Hydrology - Water Resources Report 27

Impact of Removing the Granby Dam on Water Levels in the Congaree River

By
Andrew Wachob
2002

SUMMARY OF FINDINGS

The Granby Dam and Lock structure was constructed at the beginning of the 20th Century to provide at least a 4-foot navigation depth in the Congaree River up to the city of Columbia. Unused for many years, the structure remains in place as a short dam and an open-lock passage that is used by boaters to bypass the dam when river levels are low. A recent proposal to remove the Granby Dam has raised concerns that removing the dam might decrease water levels within the lock, making navigation through that area difficult, especially during periods of low river flow. This study examines how removing the dam would affect water levels and depths within the lock passage and in the river's main channel.

Removing the Granby Dam from the Congaree River could cause water depths within the lock passage to decrease by as much as 4 feet. This decrease in water depth would result from a combination of lower water-surface elevation and increased sedimentation within the lock area. Removing the dam would impact water depths most significantly during periods of low flow, but even during higher flows the water depth within the lock would be less because of sedimentation.

During low flows, most of the river water is diverted through the lock, and at high flows, when most of the river water spills over the dam, a significant amount of water still passes through the lock. Because there is always considerable flow in the narrow lock, water velocities stay high (greater than 4 feet per second), which keeps the lock free of deposited sediment. Without the dam, flow through the lock would be substantially reduced, and water velocities within the lock would seldom exceed 3 feet per second. As a result, the lock would begin to fill with sediment, effectively decreasing the water depth in this channel by as much as 2 feet.

Removing the dam would also decrease water depths within the lock by lowering the elevation of the water surface. At the upstream end of the lock passage, water-surface elevations would be reduced by 1 to 2 feet during low flows; at the downstream end of the lock, the reduction would be 1 foot or less. During higher flows, the decrease in water-surface elevation would be 0.5 foot or less.

The impact of removing the dam was evaluated by comparing conditions in the river (water depths, water velocities, and flow distribution) as they are now, with conditions as they would be if the dam were removed. These conditions were determined by using a combination of surveyed water-surface and riverbed elevation data, flow rate data for the Congaree River, and computer model simulations.


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