The sewage control plant that failed four years ago performed the way it is supposed to on Sunday. Sewers can only accommodate so much rainfall and so-called 100-year storms – the ones that most infrastructure is not designed for – can and do overwhelm them.
The problem now and in the future is that 100-year storms are happening with increasing frequency. Three monster rainstorms have drenched Thunder Bay and the surrounding area since 2008. Meteorologists have been warning for some time that climate change is causing havoc in weather systems and patterns that we are used to. Things are not going to get better in the foreseeable future, given reluctance among particularly large developing nations to reduce the use of carbon-spewing energy that they are only now beginning to exploit. – Chronicle Journnal editorial
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This from the Minnesota Department of Natural resources…..
100-Year Rainstorms Defined
One of the more misleading phrases used in meteorology and hydrology is 100-year storm. The phrase implies that an intense rainstorm dubbed as an 100-year event, dropped rainfall totals heretofore unseen for 100 years, and not to be experienced again for another century. This is a logical, but incorrect conclusion to draw from the phrase. More precisely worded, a 100-year storm drops rainfall totals that had a one percent probability of occurring at that location that year. Encountering a 100-year storm on one day does nothing to change the probability of receiving the same amount of precipitation the very next day.
A better way to describe these unusual events is to refer to a one percent probability storm. However, the momentum created by repeated usage over time will assure that 100-year storm will remain in the public and scientific lexicon.
Intense rainfall events are often geographically isolated. Therefore, increased population density, improved precipitation monitoring networks, and radar-based precipitation estimation have increased the likelihood of capturing (measuring) heavy rain events. Also, improved communication allows for faster and more complete transfer of weather information. When the neighboring county is walloped by a 100-year storm, we hear about it quickly. Invariably we will vicariously “experience” the event and wonder why 100-year storms seem to be occurring every other week!
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….A better way to describe these unusual events is to refer to a one percent probability storm….
….Encountering a 100-year storm on one day does nothing to change the probability of receiving the same amount of precipitation the very next day…..
Soooo…large rain events can happen any year. In order for anyone to decide that spacing of these recent storms is exceptional, data needs to collected over a longer period than four years. Hundreds of years, maybe thousands.
If I remember correctly, flooding in the Thunder Bay lowlands due to rain is not uncommon. Also, as I said before, our city is covering more and more of its surface with concrete and asphalt. Cutting down more and more trees and brush. This all effects the amount of surface runoff that our sewer system has to deal with.
Even the height of grass in parks and lawns effects the amount of surface runoff.