Flood ProtectionCity Below Sea-LevelNew Orleans is a city below sea-level. During an extremely low stage of the Mississippi River, the city at its lowest point would be approximately five (5') feet below the level of the Gulf of Mexico because there would be no flow in the river. As the spring water from the Mississippi River system flows through the city, the levee and floodwall systems have been constructed to handle a river stage of eighteen (18'] feet above the level of the Gulf.Should it be anticipated that the water level exceed this design stage, water is diverted by other means. The first diversion to be opened would be the Bonnet Carre Spillway which would direct the river water through Lake Pontchartrain to the Gulf. If the anticipated water volume would exceed the Bonnet Carre Spillway diversion, a second diversion would be opened north of Baton Rouge, LA. This would channel the water to the Atchafalaya River, which flows into the Gulf at Morgan City, Louisiana. Click here for a French Quarter view of a ship moored in the river at the high water state. Ships generally tower over the city at low water and high water increases this towering effect by nearly two stories of a normal building.
Development Necessitates ImprovementsThe wharves which once lined the banks of the Mississippi River established the City of New Orleans as the second major American seaport. These wharves also provided flood protection to the citizens; however, initially these flood protection facilities were not of sufficient height to accommodate a projected maximum river stage. For this reason floodwalls have been added to increase the flood protection.Commercial development of these wharves began as space requirements for break bulk cargo diminished due to the use of containerized cargo. Click here to see the commercial development of the area located near the former Poydras Street Wharf in the vicinity of world famous Canal Street. Note that the developer has chosen to provide overhead pedestrian crossings to reduce the safety problems to people crossing the floodwall and the railroad tracks required for the remaining wharves.
Lakefront Neighborhoods Need LeveesClick here to see an illustration of a section of an earthen levee protecting residences along the shoreline of Lake Pontchartrain in Orleans Parish, Louisiana.The crown of an earthen levee is approximately twenty feet above the level of the Lake and as depicted above, often is of greater height than the structures being protected. The toe of the levee denotes the edge of the earthen structure. The original seawall built in the 1930's only rises about 7 feet above the Lake. Construction of the earthen levees along Lake Pontchartrain was stimulated by Hurricane Betsy," which topped the seawall. Since then, the protection elevation requirements have been adjusted based on projections of water levels by storms of greater intensity than Betsy. The critical path (or direction) at which the hurricane moves increases the water level which produce maximum storm tides in Lake Pontchartrain. The levee crown is normally 10 feet wide but the entire levee requires a width of approximately 170 feet from the flood side toe to the protected side toe. While large in size, earthen levees are the most cost effective means of providing flood protection.
Concrete FloodwallsThere are sections along Lake Pontchartrain shoreline and the Outfall Canals of the interior drainage system, where there is insufficient space to provide flood protection by the use of structures such as earthen levees. In these cases, rigid floodwalls made of concrete are constructed.In many cases, these walls, designed to withstand hurricane forces, require piles to support the weight of the wall and the lateral pressure of the water. The piles work as an interior armature to provide for wall stability when water pressure is applied. Note that the height of these floodwalls is usually greater than the structures which they protect. These floodwalls are often built along road sides and around neighborhoods. Though they serve a protective, practical purpose, their aesthetic value also must be considered. For this reason, different finishes have been created, such as brush hammer, ribbed or chizzled surface effects. Images such as birds or sailboats, illustrated above, are included to provide a decorative appearance.
Flood GatesProvisions for crossing the flood protection system include ramps and floodgates. Ramps are the most economic solution. However, there are situations which cannot facilitate the space needed for a ramp. When there is insufficient space, floodgates are constructed. Click here to see a depiction of a Swing Gate, one of three main types of gates employed in floodwall protection. The other two types are sliding and overhead gates.The drawing illustrates the intersection of Lakeshore Drive and Marconi Drive, where a swing gate is incorporated because of the space available in an established neighborhood. All gates are designed to be manually operated, since a loss of power is likely in the case of a severe storm. The gates are made of steel and when closed are latched into place against rubber seals to insure full closure. Gates vary in weight from 6,500 Ibs. to 70,000 Ibs. All movable parts such as hinges, rollers and latches are made of stainless steel to avoid corrosion. These gates have proven effective for years. Gates are closed during hurricanes, such as our most recent brush with disaster, Hurricane Andrew, as well as, during coastal flood watch conditions.
Sector Gates Maintain Marina Traffic FlowThere are occasions when waterways cross the flood protection system. Movable gates are needed to facilitate anticipated marine traffic on these waterways. Structures containing gates are used. The Orleans Levee District maintains and operates two sector gates.Bayou St. John, an original waterway to the central part of New Orleans and Bayou Bienvenue in Eastern New Orleans utilize sector gates. Click her for an illustration of the sector gate structure and floodwalls along Bayou St. John. The structure is made of reinforced concrete with hinged steel sector gates. The gates which are power operated can be hand cranked closed in the event that power is not available. When the gates are open Bayou St. John provides a thirty (30) foot wide opening with an eighteen (18) foot draft for vessel passage. The Bayou Bienvenue structure is slightly larger.
Sliding Floodwall GatesClick here for a picture of one of the largest sliding gates in the floodwall system. It is at the entrance to New Orleans Lakefront Airport. The floodwall provides protection to a highly developed area of eastern New Orleans. Because of the floodgate's size, closure of the opening is important in maintaining flood protection in the area.The gate is 70' long and 12' high. The gate's height in this location provides necessary protection for a projected high water level of 1 E3' in Lake Pontchartrain. Because of forces on the gate exerted by this amount of water pressure, the gate is a structural frame composed of steel 36" wide flange beams covered by 3/E3" thick steel plate. It weighs 63,000 pounds. Provisions have been included so that a hand winch can move the gate in place once the traffic barriers are removed. The entrance to New Orleans Lakefront Airport also provides access to South Shore Harbor. The harbor provides safe refuge for pleasure boaters and a gaming vessel.
Hard to Protect AreasThere are developed areas in New Orleans which are outside the limits of flood protection. Examples of this are the wharves and railroad tracks along the Mississippi River and the New Orleans Lakefront Airport. These facilities were built long before the flood protection system, and because of the natural slopes which fall away from waterways normal rainfall drained into the city drainage system. It is necessary to pass this rain water through the flood protection system. To prevent passage of flood water, closure of drainage system is required.Cast iron gate valves of various sizes ranging from 15 inches to 48 inches are normally used. There are closure requirements which are larger than what can be accommodated by gate valves. In this instance, specially fabricated sluice gates are required. The sluice valve structure is composed of a concrete containment box, stem, opening and gate covering. The stem controls the gate which regulates the flow of discharge through the opening. Click here for examples of a standard gate valve and sluice gate used to isolate New Orleans Lakefront Airport from the city drainage system. The Orleans Levee District maintains over 100 closure devices in this nearly 130 mile river and flood protection levee system.
SandbaggingWith all of the modern technology available, the sandbag is still considered to be the prime instrument in working to control water in a fight against flooding. The last major use of sandbags was in the 1993 midwest floods.A sandbag weights forty (40) pounds and provides one square foot of surface when placed. The Orleans Levee District maintains 22,000 sandbags in dry storage to meet known requirements in open areas such as bridge openings in floodwalls, low lying roads, levees which may have settled and other emergency uses. Shortly before Hurricane Betsy in 1965, the Orleans Levee District developed a machine to fill sandbags. The machine has air and hydraulic systems which furnish the controls required to maintain the flow of sand through the various hoppers. A complete 40 lb. bag weighed, tied and stacked can be made every five seconds. |