Abstract
Abstract. Cities are increasingly vulnerable to floods generated by intense rainfall, because of urbanisation of flood-prone areas and ongoing urban densification. Accurate information of convective storm characteristics at high spatial and temporal resolution is a crucial input for urban hydrological models to be able to simulate fast runoff processes and enhance flood prediction in cities. In this paper, a detailed study of the sensitivity of urban hydrodynamic response to high resolution radar rainfall was conducted. Rainfall rates derived from X-band dual polarimetric weather radar were used as input into a detailed hydrodynamic sewer model for an urban catchment in the city of Rotterdam, the Netherlands. The aim was to characterise how the effect of space and time aggregation on rainfall structure affects hydrodynamic modelling of urban catchments, for resolutions ranging from 100 to 2000 m and from 1 to 10 min. Dimensionless parameters were derived to compare results between different storm conditions and to describe the effect of rainfall spatial resolution in relation to storm characteristics and hydrodynamic model properties: rainfall sampling number (rainfall resolution vs. storm size), catchment sampling number (rainfall resolution vs. catchment size), runoff and sewer sampling number (rainfall resolution vs. runoff and sewer model resolution respectively). Results show that for rainfall resolution lower than half the catchment size, rainfall volumes mean and standard deviations decrease as a result of smoothing of rainfall gradients. Moreover, deviations in maximum water depths, from 10 to 30% depending on the storm, occurred for rainfall resolution close to storm size, as a result of rainfall aggregation. Model results also showed that modelled runoff peaks are more sensitive to rainfall resolution than maximum in-sewer water depths as flow routing has a damping effect on in-sewer water level variations. Temporal resolution aggregation of rainfall inputs led to increase in de-correlation lengths and resulted in time shift in modelled flow peaks by several minutes. Sensitivity to temporal resolution of rainfall inputs was low compared to spatial resolution, for the storms analysed in this study.
Highlights
Rainfall is a key input to hydrological models and a crucial issue for hydrologists is to find the importance of the spatial structure of rainfall in relation to flood generation (Segond et al, 2007)
The sensitivity of an urban hydrodynamic model to spatial and temporal resolutions of weather radar data was investigated in this paper
Analyses are based on a densely populated urban catchment in Rotterdam, the Netherlands and four rainfall events that were derived from polarimetric X-band radar data
Summary
Rainfall is a key input to hydrological models and a crucial issue for hydrologists is to find the importance of the spatial structure of rainfall in relation to flood generation (Segond et al, 2007). Many studies conducted in large natural catchments have shown that spatial variability of rainfall is important in determining both timing and volume of rainfall transformed into runoff (Obled et al, 1994) and timing of simulated basin response and magnitude of the response peak (Dawdy and Bergman, 1969; Krajewski et al, 1991; Seliga et al, 1992). It has been suggested, with much less evidence, that this is true for small catchments with shorter response times, such as urban catchments (Blanchet et al, 1992; Obled et al, 1994). Bruni et al.: On the sensitivity of urban hydrodynamic modelling
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