RETRO-MODELING THE MIDDLE AND LOWER MISSISSIPPI RIVERS TO ASSESS THE EFFECTS OF RIVER ENGINEERING AND LAND COVER CHANGE ON FLOOD STAGES

From the late 19th through the 20th century, the Middle and Lower Mississippi River (MMR and LMR) have undergone major levee expansion for increased flood protection and extensive engineering for the facilitation of navigation. As a result of the levee expansion floodplain area available for flood conveyance and storage has been reduced by ~60 % along the MMR and > 90% along the LMR. River engineering through this period significantly changed the geometry of the river channel. In addition, human settlement and utilization of the MMR and LMR valleys has altered floodplain land cover patterns, affecting the ability of the floodplain to convey floods (e.g., changes in hydraulic roughness). These changes have been associated with increases in flood stages.

In order to quantify the incremental effects of levee expansion and channel and land-cover change upon flood stages, we are developing hydraulic models of multiple historical reference conditions (i.e., “retro-models”) for three large study reaches (~200 km each): one along the MMR and two reaches along the LMR. For each reference condition, four 1-D unsteady-flow models are being developed. These models include a calibrated model of actual conditions at that each step and three “scenario” models: 1) a model with levees of the next time step, 2) a model with the channel geometry of the next time step, and 3) a model with roughness (i.e., land cover) of the next time step. Comparison of the model for actual conditions and the scenario models allows for a quantitative assessment of the impacts of each of these parameters on stage.

Comparison of the retro-model results (~1900) with near-modern (1996) modeling results for the MMR study reach (St. Louis to Commerce, Missouri) shows increases in flood stage for large events (>50-year recurrence interval) of 2.2 to 4.3 m. The corresponding scenario modeling for this reach revealed the following contributions to these increases in stages: levees 0 to 35%, changes in channel geometry 5 to 75%, changes in land cover 0 to 10%, and contribution of roughness from river engineering structures (e.g., wing dikes) 5 to 45%. Preliminary results for the Hickman, Kentucky to Memphis, Tennessee study reach on the LMR show an increase in stage of up to 1.6 m for flood flows between 1937 and 1995. The observed increases in stage are currently being independently assessed by scenario modeling.