Report on historic Dane County flooding offers prevention, management options
An analysis of last summer’s historic flooding of the Yahara River chain of lakes by a Dane County work group has yielded options for preventing future flooding of the same scale.
Shoreline properties and much of Madison’s Isthmus were submerged in floodwaters either breaching the shores of lakes Mendota, Monona, Waubesa and Kegonsa, or coming up from storm sewer drains following a massive rainstorm Aug. 20. The western half of the county was hardest hit, but as much as 10 inches of rain fell onto the watershed leading into the Yahara River and its lakes in 12 hours. Continuing precipitation and slow water flow through the system prolonged the flooding.
The Dane County Board tasked a work group of experts with studying potential solutions to future flooding. The group’s report was released Friday, and a task force will convene for the first time Monday to discuss the findings and decide on short- and long-term solutions by its final meeting April 1.
The work group, made up of officials from the state Department of Natural Resources, the county Land and Water Resources Department and UW-Madison’s Civil and Environmental Engineering Department, described seven potential solutions to prevent similar flooding, with varying levels of success.
The work group was not tasked with deciding on a solution. The task force will make recommendations to the County Board.
“My hope is that we can come together to utilize science to turn into policy recommendations that can prevent flooding,” said Sup. Michele Ritt, a member of the task force.
The solutions proposed by the work group would either alter the way the lakes were managed — such as lowering the minimum level for Lake Mendota by 1 foot, managing Lake Mendota up to its 100-year water level or removing dams from the Yahara lakes — or alter the structure of the Yahara River. That could include modifying bridges to remove pillars in the water, dredging the riverbed to create faster flow and more volume, or pumping and rerouting excess water to Bad Fish Creek, or a combination of measures.
The work group analyzed options using depth characteristics of the river and lakes, rainfall levels of 2018 and a software program that can predict outcomes using that data. The study’s results showed both the total number of days the lakes would have been above summer maximum levels and the decrease — or in some cases, increase — of the peak water levels each option would have, compared to the actual levels seen last year.
None of the options included cost estimates.
The torrential rainstorm Aug. 20 is generally seen as the start of the vast flooding on the lakes, but water levels in the lakes were already above the summer maximum levels set by the state Department of Natural Resources when the rain started to fall.
The county’s Land and Water Resources Department is tasked with maintaining water levels in lakes, which includes maintaining water flow through the Yahara River. The department decided to retain water in Lake Mendota through the summer after a storm in June flooded Lake Kegonsa. After holding back that water to ease flooding downstream, Lake Mendota was not able to retain all of the excess runoff dumped into it in August.
Dredging the riverbed and rerouting and pumping water from the chain of lakes each offered the best outcomes, according to the report, as determined by the peak water levels predicted by the software programs under those scenarios. Combined, those options would produce the best results, the report said. The peak water levels would have been about 1 foot lower under this option on lakes Mendota and Kegonsa, 20 inches lower on Lake Monona and 25 inches lower on Lake Waubesa.
These options likely come with expensive price tags, considering the equipment and infrastructure necessary. There also would be potential ecological impacts from altering the current riverbed and laying pipe from Lake Waubesa to Lake Kegonsa or Bad Fish Creek.
No easy outs
Reducing the summer minimum and maximum by 1 foot and maintaining those levels on Lake Mendota along with removing dams along the Yahara River Lakes would have led to worse flooding on some lakes, according to the report.
Lowering the Lake Mendota level would have reduced the peak level on that lake last year by about 2 feet, but lakes Monona and Kegonsa would have had levels up to 6 inches higher than their actual peaks. Lake Waubesa would have had only a 3-inch peak level reduction.
Removing the dams on the Yahara River, particularly the Tenney Dam that keeps Lake Mendota artificially high, would have reduced only the flooding caused by Lake Mendota. The peak for that lake would drop by more than 3 feet, but the downstream lakes would have peaks increased by 1 to 3 inches.