CU South underpass should be redesigned

Thank you to Mr. Daniel Johnson for his thoughtful opinion article in the Daily Camera on July 17 and for his contribution to the CU South development planning. I agree with his point that getting started on significant flood prevention measures is urgent.

I am, however, struggling to understand Mr. Johnson’s first bullet point. He wrote: “The discharge from the 500-Year detention facility could exceed the design capacity of the U.S. 36 channel underpass. CDOT will not allow the larger flow.”

The purpose of a detention pond is to detain a large volume of water arriving quickly so it can subsequently be discharged more slowly over a longer duration. A properly sized and operated detention facility should, I believe, accommodate any known downstream flow constraints. The many flood control dams built throughout the world attest to their engineering feasibility.

“CDOT will not allow the larger flow” is certainly an awkward declaration. If a 500-year flood blows thru a facility designed for a 100-year flood, CDOT will be as helpless as everyone else.

A few years ago, I saw a graphic modeling the 2013 flood progression over time. It clearly showed water constrained at the U.S. 36 underpass, spreading laterally, and then overtopping the highway and moving northeast into neighborhoods well outside the natural riparian floodplain. It seemed obvious to me that the choke point was the CDOT underpass, as if flow was blocked by debris, though I do not know if debris was the actual cause or that the underpass was just too small.

Mr. Johnson’s statement suggests the underpass cannot be redesigned and rebuilt to increase flow capacity, but I believe that is one of CDOT’S core missions and skill sets.

What am I missing here?

— Kevin Mathews, Boulder