When Hurricane Katrina devastated New Orleans a decade ago, its destructive power was unprecedented. But these days, extreme weather events are becoming eerily common. How to prevent the next big storm from walloping the Big Easy? We might need to let the mouth of the Mississippi die.
That's the startling conclusion of two winning engineering and design teams at the recent Changing Course Design Competition, which challenged participants to imagine how we can rebuild the lower Mississippi delta to ensure its long-term survival. As Scientific American explains, an important theme among these plans is letting go of the decaying wetlands at the mouth of the Mississippi river, so that the upper parts of the delta closer to New Orleans can be restored and fortified.
A Delta in Decay
We all remember the photos of huge earthen levees and concrete seawalls toppling over as Katrina barrelled into New Orleans ten years ago. But what many outsiders don't realise is that these artificial barriers were actually the city's last line of defence — the storm surge never should have hit New Orleans that hard to begin with.
The root of the problem traces way south of the city, into a sprawling tapestry of decaying delta wetlands that were unable to absorb the storm surge. The reason these wetlands are in such a pitiful state? Artificial levees — erected long ago by the US Army Corps of engineers — which line the entire lower length of the Mississippi to the river's mouth.
Bayou community south of New Orleans, via Changing Course
South of New Orleans lies a vast delta floodplain, comprised of thousands of square miles of marsh and wetland ecosystems. Here, floodwaters from the Mississippi are as natural — and critical — as the rise and fall of the sun. River water supplies fresh sediment, which carries essential nutrients for the wetland's grasses and mangroves and helps to continually rebuild the naturally subsiding marsh bed. Fresh river water is also key to maintaining the correct level of salinity in the naturally brackish wetlands.
Moffatt & Nichol's concept for a future, rebuilt Mississippi delta. Image courtesy of Changing Course
Everything changed when humans built artificial levees along the river's length to prevent its water from spilling over. The levees were intended to protect farms and delta towns from flooding, but they also drove the region's natural, flood-adapted ecosystems into steep decline. In their current state, the wetlands are simply too weak to absorb the surge from powerful storms like Katrina.
Many studies have come to the conclusion that the only way to reconstitute healthy wetlands is to cut the levees, install gates, and open those gates periodically so that sediment and freshwater can flow into the marshes once again.
Pruning the Mississippi
Each of the Changing Course Design Competition's winning designs was premised on cutting the river levees to rebuild the Mississippi delta. But recognising that the river today doesn't carry enough sediment to rebuild the entire delta, two of the teams (Baird and Moffatt & Nichol) suggest ending the river many miles north of its current mouth, allowing the bottom to be swallowed by the Gulf. This is essentially like pruning the sickliest branches off a tree to keep the rest healthy.
Conceptual model of how we might restore the Mississippi delta, by opening river levees to allow periodic flooding of the floodplain. Image courtesy of Changing Course
Baird team's plan to prune the Mississippi river's mouth (#5) and rebuild the delta further north. Image courtesy of Changing Course
Studio Misi-Ziibi's plan was the most extensive. This team would attempt to save the entire delta and rebuild the barrier islands to its south (outer ring at the bottom right of the image below) to help break future storm surges:
Studio Misi-Ziibi's plan to save the entire Mississippi delta. Image courtesy of Changing Course The big elephant in the room for any of these plans is, of course, money. The Baird Team, whose design would prune the Mississippi furthest north, estimates its plan would cost between $US4.3 billion and $US5.7 billion dollars to implement. The other two teams didn't include cost estimates, but we can expect both to rack up an even larger bill, being more ambitious in scope.
Still, the point of the competition was to generate ideas and educate that state, Army Corps, and other authorities who will eventually be tasked with rebuilding the delta one way or another. Even if none of these plans is carried out exactly as conceived, it's clear that wetland restoration is going to figure big into improving the resilience of New Orleans to extreme storms. And that's an insight that could help us protect coastlines around the world.
Picture: Mississippi river delta, via USGS / NASA Earth Observatory