## 3 Data and Methodology  
### Creating a Sub-Basin Database  
This study uses GIS and NYC's digital elevation model (DEM) (1 m resolution) to create 889 sub-basins for NYC. The study conducted by [PERSON] et al. showed that increasing the number of sub-basins in an urbanized area can improve the accuracy of flood inundation [32]. This study also adopted this method and completed the division of sub-basins through GIS. The procedure involves using the flow direction and basin tools to create a grid depicting all basins in NYC. Reclassification and merge tools were then used to merge all basins less than ten thousand square meters with other basins. For a sub-basin with an area exceeding ten million square meters, this study divided the basin into several sub-basins depending on the land cover and roads to make its area smaller. For this analysis, the sub-basins were identified based on their land cover characteristics. Generally, for rainfall-runoff analysis the study area needs to be divided into several hydrological basins that contribute to the surface runoff during rainfall events. Due to the limitation in data resolution and computational capacity, the basins are quite often aggregated into a large watershed that does not reflect the microscale variation surrounding complex structures such as WWTPs. Therefore, it was necessary to use a high-resolution land use and building footprint dataset to clearly subdivide individual basins into smaller sub-basins that represents proper surface runoff characteristics for flood damage analysis. When the area of a particular sub-basin is too large, the sub-basin can be divided into smaller parts according to the road. In addition, it can also be divided according to the land cover of the sub-basin. For example, when dividing the Manhattan area into sub-basins, after basin analysis, several sub-basins with very large areas appeared in the center of Manhattan. It was found that Central Park is in it, and Central Park is almost entirely covered by green space, so Central Park was separated into one sub-basin. Figure 1 shows the whole process of database processing using GIS. In the end, NYC was divided into 889 sub-basins. The final map of the 889 sub-basins in NYC is shown in Figure 2. There are 131 sub-basins in Manhattan, 234 sub-basins in Brooklyn, 184 sub-basins in the Bronx,207 sub-basins in Queens, and 133 sub-basins in Staten Island. Based on the sub-basins, the SLOSH model, the runoff model, and the shapefile of flood depth at different heights of sea level rise provided by NOAA were used to build the database. We utilized the high-resolution (3 ft) land cover dataset which is subclassified into seven different categories: tree canopy, grass/shrub, bare earth, water, buildings, roads, and other paved surfaces. The dataset has an overall accuracy of 96% for NYC. The land cover database was also merged with the fine-scale building footprint data to create high-resolution sub-basins for the WWTPs. We used the land cover database to subdivide the study area into smaller sub-basins, as described in Figure 1. Associated curve number values and surface roughness characteristics were also assumed based on the land use/land cover dataset. The level of damage for different land cover types varies substantially based on the surface roughness criteria. The urban land cover types were classified as most susceptible to flooding due to a low friction factor and greater surface runoff during rainfall events. Additionally, the 3 ft (0.91 m) resolution raster image of NYC and the shapefile of footprint outlines of buildings in NYC are also parts of the database preparation. From these two files, digital elevation (DEM) files of sub-basins without buildings can be created. For the DEMs of sub-basins without buildings, starting from the lowest elevation of each sub-basin \(L_{0}\), with an interval \(\Delta L\) of 0.1 m, \(L_{0}+j\cdot\Delta L\), is regarded as the flood level \(L_{j}\). The formula for calculating the flood volume for each flood level is given in Equation (1).