Latest Issues Facing Our Industry

NAIOP Chicago identifies key issues to track and monitor at the state level that effect our members, member companies, and the commercial real estate industry as a whole.

Frequency Distributions of Heavy Precipitation in Illinois: Updated Bulletin 70

What Is It?
Bulletin 70 is a publication prepared by the Illinois State Water Survey (ISWS) that provides state standards for expected extreme storms, originally published in 1989, based on rainfall data from 1901 to 1983.   Effective March 2019, ISWS has updated Bulletin 70 to more accurately represent extreme rainfall events in the state of Illinois, now taking into consideration rainfall data through 2017. Rainfall volumes and frequencies of extreme storm events have increased.

Why Does It Matter?
Rainfall amounts captioned within Bulletin 70 are used as baseline criteria from which calculations are based throughout various Chicago area stormwater management ordinances. Without any changes to actual stormwater ordinances (MWRD, DuPage County, Will County, etc.), the increased baseline rainfall data will incrementally increase the required on site detention storage resulting in additional size, space and capacity for on-site detention This can represent a meaningful INCREASE to the amount of land property owners or developers must dedicate to stormwater detention.

Many jurisdictions are adopting the Updated Bulletin 70 rainfall data as of January 1, 2020. To the extent projects pending review and approval can be submitted prior to year’s end this may be beneficial to property owners or developers to do so.

Where To Learn More: