Retro-modeling the Middle Mississippi River

Abstract

A one-dimensional (1-D) unsteady-flow “retro-model” was developed using historic (c. 1900) hydrologic and geospatial data and implemented using HEC-RAS. The objective of this investigation was to create a 1-D unsteady-flow model for the Middle Mississippi River for the beginning of the 20th century in order to assess the magnitude and types of changes in flood stages associated with 20th century river engineering. The retro-model was constructed from survey data dating to 1888–1889 and hydrologic data from 1900 to 1904. The late 19th century survey data was supplemented by a modern high-resolution DEM used to fill gaps in the historic data. Land-cover data recorded during this historic survey was used to establish floodplain roughness values based on published Manning’s n for the various land-cover types, and these roughness values were then adjusted to calibrate the model. Comparison of the retro-model results with the 2004 Upper Mississippi River System Flow Frequency Study (UMRSFFS) flood stages showed increases in flood stages of 2.3–4.7 m for large events (>50-year recurrence interval). These results confirm previous research results showing large-scale reductions in flood conveyance on the Middle Mississippi during the 20th century. Increased roughness of the floodway coupled with reduction in channel and floodplain area due to wing dike and levee construction are the likely explanation for the observed increases in flood stages. Between 1889 and 1998, channel widths through the study reach decreased ∼40%, and floodplain area for the 100-year flood decreased by ∼60%. In addition, Manning’s n values in the retro-model were lower than the values used in the UMRSFSS, suggesting that (1) the modern floodway is rougher than the historic floodway, (2) this increased roughness is not a result of explicit changes in land cover, but rather (3) the increase is a result of implicit roughness changes such as wing dike construction. The retro-model developed in this investigation provides a framework for modeling hydrodynamic and ecological responses to altered hydrologic regimes during more than a hundred years of channel modification.