Side-stream reservoir ensures water future
When Spring Hollow Reservoir in Roanoke County, Va., comes on line this winter, it may provide municipalities faced with water shortages a glimpse into the future of water systems. Like many counties across the country, Roanoke County in western Virginia relied on a patchwork of wells and water purchased from a city–in this case, Roanoke–under a tenuous agreement. Officials knew this arrangement would not meet future needs, so in 1986 they received voter approval to build a new reservoir and water treatment plant to meet demand for the next 50 years.
Unlike conventional reservoirs, however, the 158-acre, 3.3 billion–gallon Spring Hollow Reservoir, built at a cost of $73 million, is a side-stream reservoir formed by placing a dam away from a river. Water is pumped from the Roanoke River one-quarter mile to the reservoir.
Environmentalists consider this method less harmful to the river’s ecology. Side-stream reservoirs also increase water quality, according to Bob Benninger, assistant utilities director for Roanoke County. Municipalities often draw water directly from a river, putting them at the mercy of runoff from storms and spills that contaminate the river.
A side-stream reservoir supplies clean, undisturbed water.
The largest roller-compacted concrete (RCC) dam east of the Mississippi and the second largest in the United States at 243 feet high and 990 feet across at the top, Spring Hollow features a concrete cavity for impoundment that is similar in size and shape to those of traditional dams. It is, however, built differently. RCC used in Spring Hollow dam resists thermal stress much better than standard concrete and is cheaper and quicker to use because it requires less forming.
“An RCC dam is a cross between an earthen dam built with construction equipment and a poured-in-place concrete dam,” explains Project Engineer John Bradshaw of Hayes, Seay, Mattern and Mattern, Roanoke,Va.
Both the up- and downstream faces of the dam consist of precast panels, which acted as a form for placing the clay-like RCC in the middle. One, foot-thick layers were applied, spread by a bulldozer and compacted by vibratory rollers, similar to steam rollers.
When treating water, most plants use giant settling tanks to remove sediment. Although a consulting engineer recommended this method, Cliff Craig, the late Roanoke County utilities director, designed a treatment system with a solid contact absorption clarifier process like those used in small treatment plants. He based this decision on his belief that the Roanoke River runs cleaner than most water sources and the reservoir itself would work as a settling basin.
No such plants existed in Virginia, so the state’s health officials demanded proof the system would work. Craig and his staff built a pilot plant near the reservoir and, after eight months of testing, produced data verifying that the water met health specifications.
Aside from the lack of settling tanks, the filtration process at the treatment plant uses standard technology. Ferric sulfate is added to the water to coagulate sediment. Water passes through small plastic chips to remove the particles and then flows through a mixed-media filter of sand and coal. A granular-activated carbon system removes trihalomethane molecules that can create carcinogens when they react with chlorine. Finally, chlorine and fluoride are added.
At the pumping station for filling the reservoir, five vertical turbine pumps will pump a maximum of 80 mad. A computer monitors and controls flows, water levels as well as other variables.
The Army Corps of Engineers restricts the amount of water pumped from the river. From June through March, no more than 70 percent of the river’s flow can be pumped. In April and May–spawning season for trout and other fish–the limit drops to 60 percent.
Environmental groups such as the Friends of the Roanoke River lobbied for the restrictions.