Emergy-Based Assessment of Blue Carbon and Sustainability in Coastal Engineering–Wetland Systems

A comprehensive emergy-based evaluation framework was developed for coastal engineering–wetland coupled systems to quantitatively assess the carbon efficiency and sustainability of three representative engineering models: traditional hard engineering (Scenario A), ecological restoration engineering (Scenario B), and Nature-based Solutions (NbS) engineering (Scenario C). Through field investigations, engineering records, and remote sensing interpretation, key indicators including emergy-based carbon efficiency, Emergy Sustainability Index (ESI), spatial coupling degree, and path dependence coefficients were calculated, combined with system dynamics modeling. The results showed that the NbS model achieved the highest emergy-based carbon efficiency (0.547), which was 2.0 times that of Scenario B (0.271) and 260 times that of Scenario A (0.0021), with ESI values of 2.79, 0.814, and 0.084, respectively. A phased nonlinear relationship was identified between intervention intensity and system benefits, with a synergy–trade-off transition threshold of γ1 = 0.58 and a high-input dependency trap threshold of γ2 = 0.78. Spatial coupling degree was identified as the most critical mediating variable, with optimal configuration parameters of spatial coupling degree 0.68–0.75, engineering–wetland distance 12–18 m, and hydrological connectivity 0.65–0.75. Path dependence analysis revealed that the initial renewable emergy proportion significantly influenced later-stage ESI (α1 = 1.86, p = 0.004), with trap-type sample points showing 5–18% declines. System dynamics modeling demonstrated that the moderate-input scenario achieved optimal long-term performance, surpassing the high-input scenario in carbon sequestration after year 20, and under the RCP8.5 climate scenario, the ecology-priority restoration strategy maintained a carbon pool retention rate of 85%. Based on these findings, a three-level hierarchical retrofitting strategy for traditional embankments and a full lifecycle emergy management system were proposed, providing a quantitative scientific basis for engineering model selection, intervention intensity control, spatial configuration optimization, and long-term planning in coastal blue carbon restoration projects.