To understand the enormity of Conowingo Dam and its accompanying hydroelectric generating plant, a group of visitors stood inside the facility's cavernous Turbine Hall and listened to what on Saturday was just a gentle whirring two of the plant's generators along the hall's 600-foot length.
To appreciate the complexity of the generating plant itself they walked carefully through the maze of spiral stairways and tunnels and climbed across the bowls of one of the massive generating units during the dam's annual visitors day open house hosted by its owner Exelon Corp.
About 1,200 people took advantage of Friday and Saturday's open house to have an up-close look inside an engineering and construction marvel of the early 20th century, Conowingo general Manager Archie Gleason said.
Visitors were taken through the generating plant in groups of 25. Questions abounded, and several cast some wary eyes skyward when Reta White, an Exelon civil engineer, who was one of the tour guides, explained the floor of Turbine Hall is 46 feet below the surface of the river being held back by the dam.
One for the ages
Completed in 1928 at a cost of $52 million, Conowingo Dam, which spans the Susquehanna River for nearly a mile and joins Harford and Cecil counties, was the largest structure of its kind for a brief spell before Hoover Dam in Nevada followed.
Unlike the latter, Conowingo was built by private enterprise. Hoover and many dams constructed in the next 15 years during the great American dam building era of the 1930s and early 1940s were largely federal government public works projects. At the time of its completion, Conowingo was like nothing the United States had seen and, nine decades later, it endures not only as a physical testimony to "The American Century," but also as a major recreation center in the region.
Saturday afternoon, scores of fisherman lined the bank of the river below the power plant, and hundreds of cormorants stood watch along the island in the middle of the river for the same reason: the millions of fish attracted to the base of the dam in search of baitfish sweeping through the operating waterwheels from behind the dam. Fishing and wildlife photography and boating attract tens of thousands of people to the dam and its lake annually.
While Conowingo is in Maryland, the dam and power plant were built to supply cheap power to Philadelphia, which it would do for about five decades, before cheaper ways of producing power overtook hydro plants. Today, Conowingo is a backup power source for the PJM regional grid, a multi-state electric power transmission system, but the dam's status has not necessarily diminished.
Standing by one of the two small generators that produce the constant power to operate the lights and machinery inside the power plant and dam, White explained that if the PJM grid should crash with a brownout or blackout, Conowingo is a first line of defense because its generators can start producing up to 572 megawatts of electricity "in a matter of minutes," not the hours it takes to start up plants using fossil fuel or the days to start nuclear plants.
The Peach Bottom Atomic Power Station, about a dozen miles upriver from Conowingo, could use the latter's power to restart its reactors in the event of a shutdown, she said. The two are linked by a direct transmission line.
Conowingo was built by and still essentially runs on 1920s technology. White said each generating unit – there are 11, an original seven and four others added in the mid-1960s – is overhauled about every four years, but the basic guts of the operation has been the same for nearly 90 years.
At the far end of the power plant, visitors could get a look from below along the 94-foot high dam's 50 floodgates, which are topped by the Route 1 crossing, the latter which was completed about four months before the dam itself began full operation.
The dam was engineered to pass up to 800,000 cubic feet per second of water — 359 million gallons — though the rated capacity is 720,000 CFS with all 50 floodgates open.
That's happened twice, in 1936 during a then-record 100-year flood and again in 1972 when Tropical Storm Agnes sent through a record flow of 1,128,000 cubic feet per second – 7.5 million gallons per second – on the morning of June 24, 1972, according to handwritten notes and a hydrograph of the U.S. Geological Survey, cited in a new book, "Conowingo," by John R. Paulson, who also produced the 2016 Maryland Public Television documentary, "Conowingo: Power on the Susquehanna."
Paulson was on hand Saturday to sign copies of his book, which is available through Amazon and local bookstores, as is the DVD of the PBS documentary. The latter contains bonus footage of motion picture film shot during the dam's construction whose existence was discovered by executive producer Michael English during preparations for documentary.
"A miracle," said Paulson, who said the film joined a trove of archival photographs that exist from before, during and after the dam's construction.
Elevation, not flow
On Saturday, it appeared that water was gently falling from every one of the floodgates onto the parabolic apron at the base of the spillway which was engineered to minimize bank erosion immediately downstream.
But when White was asked how many gates were open, she replied, "None. What you see is water leaking from around the floodgates," the 40-ton structural steel walls that are raised and lowered by the dam's three 60-ton gantry, or overhead. As with so much of the equipment at Conowingo, the electrically operated cranes, which travel on rails along the top of the dam, are original.
White said the floodgates are in almost continuous need of repair and replacement and "we can't completely seal them," but the leakage is counted toward the dam's water release requirements from the 15-mile long, 9,000-acre lake or pool, created by the dam's construction.
There are also requirements that certain oxygen levels be maintained in the water below the dam to support fish and other aquatic life. The single generating unit running Saturday was not producing power, but was being used to maintain river flow and oxygen levels, White explained.
"We don't control the flow of the water, we control the elevation of the river we need to efficiently produce power," White said.
That ideal elevation ranges from 106 feet to 108.5 feet, she said, the elevation that is needed to drive all the water wheels that drive the 11 generating turbines. Above that level, White said, they begin opening the floodgates to attain the optimum level.
During the record-setting Hurricane Agnes period in June 1972, with the river behind the dam steadily rising toward 111 feet, then-superintendent Paul English warned, as quoted in Paulsen's book, the operators would be in "a hazy area" and "it is not known if the structure of the dam may give … ."
Much was reported then and since about a plan to blow out the dam on the Cecil County side of the river in order to prevent the water from breeching the top – at 116 feet – and potentially collapsing the structure completely.