In 2005, the marshes in the North Shore Mapping Unit sustained severe damage due to Hurricane Katrina. Hundreds of acres of emergent marsh within this mapping unit were lost, resulting in hundreds of acres of shallow open water and scour ponds averaging about 2 ft deep. USGS calculated a 1984 to 2016 area loss rate of -0.91 % per year. Currently there is one area along the shoreline that looks as if a breach is forming. This area also has a small pond immediately behind the critical shoreline. If there were a breach in this area it would allow direct connection between the fresher interior marshes and higher salinity waters of Lake Pontchartrain.
Restoration Strategy:
The overall goal of this project is to restore marshes that were lost and/or damaged due to the effects of Hurricane Katrina. Restoring the marshes should reduce salinity effects on interior emergent marshes.
The proposed features of this project consist of filling approximately 384 acres of shallow open water and nourishing an additional 65 acres of fragmented and/or low marsh with material hydraulically dredged from Lake Pontchartrain. Target settled marsh elevation would be +1.2 NAVD 88, but will ultimately correspond to surrounding healthy marsh.
Progress to Date:
This project was approved for Phase I Engineering and Design on February 9th, 2018.
This project is on Priority Project List (PPL) 27.
Neither time nor sediment should go to waste! CWPPRA and our partners believe that beneficial use of dredged material in projects is an important part of coastal wetlands restoration.
Beneficial use, in simple terms, is the act of using dredged materials to fortify our barrier islands, build marsh platforms, or nourish the coastline instead of disposing it into places that will not benefit from it. Dredging is necessary to keep important transportation channels open for commercial ships and recreational boating. When dredging a canal, sediment is often dumped in holding facilities or off the continental shelf because of the low price tag. Borrowing sediment from otherwise untouched and stable areas is not necessary when dredging already makes viable material readily available. [1]
Many CWPPRA projects that are approved for construction have implemented beneficial use of sediment. For example:
BA-39 Mississippi River Sediment Delivery – Bayou Dupont
MR-08 Beneficial Use of Hopper Dredged Material Demonstration
AT-02 Atchafalaya Sediment Delivery
TE-44 North Lake Mechant Landbridge Restoration
CS-28 Sabine Refuge Marsh Creation (Cycle II and onwards)
And many more!
Of course, some areas will not be in close enough proximity to a channel with reliable dredging, but we want to maximize beneficial use when and where possible. For CS-28-2, our partners installed a permanent dredged material pipeline to further decrease damage to coastal wetlands that temporary pipelines can cause. The permanent pipeline ensures that whenever the Calcasieu River Ship Channel needs dredging, the dredged material goes to restoring wetlands with as little detrimental influence as possible.
Sediment is a valuable resource for coastal Louisiana, and the need for sediment across the coast means that we can’t afford to waste any. CWPPRA projects strive to use sediment from as many sources as possible so that more projects have the material they need- with some creativity, a little sediment can go a long way.
Land loss and land gain are terms we throw around a lot at CWPPRA but what do they mean? Where does the old land go and where does the new land come from? To answer that, we need to understand that “land” is made of inorganic particles that we call sediment and various types of organic matter. Sand, clay, gravel, boulders, and silt are all types of sediment, and grain size is how we classify them. [1] For example, a boulder is larger than gravel, which is larger than a grain of sand, which is larger than a silt particle, etc. Sediment size influences how each grain experiences force and inertia, which leads to different rates of land loss and gain between sediments. Imagine holding a handful of sand in one hand and a handful of gravel in the other. Now imagine you blow as hard as you can on each one. More gravel would stay in your hand than sand. The same is true of sediment in water- smaller grains of sand can be picked up more easily by the forces acting on them than the gravel can.
Erosion detaches sediment from an original source, such as a cliff face or the middle of a valley. Over long periods of time, eroded particles get smaller and smaller, eventually degrading to sand or silt, depending on the mineral base. Once they get into a river or stream, their movement is connected to water flow. When water flows faster or stronger, it “suspends” and carries more sediment, while sediments in slower currents tend to settle out and “deposit” on the bottom of the lake, bayou, or swamp. Approximately 40% of the USA drains through the Mississippi River, and any suspended sediment in those waterways travels through Louisiana on its way to the Gulf of Mexico. [2]
Sediments move downstream differently depending on their size class.
Wetlands are defined by sediment type and other characteristics including salinity. In Louisiana, we have fresh water wetlands like swamps and bottomland hardwood forests, but also saline wetlands like salt and brackish marsh. Each of these wetlands types contains fine sediment particles, and they are all relatively new in the scope of geologic time. Because they are young, there are not many hard-packed substrates in Louisiana wetlands, but instead deeper layers of sediment that are compacting and subsiding. [3] Sediment replenishment is important to all the wetlands in Louisiana because new sediment is needed on top of compacting sediment to maintain elevations that support plant life and productive ecosystems. Unfortunately, sediments that should be replenishing the wetlands of Louisiana are not doing so. Instead, they are being transported out into the Gulf of Mexico or are trapped farther upstream behind dams. More information about this topic can be found in our post “The Mississippi River Deltaic Cycle”. Controlling the flow of the Mississippi river keeps sediments suspended for longer because water does not disperse or slow down as it naturally wants to. Without new sediment, marsh platforms lose structural integrity and they erode, leaving open water where marsh once was.
To answer the original question; for CWPPRA land loss is the process of sediment and marsh sinking or eroding into open water along Louisiana’s coastline and reducing the land available. Land gain describes the process of sediment depositing to form new platforms and it is much less common along our coast, but CWPPRA and their Partners in Restoration are working to restore the integrity of coastal wetlands by moving and capturing sediment, planting stabilizing species on terraces, and creating marsh in critical areas. Combating land loss is a multi-disciplinary effort, and we have a long fight ahead.
CWPPRA 