Hydrogeological modelling of an urban river restoration

Description

The Bispeengen plan in Copenhagen proposes to replace a flyover motorway with a tunnel and, in doing so, to bring the Ladegaardsaa river — currently piped beneath the road — back to the surface. The NAIAD work concentrates on the river restoration, and on a question that a surface-level appraisal would miss: what the shallow groundwater does when a piped river becomes an open one.

The modelling chain works at three scales. The greater Copenhagen model, GCOP, simulates the unsaturated zone in one dimension, the saturated zone in three and overland flow in two, delivering hydraulic head boundary conditions to the street-scale model. The Street Level Urban Model, SLUM, resolves the urban area at the scale at which the river would be restored and drainage rerouted. The two together show where shallow groundwater would rise or fall, and where drainage to the river would have to be routed — close to the sea, directly to the sea rather than to a river branch.

The hydrological results then feed a spatial Bayesian Belief Network trained on observation and model data, which produces a probability distribution of the insurance value of the measure rather than a single figure.

Applications to decision making in the insurance sector

Groundwater flooding of basements, cellars and subsurface infrastructure is a real and frequently uninsured exposure in cities, and it is largely invisible to surface-hazard models. This modelling shows what an urban river restoration does to that exposure — including the cases in which it increases it — which is precisely the information a carrier needs before treating a blue-green measure as loss-reducing.

Added value of this modelling approach

It addresses the subsurface, which urban flood assessments routinely omit, and it does so at the scale of individual streets. The Bayesian framework then carries the uncertainty of the physical modelling through to the economic result instead of discarding it.

What decision can be improved with this modelling approach

Whether a river restoration should be designed to drain shallow groundwater as well as stormwater; where subsurface damage is likely to occur; and how the insurance value of a blue-green measure should be represented when it is submitted for appraisal.