CITY

What a city setting means

In a city the hazard is generated by the city itself. Impermeable surfaces convert rainfall into runoff almost immediately, drainage networks have fixed capacities, and the areas that flood are determined by the interaction between the two rather than by the river. The interventions available are correspondingly different: sustainable drainage, green roofs, permeable paving, daylighting, parks designed to hold water, and the reconnection of the urban fabric to the water body it was built beside.

The assessment problem is one of density and of multiple objectives. The area is small, so the value at risk per hectare is high and the damage functions have to represent buildings, contents, business interruption and transport together. The space is contested, so a measure that reduces flood risk but removes parking, or that stores water but creates a maintenance obligation for a department that has no budget for it, will not be built.

How it is assessed

Hydrological and hydraulic modelling at city scale has to represent the drainage network as well as the watercourse, and to run at the resolution at which the interventions are actually sized. The damage assessment is correspondingly detailed, with the asset inventory built at building level and the damage functions distinguishing residential, commercial, industrial and infrastructure assets.

The co-benefits are usually decisive in a city. Urban heat moderation, air quality, amenity, biodiversity and the value of the space itself are all delivered by the same interventions, and they accrue to a population that is concentrated and politically present. The co-benefits valuation material provides the methods, and the multi-criteria toolkit the comparison when the objectives conflict — which in a city they usually do.

Engagement matters more here than in a rural catchment, because the people affected are numerous, organised and immediately aware of any change to the space they use. The stakeholder engagement protocol and the participatory modelling methods are the instruments, and the acceptability work identifies which attributes of a proposal determine whether it survives consultation.

Where it is used at NAIAD

Three of the demonstration areas are cities: Łódź, Copenhagen and Rotterdam. They cover a dense inland city with combined sewer constraints, a coastal capital with cloudburst exposure, and a delta city with tidal and fluvial risk together, which is the range over which the urban methods were developed and tested.