Subgrid modeling of storm surge inundation in a large coastal city considering building volumes

The applicability of current subgrid models, particularly drag force models (DFMs), for the simulation of large-scale storm surge inundation in highly urbanized coastal megacities remains unclear, particularly in terms of their performance across varying grid resolutions and their ability to capture complex inundation dynamics at the city scale. This paper proposes an enhanced subgrid model, iDFM-MD, for storm surge inundation over a coastal city, incorporating both building drag and volume effects using a wet fraction formulation based on the authors’ previous subgrid model, iDFM. The proposed model was applied to the numerical modeling of storm surge inundation targeting a large coastal city along Tokyo Bay in a pseudo-global warming scenario involving Typhoon Hagibis (2019). The obtained results were compared with a high-resolution structure-resolving model: iDFM-MD produced better estimates of the inundation starting time, depth, and extent, attributed to the consideration of building volumes. The model could reduce the mean absolute error in the water depth to 0.27 m and improve building-scale inundation classification. A sensitivity analysis of the inundation characteristics helped confirm that the model could maintain its accuracy at resolutions of up to 60 m. Our findings demonstrate the capability of iDFM-MD in providing efficient and accurate predictions of urban inundation under future climatic conditions.