RETRO-MODELING OF THE MIDDLE MISSISSIPPI RIVER

A one-dimensional unsteady-flow “retro-model” was developed using historic hydraulic and hydrologic data and HEC-RAS to assess hydraulic conditions along the Middle Mississippi River near the beginning of the 20th century. Bathymetric and land-cover data for the model were digitized from the Mississippi River Commission's Upper Mississippi River Survey conducted between 1888 and 1897. Floodplain elevations for this model were derived from a modern high-resolution digital elevation model. The bathymetric and floodplain elevation data were used to develop cross sections of the river channel and adjacent floodplains. Land-cover data were used to establish realistic 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 Flood Frequency Study (UMRSFFS) flood levels revealed increases in flood elevations of 2.3 to 4.7 m for extreme events (>50-year). These results are similar to 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 dams and levees are the likely explanation for the observed increases in flood levels. Between 1897 and 2000, river channel widths through the study reach decreased ~ 30% and floodplain area for the 100-year flood decreased by 60%. In addition, Manning's n in the retro model were discernibly 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 area, but rather (3) is a result of implicit roughness changes such as wing-dam 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 river engineering.