Toledo Bend Dam Spillway Repairs Honored by APWA Texas
Pictured at top: Fish relocation efforts occurred three times during construction so the stilling basin could be dewatered. Water Resources Engineer Dara George was among the team members who captured fish of special interest, particularly paddlefish and freshwater eels, and released them downstream.
In 2016, Toledo Bend Dam experienced the flood of record with a peak discharge of more than 200,000 cubic feet per second, resulting in damage to the spillway, likely due to cavitation. The Sabine River Authorities hired Freese and Nichols to design repairs to the concrete spillway and downstream channel following these unprecedented high flows. The repairs project was a result of the disaster and consisted of riprap replacement, structural concrete repairs and downstream river channel restoration. The scope also included a more robust riprap design, complicated Care of Water Design, cofferdam design, damage assessment, petrographic material testing (microscopic analysis and compressive strength testing of concrete core samples), computational fluid dynamics (CFD) model evaluation, and environmental site assessments.
Computational fluid dynamics model of probable maximum flood event
The team developed a CFD model using Flow-3D to calculate the movement of water, including waves and splashing at instantaneous points in time. The model was calibrated to various scenarios, including the design storm, the probable maximum flood (PMF) event, the March 2016 flood of record, and the results of a 1:100 scale physical spillway model constructed in 1965. The results of the CFD runs closely matched the observed conditions of the March 2016 event. Based on this evaluation, FNI determined that the original rock riprap did not meet current design criteria, as exhibited by the displaced downstream riprap. The repair design utilized parameters from the CFD model to increase the thickness, maximum diameter, and gradation of the replacement riprap.
Unusual Accomplishments Under Adverse Conditions
The first unforeseen storm in December 2018 required engineering judgment to place temporary high-strength early-set concrete in an area that had been demolished and had the potential to compromise the integrity of the concrete. Approximately 7,000 square feet of demolition had occurred by December 2018 (roughly one-third per the contract documents). A concerning area was identified of approximately 1,600 square feet of concrete demolished to an average depth of 10 inches, located on two longitudinal joints at spillway gate piers 6 and 8 and extending downstream approximately 75 feet. The existing concrete slab in this area was recognized to be much thinner than the other demolished areas.
Freese and Nichols, the Authorities and contractor were in constant communication as they watched the National Weather Service storm projections. They decided in the early morning hours to move forward with the unscheduled work and install temporary concrete at longitudinal joints on the spillway. The work was completed on December 19, 2018, in anticipation of imminent spillway gate operations, and the gates were opened the next day.
The 2-mile-long dam impounds Toledo Bend Reservoir on the Texas-Louisiana border.