bottomless arch pipe

We are modeling a regional detention facility with a fish friendly outlet structure. A bottomless corrugated metal pipe arch under a berm is proposed for the primary outlet. The upstream side of the berm will detain excess flow during a storm event. The downstream hydraulic control is backwater from a natural channel. We are looking for recommendations to model this outlet structure.

What is the best way to model the bottomless arch pipe?

Kristen Pleyte
W&H Pacific