Risk Scenario
A risk scenario is similar to an event scenario, but instead of running for one single event FloodAdapt runs the flood model for every event in a probabilistic event set. This is a set (put together when a FloodAdapt system is set up) consisting of specifications for different (compound) events that can lead to flooding in the area and their occurrence frequencies. A paper describing how this event set was prepared for Charleston, South Carolina can be found here.
The hazard calculation and impact & risk calculation frameworks for a risk scenario are presented in the sections below.
Hazard calculation
Figure 1 zooms in on the hazard calculation portion of the FloodAdapt workflow for a risk scenario. Referring to the figure can support the description of the calculation.
For a risk scenario the hazard calculation involves both simulating the flooding for each event in the event set and the derivation of probabilistic return period maps. FloodAdapt modifies the SFINCS model based on adaptation options and calculates the flooding for each event in the event set in the same way it does for an event scenario. This is indicated in Figure 1 by a horizontal line, above which the event scenario and risk scenario are the same, except that the flood model is called multiple times to calculate flooding for each event in the event set. This workflow is described in the event scenario documentation and not repeated here. Below the horizontal line in Figure 1, the workflow for the risk scenario hazard is unique. Once the simulation for each event in the event set has been completed, the entire set of flood maps is passed into a probabilistic calculator. This is a routine that uses the event frequencies and simulated flood maps to create a water level-frequency curve for each grid cell in the flood model. It then derives from this curve the flood depth for the return periods specified for the site at system setup. From this outPut it creates gridded return period flood maps.
Impact and risk calculation
Figure 2 zooms in on the impact and risk calculation portion of the FloodAdapt workflow for a risk scenario. Referring to the figure can support the description of the calculation.
The automatic updating of the Delft-FIAT model based on user-specified adaptation options and projections is identical to the handling in the event scenario. This workflow description is not repeated here.
For a risk scenario, FloodAdapt imports into Delft-FIAT the set of return period flood maps (calculated in the hazard calculation framework described above) and their associated return periods. Delft-FIAT has an automatic risk module built in which calculates and outputs the direct economic damages to the assets in the exposure data for each input return period flood map. It uses this output to generate a damage-frequency curve (note that the exceedance frequency is the reciprocal of the return period). The expected annual damages (risk) are then calculated as the area under the damage-frequency curve (see Figure 3). The set of return periods will be finite, so Delft-FIAT makes approximations for return periods beyond what is calculated. For return periods higher than the highest calculated, it assumes economic damages equal to the highest calculated. For return periods lower than the lowest calculated, it assumes economic damages of zero. In FloodAdapt, users can choose the return periods for which damages will be calculated, to minimize these approxmiations. For example, users can take an upper limit for the return period of 500 or 1000 years and a lower limit of 1 year.
After the return period damages and risk have been calculated by Delft-FIAT, FloodAdapt sends the output through a suite of postprocessing scripts to derive aggregated damages, spatial maps, risk metrics, and risk-related infographics. Additionally, FloodAdapt calls an Equity routine that calculates equity weights and applies them to risk estimates at an aggregated level for optional viewing in the user interface.