Tuesday, June 14, 2011

Flood Control vs. Coastal Erosion: Finding A Balance


As most of the people in Terrebonne Parish know very well, managing the Mississippi River can be a big problem. Levees and other flood control structures have made catastrophic floods less common than they once were, but there is a downside to keeping the river on a tight leash: as disastrous as floods can be, they are what created southeast Louisiana.

Before the levees and spillways were built, land here was created when the Mississippi and its tributaries and distributaries (offshoots) overflowed their banks every year, depositing layer after layer of sediment. While land was perpetually created in this way, it was also perpetually destroyed by subsidence and erosion. For thousands of years, these two opposing forces maintained a dynamic balance. The pattern of dry land and wet land shifted with the changing river channels, but the total amount of land stayed roughly the same.

This balance was destroyed when people began to prevent the Mississippi River from flooding, by building levees and other flood control systems. Since we learned to prevent most floods, the river almost has stopped depositing sediment. Subsidence and erosion keep destroying land, as they have for millennia, but the Mississippi isn't allowed to build it back again.

The amount of land lost in the last hundred years is truly mind-boggling. Since 1932, Louisiana has lost over 1,800 square miles of land--an area about the size Terrebonne Parish. Currently, our coast loses about a football field of land every 45 minutes. It's obvious that this is a huge problem, but it's not obvious what to do about it. We can't just knock down the levees and let the Mississippi go back to its old ways. Its method of creation is just too destructive. The challenge is to find ways to allow the river to create new land, but in a controlled way.

Because this is such a complex issue, the Terrebonne Parish Library Reference Department will be present a series of blog posts about it, discussing the best places to find reliable information. For now, we would like to mention three excellent introductory resources:

To get a sense of just how much the Louisiana wetlands have changed in the last few decades, take a look at the "before and after" image that appears in this article in the Huffington Post. Moving your mouse across this image will show how much the landscape has changed from 1973 to 2010.

If you want a more detailed view of wetlands loss, the US Geological Survey has just published a new map showing where and when land has been lost since 1932. Red, orange, and yellow areas of the map show land lost before 1980, while blue and purple areas show land lost since then. A couple of small green areas show how the Atchafalaya River has begun to create new wetlands and deltas in Atchafalaya Bay.

For an excellent overview of the geologic processes that created southeast Louisiana, and the processes that are now threatening to destroy it, check out this excellent animated tutorial from the New Orleans Times-Picayune.

If you would like to find out more about this topic, please contact the Terrebonne Parish Library's Reference Department. We can show you a wide range of books, videos, articles, and websites to help you understand the issues. We'll also be focusing on some of these great resources in future blog posts, so stay tuned!

1 comment:

Jamie Cloud said...

Thanks for the detailed info. Rainfall runoffs can be a serious problem especially if there isn’t a good drainage system in the area. However, erosion control products such as jute mats or geotextiles are effective in stabilising the soil and preventing erosion. It’s also a great way to help debris settle on the surface and prevent rubbish from finding its way to nearby waterbanks.