The new 100-year storm: 3 ways the Bulletin 70 update will affect your project or facility

Does it seem like the 100-year storm is happening more regularly than it should? In light of the recent rainfall data updates included in the Illinois State Water Survey’s (ISWS) Bulletin 70, 100-year storms are occurring more frequently.

Bulletin 70 was last updated in 1989. This document sets 100-year storm standards that guide floodplain policies, development regulations, and stormwater requirements across much of Illinois.

Upon reviewing the new bulletin, our team concluded that the new standards for the 100-year storm will have design and cost implications trickling all the way down from state and county programs, to municipal regulatory agencies, to developers, facility owners and design teams.  

Potential Impacts to your Project or Facility

1. Redrawn Flood Plain Maps: Flood-prone areas may be reclassified as regulatory flood plain, increasing development costs and potentially triggering mandatory flood insurance coverage.

2. Larger Detention Systems: Some local stormwater agencies are already pursuing changes to their requirements based on the updated bulletin.  As regulatory agencies change their ordinances, projects will require more costly stormwater management systems to account for larger volumes of stormwater runoff.

3. Land-use Obstacles:  Adjustments to the flood plain maps could deter property owners from developing their land if sites are deemed within expanded flood plain zones.

For more information about how updates to Bulletin 70 could impact your project or facility, contact EEA’s Technical Director, Tom Hill, at or at (847) 223-4804 ext. 23.