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Title: The dynamic loop effect on the Mississippi River : project design flood flow line
Authors: United States. Army. Corps of Engineers. Lower Mississippi Valley Division.
Huval, C. J.
Keywords: Design flood
Mathematical models
Numerical models
Dynamic loop effect
Peak floods
Flood control
Flood prevention
Mississippi River
Flow characteristics
Unsteady flow
Publisher: Hydraulics Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Miscellaneous paper (U.S. Army Engineer Waterways Experiment Station) ; HL-79-2.
Description: Miscellaneous Paper
Abstract: 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 η 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.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Miscellaneous Paper

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