The huge, floating concrete barge gate that closes half of the Gulf Intracoastal Waterway as part of the Lake Borgne Surge Barrier poses the greatest flooding danger to residents living behind the east bank’s hurricane levee system among the system’s nine major gates, according to a new study released Thursday (July 20).
The risk of the barge gate failing is still comparatively small, just a 1 in 318 chance per year, according to the study of complex risk issues by Tetra Tech, an engineering consultant firm working for the Southeast Louisiana Flood Protection Authority-East.
But when looking at what that risk could mean in terms of money, the report concluded it represented an average threat of a $695,410 financial loss per year, while the maximum cost from a single failure event could total $808 million in flood damage.
The report points out that the barge gate is the only major gate in the system whose risk of failure is significantly greater than the 1 in 10,000 probability used by the Army Corps of Engineers to judge dam safety.
The barge gate was originally supposed to be closed except in advance of tropical storms or hurricanes, with waterway traffic using an adjacent “sector gate” consisting of two pie-shaped doors that can be easily closed in advance of a storm.
But as the two gates were being built into the barrier wall, the Army Corps of Engineers completed a study of water movement through the canal with one gate open and the other closed and found it would be better to have the barge gate open most of the year.
Before the barge gate is closed, which requires the floating structure to be swung by winches to be parallel with the wall and then sunk into place, a diver must check under water to assure that there is nothing on the sill on which it will be sunk.
The gate has been problematical since it was first built. In June 2012, contractors discovered a 15-foot-long, horseshoe-shaped crack in its bottom when the contractor building it conducted a test of its ability to be sunk in place by filling several ballast tanks with water. Officials believe the barge settled on steel beam as it sank onto the sill.
A year later, the corps had problems closing the gate during tests in advance of the hurricane season, blaming an oil leak involving the windlass used to pull the gate into place.
During a similar test in June 2016, after the gate had been turned over to the levee authority as complete, divers found a four-foot-high pile of rocks on the underwater concrete slab. The rocks were sliding off a sloped portion of the waterway channel leading to the navigation gates, where rock rip-rap had been used to reduce erosion.
David Moore, an engineer with Tetra Tech, presented the results of the study to the authority at its monthly meeting Thursday. The other gates reviewed in the study all had much smaller probabilities of failure annually, ranging from a 1 in 8,157 chance for a vertical lift gate on Bayou Bienvenue where it crosses the Lake Borgne Surge Barrier, to a 1 in 793,227 annual chance of failure for the sector gate at Bayou St. John on Lake Pontchartrain.
The study points out that while the risk of failure at the Bayou St. John gate is tiny, the cost of a failure could be catastrophic, as much as $1.9 billion under a worst-case scenario, since it could result in flooding of a significant part of the city of New Orleans.
In determining the risk of failure events, Tetra Tech reviewed each gate separately. They examined the manuals governing the detailed procedure for closing the gates, including how to close them by hand when electrical equipment is knocked out, or the availability of tugs, when needed; the equipment involved in their opening and closing, including electronic and hydraulic equipment; the personnel needed for opening and closing, the routine daily, annual and long-term maintenance requirements, and the natural and man-made incidents that might be involved.
They also factored in the time it would take to repair the gate and its operational equipment, and the time it would take to recover from a failure to close.
For failures, the team reviewed the amount of water that could flow into the protected area of the levee system, the frequency of surge levels that might stress the gates, the additional effects of overtopping, and then compared the results to similar reviews conducted by the Corps’ Interagency Performance Evaluation Task Force during construction of the system. The financial estimates of losses were adjusted to present values.
Bob Turner, director of engineering and operations for the authority, said the results would be used to refocus training on how to operate and maintain the gates, and will be updated as the gates age or as changes in equipment or personnel are encountered.
