So how can a 200 year storm take place just seven years after the last 200 year storm?
That’s the question many Northeast Ohio residents are asking after last month’s huge rain event swamped some 15,000 basements in the area. This latest storm dumped between five and six inches of rain in a nine hour period. Weather charts tell us that this falls somewhere between a 200 year and 500 year storm.
The term 100 year or 200 year storm is used by meteorologists and engineers but is somewhat of a misnomer. Actually, these terms refer to the percentage of time that a storm is likely to occur. A 100 year storm has a 1% chance of occurring. A two hundred year storm has a .5% chance of occurring. We’re all familiar with the term “chance of precipitation.” A meteorologist might say “chance of precipitation is 20%.” If he would add “chance of flood is .5%” this would more accurately define the chance of a 200 year flood.
In other words, every time it rains there is a .5 percent chance that five to six inches of water could be dumped on us in a short time frame. The odds aren’t great, but like lottery winners, it can happen. Unfortunately, Northeast Ohio won the “flood lottery” twice in seven years.
Ten years ago we revised our storm water management ordinances for new development in the city. This was actually mandated by the United States Environmental Protection Agency. In 1987 amendments were added to the Clean Water Act which required the U.S. EPA to address storm water runoff in two phases. Phase I of the EPA’s Storm Water Program began in 1990 and applied to large and medium municipal storm sewer systems. Phase II of the program began in 2003 and applied to smaller municipalities, including Willoughby. As a result, Willoughby adopted new storm water regulations in March of 2003.
Part of these regulations called for larger storm water retention systems for new development. Prior to 2003 retention basins and underground retention systems were designed for 25 year storms (4% chance of happening.) Since 2003 all new subdivision and industrial construction must design retention systems for a 100 year event.
Earlier this year we began the first phase of a citywide storm water study to determine what we have to do to solve storm water problems in the city. Our plan was divided into seven phases, with the idea of completing one phase a year. Since the most recent rain event we’ve decided to expedite the plan and have it completed within the next year or so. At that time we will have a better understanding of the problems and possible solutions to many of the flooding problems that have occurred, as well as an estimate of the costs involved.
As these studies are completed I’ll share more information with you. We will also continue to maintain our systems in the best possible working order. In the meantime, we can all hope and pray that we don’t hit the “storm lottery” again anytime soon.
Have a great month.