The City's History of Flooding
From as far back as the mid-1800s, records show that Oil City, Pennsylvania, had been plagued with ice jams and ice-related flooding. These floods have caused extreme hardships for the community and heavy economic and personal losses as well. Since the early 1980s, the severity and frequency of spring and summer storm-type floods have been effectively reduced. This came about as a result of the U.S. Army Corps of Engineers' construction of the Tionesta and Kinzua dams and the Oil City Local Flood Protection Projects on Oil Creek and the Allegheny River.

Oil City is in Venango County at the confluence of Oil Creek and the Allegheny River. The project includes a floating ice control structure on the Allegheny River and a fixed concrete weir on Oil Creek. Both are upstream of the city and are designed to eliminate flood-causing ice jams on the Allegheny River at the mouth of Oil Creek. The floating structure on the Allegheny River was installed in 1982 and modified in 1983 at a federal cost of $1,110,000. It has effectively reduced ice formation on the river. The ice control structure on Oil Creek cost approximately $2.3 million and was completed in December 1989.

What Is An Ice Control Structure?

An ice control structure has been called everything from a huge fish net to a log boom to an ice dam. It is none of these. The Allegheny River structure is simply a collection of floating pontoons (a series of 20-foot-long steel boxes) connected together by 2 1/4 inch steel cables and attached to a junction plate in the center of the river. The cables are stretched across the river during the winter months and anchored at two separate locations on each shore.

The structures provide a barrier behind which the floating slush can collect, thus initiating the ice cover process at a more advantageous time and location. With early ice covers on both streams upstream from their meeting place, less ice is produced and less ice accumulates in the deep pool area of the Allegheny River.

Several factors are important to note about the river ice control structure:

• It will not act as an ice dam. Once the ice cover is formed bank to bank, the ice will support itself. When the melting ice and runoff begins, the ice control structure will be submerged by the running ice.
• Ice may continue to cover the deep pool area downstream of Oil City, but only to a fraction of the total thickness that occurred prior to the flood control structure. With the reduced thickness, the ice moves out of Oil Creek and having some place to go reduces or eliminates ice jams.
• The ice control structure does not cause upstream conditions to be different from those conditions that appeared prior to placement of the ice control structure.

The theory and concept of an ice control structure is not new. However, this is the first time that such structures have been installed in a relatively shallow, fast moving stream, such as on the Allegheny River and Oil Creek, to prevent ice jam flooding of a community.

The U.S. Army Corps of Engineers, along with the community of Oil City, acknowledge that this project is a success.

The Solution
Pittsburgh District had studied various alternatives to the problem, but, until 1982, was unable to devise a project that did not involve excessive economic, social and environmental costs, or one that even offered a reasonable chance of success. After two winters of on-site investigations and study, CRREL successfully developed a set of recommendations. These involved the construction of ice control structures on Oil Creek and also on the Allegheny River upstream from its juncture with the creek. The purpose of these structures is to reduce the build-up of slush ice in the confluence area of the creek and river. This allows the creek ice to empty into the river at the time of breakup.

What Causes Ice Jam Flooding?

Although the ice jam flooding problem had been observed for many years, it defied a scientific explanation until the early 1980s.

The U.S. Army Corps of Engineers Cold Regions Research and Engineering Lab's (CRREL) ice experts conducted an extensive investigation in the 1980s. They found that the problem stems from a combination of factors associated with river and creek characteristics, location and the ice forming process itself. Ice begins to form when water is cooled to 32oF (Oo C) and continues to lose heat to the colder air.

Tiny disc-shaped ice crystals are then formed at the water surface. In areas where the velocity of the river is slow, a surface skim of ice will begin to form. However, in the more characteristic turbulent reaches of the streams, the crystals are mixed throughout the entire depth of the flow. These small crystals, called frazil ice, are actively growing and readily attach themselves to objects in the water such as bridge piers, trash racks on hydroelectric power plants, or even rocks on the stream bed. Eventually the crystals accumulate together in clumps which rise to the surface to form very visible, randomly-shaped floating ice pans. This ice has a slush-like consistency as it floats downstream, sometimes for many miles, until it finally collects from bank to bank to form the initial solid ice cover.

Besides the hard, thick cover, CRREL found that as much as 18 feet of slush ice accumulates under the solid ice in the Allegheny River downstream of the mouth of Oil Creek. At Oil City this slush ice, similar to snow which has been placed in water, collects in the deep, relatively calm river pool created by long-since-abandoned commercial dredging. With the right combination of water and air temperatures, frazil ice continues to float downstream, collecting under the surface ice of the deep pools.

The combination of the surface ice and the slush ice severely constricts the flow in the Allegheny to a small area along the bottom of the river. When the Oil Creek ice finally breaks up and runs, it occurs before there is any significant break-up or movement of the river ice. Without a place to go because of the excessive mass of river ice, the creek ice jams. It then forms a dam at the juncture of the creek and the river, forcing large volumes of ice and water over the creek bank into the streets of Oil City.