HAZARDS: RISK MODELLING APPROACHES

Overview

Risk is what happens when a hazard meets something that can be damaged. The modelling approaches collected in this part of the guide describe how that meeting is represented for floods and for droughts across the NAIAD demonstration areas, and how the approaches were reconciled into a framework that can be applied consistently to a Nature Based Solution wherever it is proposed.

The demonstration areas do not share a hazard. Some are fluvial, some are pluvial, some are coastal or estuarine, and several face drought and water scarcity as their binding constraint rather than flooding. An approach developed for one of these settings cannot simply be transferred to another, so the work began by inventorying what each site already used, on what data, at what resolution, and with what known limitations — and then identifying the common structure underneath.

What it covers

The guidelines set out the risk chain that the project adopted as its common frame: hazard, exposure, vulnerability, and the resulting risk, with the effect of an intervention expressed as the change in that final quantity rather than as a change in any single link. They describe how hazard is characterized for rapid-onset events such as floods and for slow-onset events such as droughts, how exposure is built from asset inventories, and how vulnerability is represented through damage functions.

They also set out where the Nature Based Solution enters the chain. A solution changes the hazard itself by modifying the movement and storage of water; it changes exposure by altering what is located in the affected area over time; and it changes vulnerability by reducing the losses that a given event causes. Being explicit about which of the three is being claimed is the difference between an assessment and an assertion, and the guidelines require the claim to be located.

The modelling approaches are documented against the demonstration areas that used them, with the data sources, the spatial and temporal resolution and the validation status recorded for each, so that a reader can judge whether an approach fits their own case rather than adopting it because it exists.

How it is applied in NAIAD

The approaches were consolidated into the models used for each demonstration area, covering hydrological and hydraulic modelling of Nature Based Solutions and hybrid solutions, runoff hazard modelling, surface hydrology and water balance and allocation modelling, groundwater flow modelling where the resource is an aquifer, and catastrophe risk modelling for insured losses. Together these produce the hazard and risk inputs that the economic assessment and the insurance value calculation consume.