Two interconnected floodwater detention basins started six years ago on the County Grounds in Wauwatosa are finished, the Milwaukee Metropolitan Sewerage District announced Tuesday.
The flood basins together will hold approximately 315 million gallons of water. By comparison, the Deep Tunnel system stores 521 million gallons, MMSD spokesman Bill Graffin said.
However, the basins are not designed to work as the Deep Tunnel does. Rather than collecting and storing water during any rain or snowmelt, they will fill only when their single feeder, Underwood Creek, reaches the level of a 1 percent probability, or so-called 100-year, flood event.
A 17-foot diameter tunnel from that level on the creek bank runs for one-half mile, cutting under the DNR Forestry Center woods, and leads into the basins, Graffin said. During a severe flood on Underwood Creek, the tunnel can be opened to capture the flood crest for several hours, mitigating flooding downstream, especially on the Menomonee River as it passes through the Village area of the city.
The basins are also intended to relieve flooding further downstream in Valley Park in Milwaukee, also known as Piggsville.
The tunnels are also not intended to hold water for any length of time should they fill in a flood. As soon as floodwaters on the streams receded, the water in the basins would be released slowly into the Menomonee.
"If the basins fill to capacity, it will take four days for them to empty," Graffin said.
Back-to-back severe floods that inundated scores of Tosa homes and State Street businesses in 1997 and 1998 led the City of Wauwatosa to seek assistance from MMSD in protecting against future floods.
The MMSD embarked on a flood study of the entire Menomonee River watershed and brought forward a plan that called not only for the basins on the County Grounds but also for lowering the floodplain in and surrounding it with levees.
For that part of the watershed project, more than 60 homes and more than a dozen businesses were purchased and demolished, and Hart Park was nearly doubled in size.
The Hart Park project was finished several years ago, but it was intended to function along with the detention basins. Had a very severe flood event occurred before the basins came online, the Hart Park levees might not have held back the tide.
Now that the basins are ready, the Village should be safe from the kind of flooding that twice turned State Street into a navigable waterway more than a decade ago.
On July 22 of last year, parts of State Street flooded again in massive rains that inundated large areas of metro Milwaukee. But it was determined that none of the water outside the Hart Park levees was either river water or backups from storm sewers. It was surface water that came from the sky so fast it couldn't get into the storm sewers fast enough.
MMSD has published a 2010 year in review video that offers some astonishing footage for those interested in reliving that event.