Morganza Spillway

The Morganza Spillway is a hydraulic structure on the Mississippi River above Morganza on the west bank of the river in Louisiana. It lies on the river mile 280, ie approximately 450 km upstream from Head of Passes in Pointe Coupee Parish southern area of ​​and is operated by the United States Army Corps of Engineers. The building is designed to reduce at high water the level of the river and during extreme flood events to direct the water to the cities of Baton Rouge and New Orleans over, by passing it into the Atchafalaya River and the Atchafalaya Basin. This is also the potential risk be countered that the Mississippi River permanently changes its course after a flood, what enormous ecological and economic consequences.

Operation

The Morganza Spillway is a controlled spillway, because doing a series of lock gates is used to control the amount of water that can flow into the relief areas on both sides of the building. It consists of a reinforced concrete weir, two roll weirs, seventeen scour indicators, and 125 provided with sluice gates valves, with which approximately 17,000 m³ / s of water can be derived from the Mississippi River. The building was completed in 1954 and is maintained by the United States Army Corps of Engineers. In the case of flood, it is checked that foundations are not undermined and become unstable. U.S. Highway 190 crosses the relief channel on a roughly seven -mile bridge. An older two-lane bridge leads directly about the building and forms part of the Louisiana Highway 1

Operation during flood

Both sides of the Morganza Spillway are usually above the normal water level and therefore are usually dry. Thus, the water can be discharged through the discharge channel, the water level must exceed the high water mark in the Mississippi River. The Corps of Engineers is considering the opening of the Morganza Spillway when the discharge flow of the Mississippi River at Red River Landing, Louisiana 1,500,000 cubic feet exceeds ( 42,000 m³ / s) and continues to rise.

Since 1963, usually water from the Mississippi River is diverted at high water in only one place, at the Old River Control Structure ( ORCS ), where normally 30 % of the flow into the Atchafalaya River is derived. During the Mississippi flood in 1973, this building has been damaged due to the high discharge rate, so in addition to the Morganza Spillway was opened to relieve the Old River Control Structure, later the Army Corps built directly below the ORCS the Old River Control Auxiliary Structure ( ORCAS ), so that at extreme flood additional flood gates the derivation of water allowed.

The Morganza Spillway, about 50 km downstream is of ORCS and ORCAS was built to conduct additional water from the Mississippi River into the Morganza Floodway, which leads further down in the Atchafalaya Floodway before the united water from two discharge channels in the Gulf of Mexico flows. In an extreme flood event, the flood can flow through the Atchafalaya Basin also. The derivation of the water from the main arm of the river leads to the lowering of the water level in the Mississippi River below the building and reduces the pressure on the levees and other structures for flood protection above and below the two plants. The spillway at Morganza should also make it mitverhindern that the Mississippi River permanently changes its course and flows away from Baton Rouge and New Orleans over the Atchafalya River to the Gulf of Mexico.

The Morganza Spillway passing first the water reaches the discharge polder, extending from the dikes on the Mississippi River to the eastern dyke at the Atchafalaya River. The polder has a length of 30 km and a width of up to eight kilometers. These include a stilling basin, an inlet channel, a flow channel and two dikes that divide the polder from the environment. The derived water reaches the Atchafalaya River Basin Floodway near Krotz Springs.

If water from the flood- leading Mississippi River Morganza Floodway in the Atchafalaya Basin and thus derived the will, there is a flooding of a large area of southern Louisiana. For this reason, the weir will only be unloaded when it comes to areas below the discharge polder flooding, regardless of whether the weir is opened or not. This " natural" flooding and the amount of discharged water will determine the extent of the flooding in the Atchafalaya Basin. Risk are Morgan City, several other smaller settlements, many farms and extensive wetlands. The people know that it is a natural flood plain in the region, and all concerned are reminded every year in writing the fact that the operation of the spillway was possible. Any decision to open the weir must be carefully planned, so that those affected accordingly warned and life and property can get to safety. Part of this planning process is the development of scenarios by the Army Corps of Engineers, the basis of which the person concerned can discuss how much water, if any, to be derived from the Mississippi River.

Past flood events

Morganza Spillway 1973, the first time was open: This was done in order to reduce the water pressure on the Old River Control Structure. The floodgates were opened so that the spillway took up about half of its planned maximum flow rate. From the building appeared minor washouts, and it caused minor damage to the stilling basin. The Atchafalaya Basin was partially flooded. After the tsunami of 1973, the building was repaired.

In 2008, the apex height of a flood on the Mississippi River reached higher than the dikes near the Morganza Stillway, but this has not been opened. Local residents and soldiers of the National Guard of the United States tried to raise the dikes by sandbags, but resulted in additional rainfall to the fact that the flood swept over the sandbags and flood inundated about 1,200 acres of farmland within the discharge channel.

Floods in 2011

Before 13 May 2011, the Army Corps of Engineers was considering four scenarios, which were assumed in all four variants thereof that the Bonnet Carre Spillway would be fully open at New Orleans.

• Scenario 1: Morganza Spillway is half open, so 8500 m³ / s would be derived
• Scenario 1A: as # 1, but Morganza Spillway would be opened only to 25 per cent, so 4200 m³ / s would be derived
• Scenario 2: Morganza Spillway will be closed and the Old River Control Structure, the flow rate of the Mississippi River to River Atchafayala is limited to 30 percent
• Scenario 3: Like # 2, but can be derived at the Old River Control Structure extra 4200 m³ / s

Scenario 1a

Scenario 2

Scenario 3

The analyzes showed that it is definitely in the Atchafalaya Basin flooding, regardless of whether the Morganza Spillway is opened or not. On 13 May 2011 the Army Corps of Engineers decided to open the Morganza Spillway to 25 percent of its capacity, thus we used scenario 1A.