Multiwavelength optical packet and circuit integrated (OPCI) networks featured by the agile wavelength resource adjustment have been developed. With quick adjustment of wavelength resource, e.g., in emergency cases like disasters, the capacity of optical packet switching (OPS) portion of OPCI network where is closed to the disaster area can be extended quickly first so as to accommodate the significantly increased packet traffic. Here, we are motivated to investigate the efficient way for taking advantage of the emergently extended wavelength resource (for OPS) in the most congested part of the OPCI core network (i.e., outside of but close to the disaster area). In this paper we propose a waveband (WB) assignment scheme for multiwavelength-based OPS network which mixes the dedicated WBs and shared WBs assignment to isolate the heavy traffic flows as many as possible, and minimize the packet congestion efficiently. Simulation results reveal that our proposal can significantly reduce the packet loss, hence enhance the robustness of OPCI networks in emergency situations.