Jefferson County - East-West Gateway Coordinating Council

A Regional Overview All-Hazard Mitigation Plan 5
group recommended that the 1-percent-annual-chance flood (also referred to as the 100year
or “Base Flood”) be used as the standard for the NFIP.
The 1-percent-annual-chance flood was chosen on the basis that it provides a higher level
of protection while not imposing overly stringent requirements or the burden of excessive
costs on property owners. The 1-percent-annual-chance flood (or 100-year flood)
represents a magnitude and frequency that has a statistical probability of being equaled or
exceeded in any given year, or the 100-year flood has a 26 percent (or 1 in 4) chance of
occurring over the life of a 30-year mortgage.
Floodplains are a vital part of a larger entity called a watershed basin. A watershed basin is
defined as all the land drained by a river and its branches. In some cases, flooding may not
be attributed to a river, stream or lake. It may be the combination of excessive rainfall,
snowmelt, saturated ground and inadequate drainage.
Likely Locations
In certain areas of Jefferson County, steep slopes of the region induce high velocities as the
water flows downhill and downstream, in many cases producing flash flooding conditions.
Because some areas in Jefferson County are located in low areas, and therefore, often in
the floodplain, floodwaters have the potential to affect or even severely harm portions of
the community, especially if the floodwalls or levees fail.
There are no federal levees on the Missouri side of the Mississippi River; however there are
federal levees on the Illinois side that extend as far south as Ste. Genevieve County,
Missouri. During floods, these levees would force higher floodwaters to inundate the
Missouri side of the river. The failure of the federal levees would lower the floodwaters
and reduce the flooding impact on the Missouri side. These conditions that exist in areas
where flash floods are a problem make response operations and evacuation very difficult,
adversely affecting the safety of the residents.
Type of Damage
Damage incurred as a result of flooding includes the inundation of residences,
outbuildings, businesses, churches stormwater, mud, rock, trees, debris, trash, and
chemical pollutants. Depending upon the severity of the flood and the volume and rate of
flow of the water, floodwaters may be capable of carrying vehicles, whole or parts of
buildings, etc. Wherever they reach, floodwaters leave behind layers of thick muddy ooze.
During spring and summer 1993, record flooding inundated much of the upper Mississippi
River Basin. The magnitude of the damages -- in terms of property, disrupted business, and
personal trauma -- was unmatched by any other flood disaster in United States history .
Property damage alone was over $20 billion. Damaged highways and submerged roads
disrupted overland transportation throughout the flooded region. The Mississippi and the
Missouri Rivers were closed to navigation before, during, and after the flooding . Millions