Modelling flood water ingress in buildings: towards a simplified, orifice-based hydraulic model

Due to climate change, previously safe buildings will face flood risk, while for others risk will grow. The trend can be inverted by retrofitting buildings to achieve low losses (e.g., economic loss, downtime). Retrofit should focus on property flood resilience (PFR) measures to restricting water entry (e.g., flood skirts) and/or reducing its impact (e.g., water-resistant plasters). Currently, PFRs are selected with prescriptive checklists, rather than based on an explicit model of building water ingress (only possible with research-oriented computational fluid dynamics). Consequently, there is no guarantee on the effectiveness of the selected PFRs, nor the trade-off between their cost and reduced future losses. This work shows the preliminary implementation of a simplified water ingress model overcoming this gap.

For a selected hydrograph (i.e., time-variant depth and velocity of the exterior water), water ingress through a building envelope is modelled with a 1D dynamic flow model, using a quasi-steady, fixed-step, explicit Euler scheme. Each ingress pathway is treated as an orifice-like opening, with flow regulated by both hydrostatic water head difference and a velocity-dependent correction to account for drag effects. Calibration of the opening areas and discharge coefficients is based on available experimental data. PFR measures are explicitly considered: for example, a waterproofing membrane renders inactive all orifices below a certain height, while causing a sudden influx if a calibrated pressure strength is exceeded. After aggregating the flows, the interior water height is calculated separately for the building and the basement using mass conservation.

The model is illustrated for an archetype consistent with a ~1980s terraced masonry building in the United Kingdom. Its materials, plan dimensions, height, and water entry points are characterised according to relevant statistics. Inventories of finishes and contents are derived using public commercial listings (e.g., Zoopla). Apart from the as-built configuration, three retrofit solutions are defined considering combinations of PFR measures (e.g., waterproofing membranes, self-closing airbricks, flood doors/windows, non-return valves, raising power sockets and contents). The flood hazard profile is characterised consistently with an ideal site exceeding 1% annual flood probability. Using several hydrographs, the probability distribution of peak interior water depth is computed. The results are used as inputs of an analytical, component-based flood vulnerability assessment. Expected annual economic losses are finally calculated and compared.

The preliminary results show that this approach is simple enough for the early design phase yet accurate enough to allow identifying the marginal benefit/cost of PFR measures. However, benchmarking against refined computational fluid dynamics models is identified as a required step to fully validate the approach and generalise its results.