East Isthmus Yahara Watershed Study Flood Mapping

The isthmus is low-laying. It is covered in dense urban development. It drains to the Yahara River or Lakes Mendota or Monona. When the lakes are high, stormwater from the river and lakes back-up into the storm sewer. This does not allow the storm sewer to drain water away as quickly as usual.  As such, the isthmus has a higher chance of flooding when lake levels are high.

Because of this, the City studied the impact of high lake levels on flash flooding in the watershed. This storymap describes how it was done.

The lake levels, themselves, are being addressed with other efforts. To learn more why lake levels rise, please visit another storymap, the  High Lake Level Flooding Story Map .

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.  And, if are on East Washington Avenue and are walking towards the Capital, you again have to walk up a hill. These hills essentially create a three-sided bathtub. The bathtub is the Isthmus.

To make sure the bathtub did not fill up and flood the Isthmus, a storm sewer system was installed as the area developed. When lake levels are normal or low, the storm sewers drain the Isthmus, like a drain in a bathtub.  Yet, when lake levels are high, the storm sewers cannot drain as fast, like a clogged drain in a bathtub.  The Isthmus area is unique in the City of Madison.  Most of the other areas are not like this.  So, the Isthmus faces elevated flood risks.

These two connected concepts impact how the watershed study model was built:

1. Lake water backing up in the streets through storm sewers
2. Increased risk of flash flooding

Most rivers have a "slope" to them.   A slope is where one side of something is higher than the other side of it.   This slope is what causes the river to flow in one direction.  The Yahara River has a slope.  The water level is higher at Tenney Park near Lake Mendota than it is at the Yahara Place Park near Lake Monona.

We can graph the slope of the water levels.  That graph is called a "profile".  The profile of the Yahara River is higher near the Tenney Locks and decreases along to river to Lake Monona.  At Lake Monona, the water level matches the water level of Lake Monona.

Something to note: we can call water levels "water surface elevations."  You may see that term in this Story Map and other places.

The City wanted to see the impact of different lake levels on flash flooding.  To do so, we combined different high lake levels (or “scenarios”) with our "typical" design storms.  These scenarios ranged from what the typical lake levels are to very high lake levels.  The scenarios were evaluated using the computer models created for the watershed study.

The City modeled 4 different scenarios to show a variety of profiles of the Yahara River. The scenarios were generated with a combination of Yahara River discharges and Lake Monona elevations.

"Discharge" is the flow of water.  The Yahara River discharge is approximately the amount of water that is being let through the Tenney Locks from Lake Mendota. A way to measure discharge is in cubic feet per second (cfs). You can visualize this by drawing a line across the surface of the Yahara River and imgaining a specific number of basketballs (~about 1 cubic foot) passing under the string per second. For the Yahara River, a normal or low discharge is ~250 cfs (or 250 basketballs crossing under a string at the water surface).

The following scenarios were chosen to include in the watershed model:

• Scenario 1: Yahara River discharge 250 cfs, Lake Monona elevation 846'
• Scenario 2: Yahara River discharge 700 cfs, Lake Monona elevation 846'
• Scenario 3: Yahara River discharge 700 cfs, Lake Monona elevation 847.5'
• Scenario 4: Yahara River discharge 700 cfs, Lake Monona elevation 848.5'

Something to note, The  Target Summer Maximum  level for Lake Monona is 845'-- this is the level Dane County tries to keep the lake at, or below. However, there is often too much water entering the system, and Lake Monona exceeds the Target Summer Maximum for a portion of most summers.

Dane County operates the Tenney locks.  The County opens them to let more water out of Lake Mendota and closes them to let less water out of Lake Mendota. Typically, when a rain event is forecasted, the County will close the gates a little. This reduces the water going through them.  This, in turn, reduces the discharge through the river.

The City used 700 cfs for discharge in Scenarios 2-4. This discharge is close to the highest discharge that can come through the Tenney Locks. A high discharge will create higher water levels in the Yahara River, especially on the north side of the Isthmus.  Normally, when there is a rain event, much less flow is going through the Tenney Locks. This lower flow creates lower water levels in the Yahara River.  By using the higher flow in the computer modeling, the results will be more conservative.  

Each scenario was included in the watershed model and run with our standard modeling design storms. This means that Scenario 1 was added to the watershed model, and all the design storms were simulated through the model with Scenario 1 as the starting condition--this was repeated for each scenario. This combined two things that are usually independent of each other - high lake levels and flash flooding. The short, intense rainfall events are generally the rain events that cause flash flooding.  These are the City's design storms.   High lake levels are generally caused by longer-term wet weather patterns. By combining them and running different the scenarios, we can see what the impact on the Isthmus will be.

As a standard part of the City's watershed studies, different size storms are run through the watershed study models.  The different size storms help the City understand flood risk. No other watershed in the City has the unique characteristic of the Yahara River. 

The City had to choose one scenario to include in the citywide watershed study flood risk map. It is most probable that storms will occur when the Yahara River has a lower water surface profile.  This is because this is the most common river condition. Therefore, the most comparable scenario is Scenario 1, which is a Yahara River discharge of 250 cfs, and Lake Monona elevation of 846'. We can use Scenario 1 to conservatively estimate of flood risk and be in line with our other watershed studies.

This map exists to help you quickly get information about general flood risks. This map does not identify all areas that may flood or predict future flooding. Do not use this map to make official flood risk determinations for insurance, lending, or other purposes. This is not an official FEMA federal Flood Insurance Rate Map or the state or local equivalent. The City of Madison assumes no liability for any errors, omissions, or inaccuracies. The City also assumes no liability for any decisions or actions a user might take based on this map.

To understand the impact of higher Lake Monona levels on Isthmus flash flooding, we looked at what would occur in the worst-case scenario.  The worst-case scenario is that Dane County would need to let lots of water out of Tenney Locks while Lake Monona was very high.

The worst-case scenario is evaluated in Scenario 4. This scenario assumes a Yahara River discharge of 700 cfs with Lake Monona at elevation 848.5'. 700 cfs is close to the highest discharge ever recorded.  Lake Monona at elevation 848.5 is the peak lake level in 2018.

This worst-case scenario creates a Yahara River profile that is higher than it was at any point in the late summer/fall of 2018.  But it provides a "what if" scenario to understand what happens when flash flooding occurs with high lake levels. 

In the fall of 2018, as lake levels were rising, the City recommended that many homes and businesses install sandbags. The sand bags protect structures from rising waters. They also help protect from flash flooding that could occur if a large storm event happened at the same time.  

The City is able to use the watershed study model to predict the areas that are most at-risk for flash flooding when lake levels are high.

The watershed study models help us make more informed decisions and be better prepared. In the event of future high lake levels, the City would look at the scenario that most closely matches the current conditions and evaulate risk.  Using this information, the City can communicate more accurate flash flood risk with the public.

Please note that the City and Dane County are working to mitigate future lake level flooding. There are a variety of initiatives including:

1. Dredging downstream sections of the Yahara River
2. Increasing aquatic vegetation harvesting
3. Setting low building openings on new development so new buildings are not built in flood risk areas

To learn more about how high lake leves are being mitigated, please visit the  High Lake Level Flooding story map .

•  General Flood Risk Map 
•  Prepare Your Home for Lake Level Flooding  
•  Groundwater Information 
• For more information on the flash flooding that occurred during the August 20-21, 2018 storm, please visit the  Flash Flooding Resilience Story Map .