It seems as if there is increased flooding in the Chicago area in recent years. Is this true?

Dear Tom,

It seems as if there is increased flooding in the Chicago area in recent years. Is this true?

— Al Pondroski, Chicago

Dear Al,

Flooding IS increasing in metropolitan Chicago and it’s the result of two factors. First, rain of any given amount now produces higher river crests than ever because urbanization increases the runoff of rainwater (and even snowmelt) up to six times what would occur on natural terrain. As the urban area grows, roofs, concrete and other impervious surfaces make more land impervious to water absorption.

Secondly, the frequency of flood-producing rains is increasing. There were 31 precipitation events of 3 inches or greater in the first half (73 years, 1871-1943) of Chicago’s rainfall data and 48 such events in the second half (1944-2016). Fourteen events have occurred in the last 20 years (1997-2016).