Abstract
Abstract. On 26 August 2010 the eastern part of The Netherlands and the bordering part of Germany were struck by a series of rainfall events lasting for more than a day. Over an area of 740 km2 more than 120 mm of rainfall were observed in 24 h. This extreme event resulted in local flooding of city centres, highways and agricultural fields, and considerable financial loss. In this paper we report on the unprecedented flash flood triggered by this exceptionally heavy rainfall event in the 6.5 km2 Hupsel Brook catchment, which has been the experimental watershed employed by Wageningen University since the 1960s. This study aims to improve our understanding of the dynamics of such lowland flash floods. We present a detailed hydrometeorological analysis of this extreme event, focusing on its synoptic meteorological characteristics, its space-time rainfall dynamics as observed with rain gauges, weather radar and a microwave link, as well as the measured soil moisture, groundwater and discharge response of the catchment. At the Hupsel Brook catchment 160 mm of rainfall was observed in 24 h, corresponding to an estimated return period of well over 1000 years. As a result, discharge at the catchment outlet increased from 4.4 × 10−3 to nearly 5 m3 s−1. Within 7 h discharge rose from 5 × 10−2 to 4.5 m3 s−1. The catchment response can be divided into four phases: (1) soil moisture reservoir filling, (2) groundwater response, (3) surface depression filling and surface runoff and (4) backwater feedback. The first 35 mm of rainfall were stored in the soil without a significant increase in discharge. Relatively dry initial conditions (in comparison to those for past discharge extremes) prevented an even faster and more extreme hydrological response.
Highlights
Flash floods, defined here as extreme floods generated by intense precipitation over rapidly responding catchments, have recently drawn increased attention, both from the scientific community and from the media
In this paper we report on the flash flood triggered by an exceptionally heavy rainfall event on 26 August 2010 that occurred over the 6.5 km2 Hupsel Brook catchment
We present a detailed analysis of the synoptic meteorological situation leading to the event (Sect. 3.1), the rainfall accumulations as measured by rain gauges, weather radar, and a microwave link (Sects. 3.2 and 3.3) and the extreme value statistics of the rainfall accumulation (Sect. 3.4)
Summary
Flash floods, defined here as extreme floods generated by intense precipitation over rapidly responding catchments, have recently drawn increased attention, both from the scientific community and from the media Their often devastating consequences, both in terms of material damage and loss of life, have triggered a number of European research projects (e.g. FLOODsite, HYDRATE, and IMPRINTS) to study the meteorological causes and hydrological effects of such events. In case of extreme rainfall, rapid runoff generation due to overland flow can trigger flash floods in lowland catchments (Van der Velde et al, 2010) Lowland areas, such as the densely populated delta region of The Netherlands, are typically associated with large-scale flooding of the Rhine and Meuse.
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