Impact of Agricultural Levees on Flood Hazards

摘要

This report provides an assessment and methodology for determining the impact ofagricultural levees on computed flood elevations. By accounting for flood hydrograph stage, flood hydrograph volume, and the conveyance and storage of floodwaters in the protected area behind the levee system, the effect of agricultural levees on floods of a magnitude greater than the levee design can be determined. A dendritic, one-dimensional dynamic wave model was used to computer the hydraulic routing characteristics for the flooding simulations. The methodology was applied to determine these effects specifically along the Missouri River from River Miles 187 through 235. Site-specific results from this application indicate that the level of flood protection provided by the existing agricultural levee system is less than the 10-year flood. Individual reaches of the existing agricultural levee system exceed the computed 100-year flood elevations; however, the levee systems taken as a whole do not provide a consistent level of protection. Overtopping of the existing levee system fills floodplain storage over the duration of the simulated flood hydrographs. Computer flood elevations for floods of magnitude greater than the levee design profile are not affected by the temporary loss of overbank storage and conveyance.