Waller Creek will still flood. It is impossible to engineer a flood prevention system that can eliminate flood risk 100% of the time. Nature is unpredictable and history is full of disasters that were thought to be prevented by engineering solutions. There is some calculus involved in sizing a system to prevent the majority of floods vs the increased cost to size it larger. I could not find the design specs for the tunnel (I'm sure they are out there somewhere), but I am thinking it was designed to contain a 500 year flood. That is, there is a 0.02% chance of a flood of that magnitude happening in any given year. The thing is, in the past decade or so, we have had at least three 500 year floods, and those will become larger and more common as climate change accelerates. Structures lower in the creek bed will flood more often than those higher in elevation. It's just a guess, but I imagine that new board walk is still within the 100 year flood plain. The best information I was able to find easily is this:
"Note that the hike and bike trail along the creek is still subject to flooding. Flooding may occur quickly along the trail. Users should be aware of their surroundings and be cautious using the trail in heavy rain."
https://www.austintexas.gov/departme...ict-and-tunnel