Here’s a question I’d like to know the answer to. Hurricane Gustav dealt New Orleans a glancing blow, passing it by to the west. Yet as the world saw, the city’s Industrial Canal — a large ship channel running north-south close to neighborhoods — filled nearly to the top, and there was some alarming, if mostly harmless, overtopping due to wind and waves.

Why did this happen, and what does it say about the city’s vulnerabilities in future storms — and Louisiana’s disappearing coast?

We sort of know the answer to this. The Industrial Canal is connected to the open Gulf of Mexico via two other channels, the Gulf Intracoastal Waterway and the notorious Mississippi River-Gulf Outlet. The latter is slated for closure because of its role in coastal erosion and (though this is controversial) as a conduit for storm surges into the city. Both these waterways run adjacent to open marshes that are rapidly eroding, and Lake Borgne, which is essentially a large lagoon open to the Gulf. So it’s very easy for a lot of water to roll right into the heart of New Orleans. It happened in Katrina. Now it has happened in Gustav. Another foot or two and there would have been some serious flooding.

The Army Corps of Engineers plans to build a gate across the MRGO and Intracoastal where they meet in a giant "V." That area also became notorious after Katrina because storm-surge water flowing into the vertex of the V can rise up very high and overtop levees. And from that point, it’s a straight shot to the Industrial Canal, alongside neighborhoods of the St. Bernard, the Lower Ninth Ward, and eastern New Orleans.

But if the large flow into the Industrial Canal, and its near overtopping Monday, is due partly to the diminished protection of disappearing marshes, this whole area may be getting even more dangerous. Building a gate may offer a good brute-force solution, but it also seems we’re getting closer to a New Orleans-as-Venice scenario, in which Gulf waters are lapping against levees, gates, and other structures — a risky scenario indeed.

To keep this from coming to pass, it’s necessary to speed up coastal restoration, and integrate it with levees, gates, and other structures — something we haven’t really learned how to do yet.