Flood Plain Mapping using HEC-GeoRAS and ArcMap 10.3

[Mohammed_Hasif_MeeraSyed_Ali]

How can I get complete bounding polygon of XS CutLines for flood plain mapping (Flood Inundation Mapping) using HEC-GeoRAS and ArcMap 10.3?

When I tried to perform this task, the bounding polygon formed was in an unexpected manner:

I want to get the bounding polygon without the gap that is visible around the river junction.