Multiple Lines of Defense Strategy
Multiple Lines of Defense Strategy
MULTIPLE LINES OF DEFENSE STRATEGY
The coast has always been our first line of defense against hurricanes for southeast Louisiana. Recognizing this, Pontchartrain Conservancy (PC) developed the Multiple Lines of Defense Strategy designed to help save our coast. In the simplest terms, this strategy highlighted below shows how natural features of our coast (like barrier islands, marshes, and ridges) compliment man-made features (like levees) to protect the Greater New Orleans area from hurricanes. Click here to see a video about the Multiple Lines of Defense.
There are 11 lines of defense that help to reduce risk and protect communities from storm surge. Five of these defenses are natural features on the landscape. Many of our natural lines of defense are disappearing due to Southeastern Louisiana's land loss crisis. There are also six man-made lines of defense. The natural lines of defense generally protect the man-made lines of defense which protect the people. Simply put, wetlands protect our levees which protect our people. The 11 lines of defense are described below. The Multiple Lines of Defense Strategy was developed in 2006 by PC. It describes the various features on the landscape that reduce the risk of damage from storm surge to local communities, infrastructure, and economy.
Natural Lines of Defense
The offshore shelf ranges in depth from 300 feet at the shelf edge to zero depth at the Gulf shoreline. Its width varies from a few miles to hundreds of miles. The primary benefit of the shallow shelf is to dramatically reduce wave height and wave energy from an approaching tropical system. A negative aspect of the shelf is that it will promote higher storm surges inland. The variable influences on storm surges due to the geometry of the shelf needs to be considered for storm surge analysis. Also, dredging activities on the shelf should avoid increasing shoreline erosion by wave refraction around dredge holes. The Gulf fisheries and the oil and gas industry are key economic aspects of the shelf. Map of the Gulf of Mexico showing the shallow continental shelf off of the coast of Louisiana. The shelf can provide the first line of defense against deep water currents in storm surge as it moves from the deep waters of the gulf into coastal waters.
The Louisiana barrier island shoreline is characterized by fragmented barriers or shoals with low vertical profiles and low sand content. However, barrier islands provide an important wave barrier for interior sounds and coastal marsh. The primary benefits of barrier islands are the near-complete reduction in wave height and the slight reduction in storm surge further inland. A negative aspect of barrier islands is their ephemeral nature and unpredictable local impacts to them from hurricanes. There are many barrier islands off of the Louisiana Coast which help dampen storm surge waves (left). Chandeluer Islands are a large chain of islands in the Pontchartrain Basin that have been slowly eroding away over time (right).
The primary benefit of the sounds or bays is to provide a relatively shallow water buffer to deep water currents. Sounds do have a negative aspect during storms by allowing waves to re-generate on the on the sound side of barrier islands. Also, sounds may cause storm surge and wave erosion on the back side of barrier islands.
Sounds, bays and other open bodies of water provide a line of defense buffering deep water currents. However, they can also allow storm surge waves to re-generate after they have been dampened by barrier islands or other lines of defense.
Marsh landbridges are areas of emergent marsh with relative continuity compared to adjacent bays, sounds or areas of significant marsh/land loss. Ideally, landbridges connect other elevated landforms such as natural ridges. Since some ridges are developed and have adjacent levees, marsh landbridges may also bridge adjacent levee systems and economic corridors. Landbridges impede storm surge movement inland and protect other emergent marsh areas that may perform the same function. Some landbridges are threatened themselves by various processes of marsh loss and need to be sustained through restoration and maintenance.
Marsh landbridges are a solid line of defense that protect many communities in south Louisiana. A series of landbridges, which occurs in the Pontchartrain Basin, provide multiple levels of risk reduction for interior populations. Marsh degradation threatens to erode these landscape features away, reducing protection.
In southeast and central Louisiana, most natural ridges are the natural levees of abandoned distributary channels. In southwest Louisiana, most natural ridges are chenniers running parallel to the Gulf coastline. Natural ridges may have continuous elevation of several feet and, therefore, will impede overland flow across the ridge and potentially reduce storm surge. Natural ridges are most effective when they have at least 6 feet of elevation and well drained soils to maintain upland forests. Forests will also slow the movement of overland flow and may also provide a wind barrier. Natural ridges tend to be the economic corridors across the coast including primary state highways and coastal communities. These highways are also likely to be evacuation routes.
In southwest Louisiana, the cheniers that formed parallel to the coast line provide a line of defense, like those seen in Cameron Parish (top left). They formed by the reworking of the deposited sediments into ridges that were high enough for trees to grow (top right). Sediments then were deposited again and the cycle continues. Ridges also formed along the edges of abandoned or existing bayous, such as the Bayou la Loutre ridge in the Pontchartrain Basin (bottom). The ridge is where the banks of Bayou la Loutre were and the rock dam that closed the MRGO reconnected this ridge where the MRGO cut through it.
Man-made Lines of Defense
Railroads, highways and spoil banks may run parallel to the coast and locally provide a manmade ridge several feet in height. These foundations may have settled and may need improvement to provide reliable transportation routes without chronic flooding.
