High Lake Level Flooding
Why the City of Madison is vulnerable to lake level flooding, and the response to historic lake level flooding in August, 2018.
Historical Flooding Context
The threat of rising lake levels
The soils beneath our roads and homes hold historic information about our neighborhoods. Based on the amount of hydric soils present around the lakes, we know that many areas were historically prone to flooding. While high lake levels haven't been a significant issue for residents in the recent past, hydric soils were an indicator the City used to prepare for high lake levels.
In 1892 the City of Madison was developing on the Isthmus. At that time, there were wetlands east of the capitol in the East Washington Ave corridor, in the Fordem Ave railroad corridor, and along the rivers and lakes.
As the city developed, wetlands were filled.
The city continued to develop over wetlands in 1906.
The soils beneath our roads and homes hold historic information about our neighborhoods. The turquoise areas on the map show hydric soils.
Hydric soils develop in areas that have flooded or been saturated. Therefore, by looking at where hydric soils exist, we know these areas have flooded historically.
The lowest lying areas of Madison are shown in dark blue. As we would expect, these correlate with the areas that have flooded historically.
A citywide terrain model is updated every four years. This model can be used to help estimate where lake level flooding may occur as water levels increase.
Animation showing flooding extents as water level increases
This animation uses the City's terrain model to show the impact of rising lake levels on areas near Lake Monona.
You can see that as the water rises it spreads to low-lying areas adjacent to the lakes.
And also begins rising in the middle of the Isthmus.
...and continues to rise and flood more areas...
While the water should never get as high as shown here, the map clearly depicts the lowest lying areas of the City.
Threat of Lake Level Flooding on Isthmus
If you are on East Washington Avenue and head toward Lake Monona, you have to walk up a hill. If you are on East Washington Avenue and head toward Lake Mendota, you also have to walk up a hill. The Isthmus is essentially a three-sided bathtub.
To make sure the bathtub did not fill up and flood the Isthmus, storm sewer was installed as the area developed. It drains the Isthmus, like a drain in a bathtub, when it rains during normal conditions. However, when lake levels are high, the Isthmus area, and other areas, that drain to the lakes, face three types of elevated flood risks:
- Standing lake water in the streets via storm sewers
- Increased risk of flash flooding
- High groundwater that results in basement flooding
Standing lake water in the streets via storm sewers
Storm sewers are necessary to drain the Isthmus during rain events. Rainwater that doesn't soak into the ground flows as runoff into the storm sewers that act like water slides to the Yahara River.
During very wet years after large rain events, large amounts of water released from Lake Mendota cause higher water levels along the Yahara River.
Sewers act in reverse allowing water to travel “up” them.
A typical storm inlet that moves water out of the street.
Image of high water on the Yahara River between E Main Street and East Washington Ave (August 2018).
When water levels in the lakes or river is higher than the area of land that the sewer is designed to drain, the lake water moves up the sewer and pools in the street. The lake water in the street is at the same elevation as the lake!
Main Street flood water is part of the lake. The flooding is a result of lake water traveling up the storm sewer (August 2018).
Farther from the river (~1 mile), the storm sewers still allowed lake water to flow back onto East Mifflin and Livingston Street (August 2018).
Increased risk of flash flooding
When storm sewers are full of lake water they cannot effectively drain rain water
This can lead to flash flooding in places that normally do not flood.
Sandbags were placed around infrastructure, homes and businesses on the Isthmus to provide protection from the increased risk of flash flooding (August 2018).
High groundwater that causes basement flooding
Additionally, when the City was built on wetlands, much of the area that was filled in was filled with soil that drains quickly. That works both ways in that water can move back through the soil quickly, as well.
The groundwater table is connected to lakes and rivers at the shoreline.
Therefore as the water level increases on the rivers and lakes, the groundwater level increases. When the groundwater table increases above the bottom of people's basement, the water finds its way into the basements through cracks or joints.
Flooding in August, 2018
In the summer of 2018, large amounts of rainfall and inefficient movement of water out of the system led to flooding along the Yahara chain of lakes [1].
Earlier that year, in May 2018, the Dane County Regional Airport received the second highest amount of rainfall on record [1].
Despite all four lakes being at summer minimum at the beginning of the season, all four lakes rose above their summer maximum levels with Lake Kegonsa reaching a historic high in July. The flow from Tenney Dam was decreased to minimize flooding in Lake Kegonsa [1].
In early August, the watershed received higher than average amounts of rain causing waterlogged soils [1].
This culminated in an intense rain event on August 20-21, with parts of the watershed receiving over 10 inches of rainfall. This was followed by many more rain events in August, September and October. This flooding greatly impacted the City of Madison, as well as surrounding communities [1].
