Runoff hazard modelling

Description

In Mediterranean catchments a large share of flood damage is caused not by a river leaving its banks but by rain falling on a surface that cannot absorb it. Runoff hazard modelling addresses that case: a two-dimensional rainfall-runoff production and transfer model, spatialised and based on hourly rainfall, which routes surface water across the terrain and identifies where it accumulates.

In the Lez catchment the runoff model is used to test the effect of urban Nature Based Solutions — retention, infiltration and de-sealing — by running the same rainfall events with and without the measures and comparing the volumes and the depths that reach the city. The model is adapted to the catchment scale by refining the digital terrain model slopes so that water spreads realistically along short watercourses, and its results are validated against floodmarks and observed extent from past events.

Runoff hazard is also the input to the damage modelling performed with the same instrument at national scale, so the physical result and the loss estimate are produced in one chain.

Applications to decision making in the insurance sector

Runoff is the hazard for which insurance penetration is lowest relative to the damage caused, because it falls outside the mapped flood zones that underwriting and public compensation schemes generally rely on. A model that identifies where runoff accumulates, and how much of it a measure removes, allows that exposure to be assessed and priced — and identifies the cases in which an urban NBS reduces a loss that would otherwise be borne by the property owner or by the state.

Added value of this modelling approach

It explains a category of loss that conventional flood mapping systematically omits. In several of the NAIAD catchments a substantial fraction of the recorded damage came from runoff rather than from overflow, and the model is what makes that visible.

What decision can be improved with this modelling approach

Where urban measures should be placed to reduce runoff damage; how much a de-sealing or retention programme reduces the hazard; and whether a runoff exposure is significant enough to be reflected in the terms of cover.