I have heard before that its okay to model a culvert as a bridge, if there is a large opening when compared to the deck thickness. Do you think a 12' span x 9' rise RC Box culvert with 2 ft thick deck is a good candidate for this concept? Also, how would one recognize when to model one, or the other? Is there some threshold of Span vs Deck as a cutoff between the two?
Background on the project:
Existing condition is a bridge, ~11.5' span. Proposed is the RC box culvert mentioned above. Steep channel >5% with flat area (from scour) local to the structure. Using mixed flow regime due to the flat area and flow wanting to transition to sub-critical at the structure.
When the proposed conditions are modeled as a culvert, we see WSE increases downstream of the structure. When modeled as a bridge, we see decreases downstream.
If you don't think this culvert is a good candidate to model as a bridge, do you have any suggestions to reduce the proposed water surface downstream of the bridge?
You would just have to understand that the equations used to solve both are different. Does the bridge overtop? Weir and orifice flow calculations will differ between the culvert and bridge calculations. I assume that this is a steady state model?