2012, cropped [They moved the position of the siphons depending on what part of the dam they were working on.]
Since the current Low Level Output (LLO) has been unusable since the 1960s because the inlet is plugged with sediment, a new LLO is part of the reconstruction plans. "NYCDEP is considering the installation of a warning system to notify downstream communities should an emergency release be necessary." Regulations allow four months to reduce the reservoir to 10% of normal pool level. That struck me as rather long if you are emptying the reservoir because you saw a crack in the dam. But high flow rates have the risk of causing flooding downstream and lowering the water surface too quickly. The maximum daily drop of the water level should be in the range of 1 to 2 feet per day to avoid inducing landslides in the sides of the reservoir. The design capacity is 2,500 cfs when the reservoir is 100% full, which reduces to 1,700 cfs at 10% capacity because of the lower head over the LLO inlet. They did test borings for a tunnel for the LLO and determined it is a combination of soft soil and boulders, which is basically optimally bad for tunneling.
Work is proceeding on the $142 million release outlet project. The current plan is that it should be finished in 2023.
CSengineering, cropped Micro-tunneling machine breaks through to complete land leg for new release works
They are going to turn the 220' by 5.5' spillway notch into an asset by filling it with
pneumatically operated crest gates. This will help them lower the reservoir to absorb some of the melting of the observed snowpack. This will reduce the amount of flooding that happens downstream. It is rather barbaric that a dam of this size hasn't already been able to reduce flooding. They still don't plan to eliminate flooding. They would lower the reservoir to capture just half of the predicted Snowpack Water Equivalent. They plan to "attenuate" the flooding "that would result in fewer structures being impacted in downstream communities." To be fair to NYCDEP, if the dam was not there the communities would probably see bigger floods because the dam does capture some of the wet season water when it refills.
Project Description
Project Description
Project Description
There is a lot more to the project such as creating access roads and a staging area that includes a portable concrete batch plant, disposing of the spoils, restoring the Scenic Public Overlook Area, site restoration, noise and air emissions abatement, and getting 19 permits and approval. I find those aspects boring and won't describe them. But one fact of note is that 90,000 cubic yards of the old facing stone is going to have to be disposed. High-strength concrete will be used for the new facing.
Another project in the area is the Shandaken Tunnel Intake. Three of the eight sluice gates in the intake chamber are currently jammed. They will be replaced. Also a pipe that can be moved up and down will be added so that water can be drawn from different layers depending on the turbidity and temperature of the layers. This allows them to draw the colder waters at the bottom of the reservoir when needed to help the health of the trout in the Esopus Creek. The cold reservoir water has created fisheries that "attract anglers from around the world, boosting the local economy." Furthermore, the design of the LLO has been changed to include a High Level Outlet so that warm water can be released before cold water is released because the Schoharie Creek never developed cold-water fisheries. [HudsonValleyOne]
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