Flash Floods, Sirens and Combined Sewer Overflows

We’ve been working on sleep training with our six-month-old and, to be diplomatic, progress is slow. Earlier this week, we had one of those nights when he was awake crying for over an hour at 1am, then up again for another five to ten minutes at 4am. We had just settled him back down and fallen into a hazy sleep when my brain began to register a new, but equally annoying noise.

Crying? No. Meowing? Not this time. Alarm clock? Not yet. Finally I awoke enough to realize that it was a tornado siren and leapt out of bed, grabbed Charlie, and ran down two flights of stairs to the basement. Our dog Macy barked at the top of the basement steps and I had visions of her flying through the air like that cow in Twister. Eventually, my husband checked the weather report and saw that it was only a severe thunderstorm warning and by then, the sirens had already ended, leaving only a wide-awake baby and a slightly dazed dog to deal with.

The weather in our area has been unpredictable lately, to say the least. We’ve been rocked by several recent storms with high winds, heavy rains and even hail. Oftentimes the storms are spotty, drenching one town and leaving the next high and dry. So far, the metro area has been mercifully spared the flooding that has devastated Duluth and other towns along the North Shore, but as I write, most of central and southern Minnesota is under flash flood warning and already the St. Croix River is running fast and high.

When I speak to people about preventing stormwater pollution, I often hear two questions. "Why do we have storm sewers, if they are such a big source of pollution?" and "Why don’t we route stormwater through treatment plants like we do with our sanitary sewers?"

In the Twin Cities metro area, most developed communities have curbs and storm sewers on almost all of their roads. Storm sewer inlets connect to belowground pipes that carry rainwater and melting snow (aka stormwater) quickly off of our roads and into nearby lakes, rivers and streams. In more recently developed areas, this water is often carried first to stormwater ponds, which provide an opportunity for litter, sediment and other pollutants to settle out before the water reaches the nearest water body, but in many places the stormwater is not treated at all.

Although the Environmental Protection Agency has deemed stormwater pollution to be the single greatest threat to lakes, rivers and streams across the country, storm sewers are vital for public safety and greatly reduce flooding in urbanized areas where much of the land is covered in impervious surfaces. In many Lake Superior communities, storm sewers and sanitary sewers are combined in older parts of the cities, which means that stormwater flowing into these combined pipes goes to wastewater treatment plants before ending up in Lake Superior. The scenario sounds good, but can actually be disastrous when excess rains like we have experienced lately overwhelm the sewer systems and cause overflows of untreated sewage to basements, roadways and the lake. Recently, Duluth has achieved a  91 percent reduction in sewer overflows to Lake Superior during intense storms due to pricey efforts that were made to disconnect basement drains from sewage systems, replace leaky lines across the city and add large overflow storage basins. The City of Superior has also spent considerable sums of money to separate their sewer systems in parts of the city and construct overflow facilities that store and treat combined wastewater during large storms.

It is virtually impossible to prevent flooding when more than six inches of rain fall within a 24-hour period, as has been the case in the Duluth area this week. The impact of heavy rains is especially pronounced, however, in urban areas where impervious surfaces such as rooftops, roads and parking lots prevent water from soaking into the ground, as well as in agricultural areas, where drain tiles direct water off of farm fields and into a network of ditches that quickly lead to streams and rivers. Small scale practices like raingardens, which are very effective at capturing stormwater from the smaller storms that make up 90% of our annual rainfall events, do little to mitigate flooding during big storms. Many stormwater engineers now advocate for larger pipes to handle bigger storms, as well as low impact development practices, which mimic natural hydrology by reducing impervious cover and creating more opportunities for water to soak into the ground before reaching storm sewers, drain tiles or roadside ditches.

As we brace ourselves for another round of storms, keep your ears perked for sirens and your eyes peeled for flooded streets. Stay safe!