Flooding

Risk assessment

With the rain over the weekend, the lakes have risen. Dane County appears to be holding water a bit more on Mendota to protect Monona. Even with that, Monona has elevated above the 847.00 number, which is the elevation that we have noted as resulting in significantly reduced storm sewer capacity in low areas of the Isthmus causing a susceptibility to flooding during intense rain events like those experienced last weekend.

We will continue to watch closely the lake levels.

Recall typically 1 inch of rain in the Yahara Watershed typically gets you a 3-inch rise in the elevation of Lake Monona.

Lake Mendota Today

Elevation = 850.92 feet
Change from one week ago = up 0.23 feet
Change from one month ago = up 0.54 feet
Above/below summer maximum: above by 0.82 feet
Above or below 100-year elevation: down 1.88 feet

Lake Mendota

Surface Area = 9,842 surface area acres
Summer Min = 849.6 feet
Summer Max = 850.1 feet
100-year Record = 852.8 feet

Lake Monona Today

Elevation = 847.15 feet
Change from one week ago = up 0.25 feet
Change from one month ago = up 0.51 feet
Above/below summer maximum: above by 1.95 feet
Above or below 100-year elevation: down 0.55 feet

Lake Monona Statistics

Surface Area = 3,274 surface area acres
Summer Min = 844.7 feet
Summer Max = 845.2 feet
100-Year Record = 847.7 feet

Further description of the numbers above:

Mendota is at 850.92, which is up from 850.69 last Thursday night. The summer max by WDNR order is 850.10 (so we are approximately 10 inches over summer max). The 100-year elevation is 852.8, but we can only currently effectively rise to approximately 852.0.

The historic high on Lake Mendota is 852.74 and was reached on June 6, 2000

Monona is at 847.15 up from 846.90 last Thursday night. The summer max by WDNR order is 845.20 (so we are approximately two feet over). The 100-year is 847.7.

The last posted updated Historic High on Lake Monona is approximately 848.53 on September 6, 2018.

Both lakes are likely to continue to rise for another day or so.

The problem low ground elevations in the Isthmus area are at about elevation 848.0 to 849.00. These areas drain to Lake Monona and/or the Yahara River mostly at East Washington. The closer Monona rises to 847.00 the more ineffective our drainage system in these low areas becomes at addressing high intensity summer storms (1" or more per hour).

If there is any urban flash flooding in any areas of the City tonight, residents and visitors should remain cautious when driving and should report significant flooding to City Engineering Operations 608-266-4430.

Never drive through flooded streets. Stay calm, find higher ground, and take some time to think about your route options. It is not worth risking your life or damaging your car. Street flooding will typically recede quickly after the intense rains have stopped.

A few other points to keep in mind, should more rain move in.

• Don’t drive through standing water.
• When intense rain is predicted, avoid parking in low lying areas of the Isthmus and the near east side.
• Avoid parking in other areas known to have flooded in the past.
• Know where the nearest sandbag fill locations are.
• Report flooding when possible on the flooding form.

It is the City’s priority to keep residents safe and keep the community connected with updates as crews continue to monitor the potential for flooding.

The Engineering Division also has a podcast episode featuring engineers talking about the 2018 flooding event and ways to flood proof your home. Listen on iTunes and Google Play, by searching “Everyday Engineering.”