Heidi:By retrofit, I assume you mean use much of what's already there, and modify it to perform better. If so, I offer an example of a project I worked on.
In the 1980s, the 8.6 square mile community of Skokie, IL was faced with frequent surcharging of combined sewers which caused widespread basement flooding. The problem was viewed as stormwater out of control.
The solution was to intentionally and temporally store stormwater on the streets. This was accomplished mostly with a system of mild cross-street berms and flow requlators in catch basins.
The successful solution cost 38% of the cost of the next best solution -- sewer separation.
The project is described on pages 139-142 in my 2017 book
Introduction to Creativity and Innovation for Engineers, Pearson.
Hope this helps.
Stu Walesh
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Stu Walesh PhD, PE
Consultant - Teacher - Author
219-242-1704
www.HelpingYouEngineerYourFuture.com
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