## 3 Results and Discussion  
### Trade-Offs Between DDL and LCC in a GRE1 Context  
The trade-offs between centralized (\(0\leq DDL<50\%\)) and decentralized (\(50\%<DDL\leq 100\%\)) GREI systems arise primarily from variations in discharge outlet configurations, pipe specifications, and fluid flow distribution within the network. These factors directly influence the capital expenditure associated with construction. Construction costs encompass excavation for manholes, trenching between nodes, pipe installation, backfilling, and leveling, all of which vary with the \(DDL\). To streamline material costing and accounting, a uniform \"mean\" pipe diameter was adopted across all configurations. Manhole depth, a critical factor in GREI design, significantly impacts overall construction costs. Manhole depth determines pipe-laying complexity and flow capacity, with deeper manholes requiring more extensive excavation, structural support, and associated costs. Flow capacity is calculated as the product of pipe flow velocity and cross-sectional area (assuming full-pipe flow), and manhole depth directly reflects the excavation effort and cost. As such, manhole depth serves as a reliable indicator of construction costs. Table 1 presents data on pipe diameters, manhole depths, and their associated material and construction costs. The optimization of GREI networks across various \(DDL\) levels focused on minimizing LCC by reducing pipe diameters and excavation depths while maintaining flow rate requirements. The optimization aimed for the smallest feasible pipe diameter, the shallowest possible manholes, and minimal longitudinal gradients to achieve cost-efficient designs. For practical classification, threshold \(DDL\) levels of 40% and 60% were established to distinguish primarily centralized (\(DDL\leq 40\%\)) from significantly decentralized (\(DDL\geq 60\%\)) drainage networks. Table 1 highlights a consistent decrease in LCC with increasing \(DDL\) levels. Decentralized networks (\(DDL\geq 60\%\)) were characterized by smaller \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \multirow{2}{*}{\begin{tabular}{c} **DDL** \\ **(\%)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Max. Pipe** \\ **Diameter** \\ **(m)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Max.** \\ **Mean Pipe** \\ **Diameter (m)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Mean** \\ **Manhole** \\ **Depth (m)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **LCC of** \\ **Pipes** \\ **Depth (m)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **LCC of** \\ **UnSD M)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **LCC of** \\ **Monholes** \\ **(USD M)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Total LCC** \\ **(USD M)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **DDL** \\ **(\%)** \\ \end{tabular} } \\ \hline
0 & 2.0 & 0.9 & 7.1 & 2.3 & 91.7 & 5.0 & 96.7 & 0 \\
20 & 2.0 & 0.9 & 6.6 & 2.4 & 81.9 & 4.9 & 86.8 & 20 \\
40 & 2.0 & 0.9 & 6.1 & 2.3 & 80.1 & 4.8 & 85.0 & 40 \\
60 & 1.5 & 0.9 & 6.0 & 2.3 & 76.2 & 4.5 & 80.7 & 60 \\
80 & 1.5 & 0.9 & 5.1 & 2.2 & 70.9 & 4.2 & 75.2 & 80 \\
100 & 1.5 & 0.9 & 5.8 & 2.4 & 70.2 & 4.5 & 74.7 & 100 \\ \hline \end{tabular} Note: The mean manhole depth serves as an indicator of construction costs.  
\end{table} 
Table 1: Mean pipe diameter, mean manhole depth, and associated unit material and construction costs of the pipe network for optimized GREI systems across various degrees of decentralization (\(DDL\)). pipe diameters and shallower manholes, aligning with findings by [PERSON] et al. [44] and corroborating recent research by [PERSON] et al. [45].