Flooding

Risk assessment

Dane County appears to be holding on Mendota to protect Monona. Monona continues to be at 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.

The extended forecast for the coming week is predicting up to about three more inches of rain. I am not seeing predictions that this rain will occur in large thunderstorm activities, so localized flash flooding is not currently an issue. However, continued high lake levels will remain a problem.

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 = 850.93 feet
Change from one week ago = up 0.01 feet
Change from one month ago = up 0.30 feet
Above/below summer maximum: above by 0.83 feet
Above or below 100-year elevation: down 1.87 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 = 847.01 feet
Change from one week ago = down 0.14 feet
Change from one month ago = down 0.02 (UNDER CHANGE FROM LAST MONTH)
Above/below summer maximum: above by 1.85 feet
Above or below 100-year elevation: down 0.69 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 850.93, which is up from 850.92 earlier this week. 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 847.01, down from 847.15 earlier this week. The summer max by WDNR order is 845.20 (so we are approximately one foot, ten inches over). The 100-year is 847.7.
This Historic High on Lake Monona is 848.53 on September 6, 2018.

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).