The unusually high 1973 and 1975 flood seasons on the Mississippi River revealed apparent significant reductions in flood-carrying capacity. In addition, a heretofore unappreciated phenomenon known as loop effect appeared to have a large impact on stage-discharge relations. As a part of a general reanalysis of the design flood flow line, a study was initiated to help quantify the dynamic loop effect. The purpose of the study was to estimate the loop magnitude at the design flood to be added to the separately evaluated steady-state design flood flow line. A previously developed numerical model for calculations of variable stage-discharge relations was used. The study showed that this model could not account for all of the loop magnitude, probably due to flow complexity and model assumptions. An ad hoc method to simulate the total loop was implemented involving double-valued, variable Manning's ? corresponding to the rising or falling phase of the flow hydrographs. The method of simulation suggests a higher channel resistance at falling river stages. Extrapolation to the design flood is based on the assumption of similarity of river behavior to the 1973 flood. This assumption is consistent with the steady flow line computations using river resistance extrapolation from the 1973 flood data to the design flood. The modified model was applied to three representative gaging stations to quantify the loop magnitude along the lower river. Results of the study indicate that the dynamic loop allowance should be as follows: (1.) Vicksburg - 0.9 ft (2.) Helena - 1.0 ft (3.) Baton Rouge - 0.5 ft. In summary, an allowance of approximately 1.0 ft should be added to the flow line from about Helena to Red River Landing. An amount of 0.5 ft should be sufficient from Red River Landing to about Baton Rouge.
The Dynamic Loop Effect on the Mississippi River: Project Design Flood Flow Line
Miscellaneous Paper ; HL-79-2
1979
6660361
Report
Electronic Resource
English
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