Analysis of the factors that influence dry weather and flood flows in an urbanized drainage basin

This paper presents an analysis of the factors involved in the response of an urbanized drainage basin to a rainfall event. It covers 43 episodes occurring on the Merly basin in Bordeaux, which has a surface area of 2,23 ha. The network is a combined system. An examination of flow records in the absence of precipitation reveals the existence of a permanent low level of flow during dry weather whose rate depends on the level of ground moisture. The transfer function of the drainage basin has been computed using a numerical technique used in rural hydrology the (DPFT technique). This has given a hydrograph which although uninterrupted nevertheless depends on the characteristics of rainfall. The intensity of rainfall and the rate of transfer are related: on the other hand the amount of rapid underground drainage would seem to be negligible. In the case of moderate and heavy rainfall, the coefficient of flow is independent of the characteristics of the rain, being close to the imperviousness coefficient. The same mechanism of run off transfer would therefore seem to be involved. For lighter rainfall, flow is above all determined by the initial surface state of the drainage basin.