More big rainstorms? It’s not your imagination.

A stormwater outfall in the Mississippi River during pouring rain.

It was an early Wednesday morning in June and I was boarding a boat on the Mississippi River at Bohemian Flats along with 60 or 70 other people from around the Twin Cities area. It was starting to sprinkle and heavy rain was rolling in, a fact that neither surprised nor disappointed most of the people on the boat. We would spend the next three hours traveling downstream and back through the Minneapolis River Gorge, learning about efforts to stop the spread of invasive Asian carp, as well as the implications of new weather data in terms of flood prevention and stormwater management. Strange as it may sound, many of us were actually looking forward to seeing storm sewer outfalls in action.

In developed areas, a network of underground pipes carries stormwater off of roads when it rains in order to prevent flooding. The pipes typically empty into nearby lakes or rivers, though in newer developments the water sometimes travels to a stormwater pond first. Cities use rainfall data from the National Oceanic and Atmospheric Administration (better known as NOAA) to determine how big stormwater pipes and ponds need to be to handle large storms. In recent years, however, many communities have been caught off guard when rainstorms hit that were much larger than expected, leading people to wonder, “Are big storms happening more often than they used to?”

As Steve Klein, Vice President with Barr Engineering, explained during our Mississippi River boat trip, the simple answer is yes. NOAA recently updated its precipitation frequency estimates based on more years of data collection, better technology, and more weather tracking locations. The new estimates, known as Atlas 14, replace ones developed back in 1961 and they show that rainfall in Minnesota has increased by 20-25% for 50 and 100-year storms (events that are predicted to happen once every 50 years and once every 100 years, on average), while smaller storms like 2, 5 and 10-year events have remained relatively unchanged. At the Minneapolis–St. Paul International Airport, for example, the 50-year rain event has increased from 5.3 inches to 6.4 inches, while the 100-year storm has increased from 6 inches to 7.5 inches.

Because of changes in rainfall patterns, communities across Minnesota will be faced with some difficult decisions in coming years. Many have already begun to plan accordingly for new and redeveloping areas, but the bigger question is what to do about already built stormwater infrastructure. The cost to retrofit existing pipes and ponds is astronomical, but so too are the costs to repair damage from mega storms like the one that hit Duluth last year. Another looming question is how to prepare for the future when climate change scenarios predict these big rain storms will become even more frequent.

As we cruised down the river, we could see chocolate brown stormwater from city streets in Minneapolis and St. Paul dumping into the Mississippi from storm sewer pipes, some as large as 10 feet in diameter. Is the only answer larger pipes to move more stormwater into our lakes and rivers? Then Leslie Yetka, Education Coordinator with Minnehaha Creek Watershed District, offered a ray of hope. A study the watershed district conducted in the City of Victoria showed that that community is well prepared for the changing weather because they have designed stormwater infrastructure for 100, instead of 10-year, rain events and they have also preserved wetlands and open space in flood prone areas, meaning homes and businesses are less at risk. Yetka hopes that more cities will begin to think critically about how to prepare for the stormy weather ahead and consider “green infrastructure” solutions in addition to stormwater pipes and ponds.

For water resource professionals, engineers and local decision-makers interested in learning more about projects and cutting-edge research related to stormwater, green infrastructure and low impact development, the Twin Cities will host the International Low Impact Development Symposium, Aug. 18-21 at the RiverCentre in St. Paul. Find more info at