Hydrological and hydraulic modelling of NBSs and/or hybrid solutions to identify their impact on flood extent

Description

To know whether a Nature Based Solution reduces flooding, the flood has to be simulated with and without it, and the resulting extents compared. This is the modelling pair that does that: a hydrological model that converts rainfall into runoff under the changed land cover and soil conditions, and a hydraulic model that routes that runoff through the channel and floodplain and produces the inundation extent.

The two are used together because a measure rarely acts on only one of them. Retention areas and re-meandering change the routing and the channel geometry; re-vegetation and changes in soil management change the runoff generation. Modelling only one side of the pair produces a result that is internally consistent and physically incomplete.

The approach was applied in the NAIAD demonstration catchments — in the Glinščica, where the measures under consideration were retention areas, re-meandering and riparian re-vegetation, and in the urban demonstrations, where the question was instead the effect of distributed interventions on surface runoff and on the load reaching the drainage network.

Applications to decision making in the insurance sector

A change in flood extent is the most direct physical statement of what a measure does to insured exposure: it says which properties stop being flooded. That is the evidence a carrier needs, and it is best produced by exactly this comparison rather than by an assumed percentage reduction.

Added value of this modelling approach

It produces results that can be read by non-modellers. A pair of inundation maps with and without the measure communicates the effect of an intervention to a landowner, a council or an underwriter in a way that a table of discharges does not.

What decision can be improved with this modelling approach

Which measures actually reduce the flooded area and by how much; how large a retention volume has to be to make a difference at the location where the damage occurs; and whether a proposed measure would transfer flooding downstream rather than remove it.