Current municipal drainage networks across the state were largely constructed using historical guidelines derived from a 1961 federal standard known as Technical Paper 40 (TP-40), as read in Chapter 3 of the MNDoT Drainage Manual. This now outdated baseline assumed that rainfall patterns would remain static within stable, predictable limits.

Today’s scientific reality has completely broken those mid-century models. In 2013, the National Oceanic and Atmospheric Administration released NOAA Atlas 14 (Precipitation-Frequency Atlas of the United States), which updated Minnesota’s rainfall data by doubling the amount of historical time they looked at and gathering info from hundreds of new locations. The results were stark: the volume of water falling during major storm events had systematically intensified across the entire state. Because our underground infrastructure was built using the smaller rainfall assumptions of the 1960s and 70s, modern downpours routinely overload our legacy systems.

The Consequence of Overwhelmed Drainage

When an extreme downpour exceeds a storm sewer’s designed hydraulic capacity, the resulting flooding is a mathematical certainty. Forcing 21st-century precipitation volumes into systems engineered for 1960s weather patterns inevitably results in systemic back-ups & delays.

Fixing this problem is an immense engineering challenge. It requires local governments to dig up and upsize miles of subterranean concrete pipe, reconstruct containment basins, and replace restrictive culverts. Under traditional funding models, this places a massive, multi-million dollar unfunded mandate squarely on the shoulders of local municipalities and property owners.