assume you mean Arizona? that policy is nothing new, arched culverts are not generally required and circular are definitely preferred by District O&M. For retrofitting, arched and ovals may need to be used and shouldn't be hard to justify if they are really necessary. For larger crossings, boxes or bridges have always been preferred over con-arch or other systems. I believe Scottsdale, Tucson and other municipalities do allow con-arch

Yes Arizona, I was just wondering if anyone new of any technical/functional/operational reasons for their preference significant enough for them to state it in their hydraulics manual.

well, technically the hydraulics are a bit more complicated since the cross sectional shape can vary quite a bit. Structural analysis is also somewhat more complicated by the shape. But, with HEC-RAS and FEA to help with the analytics, I don't see how this is insurmountable. I'm not aware of any performance issues with the existing con-arch structures in the area, so I don't believe that is the issue. The city of tucson design memo linked below discusses some issues with arches.

City of Tucson
https://www.tucsonaz.gov/files/transportation/engi...

Marana, Arizona
http://www.psomas.com/drainage-crossing-design-cha...

Litchfield Road, ADOT underpass
http://www.huntercontracting.com/index.php/gallery...

Unless GREAT care is taken during installation make sure the seals between pipes are actually in place properly. Round pipe is easier.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

I was looking at various culvert options with HY-8, and it calculated less flow capacity for an elliptical pipe than a circular one with the same rise and a shorter span. I never did figure out why. I ended up recommending a concrete box.