Dear Chi Li Lin :

Note that HEC-RAS does not include the sediment transport modeling that
HEC-6 has. However, I would recommend looking at SWMM or HSPF. It can be
used to model a detention pond, and you can determine the time of
retention within the pond for settling of the sediment. Neither HEC-RAS
or HEC-6 can do that.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com