This article goes to the heart of the problem. Sea level rise pushes the wetlands inland. If you have barriers, the wetlands drown. The only thing the coastal restoration folks care about is building levees, but if they only push for levees, the environmentalists will kill the projects. Thus the myth of coastal restoration had to be created – the idea that you could build wetlands in front of the levees so there would be wetlands left after you build the levees. The related myth is the myth of coastal erosion. Erosion implies you can put the dirt back There is very little erosion. Pretty much all of the land is still there, it is just covered with water. The drivers of this inundation are subsidence (with faulting just being one contributor) and sea level rise.
The key to understanding the problem is recognizing that the narrow strip between the dry land and the water – the intertidal zone where the wetlands live – is not the delta. The delta is the mountain of land built up over hundreds of millions of years and extending upriver for about 200 miles and out into the gulf to the edge of the continental shelf. That delta is always building, it is even building now. But the location of the intertidal zone is not determined by moment to moment sediment, but by relative sea level. The intertidal zone has been in about the same place for the last 5-6k years because sea level has been pretty constant. Even during that period, the river moves and lobes come and go, with the same basic area, as plants and sediment build up and then subside away locally. Look at the Chandeleur Islands. Those are not barrier islands, they are old edge of the coast. The land between them and the current coast was mostly lost long before there were dams and levees on the river.
Sea level has risen 8 inches in the last 100 years. That is really fast in geologic terms. Combined with subsidence, that completely accounts for the land loss – it has just be drowned. Look at those trees in open water, when they were seedlings, that was not open water. But if the land were eroding, the trees would fall over. Sea level will continue to rise and the rate will likely increase, and subsidence will continue. Just as in every past period of sea level rise, the intertidal zone will migrate inland, at least until it hits a levee. Piling up sand and dirt at the edges with diversions and dredges will make no difference. Look at the elevation map and the flooding from Isaac – the inland areas are sinking below sea level. To have a coast, you have to land behind it that is higher than sea level. When people point to the Dutch, they forget three things.
1) The Dutch destroyed all their coastal wetlands a long time ago.
2) The Dutch do not get hurricanes – their 10,000 storm would be a small hurricane.
3) The Dutch do not have anywhere else to go, so they spend several % of their GDP on levees and just accept the environmental damage.
But for the subsidies from the National Flood Insurance Program, and the federal government making disasters good business, LA would have faced these realities long ago and would have been moving away from the coast since Betsy. Sadly, the likely course will be levees and diversions to destroy the wetlands, and then it will all get swept away because you really cannot hold back the ocean in the long term. All of this coastal restoration is very short term. The only question is whether ocean rise is high enough to wipe out the coast by 2100. There is no question that it will do it by 2200. The real question is whether we just wait for the disasters or whether we try to move inland in an orderly way and preserve some of the culture.
Assuming that NO is lucky and dodges hurricanes for another 40 years, the commercial insurance market, which is not subsidized by the feds, will slowly price most businesses out of the city and continue its economic death spiral. The only thing propping up the economy now is residual federal Katrina spending and debt financed projects like the hospital construction. When that is complete, the cost of paying the bonds will be joined by the huge operating loss of the facility.