3.5 Hydraulic Models on the Mississippi River

The Corps of Engineers has used several models and procedures for the 300 mi (483 km) reach of the Mississippi River that comprises the limits of the St. Louis District, from the mouth of the Ohio River to near Hannibal, Missouri.

1950s/1960s - During this period, hand computations and mechanical calculators were used to perform standard step backwater computations to determine design water surface profiles for levee design events. Days or weeks of work were required by the engineer to compute a profile over a long reach of river. These computations were sometimes confirmed or modified based on results of the Mississippi Basin (physical) Model (MBM) of the Waterways Experiment Station. The levees and floodwalls near St. Louis, as well as lower levees protecting farmland downstream of St. Louis, were analyzed and designed with these tools. The first rudimentary computer programs to perform water surface profile computations for this reach were used in the mid to late 1960s.

1970s - The large flood on the Mississippi in 1973 set new records for much of this reach of the river. Following this event, discharge data were analyzed statistically (discussed in Chapter 5) to update estimates of the peak discharge for hypothetical flood events, such as the 100-year average recurrence interval event. These steady flow discharges for the 5-year through 500-year average recurrence interval floods were used in the MBM to compute peak water surface profiles over 300 mi (483 km) of the river. These results served as the best estimates of hypothetical flood profiles into the 1990s. Flood insurance studies, feasibility reports, and other studies all employed the results of these physical model tests for the Mississippi River.

1980s - The hydraulic needs of the USACE required a more available source for computation of hydraulic profiles than the MBM. Consequently, surveyed cross-section data for the 300-mi (483-km) study reach of the Mississippi River floodplain was collected in the 1970s for use in the development of a HEC-2 model of the river. Many additional sections were interpolated from both the nearby survey data and USGS topographic maps. Hydrographic survey data (soundings of the channel beneath the water surface) were incorporated to obtain geometric descriptions of the channel. An HEC-2 model was built for the 300 mi (483 km) of the Mississippi River within the boundaries of the Corps of Engineers' St. Louis District and calibrated to several recent large floods. The model was used throughout the 1980s to refine the frequency profiles computed with the MBM, to evaluate proposed changes in the floodplain, such as levee raises, and to establish a regulatory floodway on both sides of the Mississippi River for the U.S. Federal Emergency Management Agency. Two-dimensional hydraulic modeling was also performed for a short reach of the river just upstream of the mouth of the Missouri River for the new locks and dam that would replace old Locks and Dam 26 at Alton, Illinois. The TABS2 package was used to study flow patterns around the first-stage cofferdam. Calibration and verification data for this numerical modeling effort were also obtained from physical models in conjunction with two-dimensional modeling (hybrid modeling).

1990s - Following the Great Flood of 1993, which broke all discharge and stage records over nearly this entire 300-mi (483-km) reach of the Mississippi River, it was apparent that an unsteady hydraulic model of the river was needed. Barkau's UNET program was selected for use and the initial unsteady flow model, using 1970s survey information, was developed in 1994 to study various alternate scenarios for the 1993 flood, such as 1993 flood elevations for no levees, higher levees, more flood storage, and so on. Following the 1993 flood, close-contour-interval aerial mapping was obtained of the entire floodplain throughout the reach. The new mapping was later incorporated in the model and the model was calibrated to both recent flood and low flow events. The HEC-UNET model has been successfully used for river forecasting to better regulate USACE reservoirs and navigation works. By 2002, the HEC-UNET model had been applied to 100 years of daily discharge data to perform period-of-record hydraulic routing throughout the reach. Period-of-record inflows to the HEC-UNET model were developed using the HEC-HMS program to compute continuous discharge hydrographs for all tributaries to the Mississippi. This effort will be used to determine revised frequency relationships both with and without upstream reservoir effects and to update the stage-frequency relationships currently in use for the Mississippi River.