Q: What does a "100-year flood" mean?
A: The term "100-year flood" is often used as an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year. Likewise, the term "100-year storm" is used to define a rainfall event that statistically has the same 1-percent chance of occurring. In other words, during the course of 1 million years, these events would be expected to occur 10,000 times. Just because it rained 10 inches in one day last year doesn't mean it can’t rain 10 inches in one day again this year. (USGS)
Q: What is meant by the term "flood stage"?
A: The National Weather Service, based on the desires of the local community, establishes the "flood stage" gauge height for any given community. The flood stage gauge height is often the stage where damages begin to occur. Many communities desire to use the flood stage gauge height as an early warning alert, prior to the onset of significant damages. Significant damages may not occur until river levels are several feet above flood stage. Additionally, conditions along some rivers may have changed since the gauge and flood stages were established and reaching the flood stage may or may not result in actual damages. Again, stages are site-specific, so feet above flood stage at one location can’t be compared to another.
Q: What is meant by the term "river stage"?
A: A site-specific measurement of river-level referenced as the height in feet above a designated zero reference point, called the gauge zero, at the site. The zero reference point is sometimes, but not always, chosen as the elevation of the river bottom. Normally, stage values are always positive. Drought conditions could cause the river level to fall below gauge zero, and the stage reading at that time would be negative. Since each gauge was established independently at each location, the stage reading is good for that location only and cannot be compared to other locations. For example, a stage of 30 feet at Fargo, N.D., cannot be compared to a stage of 30 feet at Grand Forks, N.D. The only way direct comparisons between two gauges can be made is by converting river stage to elevation by adding the stage to the gauge zero elevation.
Q: What can the Corps of Engineers do to assist before and during a flood?
A. When all available local/state resources have been exhausted, the Corps can provide flood fighting supplies, such as sandbags and polyethylene, as well as contingency planning support, technical assistance and emergency, temporary levee construction. A governor's request must be made to engage Corps resources. Ultimately, the Corps is there to SUPPORT local communities and not lead the flood fight.
Q: If the Mississippi River is going to flood, how come the Corps of Engineers doesn’t use the locks and dams to draw down the river before the flood?
A: The locks and dams on the Mississippi River cannot be drawn down before a flood for two reasons. First, doing so would not result in lowering flood elevations. The pools created by the locks and dams do not contain sufficient storage capacity to accommodate flooding events. Even if each pool were to be completely emptied, it would take only a matter of hours to refill them. The second reason is that it is illegal. The Anti-Drawdown Law of 1934 directs the Corps to operate the pool levels year round for navigation. The reason for this law is to protect the environment.
The Mississippi locks and dams were built for navigation. When the water is high enough for vessels to operate on the Mississippi River without the channel, the gates on the dams between the piers are raised completely out of the water so that the river flows free as an open river.
Q: How come the Corps of Engineers still releases water from its Red River of the North reservoirs during a flood?
A: To prepare for spring floods, the district draws down each of its Red River reservoirs during the winter. However, a reservoir is similar in operation to a bathtub. If the amount of water entering the reservoir from the basin is greater than the amount of storage available, the reservoir fills up. Once the reservoir is full, all additional water is passed on downstream potentially resulting in even more severe flooding. Our goal is to have the reservoir full by the end of the flood not in the middle. If it fills up in the middle than the peak inflow is passed on to downstream communities – by releasing during the flood, the peak outflow can be kept lower.
Q: Does the Corps of Engineers maintain all of the levees across the U.S.?
A. Nationwide, the Corps of Engineers monitors about 14,000 miles of levees. There are more than 100,000 miles of non-federal levees across the country. The St. Paul District monitors 165 miles of federally constructed and 15 miles of non-federally constructed levees. The Corps inspects these systems annually to identify maintenance deficiencies and operational problems as part of its Inspection of Completed Works program.
Q: How come the Corps of Engineers has to build emergency levees in the Fargo-Moorhead area each year? Why not leave the levees up, so the government doesn’t have to rebuild them year after year?
A. Fargo, N.D., and Moorhead, Minn., do not have permanent flood damages reduction project that other cities, such as Minnesota’s St. Paul and Rochester, have. The levees the Corps builds in these cities before a flood fight are temporary and emergency in nature. They are not built to the same engineering standards that a permanent levee would be built to. Further, when a community requests these emergency levees be built, they agree to take them out after the threat has passed. The purpose in this is to prevent communities from feeling secure behind an emergency levee built in haste.