It's not as simple as you may think. There have been problems with coastal erosion in south LA long before anyone even brought up the idea of global warming. As another poster mentioned, creating canals through the marshes lets salt/brackish water come further inland than it would have (especially with hurricanes). The salt water kills some of the vegetation that holds the marsh together. Another big factor is dredging years ago. Dredging was done as part of creating canals, but also to gather shells that were used for decades to make 'gravel' roads and such. Probably most significant of all (as pointed out by others) was putting in river control structures (primarily levees) long ago by Army Corps of Engineers. The prevents the natural build-up of silt, sediment, and alluvial soil that built up the river plains and deltas.

All of these things occurred long before there was any discussion of global warming. The state can be blamed for excessive dredging and cutting canals, but hindsight is 20/20. Putting in river control structures was considered the right thing to do back then (Mississippi River levees were built in 1800s), and certainly was not only done by those in LA.

Now comes global warming/climate change and it complicates the picture even more. The solutions to all of these problems are not mutually exclusive. Coastal erosion exacerbates the problem of rising seas for coastal areas.

Another tricky factor is where people should and should not live. It's easy (and logical) to say people shouldn't live in areas prone to heavy flooding, and if they do they should accept the possibility of flooding as a consequence of their decision. For example, the lower 9th ward in New Orleans that was completely flooded in Hurricane Katrina was known to be susceptible to major flooding (it had happened at least once before in the 1920's or thereabouts). The majority of the people that live there are poor, their families have been there all their lives, and most probably don't have the means to pick up and go someplace else.

Of course LA residents are responsible for many of these things that have occurred over very long periods of time. But when you consider the full story and where they are today, it's not like the problems in the coastal areas are only the result of global warming.