## 2 Materials and Methods
### Methodology
#### 2.2.6 Scenario Modelling  
Potential solutions for the catchment were investigated, mainly focusing on the built-up area and increasing retention capacity. The scenarios cover different combinations of NbS, including green roofs, permeable paving and ponds (Table 1). The prevailing method for modelling design discharges for infrastructure design is to use one set of parameters, calculated from the calibration and validation process. In this case, the calibrated parameters repeatedly failed the validation process. As the focus of this study is changes in peak discharge and runoff, the decision was made to model each event individually with different parameters. Therefore, the model was calibrated to each rainfall event individually using the method outlined in Section 2.2.4. The use of different CN values for each event can be substantiated by the fact that the CN derived from observed rainfall-runoff data varies significantly between storm events, since the selected storm events vary greatly in terms of total rainfall amount, intensity and duration [32; 68; 69]. Significantly, [PERSON] et al. (2024) [32] showed that the CN of the studied catchment varied greatly between storm events, ranging from 63.6 to 97.4, with observed seasonal variability. This correlates with the CN parameters determined from the calibration in this study (Table 2), which show a similar range. This suggests that the catchment is reactive, meaning heavy rainfall events can alter the hydrology, which changes the runoff patterns. Modelling each event with different parameters allowed for accurate representations of the varied hydrological response and discharges for each event, thus achieving a better representation of the impact of each NbS scenario. For Scenarios 1-3, which involved green roofs and permeable surfaces, adjustments were made to the CN parameter and the impervious % to represent the change in land cover. Based on a literature review, a CN of 90 was selected for green roofs [70; 71] and 62 for permeable paving [24; 72]. The areas where land cover would change in each scenario were calculated, and the updated CNs for the new surfaces were incorporated into the sub-catchment CN calculation (Table 2). The implementation of green roofs utilised only a change in impervious %, as the CN of green roofs is the same as that used for the built-up area. A calculation of the reduction that each NbS results in in the CN and \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Scenario** & **NbS** & **Area (m2)** & **Description** \\ \hline \multirow{2}{*}{**Original**} & Calibrated & \multirow{2}{*}{-} & Model without NbS (i.e., baseline scenario, current situation) \\ \cline{3-4}  & Model & & \\ \hline \multirow{3}{*}{**Scenario 1**} & Permeable & Permeable alternative to traditional \\  & Streets & & impervious surfaces, which allows \\  & & & stormwater to infiltrate through the \\  & & & pavement into the ground below \\ \hline \multirow{3}{*}{**Scenario 2**} & Permeable & \multirow{3}{*}{1967} & There are parking areas (Figure 3) near the \\  & Parking & & hotel, where implementing permeable \\  & & parking can reduce discharge and increase \\  & & & infiltration rates \\ \hline \multirow{3}{*}{**Scenario 3**} & Green Roofs & 2185 & Implementing green roofs on flat roofs \\  & & & (Figure 3) \\ \hline \multirow{3}{*}{**Scenario 4**} & All NBS East & \multirow{3}{*}{5610} & Incorporating all NbS implemented in east \\  & Basin & & sub-catchment (i.e., Scenarios 1–3 \\ \hline \multirow{3}{*}{**Scenario 5**} & \multirow{3}{*}{Ponds} & P1: 250 & Three retention ponds to prevent the \\  & & P2: 212 & possibility of flooding during heavy \\ \cline{3-4}  & & P3: 165 & rainfall events from runoff (Figure 3) \\ \hline \multirow{3}{*}{**Scenario 6**} & All Basin NBS & \multirow{3}{*}{6237} & Implementation of all NbS from west and \\  & & & east sub-catchments \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Scenario description table. impervious % is then applied to the baseline parameters of each event.