Contrary to earlier proposals, lowering lake levels may not in fact be the best way to lessen the impacts of future flooding, according to a new report.
The 2018 Yahara Chain of Lakes Flooding Technical Working Group released its report Friday, at the request of the county, following the late summer flooding that wrecked more than $154 million in damages county-wide. The working group was composed of city and UW-Madison engineers and water resources managers.
Its report investigated several different adaptation and mitigation strategies — including lake level manipulation — but ultimately it found that the most successful way to prevent flooding was through some combination of dredging and pumping.
“The adaptation scenarios of lowering Lake Mendota provided little benefit to flooding,” the report said. “The mitigation scenarios of dredging and pumping produced the best results for lowering flood levels, especially when used in combination.”
The report also investigated how bridge modification, maintaining Lake Mendota at a safe level, and removing dams would have changed the outcome of the 2018 floods.
Earlier, Mayor Paul Soglin had called for a one-foot decrease in Lake Mendota, implying that the extra space could be used to store more floodwater.
In all, the models predicted clearing debris through dredging and and expediting flow through pumping could have lowered flood levels anywhere between seven and 21 inches. Meanwhile, lake level lowering may only have accounted for a two-inch flood level decrease.
The report went on to caution that lowering Mendota may actually pose consequences to lakes downstream, with water levels rising above summer maximum levels more often. As a result, the lower lakes have higher fluctuations in lake levels due to the Tenney Dam quickly releasing water and not being used to buffer water to the lower lakes.
Lowering Lake Mendota by a foot would have negative environmental impacts, too. Resulting water level fluctuations could dislocate wetlands and impact state threatened species, such as Sheathed Pondweed, White Lady’s Slipper, and Tufted Bulrush.
Maintaining steady flow tested as a more effective option, since water movement through the chain of lakes is currently impacted by several flow limitations in the rivers, according to the report.
Examples the report mentioned included narrow bridges and sediment deposits.
“Debris in the river, such as tree trunks and boulders, causes friction and slows water flow. Aquatic plants also cause friction and reduce water flow,” the report said. “The flows through the lower lakes are limiting efficient release of water and are still prone to flooding.”
While managing flow may be critical to controlling floods, the report emphasized that flooding can be prevented from the land too — before stormwater has even entered the lakes.
Preventative measures, like containing stormwater on land and limiting development in wetlands, could be one solution, the report said.
The county will turn to the report through out the spring, to guide it as it makes plans to address future flood risks. Beginning Feb 4, the public is invited to attend these discussions.
