The '1-percent annual exceedance probability' is the statistical probability that a similar flood event will occur in a given year.
(TNS) - It was the last week of March 1950, and South Bend, Ind., Mayor George A. Schock watched as the St. Joseph River rose out of its banks, forced out by heavy rains.
Riverside Drive was inundated, the water lapping at the steps of houses perched on the embankment. Floodwaters crept perilously close to houses on Emerson Avenue near Northside Boulevard after filling the avenue and edging over sidewalks. Parks near the river “bore the brunt of the overflow,” a Tribune article at the time said.
Schock said the city had no plans for flood emergencies. South Bend had never faced such a threat from the river, he said.
Less than a week later, it happened again. A second, more devastating rise and crest of the river sent floodwaters back into the same areas. Schock said there was little city departments could do to spare the Emerson Avenue residents a second flood in as many weeks.
“A flood of such proportions was wholly unexpected,” Schock said, according to a Tribune report on April 5, 1950. “After the river crested last week engineers assured me the channel would be able to hold it.”
Last month, 68 years later, a strikingly similar scene unfolded.
Riverside Drive once again became nearly indistinguishable from the river, aside from homes that were once again waterfront properties. Floodwaters again surrounded homes on Emerson Avenue, prompting firefighters to make boat rescues of stranded residents. Segments of numerous streets closed to traffic as the floodwaters inundated them.
“We’re now at a 500-year flood level and the forecast is suggesting that we’ve got about another foot to go,” Mayor Pete Buttigieg said on Feb. 21 along Michigan Street, near the submerged Leeper Park.
The river crested that night at 12.7 feet with an estimated streamflow of 22,700 cubic feet per second, according to the National Weather Service. It is the highest mark ever recorded at the river gauge, located at the city wastewater treatment plant on Riverside Drive. The mark obliterated the previous record of 10.9 feet and 19,500 cubic feet per second set in March 1982 and equaled in January 1993.
The scenes that took place last month have happened time and again — Riverside Drive submerged, homes along Emerson Avenue threatened, Leeper Park covered in water. Dramatic images aside, putting this year’s flood into perspective with the floods of history is not an easy task. Should the definition of a “500-year-flood” change? The lack of data is the first problem.
For starters, the South Bend river gauge was not installed until the early 1980s. It is also not tied into the U.S. Geological Survey’s network of river monitors. That means making accurate comparisons of river crests over decades is a tall order.
Elsewhere along area rivers, the gauges have a longer history. Gauges along the St. Joseph River date back to 1930 in Niles and 1947 in Elkhart. The gauge along the Elkhart River in Goshen has been in service since 1931. On the Yellow River in Plymouth, there has been a river gauge since 1948. All are tied into the U.S. Geological Survey.
The origin of the proverbial “100-year flood” lies with the U.S. Geological Survey. According to a report from the USGS Water Science School, the federal government in the 1960s sought a method for putting context around flooding events knowing that the circumstances that cause floods vary and not all floods are equal in magnitude, duration or effect. The government’s solution was to use a 1-percent probability as the basis for the National Flood Insurance Program.
The “1-percent annual exceedance probability” is the statistical probability that a similar flood event will occur in a given year, said Tom Weaver, with the US. Geological Survey Upper Midwest Water Science Center in Lansing. In essence, it means there is a 1 in 100 chance of a flood meeting or exceeding the 1-percent probability. Similarly, a 500-year flood means there is a 1 in 500 chance of that level being met or exceeded in a given year.
Cara Grabowski, director of marketing for South Bend’s Department of Public Works, said it was this U.S. Geological Survey guidance, as well as the Federal Emergency Management Agency’s National Flood Hazard maps, that formed the basis of the mayor’s assertion that South Bend experienced a 500-year flood last month.
“To the best of our knowledge, this flood is unprecedented in the history of our city. Just anecdotally, I can remember once in a while seeing the river start to jump the banks on Riverside at Leeper Park,” Buttigieg said in an interview Friday. “I think most of us can remember seeing Howard Park go under once in awhile when there’s a spring melt and high water, but this was nearly two feet above the highest level in recorded history.”
Setting the stage
In order to send the St. Joseph River out of its banks at a historic clip, a variety of factors had to come together.
First, it was an unusually snowy February. According to forecasters with the National Weather Service northern Indiana office, South Bend saw 29.6 inches of snow during the month — 14.6 inches above normal. The largest storm, which came just 10 days before record-breaking rainfall and flooding, left a heavy, wet snowpack of greater than a foot, with an estimated 1.5 to 1.75 inches of liquid in the snowpack.
