The impact dynamics of dilute debris flows (typically volumetric solid fractions<50%) have been extensively investigated within the framework of hydraulics. For dense debris flows, the impact mechanisms have been poorly studied. From a geotechnical viewpoint, the feedback between granular dilatancy and pore-pressure response may play an important role in the dense debris-flow regime. In this study, the impact behavior of dense and dilute debris flows in an instrumented flume is analyzed. The basal stresses (normal/shear stresses, pore-fluid pressure) and impact pressure on a rigid barrier are measured. A time-dependent creeping mode is observed for the impact process of slow-moving dense debris flows, which cannot be accurately estimated using current debris-flow load models. At the grain scale, this macroscopic creeping mode is a result of the feedback between granular dilatancy and pore-pressure response. This feedback can be further characterized by examining the timescales associated with pore-pressure generation and dissipation. The regulation of pore-pressure feedback on basal shear stress, impact load, and state of static deposit is revealed. Finally, a tentative phase diagram is proposed for dense and dilute debris-flow impact. The proposed framework complements the theory for debris-flow impact loads.