NYC water supply: Cannonsville Dam sprung a leak

(Satellite, the spillway is a text book example of a tumble bay)

Cannonsville Dam is the westernmost dam in NYC's water supply, and it captures water from the Delaware River, which flows through Trenton, NJ and Philadelphia, PA. "Placed into service in 1964, Cannonsville Reservoir was the last of New York City’s 19 reservoirs to be built. Water diverted from Cannonsville Reservoir for drinking water enters the West Delaware Tunnel and travels 44 miles to the upper end of Rondout Reservoir. From there, it is carried in the 85-mile-long Delaware Aqueduct. Water is released downstream from Cannonsville Reservoir under the terms of the 1954 U.S. Supreme Court Decree, and a flow program, known as the Flexible Flow Management Program, agreed upon by New York City and the states of Delaware, New Jersey, New York and Pennsylvania. All other reservoirs in the city’s Delaware System have continued to meet their downstream release requirements under the Flexible Flow Management Program while the condition at Cannonsville is repaired."  [Aug 2] It holds an eighth of the system's water supply. [NYtimes]

Cannonsville Public Meeting, page 3

New York City Department of Environmental Protection

It is a 175' tall earthen dam that can store 95.6 billion gallons with a two-stage emergency spillway that is 800' long.

(When this photo was taken, I wonder if the river release was kept low to deliberately fill the reservoir to test the spillway or if there was a spring season wet enough to cause the overflow.)

Cannonsville Public Meeting, page 4

If I'm reading the diagram below correctly, water will start going over the spillway at an elevation of 1150'; and, at the maximum expected spillway flow, the water level would be 1155'. So the water should remain 20' below the height of the dam.

Cannonsville Public Meeting, page 5

Soil sample bore holes were drilled as part of a study to build a 14Mw power plant next to the existing release chamber. (It is interesting that the above diagram of the dam doesn't include the intake for, and location of, the discharge pipe to runs to the release chamber. This pipe the normal way of releasing water so that the reservoir level should seldom reach the emergency spillway.)

Cannonsville Public Meeting, page 8

If you look at the dam diagram, you see by the "Assumed rock surface" graphic that they don't even know where the bedrock is for this dam. It is built on glacial till. So before they build the hydro-power plant, they need to understand what type of soil+rock mixture is under it so that they can design the pilings needed for the foundation. They do this by boring holes to sample what is underneath. One of the holes created an artisan spring under the rock drainage at the tow of the dam. What I can't tell is if that was expected and they normally then plug the rock drainage with Bentonite or if this spring was a surprise and their efforts to plug it with Bentonite failed. Now they have Bentonite and bore hole material flowing with the water through the rock and into the river.

Cannonsville Public Meeting, page 10

Evidently the booboo was on July 8 and the presentation to the public was July 23. While they are working on repairs, they have maximized the release to the NYC water supply at 970 MGD and maximized the river diversion at 470 MGD. They are also "stockpiling materials and equipment for emergency on-call repairs." [Cannonsville Public Meeting] This is lowering the water level about 8 or 9 inches a day. It has raised the Delaware River about a foot and reduced the water temperature, which is good for the trout. [WaterShedPost]
They have also released the inundation map to the public even though it is marked "FOR OFFICIAL USE ONLY - NOT FOR DISTRIBUTION." This map indicates the impact of the worse case scenario of a breech with a full reservoir. It is not surprising that the whole town of Deposit would be under water. It is surprising that several blocks and bridges in Philadelphia would also be under water.

The photo below shows the water turbulence in the release chamber caused by the maximum release flow and the turbid water to the left caused by the aquifer puncture by the boring hole. You can see a relief-well drilling rig on the upstream side of the road at the base of the dam.

The repair has two stages. The first is to drill relief wells upstream with proper screens and casings so that clean water will flow through the rock embankment. These clean holes will relieve the pressure on the dirty holes and stop the release of sediment into the river. The second stage is to grout the soil sample holes and then drill grout holes around the dirty holes to create a grout curtain with pressurized grout. [July 23]

nyc.gov Flickr

nyc.gov Flickr
One of the two rigs drilling the relief wells.

By July 29, a sediment analysis was complete. It determined that the sediments are coming from the bore hole site and not from the dam. When the first relief well started flowing, the turbidity in the river was reduced. So the relief plan was working. "Engineers are prepared to install as many as 8-10 relief wells, but they expect it may take fewer to end the turbid discharge." [July 29]

Four relief wells were enough to stop the turbid flow, and outflow rates started to shift to normal rates on July 29. In this case, normal was zero for drinking water because they switched to other reservoirs for drinking water to reduce the rate this reservoir was being drained. The river flow was reduced from 1500cfs to 500cfs between Aug 2 and Aug 5 per the advice of fisheries biologists. Inflow was 300cfs. At the time of the July 23 presentation, the water level was 1141.97' with 47.2 billion gallons of warm water and 37.5 BG of cold water. As of Aug 2, 25 BG of cold water remained. (I never found information on how much warm water was left. Nor if the water intake has inlets at different levels to control the temperature of the water that is released.) [Aug 2]

The second stage of repairs was finished Aug 21. Now they will gradually close the relief wells and verify that the dam's instrumentation shows values returning to levels comparable to what they were before the test bore holes were drilled. [Aug 26]

After two weeks of monitoring the instrumentation and the flow in the river, the repairs have been declared successful. The reservoir is expected to fill to normal capacity by late Spring, 2019. [Sep 17]

nyc.gov Flickr
"This photo shows the cloudy discharge downstream of the dam just a few days after it was discovered."
[It also shows the turbulence in the release chamber with a high release rate. In hindsight, the dam was not in danger and they did not need to release the water. But it makes sense that when there is a sign of trouble that they would drain first and think second. Nonetheless, it did take a week before they decided to start draining the reservoir even though it was close to capacity because of a wet June. [NYtimes]]

nyc.gov Flickr
"This photo shows the cloudy discharge after one relief well was installed and pumping. The discharge was reduced, but some remained along the grass and the stone abutment near the middle of the photo."

nyc.gov Flickr
"This photo shows the same area downstream of the dam on Aug. 7, after the cloudy discharge was successfully halted by the relief wells. Notice the water is so clear that rocks on the river bottom can be seen easily."

The 14MW ($2M per year) capacity of the proposed hydro-power plant is impressively small. Modern coal plants and nuclear plants are over 1000MW. 14MW is just a few windmills in a windmill farm. At an estimated cost of $72M, it was estimated that it would barely pay for itself assuming a 50-year lifespan. After this repair bill, it strikes me as a looser because of the risk of building a foundation over a pressurized aquifer.