A conceptual model is set up for the description of individual spatial rain storms. The smallest building-block of a spatial rainfall structure is the individual rain cell, and can be assumed Gaussian-shaped. The rain cells manifest spatially in a clustered way within small mesoscale areas (the rain storms) and large mesoscale areas (the frontal bands in winter). This description is based on a detailed analysis of the observed cell cluster patterns by a dense network of rain gauges at the city of Antwerp. The statistical properties of the model parameters (rain storm direction, rain storm velocity, number of rain cells per unit rain storm area, cell peak intensities, cell spatial extent, etc.) were analysed based on a large number of rain storms. Probability distributions were derived for these parameters. Also a stochastic model was derived for the spatial distribution of rain cells within rain storm areas, and of rain storms within frontal rainstorm bands. Also the distribution of the dry weather flow periods was studied.

A stochastic spatial rainfall generator is developed for use at the small spatial scale of urban and small hydrographic catchments. The generator is based on a spatial rainfall model of the conceptual and hierarchical type. It describes the spatial rainfall field in a macroscopic physically-based way by distinguishing rainfall entities with different scales: rain cells, cell clusters, small and large mesoscale areas (or rain storms). For applications at small spatial scales, the individual rain cells need a detailed description. Data of a dense network of rain gauges at Antwerp, enclosing 5940 rain cells in 807 rain storms, are used to derive such description. For separation of the rain cells in the rainfall time series, an algorithm is developed based on the identification of increasing and decreasing rain cell flanks. The rain cells observed at different rain gauges are linked together by applying criteria for testing the similarity in rain cell properties. After separating and linking the rain cells and storms, the spatial rainfall model is calibrated to many storms by two methods (e.g. Kalman filter). The derived model structure and model parameter distributions apply to the stochastic generation of long-term time series of spatial rainfall. The model is tested by comparing intensity-duration-frequency relationships and temporal scaling properties of the generated and historical rainfall series.

In hydrological and hydrodynamic modelling of urban catchments, the spatial variability of rainfall is often neglected. This spatial variability encloses two aspects: (1) the spatial variability of the statistical properties of rainfall, and (2) the non-uniform spatial distribution of rainfall over the modelled catchments. In a research project for the Ministry of the Flemish Community (Belgium), the influence of this spatial rainfall variability on the results of modelling applications is studied. At the same time, most efficient methods to reduce this influence are determined.
The results of the research can be applied directly in Flanders. They consist of a combination of unified IDF-relationships, spatial correction factors (generally applicable formulas), a stochastic simulation model for spatial rainfall (software) and a methodology for improving the spatial correction factors in a case-specific way by performing simulations with the model.

Design storms for combined sewer system design are generally applied as a uniform input. However, the spatial variability of the rainfall can play an important role. The spatial variability of the rainfall over the catchment can be large at given time steps, and the dynamic influence of a storm moving over the catchment can result in a significant difference in peak water levels and peak discharges in the system.
When a storm is moving over an urban catchment in the same direction as the main flow direction in the combined sewer system, higher peak discharges and water levels often result. If there is a predominant wind direction during wet weather, this can cause a bias in the calculation results. This means that combined sewer systems with a mean flow direction equal to the predominant wind direction during wet weather will have a lower safety level than predicted with the uniform rainfall input. For this reason, a procedure is worked out to simulate moving design storms over a combined sewer system in different directions, and to combine the results statistically in order to calculate the return period of peak discharges or water levels.

Publications:

Moving design storms for combined sewer systems' (2002) at 9th International Conference on Urban Drainage, Portland, US.

The use of uniform rainfall in hydrological modelling applications, based on point rainfall data, can lead to large systematic errors in the model input volumes. Two effects are playing a role here: the spatial variation of the rainfall intensity within a storm is often large, and, secondly, the rain gauge station most often does not observe the peak intensity of the storm. By the use of areal correction factors to the rainfall model input, the systematic errors (bias) in the model input can be removed. These areal correction factors were derived based on simulations with the spatial rainfall generator and statistical analysis. The correction coefficients depend on the catchment area, the storm duration and the rainfall intensity. The results show that for small catchments and low rainfall intensities rainfall input volumes are systematically underestimated by the use of uniform rainfall input; the areal correction coefficient is larger than 1. For increasing catchment areas and higher rainfall intensities, the correction coefficients become lower than 1. Two types of correction coefficients have been derived: coefficients to correct the rainfall input volumes over a catchment obtained from historical point rainfall data, and coefficients to correct design storms.

Publications:

Vaes G., P. Willems & J. Berlamont (2005). Areal rainfall correction coefficients. Atmospheric Research, doi:10.1016/j.atmosres.2004.10.015

Radar rainfall has received growing attention in the urban drainage community since its first appearance on the Urban Storm Drainage Conference in 1984. Operational applications of radar rainfall in urban hydrology are however still very limited. The following aspects of the radar data have to be taken into account in the applications: the type of radar, the application requirements on the radar data, the achievable radar data quality, forecast possibilities and limits. Quantitative applications of weather radar to urban hydrology are becoming more mature with better understanding of limitations and achievable accuracy. Research in the US and Europe has led to operational applications of radar to site-specific forecasting of floods and management of sewer overflow during wet weather. The uncertainties and methods for obtaining accurate rainfall measurements at high resolution and with areal extent consistent with urban catchments is the main focus of the research. The radar data is used in a way complementary to rain gauge data to improve the rainfall input to hydrological simulation models. Radar images give detailed information on the spatial coverage of rain storms over the catchment under study, while rain gauges have a higher accuracy for the point rainfall registrations.
In cooperation with the water company Aquafin, the use of a Local Area Weather Radar for urban drainage appications is being investigated. One radar of this type has been installed in the city of Leuven since 2008, and first tests are being carried out.

Publications:

Einfalt, Th.,Arnbjerg-Nielsen, K., Golza, C.,Jensen, N., Quirmbachd, M., Vaes, G., Vieux, B.,(2004), 'Towards a roadmap for use of radar rainfall data in urban drainage', Journal of Hydrology, 299, 186-202

Goormans, T., Willems, P. & Jensen, N.E.(2008), 'Empirical assessment of possible X-band radar installation sites, based on on-site clutter-tests', Proc, 5th Eur. Conf. on Radar in Meteorology (ERAD 2008), Helsinki, Finland, 30 june- 4 july (CD-Rom proceedings: ISSN 978-951-697-676-4)

Goormans, T., Willems, P.(2008), 'Correcting rain gauge measurements for calibration of an X-band weather radar', 11th Int. Conf. on Urban Drainage, Edingburgh, Scotland, 31 august-5 sept 2008, 10p.