## 3 System Development: Methodologies and Analysis
### Flood Hazard and Impacts Analysis  
To facilitate the rapid representation of inundation hazard and exposure for a given forecast flood magnitude, it was necessary to compile a library of precomputed scenarios corresponding to the given forecast flood event. This was particularly crucial due to the flashy nature of the Vaisigano catchment and current computational limitations in simulating real-time, on-the-fly, hydrodynamic simulations of flood hazard intensity. To this end, the multi-hazard model framework supported by RiskScape software [36] was configured to analyse building and road exposure to flooding hazards. The RiskScape software engine combines spatial layers representing hazards and exposures (i.e., elements at risk) with vulnerability functions in a model workflow to quantify the impacts from hazard events. Here, we used flood hazard layers represented by available TuFlow 2D flood inundation modelled depth and velocity hazard intensities (2 m grid resolution) for the 2-, 5-, 20-, 100- and 1000-year return period flood scenarios [22]. These were intersected with exposure layers representing building polygon and road polyline features to calculate metrics on the exposure and threat to safety at building and road locations (Figure 5). The hydraulic model was configured by [PERSON] et al. (2019) [22] using a rectangular grid domain (5.3 \(\times\) 7.5 km), with base topography and nearshore bathymetry derived using the 5 m LiDAR digital elevation model (DEM) produced in [37]. DEM modifications enabled hydraulic features such as river channels and infrastructure (e.g., road crests, bridges, culverts) to be represented in the model outputs. Variable roughness was applied to represent different land uses and ranged from [PERSON]'s coefficients of 0.02 (roads, car parks) to 0.3 (buildings). The model was calibrated against the 2012 and 2018 flood events, with the results showing good modelled-to-observed water levels within a desirable tolerance of 0.25 m [22]. The flood hazard layers are representative of existing mitigation works in the Vaisigano catchment (e.g., levee segments) as well as the rainfall and river stage-flow relationships described in Section 3.2 and 3.3. In addition, each hazard layer represents the estimated flood hazard intensities coinciding with storm/high tide to provide conservative indications of expected impacts to a forecast flood event. Digital building outlines used in this analysis were obtained from the 2009 Pacific Catastrophe Risk Assessment and Financing Initiative (PACRAFI) database available for Apia [38]. This polygon dataset was manually updated in QGIS software using 2020 Google satellite base imagery to include outlines of new buildings constructed since 2009, and exposed to the 1000-year maximum available scenario. Road polylines were obtained from the Open Street Map (OSM) database [39], and split into 10 m segments. These provided the input exposure layers for RiskScape to calculate flood exposure metrics (building count and road length) and human threat to safety at building and road locations (Figure 5). Threat to safety functions represent the human stability response to maximum flood depth-velocity (DV) at building locations [40]. Here, ordinal categories (e.g., none, low, medium high, extreme), describe the relative threat to a person's safety at ground-level for each building location and 10-m road segment. Similar categories determine the safety of persons located in a vehicle (e.g., sedan, truck, fire-engine) at the time of maximum flood DV on roads [41]. Human threat to safety for each flood event was reported at feature- and village-scale web-supported GIS files for ingestion and display as decision-ready maps and tables on the FDSS portal. Figure 5: Conceptual representation of the impact model workflow in RiskScape.