Providing Some Relief
By Kevin Noble, PE, LEED AP, FPE
Lately in northeast Ohio, our conversations seem to include something about the weather. “Can you believe how hard it rained yesterday?” “This is the worst flooding I have ever seen in my yard!” “That storm just seemed to come out of nowhere.” Are the frequencies, intensities, and durations of the past year’s storm events related to climate change, or is this just an abnormality to a typical weather pattern? Do we need to change the way we design projects to be better prepared to handle stormwater runoff? The jury is still out on the answer to the first question. But regardless of the answer, it makes sense that we make considerations in our designs that would minimize health and safety issues during heavy storm events.
Due to economics, many public storm sewer systems are sized to handle up to a 10-year storm event. In general, a 10-year storm refers to rainfall totals that have a 10% probability of occurring at that location in that year. Published data from the National Oceanic and Atmospheric Administration indicates that for Akron, Ohio, a 10-year storm event is equal to 1.39 inches of rainfall in 30 minutes or 3.53 inches in 24 hours. So if your rain gauge collects over 1.39 inches after a 30-minute heavy downpour and at least 3.53 inches after a full day of rain, this is considered at least a 10-year storm event. Once rainfall amounts reach these 10-year levels, it is possible that the closed storm sewer systems that the storm water runoff is being conveyed into will start to backup, creating localized flooding.
What can we do during design of projects to minimize the impact of the flooding when it occurs? An obvious answer would be to upsize any new storm sewer piping on the project site in order to handle larger storm events, maybe up to a 100-year storm event. This would reduce the probabilities of a flooding event onsite, but would come at a higher construction cost. Additionally, if this upsized system was connected into an existing public system that could only handle a 10-year storm event, the larger, new system would be subject to backups from the existing smaller system.
A recommendation is to always provide an emergency overland relief path. This is an onsite route that would be used to allow excess storm water runoff that can’t be handled by a closed storm sewer system to flow through the site into an adequate outfall, while not impacting structures and the mobility of vehicles and pedestrians. This route could consist of a grass swale, large underground conduits, and/or large water impoundments that would provide a relief path for this excess runoff. This route should be noted on the design plans and should be free of large landscaped beds and other natural or manmade objects that would impact the ability of this route to adequately convey the excess storm water runoff. For more information on emergency overland relief paths and their sizing, please feel free to contact our office.