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 90miles 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.
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 2250000cuft/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 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 2850000cuft/s with a flood stage of 74feet. Adding the flows of the Yazoo and Red Rivers, the Weather Bureau estimated a flow of just north of Baton Rouge at Red River Landing, Louisiana.
The Ohio River flood of 1937 caused the flood stage at Cairo to reach 59.5feet despite a flow of only . In response, the United States Congress ordered the MR&T to review of the flood control plan. Maximum flows were changed to at Cairo, at Arkansas City with the White River backwater area levee overtopped, and Red River Landing unchanged at .
The current project design flood was developed at the request of the Senate Public Works Committee in 1954.[1] 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 at Cairo, at Arkansas City, and 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.
| River | Flow (ft³/s) | Flow (m3/s) | 100 year Flood Stage | ||
| Mississippi R. | 150,000 | 4247 | |||
| St. Louis | Missouri R. | 100,000 | 2832 | ||
| Ohio R. | 1,760,000 | 49,838 | |||
| Paducah | Tennessee R. | 490,000 | 13,875 | ||
| 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 | ||
| Mississippi R. | 2,410,000 | 68,244 | 53.9 ft | ||
| Mississippi R. | 2,890,000 | 81,836 | |||
| Mississippi R. | 2,710,000 | 76,739 | |||
| 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 |