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By: Paul Berringer, District Adm. - Clermont County SWCD
Doesn't it make sense that a 100-year storm event should occur approximately once every 100 years? By definition, a "100-year storm" is an event of the magnitude that has a 1 percent chance of occurrence in any given year. Perhaps more accurately, it should be called a "1-in-100 chance storm." The law of averages states that this type storm should have a return rate of approximately once every one hundred years. But aberrations can and do occur. Thus a coin toss can turn up heads five times in a row. And, although it's really bucking the odds, we can have two 100-year storm events in the same year.
Storm events must be reported relative to some standard time period and geographical area to be meaningful. For the Hamilton County area, a 100-year storm is considered to be 6.04 inches of rainfall over a 24 hr. period, or 2.84 inches of rain over a 1 hr. period. Actually, 100-year storm events are reported for rainfall durations starting at 5 minutes and running all the way up to 10 days. The most current source of data is the Rainfall Frequency Atlas of the Midwest, published in 1992. These forecasts are based upon statistical calculations of actual rainfall data collected locally over many years.
The 100-year storm rainfall amount can vary greatly depending upon the region of the country. For instance, if you live in Death Valley, CA., a 100-year storm in 24 hrs. would yield a paltry, though probably welcome, 2 inches of moisture. On the other hand, if you resided in New Orleans, LA. Or Miami, Fla., a storm would have to drop 15 inches of rain in 24 hrs. to be classified a 100-year event. But then, hurricanes tend to do that!
On July 17th and 18th, parts of Clermont County received 3-7 inches of rain in a 24 hr. period. In fact, many areas locally received the bulk of that rain in very brief, hard-hitting storms that were probably only 1-3 hrs. in duration. No doubt, that means that some areas were hit with the equivalent of a 100-year storm event.
So, does a 100-year storm equal a 100-year runoff event or flood? Not necessarily. Several factors are involved here. First off, a 100-year storm in a small drainage basin or watershed, say 1 sq. mile, will have a more significant effect on streamflow that the same storm in a 50 sq. mi. basin. Also, soil that is already saturated from previous rains has a lower capacity for infiltration, thus causing more runoff and increased flooding. Other factors influencing runoff include type of groundcover and vegetation, and terrain or topography.
But probably the greatest concern today is how rapid urbanization is impacting runoff and flooding in our communities. Urbanization results in greater amounts of impervious surfaces (roads, parking lots, buildings, etc.) that increase the amount and velocity of stormwater runoff. Development also often results in the destruction and filling of flood plains and wetlands, both of which perform invaluable functions in naturally controlling flooding. In fact, recent studies show that with just 20 years of urbanization, the chances of a community sustaining a 100-year flood event can more than double.
Generally, short lived, but intense bursts of rainfalls usually do not cause widespread flooding (like the 1997 flood of the Ohio River). However, violent storms like those that recently moved through the tri-state area can still be extremely dangerous. They produce runoff amounts that are too large for storm drains and small receiving streams to carry. And they can cause debris flows and mud and rock slides, which then clog streams, drain inlets and culverts, and result in substantial secondary flooding.
So what can you do to protect yourself and your property. First of all, look before you buy! Don't inherit someone else's flooding or drainage problems. If you already own a problem home, there may be some on-site solutions that can help, including such things as surface regarding, additional catch basins, subsurface tile drainage and sump pumps. But remember, as you go about fixing your drainage problems, make sure you don't force additional water upon your neighbors. That would be a violation of Ohio's Drainage Laws.
From a maintenance standpoint, keep storm drain outlets, drainage culverts, and ditches free of sediment and debris. Maintenance of public storm sewers and road ditches is the responsibility of the local municipality, township or county that owns the roadway or system. Report any problems to the proper authority. If you own property along a stream, occasional maintenance may also be needed to prevent logjams from reaching proportions that might cause flooding. Contact the Soil & Water Conservation District if you would like more information about solving drainage problems around the home. The District also offers several publications that homeowners will find useful, including one entitled "Look Before You Buy."
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