Flooding

Risk Assessment

At this time, low areas in the Isthmus are at increased risk for urban flash flooding during the type of storm we saw last night. Areas include but are not limited to Mifflin/Blount, Mifflin/Livingston, Johnson/Third, Johnson/Fourth. These areas are all low in elevation compared to the lake levels (home elevations are about 850 feet and adjacent street elevations in these areas are about 848.5 feet) these areas are also enclosed depressions (no way for drainage out but the pipe system).

The extended forecast looks good– so we don’t believe there is any reason to be concerned beyond what we’re seeing right now.

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 = 850.63 feet

Change from one week ago = up 0.25 feet

Change from one month ago = up 0.15 feet

Above/below summer maximum: above by 0.53 feet

Above or below 100 year elevation: down 2.17 feet

Lake Mendota Statistics

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 = 847.03 feet

Change from one week ago = up 0.39 feet

Change from one month ago = up 0.46 feet

Above/below summer maximum: above by 1.83 feet

Above or below 100 year elevation: down 0.67 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 numbers above:

Mendota is at 850.63, which is up from 850.33 last week. The summer max by WDNR order is 850.10 (so we are approximately 1 foot, 6 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 847.03, up from 846.46 last week. The summer max by WDNR order is 845.20 (so we are approximately 1 foot, 10 inches over). The 100-year is 847.7.

The last posted updated Historic High on Lake Monona is approximately 848.40 on September 4, 2018.

The problem low ground elevations in the Isthmus area are at about elevation 848.0-849.00. 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).