2. Framework of emergy evaluation for assessing urban vulnerability to flooding From a systems perspective, cause–effect relationships between exposure, sensitivity, and adaptive capacity for assessing vulnerability are linked through energy and material flows between natural and social systems (Huang, Chang, & Yeh, 2011).In this section, the concepts of exposure, sensitivity, and adaptive capacity are redefined from an ecological energetic viewpoint.Emergy evaluation and indices for vulnerability assessments are also proposed.
2.1. Emergy basis for vulnerability assessmentOn the basis of general system theory and the laws of thermo-dynamics, Odum (1983, 1996) developed the emergy concept toaccount for the different energy qualities inherent in energy andmaterial flows of complex ecological economic systems. Emergyis defined as “all the available energy that is used in the work ofmaking a product and expressed in units of one type of energy”(Odum, 1996). The emergy value of a flow of storage can be derivedby multiplying its energy content or mass by its transformity:
Emergy (sej)
=
energy (J)
×
solar transformity (sej/J) (1)Emergy indices can also be developed and calculated by aggregatingresource flows to synthesize system performance and to measuresustainability. In this research, emergy is incorporated to evalu-ate the contribution of energy and resource flows with a commonemergy unit for vulnerability assessment.The emergy values of the constituents of vulnerability wereinterpreted as follows (Chang & Huang, 2011; Huang, Chang, & Yeh,2011):Exposure is the total emergy of the extreme climate events actingupon the area.Sensitivity is the amount of stored emergy that is likely to beaffected by an extreme climate event.Adaptive capacity is the system’s ability to attract emergy inflowsto recover and to adjust from the impacts of hazards.Exposure and sensitivity can be combined into potential impact