As Ohio and the Midwest have been inundated with rains, the staff at the Delaware Soil and Water Conservation District has been inundated with calls for assistance. Too much water from rain (and snowmelt) can cause ponding in yards, erosion, backed up stormwater drains, wet basements, flooding, and more. After a significant storm event, we may hear references to a 100-year flood or 100-year storm, but what do those phrases really mean?
A radio or television broadcaster may say something like, “This storm resulted in a 100-year flood on the Beautiful River (I made up that name so don’t try looking it up on your mobile device), which crested at a stage of 17 feet.” Many people mistakenly think that this peak height of the Beautiful River happens only once every 100 years. Instead, it means that there is a one in 100 chance, or 1% chance, of such a flood occurring in any given year; however, two 100-year floods can happen two years in a row, or a month apart. It all depends on how much rain is falling or how much snow falls and how quickly it melts.
This terminology was created in the 1960s by the federal government to place floods in context based on magnitude, duration and effect. Storms are also referenced in the same terms. These levels are statistically computed using past and existing data, and as more data is collected, accuracy improves. The terms are useful for scientists and engineers to determine the risk to life and property, to study the environmental impacts of floods and storms, and to design drainage and flood prevention practices.
Does a 100-year storm always cause a 100-year flood? No, because the following several factors influence the relationship between rainfall and streamflow:
• Extent of rainfall in the watershed. As we have seen repeatedly in the last 18 months, rainfall is not uniform throughout a watershed and in some cases, parts of the watershed remain dry while other parts get sprinkles, showers or downpours.
• Soil saturation before the storm. Existing conditions prior to a particular storm can influence the amount of stormwater runoff into the stream. Dry soil allows for more rainfall and snowmelt to seep into the ground, reducing the amount of runoff to the nearby stream.
• Size of the watershed and the duration of the storm. Generally, a stream with a larger drainage area requires storms of longer duration for a significant increase in streamflow to occur.
It might be helpful here to explain how rainfall amounts and duration of a storm are related. A roughly one inch rain falling throughout 30 minutes is considered a two-year storm, but that same one inch rain falling in five minutes is considered a 1,000-year storm. A nearly four inch rain over 10 days is referred to as a two-year storm; however, that same amount in 12 hours is a 25-year storm; in six hours is a 50-year storm; and in three hours a 200-year storm.
If you are interested in local rain and snow amounts, check out the CoCoRaHS network, the Community Collaborative Rain, Hail and Snow Network at www.cocorahs.org. The mission of this organization is “volunteers working together to measure precipitation across the nations.” Delaware County has four stations, and it is quite interesting to compare them to each other and to other locations around Ohio and elsewhere.
We get calls from residents with questions and concerns about their water issues and drainage. We can provide suggestions to minimize the impacts of too much water, such as surface grading, relocation of existing farm tile, directing downspouts away from the building foundation, keeping stormwater surface inlets clear, and appropriate landscaping. In some cases, neighbors with similar issues may find that working together produces a more satisfying and long term resolution. To learn more, please visit our website at soilandwater.co.delaware.oh.us or call us at 740-368-1921. Remember, we all live downstream!
Bonnie Dailey is deputy director of the Delaware Soil & Water Conservation District. For information, go to www.soilandwater.co.delaware.oh.us.