## 3 Results and Discussion
### Allocating Space for Green Infrastructure Development  
Figure 7 compares the spatial allocation of GI at the sub-catchment level across four optimal GGI designs (Designs A, B, C, and D). Each design integrates a unique combination of location, scale, and types of grey and green infrastructure, illustrating the effectiveness of GI integration in reducing the scale of GREI through peak runoff mitigation. Figure 7 visualizes the spatial configuration of GREI and GI elements for the four design types. GI types are distinguished by color and size. Pipe thicknesses reflect diameter, and manhole size reflects depth, enabling the visual comparison of system complexity and investment scale. This integration results in a more cost-effective GGI system compared to GREI-only designs with equivalent _DDL_ (Figure 3). Notably, average pipe diameters and manhole depths in GGI designs decrease by 0.2-0.3 m compared to GREI-only systems (Table 3). Among the designs, Design D achieves the smallest main trunk pipe diameter of 1 m, underscoring its cost efficiency.  
Optimization-driven solutions (Designs A, B, and C) predominantly allocate space to PPs over BCs (Figure 7a-c), largely due to the lower cost per unit area of PPs (Table 3). This strategic allocation enhances hydraulic and hydrologic resilience by effectively pairing appropriately scaled GREI with cost-efficient PPs. In contrast, Design D incorporates a larger proportion of BCs while maintaining the lowest total LCC (Figure 7d). Design D allocates 2.56 hectares to BCs, approximately 48% more than the other optimized designs (Table 3). In scenarios where GI implementation is capped at 10% of each sub-catchment, BCs outperform PPs in mitigating surface runoff, making them particularly advantageous in densely urbanized areas. Designs A, B, and C, with their emphasis on PPs, are particularly suited for high-density urban areas where space constraints and cost efficiency are critical  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{3}{*}{\begin{tabular}{c} **Optimal** \\ **GGI** \\ **Design** \\ \end{tabular} } & \multirow{3}{*}{\begin{tabular}{c} **DDL** \\ **(\%)** \\ \end{tabular} } & \multirow{3}{*}{\begin{tabular}{c} **GREI Design Parameters** \\ **(\%)** \\ \end{tabular} } & \multirow{3}{*}{\begin{tabular}{c} **GIG Design Parameters** \\ **(\%)** \\ \end{tabular} } & \multirow{3}{*}{\
\begin{tabular}{c} **LCC of GREI** \\ **(USD M)** \\ \end{tabular} } & \multirow{3}{*}{
\begin{tabular}{c} **LCC of GI** \\ **(USD M)** \\ \end{tabular} } \\  &considerations. On the other hand, Design D minimizes LCC, favoring decentralized systems with a higher proportion of BCs. While this design requires more land, it is optimal for newly developed or low-density areas where land availability is not a constraint and ecological co-benefits are valued.  
These findings reinforce the value of a balanced urban stormwater management strategy. By integrating GI to complement GREI, GGI systems demonstrate superior performance in both cost efficiency and resilience. The results align with existing literature, underscoring the indispensable role of integrated GGI in enhancing flood resilience and presenting GGI as a viable, economical solution for urban stormwater management [41; 50].