No such thing as a one-size-fits-all solution
What do you do when your design can’t meet the project constraints? I recently worked on a project designing a detention basin for an airport development. On the surface, it seemed like a straightforward, easy design. Upon digging deeper, I found the obvious solution was not right for the project. Ultimately, thinking outside the box and trying an atypical strategy gave the client a successful solution and ended up saving them a lot of money.
I started off using one-dimensional unsteady modeling in HEC-RAS to design the proposed drainage channels and detention basin, as is typical for this kind of project. The highly-restrictive site limitations, however, made this strategy less effective. There was very little slope on the site, the proposed detention basin location was in the FEMA floodplain of the downstream drainage channel, and there was also a significant backwater effect in the drainage channel.
Here is where the limitations of 1D modeling come into play. When water enters a HEC-RAS Storage Area, it is immediately available for outflow, even if the model is running in unsteady mode. Because everything was happening instantaneously in my model of the detention basin, I was coming up with a large basin that barely fit in the available space. To further compound the problem, the detention basin was above ground, and there was no available fill at the site. Every cubic yard of dirt was going to be expensive.
I needed a new approach. I decided to see if 2D flow modeling would improve my results. I had little faith; the detention basin was so small that hydrodynamic routing within the basin would make very little difference, right? To my surprise, the basin easily handled the inflow thanks to the more accurate routing. A few design iterations later, I had a much smaller detention basin. In fact, the final basin size was even smaller than originally anticipated.
In the end, sticking with the typical design strategy would never have been successful. Trying an alternate strategy, even one that at first seemed a little unusual, ended up paying off big time—for myself and for the client.
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