For more information on the flash flooding that occurred during the August 20-21, 2018 storm, please visit the Flash Flooding Resilience Story Map .
Lake levels in August 2018 rose to historic levels
Graph shows significant lake levels increases after the August 20, 2018 rain event.
Response to 2018 High Lake Levels
The City of Madison and Dane County responded quickly to high lake levels in 2018.
City of Madison response
- Protected critical infrastructure including sand bagging and rubber “sealing” manhole covers to prevent the sanitary sewer from becoming overwhelmed.
- Stockpiled sandbag locations around the City for residents to utilize.
- Communicated risks to the public.
- Ensured Isthmus flash flood risk area, and low laying areas adjacent to lakes were properly sandbagged to prevent flash flooding damage.
- Reinforced critical shorelines to prevent infrastructure failure.
- Matched volunteers with residents who needed assistance.
Large sandbags and pumps were set up on the Capitol City bike path at the Monona Terrace to keep the major commuter path open during high lake level flooding (August 2018).
Dane County response
- Completed additional weed cutting to increase flow out of lower lakes.
- Operated Lake Mendota in a manner to prevent failure at the Tenney locks and keep enough storage in Lake Mendota for additional rain events.
- Water was released from Lake Mendota to Lake Monona in a controlled manner (1-3 inches per day), which allowed people to prepare.
High lake level flooding at Tenney Park (August 2018).
Environmental factors we are unable to control
While the City and County's efforts prevent a lot of lake level flooding damage, there were a number of factors that could not be changed.
- The size and shape of the "outlet" (where the water flows out) of Lake Monona and downstream lakes. This makes it difficult to move more water through the system.
- Quantity of water coming into the system; the water either needs to be stored in Lake Mendota, or moved into downstream lakes.
High lake levels impacting private piers (August 2018).
The City's Lake Level Flooding Resilience Prior to August 2018
While no one predicted that we would receive the amount of rain that we did on August 20, 2018, the City has been working actively on the lake level issue with the County to prevent and prepare for lake level flooding since we began experiencing high lake levels in 2000.
Since that time, the City has participated in two Yahara Lakes Advisory Groups (YLAG) . The purpose of the YLAG groups was to make water level recommendations to inform potential changes to the DNR’s lake level orders, yet neither group led to a consensus recommendation to change the DNR lake level orders.
Following those efforts, the City began working on an internal resilience strategy to prepare for high lake levels. This included:
- Completed an exercise with citywide emergency management group.
- Created plans and conducted surveys to identify and create plans to protect critical infrastructure.
- Installed equipment and generators to allow critical services to remain operative during flood events.
Plans to Continue Improving Lake Level Flooding Resilience
City and County initiatives
A Technical Work Group was assembled to recommend concrete actions for the County to lower lake levels without needing to go through the DNR petition (YLAG) process:
- Using their Yahara Lakes Model the County was able to lead the technical group in brainstorming and modeling different solution scenarios.
- The Technical Work Group Report, found that dredging sections of the Yahara River, and pumping water out of Lake Monona, mitigated the most flooding (both separately, and as a combined program).
- The County is planning to complete dredging on the Yahara River between Lake Monona and Lake Waubesa in 2020. The dredging will allow more water to leave Lake Monona, and therefore will help keep lake levels on Lake Monona within their target summer range throughout more of the summer. You can find project updates on the Yahara River Sediment Removal Project website.
City of Madison initiatives
Modifications to Construction Projects:
- Modified the design to increase the road elevation of the 2019 East Johnson Street reconstruction project to allow the street to remain open during a flood event similar to the one experienced in 2018.
Quantifying flood risk citywide to help compare relative risk of flooding throughout the City to ensure that we are prioritizing flood mitigation projects that help the most vulnerable:
- While the Isthmus experienced unprecedented lake level flooding in 2018, the west side also experienced unprecedented flash flooding. While the flooding mechanisms are different, we can compare relative risk of flooding throughout the City.
- Quantifying flood risk will be done by completing watershed studies that will identify existing problems, develop solutions, and prioritize improvements.
- In 2018 the Mayor and Common Council approved a large budget amendment to begin completing eight studies this year.
- The City began Phase 1 of the East Isthmus and Yahara River Watershed Study in 2020. Phase 1 will create a model to show where and how frequently flooding occurs, and Phase 2 will propose solutions.
- The City plans to conduct watershed studies citywide within the next five years.
Investigating large-scale solutions:
- Creating a model to show how dredging on the Yahara River between Lake Mendota and Lake Monona would help lower the Yahara River levels during high lake level flooding. This may help mitigate flooding throughout the isthmus, as many of the isthmus areas drain to the river not the lakes.
