Abstract
Resistance of the overbank flow is decomposed into surface resistance and form resistance. Form resistance caused by tall vegetation in the floodplain is explicitly formulated by the form drag of trunks and momentum loss due to macroturbulence generated by a branch-leaf complex. Hydrodynamic parameters in a formula aredetermined by the critical review of existing knowledge on coherent structures in turbulent open channel flows. An application of the proposed formula to the overbank flow through floodways gives good agreement with observed discharge without any tuning of coefficients.
