When designing stormwater management for a project, it is usually performed by analyzing the pre-development conditions (or coverages) to the post-development conditions. The net difference in volume and peak run-off rates are what we are controlling. Different coverages allow different amounts and/or rates of penetration of run-off back into the ground. Our improvements are typically “impervious,” which do not allow any run-off to seep back into the ground and are therefore the major contributor to the excess run-off that we are trying to control. We design to the 100-year storm event.
Although most people think designing for such a historic rain event is overkill and the facilities capacity will never be used, mother nature has a few tricks up her sleeve’s that increase this probability. The first and most common is frost. As indicated above, impervious coverages are the major contributor to increased run-off, but when the ground freezes, lawn areas become impenetrable as well, temporarily becoming “impervious.” Suppose we get a significant rain event late in the winter season. In that case, the project site becomes 100% impervious, and the run-off volumes and rates increase dramatically over those used under normal conditions. To put this in perspective, a 1-acre site with 10,000 sf. of impervious and the remaining grass will produce 13,812 CF of run-off and a peak flow rate of 6.8 CFS in the 100-year storm event. This same site, under freezing conditions, will make 13,153 CF of run-off and a peak flow rate of 5.6 CFS in just a 5-year storm event. And in the 100-year storm event, these frozen conditions would produce 25,015 CF and a peak rate of 10.35 CFS, which would quickly overwhelm the designed stormwater facilities.
Another weather anomaly that can wreak havoc on our stormwater designs is hailstorms. Witnessed first-hand, the hailstorm we had here just a few summers back produced so much hail that within minutes it had filled up the storm sewer system in our parking lot and acted as an ice dam and stopping the conveyance of run-off altogether. As the hail turned to rain and intensified, the run-off just washed right over the top of all the inlets and continued down through the parking lot, and savagely bombarded the garage door to our lower-level access. Our site is only 2 acres in size, and the amount of run-off was mind-boggling so imagine multiplying this over 1 square mile of a heavily developed area.
It is not economically feasible to design everything for Armageddon-type conditions. Still, it’s essential to know that they can happen and that damaging floods are not necessarily the result of poor engineering design but a result of nature’s unlimited power.