Hec-Ras / culvert with overflow to pond questions

[flem1014]

I need to calculate the 100-year floodplain for a project I am working on. The stream flows into a long culvert (500 LF +/-) which is undersized. The overflow from the stream/culvert continues overland to a detention pond where it is detained (along with additional runoff from overland flow) and eventually released to the same stream where the aforementioned culvert discharges too. Assuming the overflow area as a bridge isn't reasonable due to the large width and the additional drainage area contributing to the pond. Whats the best way to calculate the floodplain along the stream and pond?

Thanks in advance for any help.