South Bend also obliterated its all-time rainfall record for the month of February with 8.08 inches — 6.13 inches above normal. The previous record, set in 1976, was 5.23 inches.
The three-day rainfall from Feb. 19-21 that triggered the rapid melt of the snowpack and the intense rise of streams, creeks and rivers set a number of other records. The 3.73 inches that fell on Feb. 20 became the all-time rainiest day in February. In addition, it is the single-rainiest day ever in South Bend between the months of December and May.
“This was a large frontal system that stalled out in our area,” said Geoffrey Heidelberger, a meteorologist with the National Weather Service. “When it stalled out, it allowed the rain to ... go over the same areas over and over again.”
Similar scenes played out across the region as the St. Joseph River in Niles, the Elkhart River in Goshen and the Yellow River in Plymouth all broke crest records. There were water rescues in Osceola, Elkhart, and Goshen. Rice Field at Elkhart Central High School was submerged, as it was in 1985.
In South Bend, there is a history of large-scale flooding along the St. Joseph River.
In Tribune coverage of the 1993 flood, Doris Henderson recounted flooding at her Emerson Avenue home during the floods of 1982 and 1985 and said she wasn’t moving.
“It’s not like it was in 1982 or 1985,” she said in a story published Jan. 6, 1993. “It hadn’t come up to the yard yet...but it drives you nuts thinking about it.”
The February 1985 flood crested at 10.7 feet or 19,100 cubic feet per second, according to the weather service.
It was another similar story in early January 2008 when a combination of heavy snow the week before and then heavy rains sent the St. Joseph River out of its banks.
The pair of floods that challenged Mayor Schock in March and April 1950 peaked at 19,453 and 25,646 cubic feet per second respectively. Tribune coverage at the time of those floods also noted that the highest recorded streamflow was a March 1908 flood when the river’s streamflow was measured at 27,000 cubic feet per second. All three of those floods, however, were measured from the Twin Branch dam just east of Mishawaka.
There are also historical accounts of significant flooding events along the St. Joseph River in the 19th century, pre-dating the city’s incorporation in 1865.
Travis Childs, director of education at The History Museum in South Bend, said there was a sudden rise on Jan. 15, 1847 from ice flowing down the river. The river backed up from South Bend to Mishawaka and caused the river to rise three feet above flood stage in 15 minutes.
“Most citizens thought there would be no hope in saving anything in the industrial center of the city,” Childs said. “Not long afterwards the river surged to 12 feet above flood stage and water rose to the second floor of the buildings along the East and West Races.”
Residents helped close the head gates of the West Race and held back most of the water.
In another instance in June 1855, several inches of rain fell and the river reached a level of two feet above its previous high-water mark.
“The head-gates of the East Race couldn’t take the force and it collapsed sending a torrent of water, trees and debris down the race,” Childs said. “Eventually the East Race was indiscernible from the rest of the St. Joseph River.”
Childs said later in the same flood the Colfax bridge had its foundations washed away and plunged into the river.
One key issue in the wake of the floods is whether the dynamics have changed enough that scientists and government officials are asking the right questions and using the right statistics. It also raises the question of what action cities can take.
“The bigger question really is do we have the right statistics around how often these extreme rain and flood events happen,” Buttigieg said. “Has their frequency fundamentally changed or are we just in an unusually rainy time? I think the more we get our arms around that the more we use it to guide decisions in the future about planning, preparedness, land use and everything else we need to know to get ready.”
Mishawaka Mayor Dave Wood said the city’s “long-term planning and strategic investment in infrastructure over several decades” helped the city escape “serious issues” from last week’s flooding, even though some of its parks were swamped.
The city has kept its floodplains for city parks, where it builds structures with concrete that can withstand the high water temporarily. He pointed out that the street lamps, built to be waterproof, in Beutter Park stayed on even while the park was flooded. And the Riverwalk is built to “shed water quickly,” he said.
Also, the city doesn’t place its own buildings in the floodplain, Wood said, and discourages homes and other development in the floodplain.
Buttigieg pointed to the last year and a half in South Bend as evidence that “something is changing.” In that span, the city has seen a 500-year flood and what the weather service deemed a 1,000-year rain event, when 8.49 inches of rain fell in a 24-hour period between Aug. 15 and 16 of 2016.
“We’re dealing with either a remarkable coincidence statistically or perhaps the beginning of a more permanent change in our vulnerability to flooding,” he said.
Tribune reporter Joe Dits contributed to this report.
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