REGIONAL IMPACTS OF LEVEE CONSTRUCTION AND CHANNELIZATION, MIDDLE MISSISSIPPI RIVER, USA

摘要

Engineering modifications of the MMR are linked to changes in the cross-sectional geometry and flow regime. These changes have been manifested in declining stages for low discharges and rising stages for water levels starting at 40-65% of bankfull and above (Fig. 6). The decline in stages is associated with increasing flow velocities over the same period of time, and likely result from progressive channel incision as previous workers have suggested. The increase in stage at higher discharges is clear at all three stations and supports the conclusions of other researchers who have found that flood stages on the MMR have increased over time. These increases in stage are associated with proportional decreases in flow velocity, consistent with the hypothesis that dike construction has played a major part in forcing stages higher. The fact that the rising stage trends continue to overbank conditions at all the stations suggests that levees also contribute to this change. This conclusion is consistent with modeling results suggesting that part, but not all, of the observed increases in stage are caused by the exclusion of flood discharges from the floodplain. The specific-gage technique eliminates the range of upstream factors that do affect the magnitude of discharge delivered to the study reach but which have little or no impact on the stage-discharge relationship at the gaging stations. Ephemeral factors, such as water temperature and rising versus falling limb of flood hydrographs, do affect the stage associated with a given discharge on a specific date, but these effects rapidly average out and disappear when the specific-gage and related techniques are utilized. The stage trends over time for each station (e.g., Fig. 2) are remarkably free from scatter, and the long-term patterns thus revealed are unambiguous. Anthropogenic increases in flood stage have broad scientific, technical, political, and legal ramifications. Design heights for levees and other structures are based on past hydrologic records, and systematic changes should be factored into such designs. Most importantly, stage is the parameter that directly controls flood damage when such events occur. Floodplain managers should recognize that rising stage trends translate into expanded hazard zones, increases in probable damage, and heightened flood risk. Engineers, planners, and politicians should seek to reach an equitable balance between 1) the local benefits of flood control and navigation improvements and 2) the elevated flood risk that these structures can cause.