Modeling two bridges within 50 feet of each other

[acf52010]

This site is a great resource for young engineers, like myself. I am modeling two bridges within 50 feet of each other. The WSEL for the 100 year storm event is lower then the 50 year WSEL through the structures. I looked at the energy diagram through this area and everything seems normal. I am thinking I have the ineffective flow areas to low. I am modeling the stream using a mixed flow regime, and the bridges as a pressure weir method since both structures overtop during the 50 and 100 year events. Do I allow the stream flow to expand through the structures?

 

[francesca]

You have a 1:1 contraction ratio and a 2:1 expansion ratio. Over 50 ft, the most your flow will expand is 25' at the upstream face of the next bridge, except that it needs to contract to go through the second opening, so the maximum point of expansion is 2/3 of the way between the bridges and about 15' expansion to either side. It's not worth modeling. Bridges that close together we usually model as a single bridge. In the 50- and 100-year flood conditions, the downstream bridge will be the controlling weir and again, it's not worth modeling the area between the two abutments separately.