Abstract

The impact of urbanisation on the hydrological cycle has been discussed for several decades. Urbanisation is frequently said to cause increasing flood peaks and volumes. While this statement is certainly true for volumes, it has not been generally proven concerning flood peaks. Unfortunately, the discussion is dominated by subjective opinions rather than by scientifically-based, objective evidence.To cope with increasing floods, politicians, the public and scientists argue that local end-of-pipe storm water management practices, rather than source control measures, can largely contribute to the reduction of flood peaks.

However, agreement has been reached on the effect of urbanisation, depending on the size of the river basin and related characteristics such as location of urbanisation, duration and intensity of precipitation and so forth. Small basins up to 10 km2 in area if developed completely have a degree of imperviousness that might reach 50%. The effective impervious area of large basins, such as the River Rhine or River Elbe, is comparatively small, reaching 2 to 3 %. These numbers are based on German land use statistics. In large river basins about 12% of the land surface is typically developed, with a maximum of 50% being impervious. However, only 50% are directly connected to urban drainage systems. In the opinion of the author, it is most important to consider the size of a basin when assessing (a) the effect of urbanisation, or (b) measures to compensate resulting negative effects e.g. measures to control the sources of storm water. It is well known that spatial scales such as basin size and temporal scales such as time steps are important issues in modeling environmental processes.

Although the scale problem in urban drainage is not new, traditional urban drainage technology did not consider it - it was mostly considered separately from basin wide hydrologic processes. Fast and effective drainage has been the major objective until recently. Today, integrated concepts of storm water management and treatment are slowly but surely replacing these old concepts. These integrated concepts may be described as "centralized", "semi-centralized" and "de-centralized". Processes at these different scales occur in an integrated river basin, and are sensitive to different intensities. For this and several other reasons, such as accuracy and computational efficiency, the river basin and its related hydrological and hydraulic processes must be decomposed carefully.

This chapter introduces the problem of spatial and temporal scales in urban drainage modeling, and recommends how suitable scales may be chosen. Of major importance in this discussion is the effect of urbanisation on floods, in particular the effect of source control measures on floods, and their modeling and integration in large river basin models at the meso scale. Only distributed, conceptual approaches with adequate physical background are considered.

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