Last week, two dams failed in Michigan, the Edenville Dam and Sandford Dam located on the Tittabawassee River. The Edenville Dam, built in 1924, was the first to go causing the failure of the downstream Sandford Dam. According to data from NOAA, the rainfall that caused these failures was between 7-8 inches over 36 hours. Based on NOAA’s precipitation frequency estimates for this area, this was greater than a 100-year storm. But on top of the 100-year storm, the failure of these dams caused catastrophic flooding with more than 10,000 residents being forced to evacuate (thankfully there were no casualties). I have seen estimates say some areas of Midland, MI could be under 9 feet of water, and at one point the Edenville Dam was discharging up to 374,000 gallons per second. That is a LOT of water.
When we design stormwater management systems on our sites, we are required to design them up to the 100-year storm. But what is the 100-year storm and are they as catastrophic as this one is? Some think it is simply a storm that happens every 100 years, but weather is not that predictable. It is a storm that has a 1% chance of happening in any given year. A storm like this could happen 2 years in a row, though unlikely. When it does happen, the downstream impact depends on many factors including soil infiltration capabilities in the areas, how wet the soils are immediately before the storm, how many stormwater management facilities are in place, how much impervious cover and other improvements are in the rainfall area etc… Answering this question is not simple, but that’s where the fun comes in! When engineering stormwater systems we look at all these factors and more to optimize our systems. Stormwater management on your new development site might be a costly pain, but at the end of the day they help curb negative impacts of heavy rains.