Highway 90 across the New Orleans East landbridge is an example of a road bed that is elevated, acting as a ridge that dampens storm surge. The highway bed is at 3 feet or more elevation (yellow in figure on the left) compared to surrounding marsh which is all below 3 feet (bridge elevations are not included in the LIDAR data used to make the map). The picture on the right is an aerial view on Hwy 90 showing the higher elevation road with trees growing on either side (usually an indicator of higher elevation).
Flood gates are typically designed to withhold flood water and, therefore, remain open under most conditions. Flood gates are generally open so as not to impede navigation or natural ebb and flow of tides and aquatic organisms. Flood gates would be closed during a threat of flooding and to reduce flood tides in channels. Because of the generally low elevation of the coast, the effectiveness of flood gates may depend on the nearby topography or constructed features such as levees or spoil banks.
Flood gates are an important line of defense during storms. They are generally open, allowing navigation on canals and bayous and then are closed as a storm approaches. These gates are part of the newly build hurricane protection system built around New Orleans. There are gates in the new levee near Braithwaite (left) and as part of the surge barrier built across the MRGO (right).
Flood protection levees are designed and constructed for flood protection of municipalities or other coastal infrastructure features. Levees are generally not designed to be overtopped or to withstand significant wave erosion. Typical hurricane protection levees protect limited portions of the coast with intense economic development.
U.S. Army Corps of Engineers plan for the Hurricane and Storm Damage Risk Reduction System around the Greater New Orleans region. The new levee system ties into the Mississippi River Levees. In addition, there are local, parish level, earthen levees that also provide protection.
Pumping stations are generally within leveed areas and are used to reduce flood risk from rainfall and are not designed to pump out flood water in the case of a levee breach. Most pumping stations are not prepared with fuel, staff or other requirements to be effective to pump out flood water from a significant levee breach. Generally, these are large capacity pumps which displace water vertically above the water level on the flood side of the levee. Pumping stations are generally to protect areas of intense development.
Pump stations pump water from inside levee systems to the outside during hurricanes and heavy rainfall. The E.G. Gore pump station pumps water from the Chalmette and Arabi areas over a local earthen levee into the Central Wetlands (left, water pumps from bottom of the picture into the wetlands at the top). The newly built, GIWW West Closure Pump Station is larger than two football fields and over 100 feet tall and protects communities on the Westbank from flooding (right).
All homes and businesses in south Louisiana are subject to being flooded if they are not elevated above the normal land elevation. Even those behind levees are not 100% safe. Since there will always be the potential of a storm exceeding the limits of protection from storm surges, immovable assets such as homes and businesses should be elevated to the appropriate flood elevation risk. This is the last line of defense for immovable assets. Elevated homes also provide important side benefits such as improved protection from termites and more economic capacity to re-level or raise the houses due to settlement or increased flood risk.
Elevating houses and other structures reduces the damage caused by flooding and storm surge, which enables people to return to their houses more quickly after evacuating, reduces insurances costs and creates a more sustainable and resilient community. Many houses were elevated after Hurricane Katrina (top). Elevating structures in important both inside the levee system (bottom left) and outside (bottom right).
Evacuation routes are typically highways, but could also include other means of transportation such as railroads, air transportation, etc. Evacuation routes are the last line of defense for people or movable assets. Ideally, evacuation routes may also serve as re-entry routes for first responders and as routes to re-populate after a storm event. Evacuation routes are generally selected based on capacity to move a large number of people to safer areas as a storm approaches the coast. Some routes may be subject to flooding quickly and need to be improved.
Proper evacuation routes and plan is essential when the rest of the lines of defense are overwhelmed or fail in order to get citizens out of danger. There is an evacuation plan for the greater New Orleans regions which includes routes to the east, north and west (left). On measure that is employed is contraflow, where all lanes for the highway are opened going in the same direction (right). Despite contraflow, large traffic jams often result during evacuation due to the sheer numbers of vehicles escaping danger.
Applying the Strategy
Applying the Multiple Lines of Defense Strategy, PC reviewed the 100 plus project proposals designed in the Comprehensive Habitat Management Plan to improve the sustainability of forests, swamps, marshes, and barrier islands. The result was the identification of 10 coastal restoration project areas that provide the dual benefits of restoring coastal habitats while simultaneously enhancing hurricane protection for our region. These 10 projects grouped together are called the Pontchartrain Coastal Lines of Defense Program.
To obtain archived (no hyperlink) documents email info@ScienceForOurCoast.org
Resolution for the Coalition to Restore Coastal Louisiana
Pontchartrain Levee District Endorsement 2008
Hurricane Surge Defense Fact Sheet
MLODS Report Version 1 for Coastal Louisiana
Multiple Lines of Defense Strategy Summary Report
Multiple Lines of Defense Full Report
January 18, 2006 Workshop Proceedings
Multiple Lines of Defense Sys Eng Report Final Aug 2014
Framework for environmental assessment of alternative flood control structures on Chef Menteur and Rigolets Passes within the Lake Pontchartrain estuary, SE Louisiana