West Allis engineering report: August floods were a 1,000‑year event; 1,000‑year upgrades estimated at $23M

West Allis engineering staff presented findings on the August flooding event, saying the rainfall behaved like a "1,000‑year" storm that overwhelmed local stormwater infrastructure and that a city‑side upgrade sized for a 1,000‑year event would be costly.

The engineering department presenter (Speaker 2) told the committee the report was prepared to answer whether the city’s infrastructure failed, whether storm‑sewer valves could have been used to prevent flooding, and what it might cost to build systems able to convey a 1,000‑year storm. "We did a very high‑level analysis... it would be over $23,000,000," Speaker 2 said, adding that number is a baseline that would increase with detailed modelling and utility coordination.

The city’s sewers are designed to convey a 5‑year storm (about a 20% chance annually), Speaker 2 said, and that standard reflects tradeoffs cities make about acceptable risk and cost. The presenter added that the heaviest rainfall arrived in the event’s first hour—rain gauges near the area showed roughly 2.5 inches in that initial hour—so an intense "first flush" sent debris into catch basins and quickly clogged inlets, accelerating street and yard flooding.

Speaker 2 also emphasized jurisdictional limits. Several sections of Honey Creek, and other channels mentioned in the report, sit in infrastructure owned and maintained by the Milwaukee Metropolitan Sewerage District (MMSD). "West Allis has no control over that box sewer, its maintenance, its infrastructure," the presenter said, noting that enclosed sections operated by MMSD constrained the city’s ability to change those structures quickly.

Engineers described compounding watershed effects: earlier heavy rain upstream left the Menomonee River elevated, so Honey Creek could not discharge freely and local flows backed up. The presenter said the city inspected storm and sanitary lines under an emergency contract to televise pipes and manholes to check for damage; inspections identified some damage but the work was largely precautionary.

On potential upgrades, Speaker 2 warned of practical limits. Preliminary drawings suggested very large diameters—"a 102‑inch or a 10½‑foot diameter sewer"—which would require major trenching and coordination with existing water, gas, sanitary and communications utilities. That coordination and the need to support or relocate utilities would raise costs above the $23 million baseline.

Committee members asked staff to produce a public‑friendly summary of the report; Speaker 2 said they would work with communications to create an easier‑to‑read version. Members also questioned a planned National Avenue project: staff said they found the existing 84‑inch pipe undersized and took the opportunity to upsize to a 108‑inch sewer where feasible.

Speakers from the committee and at least one MMSD board member discussed longer‑range options such as daylighting portions of Honey Creek and noted MMSD’s expedited climate‑resiliency planning (a 2035 plan being accelerated), but there were no commitments recorded in the meeting for daylighting or major regional projects.

Procedural note: a committee member moved to adjourn; another member instead moved to accept item 31 and place it on file. The acceptance motion was seconded and no further discussion or roll‑call vote was recorded in the transcript.

Why this matters: the report explains why locally designed storm sewers—sized for more frequent storms—were overwhelmed during this unusually intense event, clarifies which infrastructure the city controls and which it does not, and establishes a high‑level cost and practical constraints for upgrading drainage to reduce future risk.