Project design flood

Flow capacity for the Mississippi river in thousands of cfs based on the 1956 project design flood.

The project design flood is a hypothetical "maximum probable" flood of the Mississippi River used by the United States Army Corps of Engineers to aid in the design and execution of flood protection in the Mississippi Valley.

The current project design flood was developed in 1956 by the Mississippi River Commission with input from the Weather Bureau. It is based on a scenario of three rain events in the lower Mississippi Valley occurring 3 days apart: the January 1937 (increased by 10%), January 1950, and then the February 1938 rain event with its center adjusted 90 miles (140 km) to the north and rainfall pattern rotated to achieve a hypothetical worst-case flood of the tributaries of the Mississippi River. Assumptions were made based on the completion of tributary reservoirs and dams by 1960. When the Commission reviewed the plan after the 1973 Mississippi River floods, the Project Design Flood in use was determined to be "based on sound technology that was still reliable by current standards. The project design flood peak discharges remained unchanged."

Once the project design flood was developed, flow rates at critical points are used to determine how high levees need to be, how deep the dredging needs to be, and the flowrates at which flood control structures need to be used.

1928 project design flood

In response to the Great Mississippi Flood of 1927, the Flood Control Act of 1928 authorized the Mississippi River and Tributaries Project (MR&T), which is implemented by the Mississippi River Commission (MRC). The MRC estimated a maximum probable flood discharge of 2,250,000 cubic feet per second (64,000 m³/s) at Cairo, Illinois that was greater than the 1927 flood but less than the 1927 flood combined with the maximum recorded discharge of the Ohio River in 1913. In a separate analysis, the U.S. Weather Bureau determined that a "maximum possible" flood would result in a discharge of 2,400,000 cubic feet per second (68,000 m³/s) based on the maximum recorded flows of the upper Mississippi, Ohio, Cumberland, and Tennessee Rivers. In a combined analysis, the higher flow rate estimate was used.

Adding the flows of the Arkansas and White Rivers, the maximum flow farther downriver at Arkansas City, Arkansas was estimated to be 2,850,000 cubic feet per second (81,000 m³/s) with a flood stage of 74 feet (23 m). Adding the flows of the Yazoo and Red Rivers, the Weather Bureau estimated a flow of 3,000,000 cubic feet per second (85,000 m³/s) just north of Baton Rouge at Red River Landing, Louisiana.

1941 project design flood

The Ohio River flood of 1937 caused the flood stage at Cairo to reach 59.5 feet (18.1 m) despite a flow of only 2,100,000 cubic feet per second (59,000 m³/s). In response, the United States Congress ordered the MR&T to review of the flood control plan. Maximum flows were changed to 2,600,000 cubic feet per second (74,000 m³/s) at Cairo, 3,065,000 cubic feet per second (86,800 m³/s) at Arkansas City with the White River backwater area levee overtopped, and Red River Landing unchanged at 3,000,000 cubic feet per second (85,000 m³/s).

Current project design flood

The current project design flood was developed at the request of the Senate Public Works Committee in 1954. The Mississippi River Commission and the Weather Bureau worked together to investigate hypothetical flood conditions. The study of a set of 35 combinations of actual storms resulted in the selection of combination called "Hypo-Flood 58A," a combination of previous storms in 1937, 1938, and 1950. The peak discharges were found to be 2,360,000 cubic feet per second (67,000 m³/s) at Cairo, 2,890,000 cubic feet per second (82,000 m³/s) at Arkansas City, and 3,030,000 cubic feet per second (86,000 m³/s) at Red River Landing.

After the 1973 Mississippi River floods, the MRC reviewed the project design flood and found that it was still reliable based on current standards.

Project Design Flood Flow Rates

Location	River	Flow (ft³/s)	Flow (m3/s)	100 year Flood Stage
St. Louis, Missouri	Mississippi R.	150,000	4247
St. Louis	Missouri R.	100,000	2832
Paducah, Kentucky	Ohio R.	1,760,000	49,838
Paducah	Tennessee R.	490,000	13,875
Cairo, Illinois	Mississippi R.	240,000	6796	63.2 ft.
Cairo	Ohio R.	2,250,000	63,713
	Birds Point-New Madrid Floodway	550,000	15,574
New Madrid, Missouri	Mississippi R.	1,810,000	51,253
Memphis, Tennessee	Mississippi R.	2,410,000	68,244	53.9 ft
Greenville, Mississippi	Mississippi R.	2,890,000	81,836
Vicksburg, Mississippi	Mississippi R.	2,710,000	76,739
Natchez, Mississippi	Mississippi R.	2,720,000	77,022
	Old River Control Structure	620,000	17,556
Red River Landing, Louisiana	Mississippi R.	2,100,000	59,465
	Morganza Floodway	600,000	16,990
Baton Rouge, Louisiana	Mississippi R.	1,500,000	42,4755
	Bonnet Carre Spillway	250,000	7079
New Orleans, Louisiana	Mississippi R.	1,250,000	35,396
	Atchafalaya Wax Lake Outlet	440,000	12,459
Morgan City, Louisiana	Lower Atchafalaya Basin Floodway	1,060,000	30,0165

See also

• Mississippi River floods
• Bonnet Carre Spillway

Further reading

• McWilliams, Charles D.; Hayes, George W. (2015). "Comparison of the 2011 Mississippi River Flood to the MR-T Project Design Flood". Journal of Hydrologic Engineering. 22 (5): E5015002. doi:10.1061/(ASCE)HE.1943-5584.0001238. ISSN 1084-0699.
• Camillo, Charles A. (2012). Divine Providence: The 2011 Flood in the Mississippi River and Tributaries Project. US Army Corps of Engineers, Omaha District. Retrieved 2015-12-16.