Cascade from dam collapse restored as smooth, slow tributary

November 24, 1993|By John Rivera | John Rivera,Staff Writer

After a storm-water dam next to the Annapolis Mall collapsed in March, a cascade of construction debris and sediment flowed down a tributary of Weems Creek, nearly obliterating it.

Eight months later, where there was nothing but mud and trees, a peaceful stream bed meanders through a thicket of trees and bushes toward the creek.

An environmental consultant who restored the tributary says it should stay that way, avoiding the constant erosion that began before the dam collapsed.

Consultant Robert Sheesley of Columbia-based Brightwater Inc. yesterday showed County Executive Robert R. Neall the results of nearly a month of work. The project should be completed in three weeks.

Instead of using concrete splashways and wire-mesh cages filled with stones to stabilize the stream, Mr. Sheesley relied on more natural materials. He redirected the flow of the stream, adding wide bends to reduce the velocity of the water. And he installed dozens of rock weirs, a V-shaped rock formation in the stream bed that directs water flow toward the center of the stream and away from the banks, further reducing erosion.

Mr. Sheesley said he designed the stream-bed modifications to repair erosion that had begun even before the storm dam collapsed. Before, the velocity of the water in the tributary was too swift, directing water to the bank and causing erosion.

The weirs that direct water flow toward the center also cause water in the center to flow fastest. Any sediment in the water is deposited along the banks, where velocity is slower.

He pointed to several trees along the bank, where soil under the roots had been eroded over time.

Already, silt was beginning to be deposited under the roots by the natural flow of the water, filling in the gaps.

The tributary's banks have been planted with grass seed and covered with straw.

In the spring, more substantial planting will go in.

"That's all new material being pushed up under the roots," Mr. Sheesley said.

Mr. Neall said he was impressed with the results of the $100,000 reconstruction.

"What he's doing makes sense from a layman's point of view, because what he's trying to do is replicate mother nature and correct a few mistakes that were made," Mr. Neall said yesterday.

Elizabeth McWethy, president of the Weems Creek Conservancy, a citizens' group, expressed satisfaction with the stream restoration project.

"I would say it has a good chance, a 90 percent chance, of success," she said.

But Geoff Thomas, a conservancy member and retired civil engineer who also toured the reconstruction, fretted that enough had not been done to shore up banks around bends in the stream. "We noticed time after time down there that on the bends he did nothing," Mr. Thomas said.

Mr. Sheesley said he did little on many bends because water velocity would not be high enough to cause serious erosion. In larger storms, the water level would rise over the shallow bank and it would not be eroded.

Meanwhile, work is progressing on the $500,000 reconstruction of the storm-water dam.

Originally constructed to control erosion from the county rebuilding project on Bestgate Road and the expansion of the Annapolis Mall, the dam's embankment collapsed after a 54-inch-wide corrugated metal pipe that fed into the tributary collapsed.

The county is replacing the metal pipe with one made of concrete. Work on the dam is scheduled for completion at the end of December, although bad weather could push that date back.

County officials, who believe poor construction led to the dam collapse, are still trying to recover the cost of the dam reconstruction from the bond company of the contractor.

The contractor has declared bankruptcy.

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