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Flash floods have long been common in Asian cities, with recent increases in urbanization and extreme rainfall driving increasingly severe and frequent events. Floods in urban areas cause significant damage to infrastructure, communities and the environment. Numerical modelling of flood inundation offers detailed information necessary for managing flood risk in such contexts. This study presents a calibrated flood inundation model using referenced photos, an assessment of the influence of four extreme rainfall events on water depth and inundation area in the Hanoi central area. Four types of historical and extreme rainfall were input into the inundation model. The modeled results for a 2008 flood event with 9 referenced stations resulted in an R2 of 0.6 compared to observations. The water depth at the different locations was simulated under the four extreme rainfall types. The flood inundation under the Probable Maximum Precipitation presents the highest risk in terms of water depth and inundation area. These results provide insights into managing flood risk, designing flood prevention measures, and appropriately locating pump stations.

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Abstract:Higher resolution topographic information contained in the topographic index of TOPMODEL is lost when digital elevation models (DEMs) with a coarse grid resolution are used; thus, the topographic index is scale dependent, demonstrating identified model parameter values to be dependent on DEM resolution. This makes it difficult to use model parameter values identified through a different resolution of TOPMODEL. The inconsistency is the result of the difference between the scale at which the model parameters are identified and the scale at which the model is applied. To overcome this problem, scale laws that govern the relationship between the resolution of digital elevation data and geomorphometric parameters of the topographic index were analysed and a method to downscale the topographic index distribution developed to account for the difference in scales between model application and parameter identification. The method to downscale the topographic index is composed of two ideas: one involves introducing a resolution factor to account for the scale effect in upslope catchment area per unit contour length in the topographic index; the other utilizes a fractal method through steepest slope scaling to account for the scale effect on slopes. This method successfully derived a topographic index distribution of a fine-resolution DEM by using only a coarse-resolution DEM. The method has been applied successfully to the Kamishiiba catchment (210 km 2 ) in Japan and has demonstrated that the downscaled topographic index distribution derived using a 1000 m grid DEM is very similar to the topographic index distribution derived via fine-target-resolution DEMs. The method is then coupled with a TOPMODEL simulation to match the scales of model application and parameter identification. It is shown that the simulated runoff from the downscaled TOPMODEL applied at 1000 m resolution of the Kamishiiba catchment, with the same set of effective parameter values derived from 50 m resolution DEM, matched the simulated runoff in the 50 m DEM resolution TOPMODEL. It was also shown that TOPMODEL coupled with the downscaling method of the topographic index accurately simulated runoff for different rainfall events in the catchment without recalibration.

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