Flood risk on the rise as rainstorms gain intensity

Flooding unsurprising at 22nd & Lyndale but heavy rainfall is increasing in Twin Cities

Hum's Liquors owner Hanh Van donned yellow rain gear and rallied his staff to clear debris from storm drains by hand after the corner of 22nd & Lyndale flooded on July 16. Photo by Jason Grote

As motorists drove home from work along Lyndale Avenue on July 16, a summer storm dumped more than 2 inches of rain in a single hour onto a small neighborhood that’s been identified as one of the most flood-prone areas in all of Southwest Minneapolis.

With the low-lying corner of 22nd & Lyndale suddenly turned into a thigh-high pond, the staff of Hum’s Liquors ventured out into the street and used their hands to pull wood and other debris from the city’s storm drains. In the shop’s basement, waterlogged cardboard boxes fell apart and two cases of wine dropped and shattered. Across the street, at the Wedge Co-op, at least five employees’ cars were damaged and one was totaled. 

Meanwhile, at the Red Dragon Restaurant and Lounge, patrons sat at the bar, nursing their drinks and pulling their feet up as the carpet disappeared below a couple inches of water. “It has flooded here for as long as I can remember,” said Pat Chan, who has bartended at the Red Dragon for more than two decades. “This is the lowest point in the whole area.”

On social media, residents joked about the “return of Lake Blaisdell,” a 40-foot-deep, leech-infested swimming hole along Lyndale Avenue that was filled in the late 19th century as the city’s population grew.

In the span of an hour, a summer storm dumped more than 2 inches of rain onto the Whittier neighborhood on July 16. Image courtesy of the National Weather Service
A flood map screening tool made by the Metropolitan Council shows why water pooled in the intersection of 22nd & Lyndale after heavy rains on July 16. Image courtesy of the Metropolitan Council

While flash floods are normal for this time of year, experts say that a warming climate means torrential rain storms are on the rise in the Twin Cities.

“What we’re seeing is that the atmosphere is able to produce more heavy rainfall events than it used to, and that’s a result of climate change, especially in the Upper Midwest,” said Craig Schmidt, a hydrologist with the National Weather Service in Chanhassen. “There’s more energy in the atmosphere, there’s more moisture in the air further north.”

A 2018 regional climate vulnerability assessment by the Metropolitan Council found that extreme rainfall leading to “unprecedented flash floods” is a main symptom of climate change in Minnesota. The report counted 14 “mega rain events” since 1866, with half of them occurring within the past decade and a half.

State climatologist Kenny Blumenfeld said that precipitation in the Twin Cities is increasingly coming on days in which it rains an inch or more. 

“We’re having not just more extreme rainfall events, but also more ordinary heavy rainfall events,” he said. 

From the turn of the century through the 1970s, about 28% of all Twin Cities precipitation was from heavy rain, which totaled about 7.5 inches per year. Those figures have been “stair-stepping up” in recent decades, Blumenfeld said, and between 2010 and 2018, 36% of the metro area’s precipitation came on days with heavy rain, totaling 11.5 inches per year. 

Eric Wojchik, a senior planner with the Met Council, said most of the Twin Cities’ stormwater infrastructure was designed to a 1960s standard. 

“A major problem is our stormwater infrastructure just can’t cope with the volume and intensity of the rain over these short durations,” he said.

Increased development can also worsen the risk of flash floods. “You don’t get water seeping into the ground as well,” Schmidt said. “The more area you have with impenetrable surface, it just leads to more runoff.”

After heavy snowfall, spring rains and July thunderstorms, 2019 to date is one of the five wettest years ever in the Twin Cities, Blumenfeld said. That’s partly due to climate change and partly due to a cyclical weather cycle that has seen southern Minnesota receive more precipitation relative to northwest Minnesota in the past decade than it did in the early 2000s.

“We’re at a wet time in an already wet year that is inside a wet multiple-year regime, which is leading to us having the wettest decade on record,” Blumenfeld said. “[This year] the ground never got a chance to dry out. It was wet from the spring flooding and then it got wet from rain and snowmelt probably [played a role as well].” 

While the Minnehaha Creek’s median flow is usually about 100 cubic feet per second at this time of year, the flow hovered around 220 throughout the first three weeks of July. After heavy rains on July 15, the flow jumped briefly to 443 and the water level at Hiawatha Avenue rose by a foot. During the July 20 thunderstorm, the creek’s flow spiked to 339 and water at Hiawatha rose by 10 inches.

The high-water mark on the Upper Chain of Lakes increased 4 inches during the July 15 rainstorm, according to the Park Board. Lake of the Isles, Cedar Lake and Bde Maka Ska have been about 1.5–2 feet above their ordinary high-water level throughout July.

In 2017, the Metropolitan Council debuted an online tool that uses state LiDAR elevation data to map which areas may be most at risk for flooding.

Showing up blue on the map: the 22nd & Lyndale neighborhood that once housed Lake Blaisdell, the southwest quadrant of South Uptown, the part of Lyndale immediately below the Lake Street Kmart, the streets north of Linden Hills Park and the corner of Windom near the Interstate 35W and Highway 62 interchange.

The flood map’s only input is elevation but “nine times out of 10 the topography’s not going to lie in terms of where the water is going to gather,” Wojchik said. “During actual events we check Twitter to see where the flood is occurring, and it is happening in the places the map says it will.” 

During the July 16 storm, Lowry Hill East resident Brendan Ireland was biking down onto the Midtown Greenway from Park Avenue — an area that shows up dark blue on the Met Council map — and saw another biker crash into the flooded trail. “He hit head first and created quite a splash,” Ireland said.

Ahead of storms, Minneapolis public works preemptively clears catch basins in low-lying areas, but city spokesperson Sarah McKenzie said the department couldn’t predict “an unusual event” like the rainstorm that battered Whittier. A vacuum truck was dispatched to 22nd & Lyndale on July 17 to suck out mulch from sewer drains.

In a time of “chronic flooding,” Wojchik said he hopes the flood map can be used by cities to predict areas of public health risk and preemptively reroute Metro Transit buses during heavy rains. Already, the Met Council has used the tool to prioritize which manholes should be sealed first and to determine how localized flooding may impact mold in Section 8 housing. 

Wojchik said flooding can be combated in cities by adding rain gardens, by enhancing stormwater infrastructure in low-elevation areas and by increasing participation in volunteer drain armies like Minneapolis’ Adopt-A-Drain program. The city asks residents to call 311 when they see flooding.

“This is an issue that’s going to continue,” Wojchik said.