Dallas' grand plan to create a community green space and recreational river from the Trinity River floodplain that borders the west and south sides of the city would be completely impractical without flood-control measures.
To control the level of the river in the region of the parks and trails and protect neighborhoods downstream, a chain of wetlands and levees is being constructed southeast of downtown Dallas. This area, where commercial and industrial development gives way to a hardwood forest of primarily green ash, cedar elm, American elm, and cottonwood trees, has in recent years been termed the Great Trinity Forest. At 6,000 acres, this is one of the largest urban forests in the U.S.
The Dallas Floodway Extension (DFE) plan, which has been estimated to cost $140,800,000 to build, is being constructed by the U.S. Army Corps of Engineers. It will be comprised of a chain of seven wetland cells, two new levees, and a 3,000-foot realignment of the Trinity River itself. The USACE is shouldering $118,125,800 of that cost, leaving $22,674,200 as the city of Dallas' share.
This DFE plan, developed jointly by the city of Dallas and the Corps of Engineers, called for a 3,000-foot long realignment of the Trinity River channel in the area of Interstate 45 to protect the bridge structure. The new channel placed the Trinity River through a wider span of the IH-45 Bridge, alleviating the hydraulic problems that were affecting the bridge substructure near the original channel.
T.J. Lambrecht Construction was awarded the contract for earthwork involved with the river channel realignment and the three lower wetland cells. The contractor, who specializes in earthmoving, completed the realignment of the Trinity River at IH-45 in 2007. The excavated dirt was used to primarily to backfill the original channel.
The DFE includes a linear length of 270 acres of swales and wetlands, known as the "Chain of Wetlands" that will produce 170 acres of water surface adjacent to the Trinity River from Cedar Creek to Loop 12, a distance of approximately four miles. The seven wetland cells, averaging about 600 feet in width, are strategically placed along the west side of the Trinity River in areas that will least impact existing trees, and will offer a secondary route for flood waters when the Trinity River is in flood stage.
The first 10-acre cell, a separate contract, was completed by Ceres Construction in 2005.
T.J. Lambrecht was awarded a second contract to construct cells E, F and G, comprising the Lower Chain of Wetlands. That project was just completed in September.
Over the course of this project, T.J. Lambrecht crews cleared 217 acres and excavated approximately 2.2 million cubic yards of material. The most significant material amounts excavated were, according to project engineer Jeff Parish, dirt — just over 1 million cubic yards; rock — 400,000 cubic yards; and landfill garbage — 800,000 cubic yards, which included 280 tons of old tires. Most of the trash was relocated to the city-owned McCommas Landfill, while the tires were recycled through CTR Tire Disposal and Nathaniel Energy Corp. The trees and brush cleared were mulched by Big Bird Land Clearing for sale as commercial mulch.
Cell F runs directly through an old landfill. In order to keep leachate from filtering into the new wetland area, 5,384 lineal feet of slurry wall (131,000 square feet) was constructed along both long sides of Cell F, some of it as deep as 50 feet, according to Ed Taylor, general superintendent for T. J. Lambrecht. The slurry wall system includes 4,600 lineal feet of 4-inch and 6-inch pipe connecting 44 little headwalls and monitoring wells.
"We usually had two crews working in separate areas," said Don Kaestner, T. J. Lambrecht project supervisor. "We excavated the most critical areas first. The river channel was a driving factor. We took the dirt out of Cell F to balance the rock taken out and build the south river bank. Then we had to cap over the landfill area."
Each wetland cell is designed with a control structure to manage water elevation and promote the establishment and growth of terrestrial and wetland plants.
"The series of lakes interconnects with a pipe system to maintain water level the year around," explained Ed Taylor, general superintendent for T.J. Lambrecht. Reinforced concrete pipes connect to weir gate structures at each lake so that the flow can be interrupted or increased during normal conditions. The wetland cells will be filled with water discharged from Dallas' Central Wastewater Treatment Plant. The water will move through the wetlands and empty into the Trinity River.
Plans for the three cells that make up the Upper Chain of Wetlands are still in the design phase and are expected to let in the summer of 2009, according to the corps.
Various species of wetland plants from the Lewisville Aquatic Ecosystem Research Facility at Lewisville Lake will be planted to provide an essential habitat for waterfowl, fish and other wildlife. About 100 acres of grasslands will fill in the area between and around the wetland cells.
About 3 miles of protective levee will be located along the Union Pacific Railroad parallel to Lamar Street to provide the Lamar area with 800-year flood protection. It will be an extension of the Dallas Floodway East Levee from the DART Bridge to the Rochester Park Levee. The Cadillac Heights Levee will be about 2.5 miles long and will provide the Cadillac Heights vicinity with 800-year flood protection as well. Design for both the Lamar Levee and Cadillac Heights Levee began this year by the USACE and city of Dallas engineers. Construction is expected to start in 2010 and be completed in 2013 or as federal funding allows.
"The city funding will not affect the timelines of the contracts to be let, as it is all bond-funded," said Rebecca Dugger, director of the Trinity River Project. "The federal money, particularly those appropriations from the Energy and Water Appropriations bill for the Corps of Engineers, is less reliable. It comes on an annual basis, and the amount is decided by the Congress."
The Trinity River Corridor Project is the largest public works project in the city's history. Dallas voters approved the $246-million project during a May 1998 bond election. When completed, the Trinity River Corridor Project will provide recreational amenities, environmental benefits and flood damage reduction, improve traffic congestion in the downtown area, as well as encourage development of the lands along the river.
Recreation facilities and trails will provide linkages between the Great Trinity Forest, neighborhoods and high-employment areas. The environmental mitigation plan will consist of the purchase of about 1,179 acres of land for forest preservation, and restoration to a native atmosphere of trees, grasslands and wildflower meadows.
Editor's Note: The Trinity River Audubon Center, located just south of the lower chain of wetlands, was completed earlier this month. (Texas Contractor, Nov. 19, 2007).
Learn more about the Trinity River Corridor project online at www.trinityrivercorridor.org.