If you have ever visited our Sea Level Rise and Coastal Impacts Viewer, you will notice that we currently do not have two states mapped yet—Alaska and Louisiana. The description on our Digital Coast page says that “maps are not currently available for Alaska and Louisiana due to accuracy of existing elevation data, the hydraulic complexity of the coast, and gaps in vertical datum transformation.” The issues we have in Alaska are for another blog post at another time. The issues we have in Louisiana are primarily due to a patchwork of elevation data sources and the hydraulic complexity of the coastal plain. This blog will give some detail on why Louisiana is so darn hard to map correctly for sea level rise impacts.
- Portions of Louisiana have the highest relative sea level rise rate in the U.S. based on monthly mean sea level observations from 1947 to 2014; the rate at the Grand Isle NOAA Tide Gauge has been 2.96 feet in the last 100 years.
- Portions of Louisiana have the highest subsidence rates in the U.S. as well. Over the last 50 years, some areas have seen over 3 feet of subsidence. This local subsidence has led to the high relative sea level rates noted in #1 above. Of the 2.96 feet of local relative sea level rise over the past 100 years, only ~0.5 feet were from global sea level rise. The rest was from local subsidence.
- With global sea level rising and the land in Louisiana’s coastal region sinking, it is well known that Louisiana loses up to 16.57 square miles of dry land a year (or a football field size loss an hour). So what is “zero elevation” is constantly changing.
- Much of the Louisiana coastal plain is not walkable/not inhabitable land. At elevated tides, much of the lower part of the state is already under water and many places are already at or below sea level.
- The Louisiana coastal plain could be called the land of meanders, canals, levees, and other natural and man-made features that make it very hard to map. Since the late 1800’s, several major flood events and resulting legislative actions to reduce flood risk have led to a very complex system of flood risk reduction measures in the form of both federal and local levees, floodwalls, and control structures. The main stem of the Mississippi River and portions of the Atchafalaya Basin, for example, are bounded by miles of levees. Some of the levees were built to protect against river flooding and some against coastal flooding. There are a mix of federal and non-federal (local) levees ranging in size and design. Many are not fully enclosed, but rather tie into higher elevations on two sides, making delineation of the levees particularly difficult to map.
- Many portions of Louisiana are actually at or below sea level today. Most of the City of New Orleans lies below NAVD88 zero (very close to present sea level). Some places in the city are many feet below sea level. Annual high water on the Mississippi River and the average lake level of Lake Pontchartrain are both well above the ground elevations in New Orleans, so the city is literally a bowl surrounded by water. Mapping the impacts of sea level in a place like this is very tricky. The area protected by levees has to be excluded from the mapping manually.
- The best available elevation data sets in Louisiana are from a variety of sources with different ages and vertical accuracies. The best available data set currently is provided by the USGS. It is called the Coastal National Elevation Database (CoNED) Project–Topobathymetric Digital Elevation Model (TBDEM). It is a 3-meter Digital Elevation Model that is a mosaic of many individual data sets with various vertical accuracies from (25cm to 7cm RMSE). In places where the older (circa 2002-2003) LA state lidar is still the best available, the mapping confidence in only on the order of +/- 2 ft. Much of the coastal plain of Louisiana floods at 2 ft above Mean Higher High Water so the elevation accuracy is a big issue. Newer, more accurate, lidar data sets are slowly replacing the older data (funding dependent), which will eventually provide much better vertical accuracies and thus mapping confidence.
So how do you map the impacts of sea level rise in a state that already has many portions at or below sea level, has very high subsidence rates and thus sea level rise rates, has poor elevation data accuracy in some places, and is chock full of complex hydrology and flood control structures? The answer is carefully, which takes a lot of time.
We have been working on it now for over two years. We have been collaborating with many Louisiana partners including LA SeaGrant, LSU, the Coastal Protection and Restoration Authority, the LA Coastal Program, and Climate Central (who also does sea level rise mapping). We now have the best elevation and levee data there is and this will pay off, as we hope to release impact maps that illustrate the realistic future flood risk in Louisiana very soon.