Usually, a two-for-one deal is a money saver.

But that might not be the case for local taxpayers when it comes to a portion of the area’s new $14.5 billion storm surge protection system.

Some 34.6 miles of the U.S. Army Corps of Engineers’ Mississippi River levees also serve as part of the storm surge system the Corps recently built with its partner, the state of Louisiana. But that twofer could eventually end up saving the Corps millions, at the expense of local taxpayers.

The hurricane protection system requires 21- to 23-foot levee heights to protect against storm surges that occur when the river level is high; river levees need to be just 20 feet high to prevent normal river flooding. So when the natural, ongoing subsidence that plagues southeast Louisiana eventually pulls those levees below the specifications for the hurricane protection design, the local levee authority must pay to raise them.

And that means the Corps may never have to pay to lift its river levees — because the local levee authorities will make sure they never sink below the 20 feet the Corps requires.

“We have to maintain our specs to qualify for national flood insurance, so we have to do those repairs and lifts,” said Bob Turner, regional director of the Southeast Louisiana Flood Protection Authority-East.

“And right now, we would have to pay all of those costs even though we’re basically doing the Corps’ job of maintaining its (river levee) specs.”

Corps spokesman Ricky Boyett said Turner’s assessment is correct. He noted that maintenance of the two systems is covered under different congressional actions. The storm surge levees are governed by the Hurricane and Storm Damage Risk Reduction System, while the river levees come under the Mississippi River and Tributaries Project.

The Water Resources and Reform Act of 2014 allows the Corps to cover 65 percent of repairs for the storm surge protection system through 2024 — but only if Congress has appropriated the funds.

“And right now, there are no such appropriations,” Boyett said.

The Mississippi levees are a vital part of hurricane protection because a storm surge can travel up the river channel.

For example, when Hurricane Isaac, a weak Category 1 storm, came ashore in August 2012, the river rose 10 feet in four hours at the Carrollton gauge in New Orleans. That posed no threat to the city because the river level was low, as it typically is in the late summer.

However, if a 10-foot jump had occurred in July of this year, when the river was at 15 feet above sea level — a record height for the summer — the river might well have flowed over even the storm surge levee heights for a few hours.

Fortunately, that scenario would have required a bigger storm than Isaac, because the stronger current of a higher river would push back against storm surge.

“We estimate if a storm the size of Isaac came ashore (with the river at 15 feet), the jump would be about only 5 feet,” Boyett said.

Still, even that rise would push the river above 20 feet. That’s why the specifications for storm surge protection require the river levees to be higher than what is mandated to guard against a river flood — and why local levee officials have to keep an eye on subsidence.

The Corps is currently raising about 15.5 miles of West Bank river levees to bring them up to the storm-surge specs, a cost covered by current authorizations.

But Boyett said the Corps’ subsidence estimates indicate 3.5 miles of East Bank levees will need a lift by 2021 — two years before FEMA will check the levees to make sure they still are high enough to qualify the areas they protect for federally subsidized flood insurance.

And by 2057, another 16 miles will need lifts, 6.5 miles on the West Bank and 9.5 miles on the East Bank.

Maintenance of the storm surge protection system is paid for by local property taxes from Orleans, Jefferson and St. Bernard parishes.