The Dredge Report: What’s up with digging down into White Rock Lake?

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Map courtesy of the City of Dallas.

Posted November 24, 2019

Sail across White Rock Lake, and you’ll see wildlife, lush vegetation and stunning sunsets, but the beauty of the lake and the health of its ecosystems are in danger of what lurks beneath the surface.

Up to 8 feet of sediment is hidden under the water, and without a dredge, the lake could fill with silt by 2034, officials from the Park and Recreation Department say. The degradation of the lake is already evident in some areas. When water levels dropped this summer because of a broken stoplog at the spillway, debris wallowing in shallow bays showed that a dredge is long overdue.

In late October, the Dallas City Council approved a more than $99,000 dredging feasibility study, kicking off a yearslong project to remove sediment and restore the health of the lake.

What is dredging? Why is it so expensive? Is there an alternative solution? We talked with Park and Recreation Department senior engineer Richard Stauffer and asked him everything you need to know about dredging White Rock Lake.

An aerial rendering of the White Rock Lake Rowhouse, just north of the Corinthian Sailing Club

What is dredging?

Dredging is the accumulation of silt and debris that reduces water depth. In 2014, the latest study from the Texas Water Development Board found that the lake contained 10,230 acre-feet of water, down from its original volume of 18,000 acre-feet when it was built in 1911. The sediment buildup also affects the lake’s ecosystems when materials, such as logs and leaves, take the oxygen as they decompose, Stauffer says. The purpose of dredging is to remove those materials and restore the lake to an appropriate depth for recreational use.

Has the lake been dredged before?

White Rock Lake is dredged approximately every 18-24 years. Since its construction, it has been dredged four times: 1937, 1955, 1974 and 1998.

When silt removal began in 1937, 588,000 cubic yards of sediment were removed, and 90 acres of land were reclaimed during the three-and-a-half-year project. In 1955, more than 15,000 cubic yards of sediment were removed, followed by 1.35 million cubic yards during the third dredging.

The final dredge removed 3 million cubic yards of sediment. It was completed in 1998 after a study found that silt affected oxygen levels in the summer, threatening the fish population. The dredge cost $18 million, half of which came from bond funds. A fortuitous connection reduced the cost of the project when a landowner in Hutchins needed his gravel pit filled. Over nine months, a dredging pump inhaled a mixture of water and silt from the bottom of the lake and sent it through 24-inch pipes to the pit.

What causes sediment buildup?

Nine tributaries feed into White Rock Lake, and much of the lake’s sediment is from the natural erosion of those streams, Stauffer says. However, as booming construction and upstream development replace farms and forests that once straddled White Rock Creek, more sediment flows into the lake.

Which areas have the most sediment?

Sediment accumulation is greatest northwest of the Arboretum near Sunset Bay where Dixon Branch, a natural stream that runs through Old Lake Highlands and parts of East Dallas, drains into the east side of the lake. The northern part of the lake also has significant accumulation where White Rock Creek empties into the water body. Those areas are the most likely to be dredged, Stauffer says.

What is the purpose of the feasibility study?

What are the dredging approaches?

One approach is mechanical dredging, which would require the lake to be drained so heavy equipment could pass along the lakebed. “That [approach] will get shot down,” Stauffer says. “There’s a lot more regulations that go into that, and it would shut down recreational activities. Plus, what would you do with all the fish?” A more likely option is hydraulic dredging, which was used in 1998. Barges suck up silt with pipes and move it to the shoreline. It would allow recreational activities to continue in other areas of the lake.

When might dredging start, and how long would it take?

It could take up to four years before dredging begins, Stauffer says. Before the project can start, the feasibility study must be completed, and a dredging approach must be selected. Construction permits and funding must also be acquired.

In 1998, dredging took just a few months because sediment was pumped to the disposal area, Stauffer says. This time around, it could take a year because the silt would have to be dried using de-watering techniques and taken via truck to a final resting place. “You can’t pump wet sludge into the back of a truck because it would spill out,” Stauffer says.

Are there more permanent options besides dredging?

Sediment forebays could be installed upstream to catch debris before it enters the lake, Stauffer says. However, the heavily wooded area north of White Rock Lake would need to be excavated. “Unfortunately, we would basically have to excavate the same volume as White Rock to have a sediment forebay,” Stauffer says. “That’s a tough sell.”

What will it cost, and where will the funds come from?

An estimated cost has not yet been determined, Stauffer says. City officials are exploring state and federal grants or another bond program to pay for the project.

What are some of the environmental concerns?

Environmental standards have become more rigorous since the 1998 dredging. Now, sediment must be tested to ensure it’s not contaminated with hazardous metals and chemicals. If it tests positive, it must be deposited in special locations.

Protecting the habitat and vegetation around the lake is another priority. Dredging would likely start in the spring after the pelican migration, and engineers would take special care in choosing where to dry sediment and load it into the trucks, Stauffer says. “We can’t be driving equipment over the Blackland Prairie around the lake,” he says. “We have to make sure we’re proceeding in an environmentally conscious methodology.”

Source: lakehighlands.advocatemag.com