articleThe Brief MMSD began a combined sewer overflow Saturday after receiving more than 3 inches of rain. The Milwaukee Metropolitan Sewerage District began a combined sewer overflow Saturday afternoon following excessive rainfall that began in the area Friday, July 31. What we know:Several MMSD rain gauges show parts of the Milwaukee area have received more than three inches of rain since Friday. MMSD averages 2.2 combined sewer overflows a year, largely because of the deep tunnel system, which opened in 1994. This is MMSD's second combined sewer overflow this year.