BREACH requires that you define the Manning's roughness value for the cross section downstream of the reservoir, and the Manning's roughness value of the earthen material that makes up the dam. The cross section roughness simply represents the on-site field conditions that the flow will encounter after it leaves the failed reservoir structure. This should be the same as what you enter into DAMBRK. The dam material roughness represents the roughness of bare earth, which can be quite small since earthen material (clay, etc.) tends to be quite smooth.

Sincerely,

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