A changing Minnesota climate has shown that more energy and more moisture in the atmosphere has the potential to create more rainfall.
Precipitation has been increasing in Minnesota over the last century, as shown in the Figure below, which illustrates historic annual precipitation, from 1865-2016.
The blue trend line in the Figure shows that annual precipitation amounts have been steadily increasing, which is compounded by increasing rainfall totals for specific, isolated storms.
These extreme rainfall trends put a strain on stormwater infrastructure and other surface water conveyance or retention efforts. Given the fact that much of the stormwater infrastructure within the Twin Cities metro was designed to convey surface water based on technical standards and rainfall estimations adopted in 1960, the increasingly short, intense rainfalls present a challenge for communities and for the Metropolitan Council.
The Fourth National Climate Assessment states that precipitation in the Midwest is projected to increase by 30% by the end of this century. Between 1958 and 2012, the Midwest had already experienced a 37% increase in larger rain events of 2.5 inches or greater.
Why Focus on Localized Flooding?
From an asset management perspective, the financial implications of inaction are well researched and documented. According to the US Federal Emergency Management Agency (FEMA), federal insurance claims for flooding damage averaged $1.9 billion a year annually between 2006 and 2015, making flooding the costliest and most common type of natural disaster in the US.
This assessment focuses on the climate hazard of localized flooding for several reasons, including:
- Increases in extreme rainfall have already occurred, and this trend shows the highest probability of continuing in the future.
- Council assets are susceptible to vulnerabilities from potential localized flooding. Until now, no regional screening tool has been created to assess potential impacts from localized flooding. The assessment allows the Council to screen regional assets for potential localized flood risk and subsequent vulnerability.
To better understand the Council’s approach to Localized Flooding, we advise readers to start with the Localized Flood Risk Introduction
prior to reading the individual system-specific chapters.